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2020,44(3):455-471, DOI: 10.3878/j.issn.1006-9895.1906.19102
Abstract:Himawari-8 is a new generation of geostationary meteorological satellites launched by the Japan Meteorological Administration. The advanced Himawari imager (AHI) sensor carried by Himawari-8 can achieve a high temporal resolution observation for 10 min/time. Level-2 aerosol optical depth (AOD) dataset from Himawari-8 satellite was compared with AERONET (AErosol RObotic NETwork) AOD at 70 sites from September 2015 to December 2017. The results show that the precision of AOD products retrieved by Himawari-8 satellite has a large spatial difference. Among them, 48 sites feature a good correlation between Himawari-8 AOD and AERONET AOD (R>0.5). The Himawari-8 AOD clearly underestimates the ground-based AOD at 22 sites. At some sites, including American_Samoa, Bandung, Birdsville, Bukit_Kototabang, Canberra, Fowlers_Gap, Jabiru, and QOMS_CAS, ground-based AODs are small and Himawari-8 satellite-retrieved AOD is large. Analysis of the absolute error (the difference between Himawari-8 AOD and AERONET AOD) shows that a good correlation exists between absolute error and AERONET AOD when the Himawari-8 AOD underestimates ground-based observations. In areas where ground-based observations are small and satellite inversion data are large, a good linear relationship exists between absolute error and Himawari-8 AOD. This provides useful basic information for the improvement and perfection of the Himawari-8 AOD inversion algorithm.
Effect of Potential Vorticity on the Formation, Development, and Eastward Movement of a Tibetan Plateau Vortex and Its Influence on Downstream Precipitation
2020,44(3):472-486, DOI: 10.3878/j.issn.1006-9895.1904.18275
Abstract:During the period of June 28 to July 1, 2016, a Tibetan Plateau (TP) vortex was generated, which developed and moved eastward to the subtropical region of China, resulting in precipitation in the lower reaches of the Yangtze River. In this study, we used the potential vorticity (PV) to understand this process based on data from the second Modern-Era Retrospective analysis for Research and Applications (MERRA-2) and precipitation data from the Tropical Rainfall Measurement Mission (TRMM). The results indicate that surface heating over the TP causes obvious diurnal variation, changing from a heat source in the daytime to a source of cold at night and directly influencing the vertical gradient of diabatic heating. Negative PV is generated near the surface in the daytime and positive PV is generated at night, which reflects a prominent diurnal cycle. When the nighttime generation of positive PV becomes very strong and cannot be compensated for by the daytime generation of negative PV, a low-vortex forms over the TP. By the time this low-vortex system moves to the eastern TP, diabatic heating associated with strong precipitation reinforces this vortex. As the low-vortex system continues to propagate eastward, the PV advection increases with height and serves as a large circulation background over the middle and lower reaches of the Yangtze River, which favors the development of rising air and results in the occurrence of heavy precipitation.
Characteristics of Stratiform Cloud over the East of Tibetan Plateau Simulated by Two Versions of the FGOALS2 Climate System Model
2020,44(3):487-502, DOI: 10.3878/j.issn.1006-9895.1903.18249
Abstract:Eastern China, on the east of Tibetan Plateau (22°N–32°N，102°E–118°E), is covered by stratiform clouds in winter time. This cloud type is unique to like latitudes of the global subtropical continent. A reasonable simulation of these clouds over the leeside of the Tibet Plateau remains a challenge for the current state-of-the art climate models. In this study, we compared the Cloud radiation forcing and other cloud properties and their radiative forcing at the top of the atmosphere from two LASG/IAP climate system models, Atmospheric Model Intercomparison Project (AMIP) of Flexible Global Ocean–Atmosphere–Land System-s2 and g2 (FGOALS-s2 and FGOALS-g2), and observations from the International Satellite Cloud Climatology Project (ISCCP). It was found that both models underestimated the strength of shortwave cloud forcing, cloud water path, and cloud fraction over Eastern China. FGOALS-s2 can reproduce the domination of low stratiform clouds over Eastern China, while FGOALS-g2 overestimated the proportion of high clouds. Such bias in the simulation of stratiform clouds is related to weak lower tropospheric stability and insufficient low-level moisture. In addition, the overestimated average cloud top height in FGOALS-g2 was explained as an unrealistic updraft, which produced stronger vertical moisture transports over the east of Tibetan Plateau.
Interannual Anomalies of Upper Tropospheric Water Vapor Mass and Its Transport into the Stratosphere over the Tibetan Plateau Area in Summer. Part II: Adiabatic and Diabatic Transport into the Stratosphere
2020,44(3):503-518, DOI: 10.3878/j.issn.1006-9895.1905.18268
Abstract:The interannual geographic patterns of the upper tropospheric water-vapor-mass anomaly are dominated by a uniform abnormal mode and an east–west dipole abnormal mode over the Tibetan Plateau (TP) regions in July–August. In this paper, we analyze the relationship between these two leading modes and the adiabatic and diabatic water-vapor-mass transport from the troposphere to the stratosphere based on the European Centre for Medium-Range Weather Forecasts Interim Re-Analysis (ERA-Interim) datasets and the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) trajectory model. Results show that when the water vapor mass is dominated by the positive (negative) phase of the uniform abnormal mode, i.e., there is more (less) water vapor mass over the entire TP area, the intensity of the South Asian High (SAH) and the upward diabatic water-vapor-mass transport are enhanced (weakened), which means both the adiabatic and diabatic water-vapor-mass transport from the troposphere to the stratosphere are stronger (weaker). The regions and layers where the adiabatic and diabatic water-vapor-mass transported from the troposphere to the stratosphere change very little from the positive to negative phases of the uniform mode, although the layers in which the diabatic water-vapor-mass transported from the troposphere to the stratosphere are slightly higher for the positive phase. When the water vapor mass is dominated by the positive (negative) phase of the west–east dipole abnormal mode, that is, when there is more (less) water vapor mass in the west (east) of the TP, the SAH center shifts westward, enhancing the adiabatic water-vapor-mass transport from the troposphere to the mid-latitude stratosphere in the northwest and northeast flanks of the TP as well as the meridional adiabatic water-vapor-mass transport from the troposphere to the tropical stratosphere in the upper layers in the south flank of the TP. However, the meridional adiabatic water-vapor-mass transport from the troposphere to the mid-latitude stratosphere in the north flank of the TP is weakened. Meanwhile, the diabatic water-vapor-mass transport from the troposphere to the stratosphere is enhanced over the TP, whereas it is weakened in the upper layers in the south flank of the TP and the lower layers in the north flank of the TP. When the opposite occurs, there is less (more) water vapor mass in the west (east) of the TP. Trajectory model simulation experiments for the positive phase of the uniform abnormal mode confirm that higher frequency trajectories enter the stratosphere adiabatically over the TP regions. Trajectory model simulation experiments for the positive phase of the west–east dipole abnormal mode are in agreement with the analyzed results, which show higher (lower) frequency trajectories entering the stratosphere adiabatically in the northwest, south, and northeast flanks (north flank) of the TP.
Analysis of the Influence of Dynamic and Thermal Effects on Sea Temperature in the Tropical Pacific Ocean during Spring
2020,44(3):519-532, DOI: 10.3878/j.issn.1006-9895.1904.18270
Abstract:The spring predictability barrier (SPB) of the El Ni?o–Southern Oscillation (ENSO) is a difficult problem in ENSO prediction. To understand how dynamic and thermal factors affect the variability of sea surface temperature (SST) over the tropical Pacific ocean during spring is very important in understanding SST changes in key areas and resolving the SPB problem. In this work, a set of monthly data, including sea surface wind stresses, sensible heat flux, latent heat flux, net longwave radiation, net shortwave radiation, and ocean current fields, coordinated with each other during 1986–2017, were generated by BCC-CSM2-MR model simulation. Based on these data, we analyzed the dynamic and thermal influences and their contributions to the variability of SST (hereafter, TS). The main results were as follows: (1) Compared with other seasons, in spring, TS presented a unique asymmetric seasonal shift from warming to cooling in the Ni?o3.4 area. This was due to a similar shift in wind stress, net energy fluxes, and ocean current, which had a robust relationship with TS. Further analyzes demonstrated that thermal effects play an important role in the variability of local TS. In contrast, Horizontal advection is dominated by negative contribution. Meridional advection always showed a negative contribution to the seasonal variability of TS. Meanwhile, zonal advection terms turned into a cooling effect from a warming effect on TS during spring, and vertical advection effect was quite weak. (2) The interannual correlation between the tendency in TS anomaly and dynamic/thermal effects showed that thermal heating, as well as zonal advection, were positively associated with the Ni?o3.4 TS anomaly in spring. However, the correlation between meridional advection and the TS anomaly changed from negative to positive during spring. (3) Quantitative analysis of the dynamic and thermal variance contributions in the Ni?o3.4 region also suggested that the contribution rate of thermal effect was ＞50%, and the corresponding correlation coefficient was ＞0.7. The contributions of zonal and meridional advection were about 10% and 20%, respectively, but were inverse to each other. Other factors contributed less.
High Resolution Numerical Simulation of Tropical Cyclone Mujigae (2015) and Diagnostic Analysis of Wind Field
2020,44(3):533-551, DOI: 10.3878/j.issn.1006-9895.1904.19105
Abstract:The process of making landfall of tropical cyclone Mujigae (1522) is simulated at high resolution using the weather research and forecasting model. The simulation was verified to agree well with a variety of observations, especially for the track, intensity, and precipitation distribution. Afterward, the kinematic and thermodynamic structures and the wind distribution while making landfall are analyzed and the wind field is diagnosed from an azimuthal momentum budget based on the simulation data. The analysis of the budget on the representative height of the near-surface layer and the top of the planetary boundary layer (PBL) reveals that the main contributors to the wind tendency are the radial absolute vorticity flux (VVOR) and the azimuthal pressure gradient force (VPGF) near the surface at an altitude of 0.4 km. Near the top of the PBL at an altitude of 1.3 km, the azimuthal pressure gradient force (VPGF) and radial advection (VVA) contribute to the acceleration of the azimuthal flow, while the inward (outward) advection of the VVOR is responsible for the strengthening (weakening) of the azimuthal wind. The analysis of the azimuthally averaged fields of the budget reveals that the VVA and VVOR are main contributors to the wind tendency before making landfall of Mujigae.
2020,44(3):552-564, DOI: 10.3878/j.issn.1006-9895.1912.19168
Abstract:This study projects the change in environmental fields and the typhoon IGP (genesis potential index) in the western North Pacific (0°–40°N and 100°E–180°) in the late 21st century (2080–2099) using outputs from the RCP (Representative Concentration Pathway) 4.5 and RCP8.5 experiments of CMIP5 (Phase 5 of Coupled Model Intercomparison Project), carried out using 19 climate models. These models are capable of reasonably reproducing modern typhoon-related environmental fields and are thus selected for the analysis. Compared with the reference period of 1986–2005, there appears to be an increase in SST (sea surface temperature) in the western North Pacific, a weakening of VWS (vertical wind shear), and a decrease in OLR (outgoing long wave radiation) in the key regions where there are significantly negative correlations between these factors and the frequency of the typhoon; this is beneficial for the formation and development of the typhoon. On the contrary, the low pressure system that extends from the mainland to the South China Sea is weakened, suppressing typhoon activities. In general, changes in the typhoon environmental fields in the RCP8.5 scenario are greater than in the RCP4.5 scenario. In addition, the signal-to-noise ratio is examined to measure consistency between individual models. This ratio is found to be greater than 3.0 for SST change and greater than 1.0 for sea level pressure in regions under the low pressure system; for VWS and OLR changes, a ratio of less than 0.6 denotes a degree of disagreement between the models. However, the models agree well with the OLR change in regions associated with typhoon activities. The aforementioned changes in the typhoon’s environmental fields are in line with the increase in IGP in the future.
Temporal and Spatial Distribution Characteristics of an Increasing Temperature Trend Caused by Unnatural Factors
2020,44(3):565-574, DOI: 10.3878/j.issn.1006-9895.1906.19106
Abstract:In this study, the relative trend in temperature change in China from 1951 to 2017 was used to construct a probability density function and exceedance probability based on long-term correlation of relevant data. The confidence limit of this trend, belonging to natural variability, was studied and calculated under a certain confidence level. We sought to determine whether the relative change trend was caused by unnatural factors (whether the temperature increase was significant), and explore the threshold values of temperature change caused by unnatural factors in different regions, corresponding transition time periods, and evolutionary trends. The results showed that: (1) 10% of the site temperatures at 160 stations were overestimated when using traditional linear regression methods to calculate trend significance. These sites were mainly located in the northwest, southwest and eastern coastal areas of China. (2) From the perspective of spatial distribution of temperature trends in the country, except for the cooling trend in the central and western regions of Xinjiang, the other regions all showed warming trends. Relative temperature changes in Shanxi, Inner Mongolia, parts of Ningxia, southwestern Xinjiang, Yangtze River Delta, and southwestern Yunnan were relatively large. Unnatural trends in Northeast China, Inner Mongolia, and the northern Shanxi Province were large, and the temperature increase was significant. (3) From the spatial evolution of the significant inter-decadal warming areas, the northern and northeastern regions of China took the lead in increasing temperature, a trend that gradually expanded to the south and west. During the period of 1966–2001, most regions in China showed an increase in unnatural factors; for 1971–2006, temperatures in the northeastern and northeastern Inner Mongolia regions began to gradually decrease, while significant warming in southwestern China began to gradually expand. The number of significant warming sites was largest during 1976–2011; from 1981 to 2016, the significant warming sites were mainly concentrated in the Yellow River and Yangtze River Basins in southern China. In summary, there were prominent inter-decadal spatial and temporal transitions in significant warming areas in China caused by unnatural factors. This paper may provide new perspectives and new ways for the attribution and prediction of temperature change in China, and strengthen the linkage of climate change research and short-term climate prediction.
2020,44(3):575-590, DOI: 10.3878/j.issn.1006-9895.1905.19115
Abstract:Spectral width of cloud droplet spectra is crucial to the parameterization of cloud optical depth, assessment of indirect aerosol effect, and the formation of precipitation in numerical simulation. Based on aircraft observations of cloud microphysics in stratocumulus on 19 July 2008, during the Physics of Stratocumulus Top (POST) field campaign, the vertical distribution of microphysical properties and cloud droplet spectra were analyzed, as were the cloud microphysical processes. Results showed that spectral width of cloud droplet spectra was basically larger near the cloud base, which is due to aerosol nucleation. Spectral width decreased as height increased in the middle layer of the cloud, which was caused by condensational growth. The increase in spectral width near the cloud top was caused by entrainment-mixing processes. The increase in vertical velocity in the adiabatic cloud increased cloud droplet number concentration by promoting the activation of cloud condensation nuclei (CCN), increased cloud droplet size, and led to the decrease in spectral width by promoting condensational growth. This ultimately led to the negative correlation between spectral width and cloud droplet number concentration as well as cloud droplet size. The decrease in vertical velocity caused by entrainment-mixing processes in cloud holes decreased cloud droplet number concentration and cloud droplet size and increased spectral width by promoting the evaporation of cloud droplets. This was enhanced by the decrease in the adiabatic fraction. Based on these observations, it is recommended that the parameterization of the spectral width of cloud droplet spectra takes into account vertical velocity, cloud droplet number concentration, cloud droplet size, adiabatic fraction, etc.
2020,44(3):591-600, DOI: 10.3878/j.issn.1006-9895.1907.19123
Abstract:In this study, the CORE-IAF (Coordinated Ocean-ice Reference Experiments - Interannual Forcing) dataset was used to force two ocean models, LICOM3 (LASG/IAP Climate System Ocean Model Version 3), and POP2 (Parallel Ocean Program version 2). The North Equatorial Countercurrent (NECC) simulated by these two models was to be found weaker than the observation. These results were consistent with the findings of Sun et al. (2019), which further suggests that surface wind stress and its curl are the most important forcing terms in correctly simulating the NECC in ocean models. At the same time, the differences in NECC dynamical mechanisms between LICOM3 and POP2 were analyzed, including wind stress, advection, and other terms. In spite of the same CORE-IAF dataset being used to force these two ocean models, the influences of dynamical forcing terms (wind stress, advection, and other terms) were not exactly the same.
Lightning Activity of a Severe Thunderstorm with Several Hail-Fall Stages in Beijing Metropolitan Region
2020,44(3):601-610, DOI: 10.3878/j.issn.1006-9895.1910.19134
Abstract:Influenced by the Northeast China cold vortex and warm-moist airflow in low level, a damaging thunderstorm with five hail-fall stages occurred in Beijing on 10 June 2016. Based on the 3D-location results of total lightning from Beijing Lightning Network (BLNET) and Doppler radar data during the STORM 973 (Dynamic–microphysical–electrical Processes in Severe Thunderstorms and Lightning Hazards) campaign in 2016, the characteristics of lightning activity and radar reflectivity structure during this thunderstorm were analyzed. The thunderstorm consisted of three isolated cells triggered in sequence and finally merged together. The total lightning frequency increased significantly during the four analyzed hail-fall stages, up to 179 flashes min?1. IC (intra-cloud) lightning flashes accounted for more than 80% of the total lightning. The ratio of PCG (positive cloud-to-ground) to CG (cloud-to-ground) lightning (PCG/CG) increased sharply before three hail-fall stages, by up to 58%. During the hailstorm developments, the area of radar echo greater than 45 dBZ increased, and the echo top exceeded 13 km. Lightning radiation sources mainly distributed in the altitude layer from 6 km to 10 km throughout the hailstorm process, which was consistent with a strong radar echo region. Moreover, the total lightning flashes increased dramatically before the three analyzed processes and passed the 2σ threshold test. Three of them were 8–18 minutes ahead of the hail-fallings, which shows that the total lightning frequency has a certain early warning ability for hail-fall processes.
Contributions of Local and Remote Water Vapor Transport to Precipitation Variations over Songhua River Basin
2020,44(3):611-624, DOI: 10.3878/j.issn.1006-9895.1909.19177
Abstract:In this study, the authors used the Brubaker model to investigate the relative contributions of local and remote atmospheric moisture fluxes to the summer precipitation over the Songhua River basin and its interannual variability. Climatologically, due to the prevailing westerly wind in early summer (May–June), remote atmospheric moisture is the dominant contributor to early summer precipitation, accounting for 78.9%. Accordingly, the local evaporation contribution is 21.1%. In late summer (July–August), the East Asian summer monsoon brings more moisture via the southern boundary, so the contribution of remote moisture increases to 86%, and that of local evaporation is reduced to 14%. JRA-55 (Japanese 55-year atmospheric reanalysis) reanalysis data can well capture the interannual variation of summer precipitation over the Songhua River basin, with the correlation coefficients of the observations in early and late summer being 0.73 and 0.83 for the period 1961–2016, respectively. This shows that the moisture flux via the southern boundary caused by the stronger southwest monsoon plays the dominant role in early summer, and the moisture fluxes via the western and northern boundaries are significantly negatively correlated with early summer precipitation. The contribution of local evaporation is not statistically significant. Moisture flux anomalies tend to occur in the early summer with the decay of El Ni?o. In late summer, a significantly positive contribution is made by moisture flux via the southern boundary and a negative contribution by local evaporation. The effect of oceanic forcing on the late-summer precipitation anomaly is not significant, and internal atmospheric variability dominates. The significant negative contribution of local evaporation is due to the significant negative correlation between the surface temperature and precipitation. When precipitation is lower than normal, the surface temperature becomes warmer than normal, so there is more evaporation and a greater contribution from local evaporation to precipitation.
2020,44(3):625-638, DOI: 10.3878/j.issn.1006-9895.1911.19183
Abstract:In the TRAMS_RUC_1 km (Tropical Region Assimilation Model for South China Sea_The Rapid Update Cycle_1 km), the initial field and lateral boundary condition scheme, including its vertical resolution, time update frequency, and completeness, were revised and tested with a typical squall line case. Preliminary analysis revealed the following aspects: (1) The mesoscale character of precipitation in the squall line was better simulated after increasing the vertical resolutions of the initial field and lateral boundary, with the increase in the vertical resolutions of the middle and low layers playing the most critical role. (2) The diurnal change of lateral boundary forcing became more accurate after increasing its time update frequency. The squall line moved south after using the high update frequency of the lateral boundary condition because of the strengthened convergence of moist flux along the coastline. (3) With the high vertical resolution and high update frequency of the lateral boundary condition, lateral forcing of the vertical speed and cloud particle had no obvious effect on the forecasting of the squall line. In general, the vertical resolutions of the initial field and lateral boundary needed to be increased. The time update frequency also needed to be increased. Meanwhile, the lateral forcing of the vertical speed and cloud particle could be omitted. The effect of the new scheme was verified against observations in April 2019 (which lasted for a month). The distribution pattern and diurnal cycle of the mean hourly precipitation were significantly improved with the revised initial field and lateral boundary condition scheme.
2020,44(3):639-656, DOI: 10.3878/j.issn.1006-9895.1912.19217
Abstract:On the basis of the summer precipitation data from North China, the Madden–Julian Oscillation (MJO) index, and a reanalysis of the circulation data from the National Centers for Environmental Prediction/National Center for Atmospheric Research using statistical methods, this study analyzed the relationship between the MJO and summer precipitation in North China in 2018 and its influence mechanism. Results show the following: (1) The MJO is closely related to the summer precipitation in North China. Although the MJO cannot move to a higher latitude and directly affect the summer precipitation in North China, the cyclone in the MJO convective region will trigger an anticyclonic circulation to the north. During the slow eastward movement of the cyclone–anticyclone pair, the anticyclone at a higher latitude will directly affect the summer precipitation in North China; that is, the MJO will indirectly affect the summer precipitation in North China. When the MJO is in phases 5 and 6, there will be an obvious precipitation weather process in North China in summer. However, the precipitation intensity is related to the amplitude of the MJO. (2) In terms of the influence mechanism. At 850 hPa, along with the eastward movement of the MJO’s cyclone–anticyclone pair, the south wind water vapor transport (corresponding to RMM1) or the southeast wind water vapor transport (corresponding to RMM2) in summer in North China will be enhanced, which is beneficial to the precipitation weather process. At 500 hPa, the propagation of the MJO disturbance to the mid–high latitude will induce the subtropical high to move to the vicinity of the Korean Peninsula and strengthen it so that it will act as a barrier to the westerly trough and be favorable to the ascending motion in North China. Therefore, this is beneficial to the occurrence of summer weather precipitation in North China. (3) The MJO can be used to provide extended-range forecasts for summer precipitation in North China.
2020,44(3):657-678, DOI: 10.3878/j.issn.1006-9895.2003.19239
Abstract:To investigate the structure and evolution of urban breeze circulation in a mountain city, we used the Weather Research and Forecasting model (V3.9) to simulate the typical urban breeze circulation from August 17 to 18, 2016 in Chongqing. We also analyzed the turbulent kinetic energy and turbulent fluxes during this period. The results show that the rural wind begins at 1500 BT (Beijing time) and increases as the heat island strengthens. The circulation reaches its maximum at 1800 BT and subsides at 0200 BT the next day. At 1800 BT, the horizontal scale of the rural wind circulation is about 1.5–2 times that of the urban area, with a vertical scale of about 1.3 km, a horizontal wind speed of about 2–4 m s?1, and a maximum rate of increase of about 0.5 m s?1. Due to the influence of the topography, rivers, and background wind, the circulation is weak and has an asymmetrical structure. We also found the turbulent kinetic energy in urban areas to be obviously larger than that in nonurban areas, which means that urban areas experience stronger transports of heat and water vapor by turbulence. With respect to the relationship between turbulent fluxes and urban breeze circulation, we found that the circulation of urban breezes transports water vapor from the suburbs to the city via turbulent motion, with the turbulence supplying momentum for dissipation of the urban breeze circulation.
Volume 44,2020 Issue 3
Study on the stable components of atmospheric circulation during the continuous heavy rainfall of Meiyu in 2016
Available online:July 09, 2020 DOI: 10.3878/j.issn.1006-9895.2006.19167
Abstract:Meiyu precipitation has significant stage changes. It is important to study the key circulation stable components that lead to continuous heavy rainfall for the analysis and prediction of Meiyu precipitation. Using NCEP/DOE Reanalysis II data from 1979 to 2016, the potential height field, wind field and relative humidity field during the Meiyu period in 2016 were analyzed, the key stable components of the circulation system were extracted, the spatial structure, evolution characteristics and the background situation of longer time scale were analyzed, which provided the basis for the extended period forecast of the periodic heavy rainfall in Meiyu area Reference resources. The results suggest that: (1) The distribution of "three poles" in the stable components of the geopotential height field is the key system to maintain the continuous heavy rainfall in Meiyu area in 2016. The "three poles" correspond to the Ural Mountains blocking high, the Okhotsk Sea blocking high, the Southeast Asia high and the West subtropical high. This configuration structure is conducive to the convergence of warm and cold air in Meiyu area to form persistent heavy rainfall. (2) In the middle and high latitudes, there is a relatively deep negative anomaly between the two deep positive anomaly areas. On the one hand, it is conducive to the South transportation of cold air. On the other hand, the stable component of relative humidity field indicates that the water vapor transportation and convergence of the northern branch of the air flow play an important role in the continuous precipitation of Meiyu area. (3) The positive anomaly area in the middle and low latitudes is zonal in the east-west direction, and mainly exists in the middle and high troposphere, but there is a positive area on the East-West side of the Meiyu area, which continues down to the lower level, and they together strengthen the convergence and transportation of warm and humid air flow in the south of the Meiyu area. (4) Through the analysis of the evolution of the stable component, it is shown that the establishment and evolution of the "three pole" system lead to the precipitation in Meiyu area showing different stage characteristics.(5) The longer time scale (60 days) circulation stable components provides an important circulation background for the key stable components of the "three pole" in the period of continuous heavy rainfall.
Impacts of An Overshooting Deep Convection over subtropical Asian Monsoon region on the Lower Stratospheric Atmospheric Composition Distribution
Available online:July 07, 2020 DOI: 10.3878/j.issn.1006-9895.2006.19148
Abstract:The rapid transport by the overshooting deep convection is essential for summertime water vapor maxima in the lower stratosphere over the Asian monsoon region. However, the impacts of overshooting deep convection over subtropical Asian monsoon region on the lower stratospheric atmospheric composition distribution have not been fully addressed. In this study, we investigate the characters of overshooting deep convections during 2016 Wuhan rainstorm by using CloudSat and Aura Microwave Limb Sounder (MLS) satellite data. The concentrations of water vapor (H2O), ice water content (IWC), ozone (O3) in the upper troposphere/lower stratosphere during overshooting deep convections are analyzed. We find one overshooting deep convection case B which occurred at 5:00 a.m. on July 4. The overshooting convection considerably reduces the ozone mixing ratio near the tropopause. For example, it leads to 32.53% decreases of the climatological mean ozone mixing ratio. The overshooting convection is farther found to enhance the moisture at the lower subtropical stratosphere through the hydrating effect which has two different ways: moistening induced by ice particles evaporation and by the convective moisture flux itself. The overshooting convection results in a stronger moisture change (about 98.15% increase of climatology) when compared with ozone change. Our study implies that the local overshooting convections over subtropical Asian Monsoon region in boreal summer are important in transporting water vapor from the troposphere into the low stratosphere.
The comparative analysis of the moisture transport anomalies in two interdecadal changes of summer precipitation in South China
Available online:June 18, 2020 DOI: 10.3878/j.issn.1006-9895.2005.19231
Abstract:Based on the daily rainfall data at 2374 stations from the China Meteorological Administration (CMA) and reanalysis data provided by the National Centers for Environmental Prediction/ National Center for Atmospheric Research (NCEP/NCAR) during 1979-2017, the characteristics of moisture transport of precipitation increasing during 1993-2002 and decreasing during 2003-2012 over South China are compared by using the modern statistics method and the physical diagnosis method. Then, the impacts of the East Asian summer monsoon circulation systems and the sea surface temperature anomalies in the key areas on these characteristics are discussed. Finally, the reason of the two decadal changes of summer precipitation in South China is explained by using moisture transport budget equation. Results show that the increase of summer precipitation during 1993-2002 in south China was mainly influenced by the eastward movement of South Asia high and westward extension of West Pacific subtropical high (WPSH), which resulted in a strengthening of the moisture transport from the Philippines and the southwest side of the WPSH and a strong convergence of moisture in the lower layer of south China .The decrease of summer precipitation during 2003-2012 in south China was mainly influenced by the westward movement of South Asia high and eastward extension of WPSH, resulting in a weakening of the moisture transport from the Philippines and the southwest side of the WPSH and a divergence of moisture in the lower layer of south China. The interdecadal variation of summer precipitation in south China is mainly related to the anomaly of dynamic divergence of moisture transport caused by the change of wind speed, it is also affected by the anomaly of thermodynamic divergence of moisture transport caused by the change of specific humidity and the anomaly of subseasonal-scale eddies caused by typhoon and other synoptic scale eddies. In addition, we found that the moisture transport anomalies were closely related to the circulation and SST anomalies.
Construction of a new dataset based on multi-source land surface flux data and its application in the boundary area of EASM
Available online:June 16, 2020 DOI: 10.3878/j.issn.1006-9895.2005.19192
Abstract:The boundary area of the East Asian summer monsoon (EASM) is a hot spot area with intense interaction between land surface process and regional climate. However, the lack of high-quality long-term flux data sets available for this region limits the study of the interaction between land surface water and heat exchange and climate. It is necessary to reconstruct a new dataset based on currently available multiple flux data and then apply it in climate research. In this study, the data sets of land surface energy fluxes in the boundary area of EASM in China were reconstructed by integrating the field observations conducted over Northern China and several gridded datasets. Based on selection of sites with good underlying surface representative and investigation of the scattering distribution of simulations and observations, a set of monthly average sensible heat, latent heat and net radiation data sets are generated by using multiple regression model. The cross validation results show that the accuracy of the constructed data set is improved compared with several original gridded data sets, and the systematic deviation of the original lattice data is eliminated to the greatest extent. Further analysis suggests that among the surface energy balance components, the response of land surface turbulent flux to summer monsoon is more significant, and the interannual variation of land surface turbulent heat flux in the boundary area of EASM shows logarithmic distribution to the duration of summer monsoon. The turbulent heat fluxes present more significant interannual variations as the summer monsoon is in a low persistent state. Weaker summer monsoon system may lead to a stronger impact of land surface processes on climate change. The new data set based on multi-source flux data fusion can provide supports for climate change research, and increase the understanding of the interaction between land surface processes and monsoon climate.
The Variation of Asian Westerly Jet Asymmetry and Its Impacts on East Asian Climate in Boreal Summer
Available online:June 16, 2020 DOI: 10.3878/j.issn.1006-9895.2005.19232
Abstract:Based on the monthly mean rainfall data of CMAP and reanalysis of NCEP/DOE ⅡReanalysis form NOAA and the monthly precipitation and average temperature data from NMC，by defining an index (IAja) that is defined to describe the zonal asymmetric variation of Asian Westerly jet in upper troposphere, we have investigated the characteristics of the intensity difference between the east and west part of the Asian westerly jet and its climate impacts on East Asia from 1979 to 2019. The main conclusions are as follows: there are prominent inter-annual variations of the zonal asymmetry of Asian summer westerly jet, existing significant quasi-period of 6~8 years and 2 years. When the zonal asymmetry of Asian summer westerly jet is typically strong (weak), the wave-like anomalous rainfall pattern signed with the positive (negative)—negative (positive)—positive (negative) signs is observed form lower to higher latitudes in East Asia sector along with negative (positive) anomalies of temperature in the area of Lake Baikal，and simultaneously significant positive (negative) anomalies in regions including the Western area of China and Northern parts of Japan. The divergent and convergent wind components by the anomalous diabatic heating as a potential vorticity source directly induce the anomalies circulation in the mid-latitudes. The anomalous anticyclonic circulation making the intensity of west Asian jet increased and the eastern segment decreased help to the zonal asymmetry of Asian summer westerly jet stronger. The formation and facilitating of zonal asymmetric anomalies of the Asian jet are affected by not only convergence and divergence in the tropics and mid-latitudes, but also by wave energy propagating eastwards in westerlies. The eastward propagation of wave energy may be related to the SST anomaly of the Northern Atlantic. These results are helpful for us to better understand the formation mechanisms of the zonal asymmetry of Asian summer westerly jet.
Evaluation of different Planetary Boundary Layer Schemes in Simulating Precipitation Caused by Southwest China Vortex in Sichuan basin based on the WRF Model
Available online:June 11, 2020 DOI: 10.3878/j.issn.1006-9895.2005.19171
Abstract:Five Planetary Boundary Layer(PBL) parameterization schemes [Yonsei University(YSU), Mellor–Yamada–Janjic (MYJ), Mellor–Yamada–Nakanishi–Niino Level 2.5 (MYNN2), Shin-Hong(SH) and the Asymmetric Convective Model, version 2 (ACM2)] in Weather Research and Forecast model (WRFv4.0) are used to simulate well-developed Southwest China Vortex(SWCV) processes that caused rainstorms in the eastern Sichuan basin in 2016.Each level of precipitation prediction are verified, and the L-band radiosonde data, which has the temporal resolution by 1 second, are used to reveal the fine structure of PBL in the midday, the difference between observation and simulation are assessed, and the reason are discussed based on the characteristics of turbulence algorithm in each scheme. Finally, a parameter of turbulence intensity are adjusted for the ACM2 scheme in order to improve the structure of PBL and precipitation in Sichuan basin. The results show that: The ACM2 and YSU schemes have relatively better Threat Score(TS) performance. Compared with other schemes, ACM2 has less false alarms. The attribute of ACM2, which can switch local or non-local algorithms according to the stability of the surrounding environment, seems to be more suitable for Sichuan basin precipitation simulation. However, all PBL schemes show a high false alarm rate in prediction of the Southwest China Vortex precipitation, especially in heavier precipitation. The fine sounding data further shows that all the PBL schemes predict higher Planetary Boundary Layer Height(PBLH) than the observations, which means the simulation has stronger mixing intensity compare to the real atmosphere. Through the parameter adjustment, the ACM2 scheme with reduced mixing intensity, the potential temperature and humidity structure in PBL is more in line with the observation. Then, the potential temperature of the lower PBL is lower, the humidity is higher, and the false alarm report of heavier precipitation is reduced, so that the precipitation simulation in Sichuan basin get some improvement. The different character of the PBL schemes in simulate the Southwest China vortex is mainly lead to different position of the vortex and the precipitation intensity, but essentially, it is derived from the local or non-local attribute and the intensity of vertical mixing. The selection based on the features of research object is the key to accurate simulation of the PBL structure.
Evaluation of the Simulated Tropical Cyclones over the Western North Pacific with IAP AGCM4.1 Based on K-means Method
Available online:June 05, 2020 DOI: 10.3878/j.issn.1006-9895.2002.19252
Abstract:As the atmospheric component of the CAS-ESM1 (The Chinese Academy of Sciences Earth System Model, version 1), IAP-AGCM4.1 (The Institute of Atmospheric Physics Atmospheric General Circulation Model, version 4.1) is developed independently by the Institute of Atmospheric Physics. The TECA (Toolkit for Extreme Climate Analysis) is used to identify and evaluate the simulated tropical cyclones (TC) over the western north Pacific with IAP AGCM4.1 during 1979 to 2012. The results show that the IAP AGCM4.1 can reproduce the TCs’ spatial distribution, track, and source reasonably compared to observation, while the model underestimates the number of TC, and only 36% of the observed tropical cyclones over the western north Pacific are simulated. Further analysis based on K-means clustering method shows that the underestimation is mostly due to inability to reproduce the northwestward-turning and westward TC. For the TCs with westward-northwestward, westward-turning and eastward-turning tracks, simulated numbers are about 39%, 48% and 85% of the observed ones, respectively. Moreover, correlation coefficients of seasonal variation between simulation and observation can reach up to 0.91 and duration biases are about 1~2 days. IAP AGCM4.1 performs well in simulating the tracks of the westward-northwestward and eastward-turning TC, with the relative biases ranging between 1%-5% for longitude of centroid, 4%-16% for latitude of centroid, and 5%-15% for latitudinal and meridional standard deviations. In addition, IAP AGCM4.1 reproduces the evolution of environmental circulation and subtropical high quite well during the lifetime of eastward-turning TC, as simulated strength and area index of subtropical high are highly correlated with observation (the correlation coefficient is 0.89). The poor simulations of northwestward-turning and westward TC are likely due to simulated biases of subtropical high.
Available online:June 05, 2020 DOI: 10.3878/j.issn.1006-9895.2004.19229
Abstract:Urbanization has an important influence on the frequency and intensity of heat waves, but the mechanism of urbanization affecting the process of high temperature is not fully understood. The Weather Research and Forecasting (WRF) model is used to simulate a summer high-temperature process in Beijing from 2 to 6 July 2010. This paper reports the main results of urbanization effect on surface air temperature of urban areas in the heat wave process. It is found that the optimized WRF model is able to simulate the temporal characteristics of the 5 consecutive days of high temperature and the variation of IUHI (urban heat island intensity) in Beijing. The impermeability of urban underlying surface determines that the 2m relative humidity of urban area is lower than that of rural area, which weakens the ability of urban area to regulate surface air temperature through latent heat. After sunset, the urban sensible heat flux decreases slowly, and cooling rate of urban area is less than that of rural area. At night, the structure of boundary layer is stable and the height of it is low, and the wind speed is small. In this case, the energy transmission between urban and rural areas is restrained, forming a strong urban heat island at night. After sunrise, sensible heat flux and latent heat flux of urban and rural land surface rise rapidly, and the stability of boundary layer decreases. In the afternoon, the urban underlying surface is in favor of the high and low value centers of sensible heat flux and latent heat flux respectively, with the ability to regulate temperature through latent heat weakened. It’s conducive to the energy vertical transfer that the stability of the boundary layer is weak. The IUHI in the afternoon is smaller than that in evening. The extreme high temperature occurred on 5 July. An abnormal continental warm high pressure controls most parts of China, and Beijing is in front of the high-pressure ridge. The Fohn effect of the northwesterly flowing over the mountains aggravated the urban high temperature except for the larger IUHI. In this process of heat wave, therefore, the obvious urban heat island effect created by the urban underlying surface in Beijing has increased the strength and severity of the extreme high temperature event.
Airborne observations of the microphysical characteristics of stratiform cloud over the eastern side Taihang Mountain
Available online:May 14, 2020 DOI: 10.3878/j.issn.1006-9895.2004.19202
Abstract:Fine cloud microphysical characteristic is the research foundation of stratiform cloud precipitation mechanism. Based on the observation data obtained from "Rainfall Enhancement and Hail Suppression project at the eastern side of Taihang Mountains”, the stratiform cloud microphysical characteristics affected by the typical westerly trough process on 21 May, 2018 was analyzed. The result show that the super cold water content at the -5℃ layer is less than 0.05 g?m-3, and the concentration magnitude is 101-102 L-1. The needle-like and columnar ice crystals are often observed in the regions with high number concentration of ice crystal, which may be related to the ice crystal fragments which may be produced by Hallett-Mossop mechanism and other mechanism together were formed by deposition under super saturation respect to ice condition. Most of ice crystal is mainly plane and dendrites in the regions with low number concentration of ice crystal. The ice and snow crystal mainly grow through the process of deposition and coalescence, with weak rimming process. The liquid water content is accounted more than 70% in the regions with the peak value of cloud water content near 0℃ layer. The particle are mainly cloud droplet with diameter between 10 to 50 μm, accompanying with a few polymer. The super cold water content is about 0.05 g?m-3 in other regions near the 0℃ layer, and ice crystal habits are predominantly polymer、rimed and graupel type. Most of the particle are spherical droplet and melting ice crystal in the liquid water layer. The vertical detection show that the ice and snow crystal number concentration increased with height above 0℃ layer. The super cold water content of the mixed layer is much lower in the stable stratiform cloud. Most of particle are mainly grew through the deposition and coalescence, and the degree of ice crystallization is much higher. The existence of a lot of liquid droplets indicates that the transformation between liquid phrase and ice phrase is not sufficient in the strongly developed stratiform cloud regions in where supercooled liquid water is relatively abundant. The particle size distribution at different temperature levels indicate that the average number concentration of ice particle in the regions poor in super cold water content is higher than that of rich in super cold water content, however average diameter is on the contrary.
Possible causes of persistently extreme hot days related circulation anomalies in Yunnan during April to June in 2019
Available online:May 14, 2020 DOI: 10.3878/j.issn.1006-9895.2004.19226
Abstract:Persistently extreme hot days broke the historical length record, which attacked Yunnan province and induced the local serve drought during April to June in 2019. Based on daily in-suit surface air temperature in China, JRA-55 reanalysis datasets and outputs of the Community Earth System Model Large Ensemble (CESM-LE) Project, we investigated the general circulation regime related to the hot weathers in Yunnan, and discuss the possible causes of the persisting hot days in 2019. Our results show the lower- and upper-level strong anticyclonic circulation anomalies directly results in the local hot weather, it induces adiabatic warming by descending motion and enhances the solar radiation heating at surface. The anti-cyclonic anomaly is one node of high and mid-latitude eastward propagating wave trains, originated from North Atlantic. The 2019 April-June mean temperature and associated anticyclonic anomalies was the highest since 1961. The anthropogenic contribution to the magnitude and occurrence probability of the temperature anomaly in 2019 is approximately 37.51% and 56.32%, with an important impact of internal variability. The negative phase of Arctic Oscillation (AO) and warm phase of El Nino and Southern Oscillation (ENSO) are important external forcing factors responsible for the persistent anticyclonic anomalies over Yunnan during April to June in 2019. The negative phase of AO causes the southward shift of the positive geopotential height anomalies over Arctic at the longitudes of Eastern Europe, favors the high-latitude wave train and anti-cyclonic anomaly over Yunnan. The warm phase of ENSO enhances the western North Pacific anticyclone and favors its westward shift, leading to the persistence of the anticyclonic anomalous circulation over Yunnan. The negative phase of AO and warm phase of ENSO jointly reinforce the intensity, duration of the anticyclonic anomalous circulation over Yunnan, and result in frequent and persistent occurrences of the record-breaking extremely high temperatures and serve drought event in 2019 under recent global warming.
Wind Characteristics Analysis of Wind Profiler Radar and Its Quality Control Method for Data Assimilation
Available online:May 09, 2020 DOI: 10.3878/j.issn.1006-9895.2004.19216
Abstract:With 0.5°×0.5° analysis fields of National Centers for Environmental Prediction/Global Forecast System (NCEP/GFS) as numerical forecast background and using surface precipitation data, quality characteristics of wind products from twelve L-band wind profiler radars in Fujian Province, including three CFL-03 radars and nine CFL-06 radars, were analyzed firstly at 00, 06, 12, and 18 UTC in a day from January to December 2017 aiming at data assimilation. Then different quality control (QC) schemes and their different effects were preliminary discussed. The results indicate that: (1) Winds detected by CFL-06 radars are obviously better than those from CFL-03 radars in the aspects of maximum detection height, effective data availability and horizontal wind quality in low levels. (2) Great differences exist in horizontal winds detected by different wind profiler radars with same model regarding its data availability, effective detection height, and vertical distribution of standard deviation, correlation coefficient and bias. These differences have no direct relationship with geographical location of wind profiler radars, i.e. coastal area or inland, and height above sea level. (3) The wind profiler radar products have obviously systematic negative bias relative to GFS u-wind field, that is, the u-winds detected by wind profiler radars are lower than GFS background field. This doesn’t meet the no-bias requirement for data assimilation. So bias corrections are necessary in data assimilation. Whereas v winds are relatively better than u wind. (4) Precipitation impacts greatly wind profiler radar detection. In precipitation days, the data availability reduces in middle-low levels but greatly enhances in middle-high levels. The standard deviations of u and v winds both increase in middle-low levels, whereas the standard deviations of v-winds increase and those of u-winds greatly reduce in middle-high levels. (5) Two QC schemes, i.e. different high-confidence range scheme and different effective detection-height scheme, are advanced up for different wind profiler radars to compare with fixed effective detection-height scheme. The results show that the two QC schemes both have obvious advantages. The QC effect of different high-confidence range scheme is much more obvious, with horizontal wind data of different radars more fully and effectively identified. The scheme does not only reduce unnecessary loss of radar data, but also further eliminates poor quality data. It also achieves good results in precipitation condition.
Available online:May 09, 2020 DOI: 10.3878/j.issn.1006-9895.2003.19242
Abstract:The long-term trend of extreme temperature has been extensively explored by previous studies. In contrast, the interdecadal variation of extreme temperature is rarely documented. Based on the daily maximum temperature, minimum temperature and daily temperature of 839 stations in China from 1961 to 2016, the interdecadal variations of winter extreme temperature index in China were analyzed. The first four wave components of the extreme temperature of each station are extracted by using harmonic decomposition, which is regarded as the interdecadal component. A station is regarded as the station that undergos an evident interdecadal variation if the cumulative variance explained by the interdecadal component is greater than 25%. The results show that the stations with evident interdecadal variation of winter extreme low temperature index are mainly located to the north of the Yangtze River, northern Xinjiang and eastern Qinghai-Tibet Plateau. The inter-decadal variation to the north of the Yangtze River and in northern Xinjiang are basically consistent after 1979. Accordingly, the period after 1979 can be divided into three periods: previous-cold period (1979~1986), warm period (1987~2007) and later-cold period (2008~2016). The inter-decadal variation of the extreme temperature indices of the stations over the above-mentioned two areas might be modulated by the interdecadal variation of East Atlantic/West Russia teleconnection type (EAWR), which corresponds to the interdecadal variation of both the frequency of blocking-like circulation over Ural Mountains and the amplitude of planetary trough over East Asia.
Research on skeleton-based objective quantization and identifying algorithm for quasi-linear convective systems
Available online:May 09, 2020 DOI: 10.3878/j.issn.1006-9895.2004.19210
Abstract:In this paper, the concept of skeleton in Computer Graphics is applied to meteorology, such as echo image preprocessing, skeleton pruning and length-width ratio quantization techniques are developed. The quasi-linear convection systems (QLCSs) in radar echo mosaics conform to the meteorological standards can automatically be identified by this method. Firstly, the detailed identifying algorithm is introduced based on a double QLCSs process in Huang-Huai area in 2016. Then, the QLCSs in Anhui Province in June 2016 are objectively identified by this method, and the moving characteristics of QLCSs are quantitatively. The comparison between disastrous weather observation and subjective identification are carried out. The results show that the shape information of meteorological echo is well preserved by using skeleton image identifying algorithm and the effective identification of QLCSs is realized base on accurately quantifying the long and short axes of convection system. On the one hand, quantitative characteristics such as the moving vectors can be applied to the classification of disastrous QLCSs, providing identifying algorithm and quantitative features for long series statistics and mechanism analysis of disastrous weather, and on the other hand, it can provide new technique for monitoring and warning of QLCSs.
Available online:April 27, 2020 DOI: 10.3878/j.issn.1006-9895.2001.19230
Abstract:In this paper, performances of the median-resolution (about 110km) climate system model （BCC-CSM2-MR）and the atmospheric model （BCC-AGCM3-MR） developed in Beijing Climate Center are evaluated by three reanalysis datasets (ERA5, JRA55 and NCEP/NCAR). Results show that high blocking tendency can be distinguished over “North Atlantic-West Europe” and “Central North Pacific”. Winter and spring blocking frequency is almost twice of that in summer and autumn. Blocking frequency in ERA5 is higher than that in JRA55 and NCEP/NCAR, especially over the North Pacific. Model evaluations show that the atmospheric model can well reproduce the main features of Northern Hemisphere blocking frequency, spatial structures and seasonal variations. Main biases are the overestimation over Europe-Asia in winter and spring especially over Ural Mountains, as well as the underestimations over the North Atlantic. The biases are attributed to the climatological biases of geopotential height at 500hPa. General performances of BCC-CSM2-MR are similar to BCC-AGCM3-MR. But winter and spring blocking over Europe-Asia especially over Ural Mountains are improved. Spring blocking and the double-peak blocking structure in summer over the North Pacific are also better reproduced in the coupled model. Therefore, air-sea coupling may help to improve the blocking frequency reproduction over Europe-Asia and North Pacific. The internal variability of climate system has great impact on the variation of blocking frequency, which will also affect model capability in blocking high prediction.
Available online:April 27, 2020 DOI: 10.3878/j.issn.1006-9895.1912.19219
Abstract:Taking a torrential rainfall occurring at Ili Valley of Xinjiang on June 26, 2015 as an example, the circulation background and unstable conditions of this rainstorm process were analyzed by using observation data and high-resolution numerical simulation results of WRF. Some results are drawn as following: (1) The precipitation occurs under the background of synoptic circulation with the “two-ridge and one-trough” pattern over the middle and high latitudes in middle troposphere and “double highs” pattern of the South Asia high in upper troposphere. Under the effect of the particular terrain of the Ili Valley, which is westward opening trumpet-shaped, the Central Asian vortex located in Kazakhstan causes westerly wind in the low layer of the Ili Valley, and the Central Asian vortex located in the Tarim basin causes easterly wind in the middle layer of the Ili Valley. The vertical shear of horizontal wind in the Ili Valley is enhanced by the interaction of two Central Asian vortexes. In the Ili Valley, affected by the topography and the Central Asia vortexes, the low-layer convergence line is formed and coupled with divergence area caused by the upper jet, which enhances the upward motion. The low-layer westerly wind transports water vapor into Ili Valley and the water vapor accumulates in the valley. The enhancement of the upward motion causes the water vapor to be lift in the Ili Valley. (2) The simulation results of WRF can basically reproduce the location, intensity and evolution process of the precipitation during this weather process, and provide data with high spatial and temporal resolution for analyzing the evolution of rainstorm process. The analysis of the simulation results shows that the divergence distribution, water vapor, vertical shear of horizontal wind and thermal stratification distribution over the precipitation area have important contributions to the generation of precipitation. Through the analysis of vertical and horizontal components of moist potential vorticity, it is concluded that the convective instability affected by the thermal stratification influences the generation of precipitation, and the symmetric instability affected by the vertical shear of horizontal wind influences the enhancement and maintenance of precipitation. The analysis of potential divergence further indicates that the convective instability in the lower layer of the whole precipitation area is mainly caused by the vertical shear part of potential divergence, while the divergence part of the potential divergence can strengthen the convective instability in the leeward slope of the small terrain. It indicates that the dynamic and thermodynamic factors are coupled with each other in the whole precipitation evolution process, which affects the precipitation intensity and area.
Characteristics of subseasonal scale zonal movement of subtropical high in summer and its relationship with precipitation in southwest China
Available online:April 27, 2020 DOI: 10.3878/j.issn.1006-9895.2003.19204
Abstract:The subtropical high of northwestern Pacific (abbreviated as WPSH) is a large-scale circulation system that affects on the climate in China. In this paper, the influence and the physical processes of the sub-seasonal east-west movement of subtropical high on the southwestern China are investigated. First, the east-west displacement index of the WPSH is defined according to the key position of WPSH movement in early summer and late summer. The index has a significant period of 10-30 days and represents well zonal shift of the WPSH. A total of 195 events (1374 days) are chosen based on standardization value of the index. The results show that the subseasonal scale zonal movement of the WPSH is a closed relation to the precipitation in the southwestern China. During the west (east) events, the WPSH move gradually from east→west→east(west→east→west), and the rainfall in most southwestern China changed from less→more →less(more→less→more). The response of southwest precipitation on the sub-seasonal scale to the WPSH change is closed related to the water vapor and vertical movement of the gas flow in the north side of the WPSH and the main area of the WPSH during the movement process of the WPSH. Otherwise, the analysis shows that, in the condition of the WPSH, the consistency of precipitation in Guizhou and Chongqing is good, but the regional difference of precipitation in Yunnan and Sichuan is very significant, especially in Yunnan. In early summer, in the process of the zonal movement of the WPSH, the precipitation in most parts of Yunnan is even contrary to that in most parts of southwest China, especially in Guizhou and Chongqing. Namely, when WPSH move westward, the precipitation is less in most regions of Yunnan, vise verse. In the late summer, the precipitation in north-central Yunnan is opposite to that in other regions.
Available online:April 27, 2020 DOI: 10.3878/j.issn.1006-9895.1911.19193
Abstract:To improve the analysis quality through incorporating the flow-dependent ensemble covariance into variational data assimilation system, the new GRAPES hybrid-3Dvar system is built, based on GRAPES regional 3DVar system which uses the statistic covariance, by augmenting the state vectors with another set of control variables preconditioned upon the ensemble dynamic covariance. The new Hybrid-3Dvar system and localization method has been verified through the single observation assimilation experiment with ensemble samples produced by 3D-Var’s control variable perturbation method. The real observation assimilation and forecast experiment for typhoon Soudelor come to the conclusions: (1) the background covariance which is represented by ensemble samples are flow-dependent and the root mean square (RMS) spread in the ensemble of momentum field and mass field are largest near the center of typhoon; (2) the analysis increments of the new Hybrid-3DVar have more detailed structure and more medium and small-scale information; (3) The analysis and 24h prediction qualities of model variables in the new Hybrid-3DVar are obviously improved in comparison with the 3DVar system, and the precipitation position predictions are more accurate; (4) The 24h forecast track of typhoon Soudelor is closer to observational one and the 48h-predicted intensity approaches the real observation as well.
Influence of Stochastically Perturbed Parameterization method on ensemble forecasting of winter precipitation in China
Available online:April 27, 2020 DOI: 10.3878/j.issn.1006-9895.2001.19157
Abstract:Precipitation ensemble forecasting has great uncertainty, the uncertainty of parameters in the physical process parameterization scheme closely related to precipitation forecast is one of the sources of precipitation numerical prediction error. By introducing Stochastically Perturbed Parameterization (SPP method) on these parameters the uncertainty of representative model precipitation forecast is the frontier research field of international ensemble forecasting. In order to understand whether this method can reflect the uncertainty of numerical prediction of winter precipitation in Chinaand provide a scientific basis for business applications,based on China Meteorological Administration’s GRAPES(Global/Regional Assimilation and Prediction System)mesoscale regional ensemble prediction model,the 16 key parameters are selected from four parameterization schemes such as cumulus convection, cloud microphysics, boundary layer and near-surface layer, which have great influences on the uncertainty of model precipitation forecast,and introducing Stochastically Perturbed Parameterization (SPP),then through the ensemble prediction experiment from December 12, 2018 to January 12, 2019, a total of 31 days, comparing and analyzing the effect of SPP method on winter weather situation and precipitation ensemble prediction.The results show that the test for the random parameter perturbation method is added,the probability prediction techniques for precipitation and isobaric elements are better than control predictions without SPP method,and the improvement effect on low-level and near-surface elements is better than the improvement of the iso-surface elements in the middle or upper floors,The precipitation prediction result is superior to the control prediction test in scoring,but because it did not pass the significance test,there are no statistically significant differences.The above results indicate: under the influence of the East Asian winter monsoon,SPP method has no obvious improvement on the prediction technique of winter precipitation in China. Analyzing the reason,it may be related to the SPP method mainly represents the uncertainty of convective precipitation forecasting,but the winter precipitation process in China is mainly related to the development of baroclinic instability, model precipitation is dominated by large-scale grid precipitation, with less convective precipitation, therefore, the improvement of winter precipitation forecast is not obvious,this provides a scientific basis for applying Stochastically Perturbed Parameterization methods in the operation ensemble forecasting model.
Available online:April 27, 2020 DOI: 10.3878/j.issn.1006-9895.1912.19163
Abstract:To investigate the response of turbulent eddies to the mesoscale heterogeneous land use change over oasis in Northwestern China, two experiments with the cases of before irrigation (BI) and after irrigation (AI) using WRF-LES model are carried out, where heterogeneity strength would be changed by irrigation over oasis. The area-averaged method is used to calculate the area-averaged flux, and the wavelet analysis is used to decompose the pattern of surface turbulent heat fluxes into multi-scale pattern. The results show that the irrigation has large influence on the vertical heat flux, soil moisture, and soil temperature. Irrigation increases the heterogeneity strength, which has large influence on the dispersion of flux pattern. The dispersion height of sensible heat flux decreases with the decrease of sensible heat flux after irrigation. The shape of turbulent eddies in AI is reticular which is similar with that in BI but the energy spectra of sensible heat flux pattern decreases after irrigation. In addition, the dispersion height of latent heat flux depends on the sensible heat flux but the energy spectra of latent heat flux pattern decreases after irrigation. From the spatial lag correlation coefficient, the height of response of vertical heat flux to the surface flux before irrigation is higher than that after irrigation. The shift distance after irrigation is less than that before irrigation. Strong surface heating results in large correlation coefficients and strong entrainment in the top of convective boundary layer.
Available online:April 27, 2020 DOI: 10.3878/j.issn.1006-9895.1912.19176
Abstract:We evaluate the ability of models to simulate aridity index (AI) and project the future dry/wet climate changes in China under the Representative Concentration Pathway (RCP) 4.5 scenario using experimental data of a high-resolution regional climate model (RegCM4) nested within three global climate models. For the reference period of 1986–2005, RegCM4 is capable of simulating the spatial pattern of AI in China reasonably, and the simulated AI slightly differs with the methods to calculate potential evapotranspiration, especially in the western high-altitude and northern regions over the country. Based on the projections of RegCM4, the AI averaged over China would decrease by 2–4% and 2–5% in the middle (2046–2065) and end (2081–2098) of the 21st century relative to the reference period, respectively, with central Northwest China becoming wetter while other regions drier. The predominant factor for the future dry/wet climate changes in China varies with regions. Precipitation is the first leading factor in central Northwest China, while potential evapotranspiration changes due to increasing temperature play the major role in the rest regions.
Observational Analysis of Planetary Boundary Layer Structure and Entrainment Characteristics under Heavy Haze Pollution
Available online:April 27, 2020 DOI: 10.3878/j.issn.1006-9895.1912.19180
Abstract:Abstrac: An unmanned aerial vehicle (UAV) was used to carry on portable instruments to measure vertical profiles of temperature, specific humidity, and particulate matter under different haze-polluted weather conditions in Nanjing from December 22, 2017 to January 18, 2018. The study is aimed to assess the impact of aerosol radiative effect on the atmospheric boundary layer (ABL) and entrainment zone structures as well as their evolution between one day with heavy haze pollution and another with good air quality. Vertical profiles of potential temperature, water vapor, and PM2.5 concentrations are characterized by performing detailed analyses on UAV-measured vertical profiles and surface observations including surface heat flux, 2-m air temperature, specific humidity, winds, and major air pollutants (e.g., O3, PM2.5) under different air pollution conditions. Results indicate that aerosols reduce surface-reaching solar radiation and surface sensible heat flux, postpone the development of the ABL, enhance the atmospheric stability near surface, decrease the ABL height, and exacerbate air pollution. The maximum concentrations of PM2.5 and the largest increase rate were observed at the top of the ABL rather than near surface. Furthermore, aerosol radiative effect imposes an important impact on entrainment and its characteristic parameters. The entrainment-zone depth increases with increasing surface PM2.5 concentrations, and entrainment rate normalized with convective velocity does not follow -1 power function with the convective Richardson number under heavy haze or PM2.5 pollution conditions, which is consistent with the findings by the large-eddy simulation studies. The study indicates that aerosol radiative effect must be included in the ABL and entrainment parameterization schemes to further improve numerical predictions of weather and air quality under heavy pollution conditions.
Evolution Characteristics of Mesoscale Convective System during the Formation of Tropical Cyclone Dujuan(1521)
Available online:April 27, 2020 DOI: 10.3878/j.issn.1006-9895.2001.19184
Abstract:As for tropical cyclone formation, there are few studies focusing on the transition from a tropical disturbance to a tropical depression, which is accompanied by the formation, merger, and development of the mesoscale convective system (MCS). In this paper, FNL data and high-resolution numerical model are used to simulate the No. 1521 typhoon Dujuan three days before its formation, and the development and evolution characteristics of MCS in the process of Dujuan formation are preliminarily explored. During the formation of Dujuan, the tropical upper tropospheric trough (TUTT) was located farther east of it, and Dujuan migrated northwestward from the southeastern edge of the monsoon gyre (MG). The decreased vertical wind shear and MG-related lower-level convergence are conducive to the formation of Dujuan in the early stage of the formation of Dujuan, more MCSs generated favorable for the development of Dujuan from the depression disturbance. In the later stage of the formation, as the convection bursts, the MCSs merge to reduce the number of MCSs become more compact. The largest MCS shrinks towards to the core region of the tropical disturbance. As a result, the formation of Dujuan is accelerated, which is accompanied by a sudden increase in the vorticity at the low level, resulting in the deep convection in the tropical disturbance. The comparison of the characteristics of stratiform and convective precipitation in the MCSs indicates the coverage of stratiform precipitation is the largest, while the precipitation rate of convective precipitation is larger than that of stratiform precipitation and the variation of the coverage of convective precipitation is more significant. The increased percentage of stratiform precipitation is closely related to the intensification of Dujuan, and the increase in the convective precipitation rate is beneficial to the intensification of MCS. Their combined effect can promote the formation of Dujuan.
The properties of convections embedded in the stratiform cloud on basis of airborne Ka radar and DMT system
Available online:April 26, 2020 DOI: 10.3878/j.issn.1006-9895.2004.19185
Abstract:Based on a compact airborne Ka-band Precipitation PMS probe Radar (KPR) and the Droplet Measurement Technologies(DMT), the dynamic and microphysical characteristics of convective bubbles embedded in stratiform clouds initiated by the huanghuai cyclone on April 22, 2018 were analyzed. At first，a total of 36 convective bubbles were observed by KPR in spring in Shandong province. The results based on the echo intensity, horizontal scale and echo-top height of these bubbles show that the average echo intensity of convective bubbles is concentrated at 20 to 30 dBz, accounting for 69%. The horizontal scale of convective bubbles is concentrated at 15 to 30 km, accounting for 61%. The echo-top height of convective bubbles is at 6 to 8 km, which is 2 to 4 km higher than the surrounding stratiform cloud. Afterwards, the microphysical parameters of convective bubbles in the cumulus mixed cloud on April 22 were counted.The results show the inside of convective bubbles is dominated by updraft with maximum wind speed of 1.35m/s, the average updraft is 0.22m/s. There is high supercooled water content in the bubbles with maximum water content of 0.34g/m3, and the average is 0.15g/m3. The ice particle concentration in the convection bubbles is 5.5 times of that outside of bubbles, and their mean diameter is 1.7 times of that outside. The images sampled by cloud image probe show that the ice particles in the front and tail of convective bubbles were mainly columnar and radial, while the ice particles in the core of convective bubbles were polymers. The growth of ice crystals depended on the accretion and collision processes, they showed columnar form when the supercooled water was insufficient, otherwise, they could quickly form graupels. The microphysical formation mechanism of precipitation in the convective bubble is different, and strongly depends on the supercooled water content. When supercooled water content was sufficient in the cloud, graupels were quickly formed, and the surface precipitation was formed after they passed through the melting layer. When less supercooled water existed in the cloud, the formation of precipitation depended on water vapor deposition and aggregation processes.
Available online:April 21, 2020 DOI: 10.3878/j.issn.1006-9895.2004.19251
Abstract:Forest canopy as an active interface between vegetation and environment transmits energy by reflecting, absorbing and transmitting solar radiation by leaves. The radiation above, within and below the forest canopy is very important in the energy balance, water and carbon cycle. Its variation with season and distribution within forest canopies is few studied in Huainan area. Using the data of total radiation measured at Huainan forest observation station from 1 July 2018 to 30 June 2019, the temporal change characteristics of solar radiation above the Sawtooth Oak canopy, and the spatial distribution and transfer of solar radiation through the canopy were analyzed, and also the albedo, transmittance and absorptance of the canopy were given. The results show that: (1) The downward shortwave radiation above the Sawtooth Oak canopy increases from spring to summer, then decreases gradually to winter. But the downward shortwave radiation within and under the canopy demonstrate a different trend with smaller value. It decreases from the early spring and increases from autumn to winter as opposite to that above the canopy. Concerning the upward shortwave radiation, whether above, within or under the canopy, the seasonal variation pattern is the same as the downward one, but the value is much smaller. (2) The downward longwave radiation above, within and under the canopy gradually increases with time from spring to summer, then decreases gradually and reaches the minimum in winter; in terms of spatial change, the radiation value of longwave within and under the canopy is higher than that above the canopy, which enhances longwave radiation and can be as large as 1.3 times under clear skies. (3) The annual average albedo above the canopy in Huainan forest area is 0.14, which is 0.01 lower than that in the temperate monsoon climate area (mainly mixed forest) in northern China (350N), indicating that the forest is denser in Huainan. (4) The shortwave radiation transmittance of the upper part and the whole canopy is mainly affected by the leaves. In summer, the average shortwave transmittance of the whole canopy (τ) is 0.1. But in winter, the leaves wither and fallen, the transmittance increases and tends to a stable fluctuation. The absorptance of shortwave radiation in the canopy was the highest in summer and decreased gradually in autumn, and decreased rapidly in winter as the leaves withered, tending to a stable fluctuation. These results could be useful for validating layered radiative transfer and photosynthesis models and for further study the energy, water and carbon cycle of the forest ecosystem.
Available online:April 01, 2020 DOI: 10.3878/j.issn.1006-9895.1911.19140
Abstract:Based on the daily snow depth (SD) data provided by the Japanese 55-year Reanalysis (JRA) project, the global reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF) and the sea temperature data from Hadley Center(Hadley), this paper analyzes asymmetric modes of the spring SD anomaly in Eurasia and its teleconnection with the North Atlantic sea surface temperature(SST), the results are verified by numerical simulation analysis. The results show that there are significant differences between the first two modes of the spring SD over the Eurasian continent, which are represented as two asymmetric forms in this paper: the first mode features a zonal uniform distribution, and the second displays an obvious west–east contrast distribution. The North Atlantic "tri-polar" and the "saddle" SST modes have significant correlation with the first and the second mode of the SD, respectively. Corresponding to the two SST modes, the wave activity fluxes in the mid-high latitudes over the northern hemisphere are characterized by two kinds of propagation characteristics: Silk Road pattern (SRP) and Eurasian teleconnection pattern (EU), which have different effects on the position and intensity of the westerly air flow in the mid-high latitudes, and thus exert different remote influences on the SD distribution in Eurasia. The Localized Multiscale Energy and Vorticity Analysis (MSE-VA) shows that the kinetic energy (KE) of source region in the North Atlantic has a transform process from bottom to top. In addition, the average KE conversions enhance over the exit region of the Westerly Jet, benefiting the high-level KE accumulation and divergence outward and creating a remote effect on downstream areas. The CAM5.1 model simulation is used to study the effects of the two SST modes on the propagation characteristics of the wave activities flux. The simulation results verified the observation results well. The SST modes may responsible for SRP and EU propagation characteristics of the wave activities flux, meanwhile, the changes of the climatic field elements of the two SST modes are consistent with the distribution characteristics of the corresponding snow depth modes.
Case Study of Hail Cloud Internal Structure Based on Rocket Sounding Date Jinhui, TIAN Xian and YUE Zhiguo
Available online:April 01, 2020 DOI: 10.3878/j.issn.1006-9895.1907.19118
Abstract:Using sounding rocket data, a new generation of weather radar data and meteorological data to comprehensively analyze the hail cloud in Baota District, Yan"an County on July 17, 2015. The results show: (1)At BT 08:00 am, low vortex in Hetao Region split eastward, with a strong cold advection, moving fast and a rising of surface temperature at BT 14:00 pm caused this hail. (2) The hailstorm conditions inside the backward position, such as temperature and humidity ,Tg convection index, IQ whole layer special humidity integral, TT（Totals totals index）are smaller than that in external natural atmosphere. The stratification stability, expressed by K index, LI（lifted index ）and SI（Showalter index ）show that hail clouds are smaller inside than outside. The thermal parameter SSI（Storms Severity Index） in the hail cloud is lower than that in natural atmosphere. The internal energy parameter CAPE（Convective Available Potential Energy）and Vm（maximum updraft velocity） in the hail cloud are significantly lower than that in natural atmosphere. The 0℃ layer height in the hail cloud is not so high as that in natural atmosphere. The - 20 ℃ high temperature layer, air flow is stronger, the whole layer partial sinking airflow. (3) Location of the rocket detection is opposite to the back of the hail cloud. The wind direction changes counterclockwise from the down to the up in hail cloud. (4)The temperature range which is near the 0℃ layer in hail cloud is 5.0℃--1.8℃. The maximum humidity area is within the depth of 1.0km, where humidity is over 80% and the maximum humidity is 87.1%. These provide water vapor conditions for the formation of hail. (5)There is a maximum horizontal wind speed of 19 m·s-1 rapid flow and thickness of 0.022 km close to the lower layer of 0℃.It is also maintain a horizontal wind speed of 13 m·s-1 or above in the temperature range of 5.0--4.8℃ and a thickness within 1.6km. These provide a dynamic field condition for the formation of hail.(6)There is a weak wind zone of ≤2 m·s-1 in areas where temperature range is -8.7--9.2℃ and the thickness is in 0.2km. Below the weak wind zone, where temperature range is -4.6--8.8℃ and the thickness is in 0.889km, it is updraft. In this place, the average rising speed is 1.79 m·s-1 and the maximum rising speed of 4 m·s-1. This configuration provides an environmental field for the growth of hail.
Characteristic Analysis of Generalized Potential Temperature and Potential Vorticity Containing Freezing Process
Available online:April 01, 2020 DOI: 10.3878/j.issn.1006-9895.1908.19154
Abstract:For comparison and analysis of spatial and temporal distribution characteristics among different types of thermodynamic variables and potential vorticity during precipitation process, based on a heavy rainfall occurring in Jilin Province during 13-14 July 2013, five different types of potential temperature including conventional potential temperature (θ), equivalent potential temperature (θe), generalized potential temperature containing condensation probability function (θGao), generalized potential temperature containing freezing probability function (θWang), potential temperature covering both condensation process and freezing process (θGu) were calculated with model output. Relationships between five associated types of potential vorticity (PVθ, PVθe, PVθGao, PVθWang, PVθGu) and precipitation were also analyzed. The results shows that generalized potential temperature introducing freezing probability function (θWang) and its potential vorticity (PVθWang ) have a better correspondence with heavy rainfall. The differences between θWang and θGao mainly concentrate on 5-11km in the mid-upper troposphere over rainfall region. θWang is always greater than θGao, with a maximum difference up to 2.5K, indicating that introduction of freezing probability function extends the application scope of generalized potential temperature and has a more suitable depiction for thermodynamic state of non-uniform saturated moist air over rainfall region. Differences among five types of potential vorticity are mainly under 12km over rainfall region. Positive and negative anomaly centers for potential vorticity PVθGaoand PVθWang defined by θGao and θWang are more visible. Compared with PVθGao, anomaly value for PVθWang is greater, and the differences can reach ±0.2PVU, which is due to the enhancement of generalized potential temperature over rainfall region caused by the introduction of freezing probability function, leading the abnormal enhancement of moist potential vorticity in freezing region.
Numerical Simulation Study on Microphysical Characteristics of Stratiform Clouds with Embedded convections in Northern China based on Aircraft Measurements
Available online:April 01, 2020 DOI: 10.3878/j.issn.1006-9895.1908.19114
Abstract:To characterize the microphysical characteristics and transformation process of stratiform cloud with embedded convections, a case study was performed using WRFV3 model based on two aircraft measurements on May 1st, 2009. The aircraft observation results showed that the shapes and formation process of ice particles existed significant difference between the regions of stratiform cloud and embedded convection . Compared to the embedded convection region, the shapes of ice crystals in the stratiform cloud was more complicated, including the needle column, capped column and dendrite type. However, the dendrite type ice crystals dominated in the embedded convection region and their growth was controlled by aggregation and riming processes. Overall, the results indicated that the basic characteristics of this stratiform cloud with embedded convections simulated by the WRF model were in good agreement with the aircraft observations, including cloud distribution, LWC and numerical concentration on the flight route, etc. The simulated results showed that in the stratiform cloud, with higher cloud water content and bigger W, could develop into embedded convection because of strong riming process. The hydrometeors of snow, graupel and rain water in the clouds account for 51.9%, 31.0%, 16.0%, respectively, cloud ice and cloud water accounts for very little. In higher level, snow and graupel grew through deposition process. In lower level, they grew through riming process and melted into rain. Stratiform cloud with lower cloud water content and smaller W, would stay to be stratiform cloud. The hydrometeors of snow, rain water and cloud ice account for 90.4%, 6.1% and 3.5%. The ice and snow grew through depositon process, and melted into rain in lower level.
Available online:March 30, 2020 DOI: 10.3878/j.issn.1006-9895.2003.19186
Abstract:Atmospheric predictability research is the basis for weather and climate prediction. Under the background of global warming, meso/micro-scale extreme weather such as heavy rain and severe convection occur more frequently in recent years, and its predictability has attracted widely attention. After a brief review of the history of atmospheric predictability research, this paper systematically reviews and summarizes the latest advances on the predictability of heavy rain and strong convection over the last 20 years. The main research methods for meso/micro-scale predictability and their differences with traditional large-scale weather predictability research are first discussed. Then, the primary initial error growth mechanism (error upscaling under deep moist convection) has been elaborated in detail and some arguments (error downscaling, error upscaling and downscaling coexisting) are discussed. The influences of errors in NWP models and convective environment to the practical predictability, as well as some recent mesoscale predictability experiments are also highlighted. Finally, this paper briefly discusses the current problems, challenges and future directions on predictability research of heavy rain and severe convection.
Diagnostic and Numerical Study on Physical Process of Strong Rainfall Associated with Rammasun (1409)
Available online:March 30, 2020 DOI: 10.3878/j.issn.1006-9895.2003.19224
Abstract:The development evolution and landfall process of supertyphoon Rammasun (1409) was investigated using high-resolution simulation data produced by the Weather Research and Forecasting (WRF) model. The diagnostic and numerical study on physical process of strong rainfall was focusing on the period of the rapid enhancement and the landfall on South China Sea of Rammasun, using the three-dimensional surface precipitation equations and the definition of precipitation efficiency. The results showed that the strong average precipitation intensity（PS） had been maintained in the main circulation area of Rammasun, and the relative contribution of PS over the land and ocean was basically reversed. The enhancement of land surface friction was conducive to more water vapor convergence to the land (positive value of moisture advection QWVA), resulting in local atmospheric humidification over the land within a short period before the landfall (negative value the local change rate of water vapor QWVL). Water vapor was rapidly transformed into liquid-phase and ice-phase hydrometeors by means of cloud-related microphysical processes, which promoted the rapid development of clouds and the intensification of precipitation intensity over the land. When the center of the circulation was located in the Beibu Gulf, the relative contribution of QWVA over the ocean was significantly enhanced. The change of the underlying surface during the landfall period led to remarkable changes in the moisture-related microphysical processes, causing significant changes in the cloud system and the strong precipitation center. The surface evaporation (QWVE) over the ocean made more effect and more obvious changes, compared with the one over the land. The average QCLL and QCIL (the local change rate of liquid-phase and ice-phase hydrometeors) in the main circulation were basically “positive – negative - positive” within the period of Rammasun moving to the coastal region of South China. The content of hydrometeors was mainly increased when the center of the circulation was located in the Beibu Gulf, but the reasons for the growth of liquid-phase and ice-phase hydrometeors were different. During the landfall period, the characteristics of the “negative - positive” change of QCLL and QCIL over the land were mainly due to the rapid development of the cloud over the land caused by the enhancement of land surface friction in the early landfall period and the consumption of heavy precipitation in the later period. The high precipitation efficiency had been maintained in the main circulation area of Rammasun. From the contact of the main circulation to the land, the precipitation efficiency over the land rapidly increased, while the precipitation efficiency over the ocean maintained a high value during most of the integration period, but reduced only after the second and third landfall.
Energy evolution characteristics of an eastward-moving convective cloud cluster producing heavy precipitation that originates from the Tibetan Plateau
Available online:March 30, 2020 DOI: 10.3878/j.issn.1006-9895.1912.19213
Abstract:Based on the Himawari-8 satellite TBB data from the Japan Meteorological Agency, the ERA5 reanalysis data from ECMWF and a new energy diagnostic method that uses temporal scale for scale separation, evolutionary characteristics of a convective cloud cluster (lasted 40 hours from 05:00 June 5th to 06:15 6th, 2016) that formed over the Tibetan Plateau (TP), moved eastward and induced heavy precipitation in downstream regions was investigated in this study. Main findings are listed as follows: the main influencing systems for the eastward-moving convective cloud cluster at different stages of its lifespan were different. Before moving out of the plateau, the cloud cluster was mainly affected by a plateau vortex and a short-wave trough. As the cloud cluster vacated the plateau, the plateau vortex dissipated, whereas the short-wave trough intensified with time, and finally the short-wave trough became the main influencing system of the cloud. The deep convection features of the eastward-moving cloud cluster were significant. The eastward-moving cloud cluster induced a series of precipitation from west to east, with the strongest precipitation occurred after it had moved out of the plateau and its convective barycenter lowered in height. The energetics characteristics of the eastward-moving convective cloud cluster experienced notable changes during its lifespan, and the associated precipitation characteristics were also significantly different. When the cloud cluster was over the TP (the first stage), the contribution from the background field was a dominant factor. The background filed provided energy for the evolution of eddy flow [by downscale kinetic energy (KE) cascade] which induced heavy precipitation directly. During the second stage, the precipitation-related latent heating was greatly enhanced, which significantly increased available potential energy (APE) of eddy flow. Under the influences of vertical motion, APE of eddy flow was released and converted to KE of eddy flow. This acted as a dominant factor for the sustainment of the precipitation-related eddy flow. When the cloud cluster moved out of the TP, the influence from background field on eddy flow enhanced again, however it was different from the direct influencing way that appeared at the first stage. In this stage, the influence from background environment favored the persistence of precipitation-related eddy flow indirectly. First, APE of the background filed transferred to APE of eddy flow through a downscale energy cascade. Then, a baroclinic energy conversion from APE of the eddy flow to its KE occurred. This acted as a dominant energy source for KE of eddy flow. Furthermore, in the third stage, a remarkable upscale energy cascade of KE was found, which reflected the feedback of eddy flow on its background field. However, the feedback intensity was not enough to affect evolution of the background field obviously.
A comparative analysis on the evolution processes of the strong and weak stratosphere polar vortex events in boreal winter
Available online:March 25, 2020 DOI: 10.3878/j.issn.1006-9895.1906.19110
Abstract:Based on the NCEP/NCAR daily reanalysis data during 1958-2017, this study comparatively analyzed the stratospheric and tropospheric evolutions during the life cycle of both strong and weak stratosphere polar vortex events (i.e., SPV and WPV events). And the atmospheric circulation and dynamical characteristics of two types of WPV events, namely with and without SSW, are also analyzed. The results showed that the formation of SPV events experienced a slow development, then a rapid intensification stage, while the WPV events are established dramatically. Compared to the SPV events, the WPV events are stronger with a higher anomaly center when they reach a peak. Moreover, the occurrence of SPV and WPV events is closely related to the positive feedback of wave-mean flow interaction. For the SPV events, a PNA-like pattern weakens the wave-1 of planetary waves during the growth stage. When the stratospheric westerly winds are strengthened to a certain extent, upward propagating planetary waves are greatly suppressed, thus the polar vortex is intensified rapidly and reaches the peak stage. For the WPV events, a wave-1 pattern enhances the upward propagating planetary waves in the growth stage, which leads to weak westerly winds soon in the stratosphere by exerting a drag on the zonal flow. More planetary waves then propagate into the stratosphere, and thus the polar vortex is dramatically weakened and even broken down. In addition, for the WPV events with SSW, enhanced upward wave-1 EP flux in the stratosphere occurres in the growth stage. Through the positive feedback of wave-mean flow interaction, both the upward propagating wave-1 and wave-2 EP fluxes are increased, which lead to the breakdown of the polar vortex. For the WPV events without SSW, the upward propagating wave-1 EP flux is weak in the growth stage, while the wave-2 flux plays an important role. Hence, the total upward propagating planetary waves are much smaller than the WPV events with SSW. For the WPV events without SSW, an EU-like pattern in the height field appears in the upper troposphere during the growth and peak stages, accompanied by strong anomalous poleward EP flux, which lead to an extremely negative AO in the troposphere. For the WPV events with SSW, a wave train from the lower latitude over the North Pacific in the height field appears in the upper troposphere mainly during the growth stage. In the later stages, the tropospheric influence of the WPV events with SSW is relatively delayed and not robust, and the magnitude of AO index is much smaller than that for the WPV events without SSW.
Investigation on a severe precipitation event that is caused by the effect of an eastward propagating MCS originated from the Tibetan Plateau and a downstream southwest vortex
Available online:March 24, 2020 DOI: 10.3878/j.issn.1006-9895.1911.19206
Abstract:Based on the automatic station observed precipitation, Himawari-8 satellite temperature of black body and European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis data, we conducted an investigation on a severe precipitation event in which an eastward-propagating mesoscale convective system (MCS) that originated from the Tibetan Plateau (TP) affected a downstream southwest vortex (SWV), and caused heavy precipitation over the Sichuan Basin (the maximum 6 hours precipitation around the Sichuan Basin was up to 88.5 mm). Main results are as follows: this heavy precipitation event was mainly induced by the effect of an eastward-propagating MCS and a downstream SWV, with the strong rainfall appeared in the coupling stage of the MCS and SWV, within the cold cloud area of the MCS. The eastward-propagating MCS lasted for a total of 33 hours, during which it changed obviously in intensity. Overall, compared with the stage before vacated the TP, after moving out from the TP, the eastward-propagating MCS lowered in the height of its centroid of convection, but enhanced remarkably in its convection intensity. During lifespan of the eastward-propagating MCS, the SWV maintained a quasi-stationary behavior around the Sichuan Basin. This vortex lasted for about 21 hours, and persisted in a shallow layer which was mainly located in the lower troposphere. The eastward-propagating MCS affect the SWV significantly. In the coupling stage of the MCS and SWV, superposition of ascending motions associated with the two systems directly induced the heavy precipitation. After that, the MCS moved eastward, whereas the SWV changed little in its location, thus the eastward-propagating MCS decoupled from the SWV. This rendered an intensity reduction of the SWV as well as a precipitation decrease associated with the eastward-propagating MCS. Vorticity budget showed that the convergence effect dominated development and maintenance of the SWV. In addition, the tilting effect was the second most favorable contribution to the positive vorticity production below 800 hPa, while the vertical transport was another dominant factor for the positive vorticity enhancement associated with the SWV above 800 hPa. Overall, these two effects were beneficial to the downward and upward extension of the SWV, respectively. Correlation analysis showed that during development of the SWV, the cold cloud area of the eastward-propagating MCS (using -52℃ as boundary) could reflect variation of the SWV intensity (vorticity) effectively, with the largest correlation (up to 0.83) appeared in the two hours advance correlation.
Available online:March 24, 2020 DOI: 10.3878/j.issn.1006-9895.1911.19132
Abstract:The real-time forecast accuracy of summer precipitation in Northeast China (NEC) is significantly low from 1978～2018. In the last second years, when the prediction of the overall distribution of summer precipitation anomaly for the whole country is relatively accurate, the prediction of summer precipitation anomaly in NEC is contrary to the actual situation. Therefore, it is necessary to analyze the reasons for the low forecasting accuracy. In this paper, the forecasting ability of dynamic models and cognitive defect on forecasting summer rainfall in NEC are discussed. Also, by systematically reviewing the climatic characteristics, influencing factors and prediction methods of summer drought and flood in NEC, and the test of the real-time forecasting, the reasons are as follows: (1) the precipitation in early summer in NEC is mainly affected by the cold eddy activity in NEC, and in midsummer it is mainly affected by the subtropical high in the West Pacific, the southerly wind in the Northeast and the circulation pattern in the middle and high latitudes. However, the main dynamic climate models commonly used at home and abroad can’t accurately reflect the key circulation systems associated with precipitation in early summer and midsummer in NEC. (2) the relationship between the summer rainfall in NEC and global sea surface temperature (SST) is weak and unstable. Especially, the influence of ENSO on summer precipitation in NEC is complicated, the relationship between them varies from decadal to decadal; (3) the summer rainfall in NEC has obvious multi-time scale variability (inter-seasonal, inter-annual and inter-decadal time-scale) influenced by different circulation systems, which makes it more difficult to accurately predict summer precipitation in NEC. Finally, some scientific problems and possible solutions are further discussed for summer rainfall forecasting in NEC, which may be helpful for the future summer rainfall prediction in this area.
Observation and Analysis on Atmospheric Ice nucleating particles with Online Continuous Flow Diffusion Chamber in winter in North China
Available online:March 19, 2020 DOI: 10.3878/j.issn.1006-9895.1911.19194
Abstract:Ice nucleating particle (INP) concentrations are reported for a mountain site on the north-western margin of Beijing, during winter 2017, with the newly purchased continuous flow diffusion chamber(BJ-CFDC). Atmospheric INP concentrations characteristics are analyzed with weather elements, precipitation size distribution measurement and aerosol observation. The relationship among INP concentration, and activation temperature, super saturation with respect to water, aerosol with size larger than 0.5 m are developed. The result shows that atmospheric INP has a large variation at different activation temperature. It ranges between 2.50～76.8 L-1 at -20℃ with an average of 18.347 L-1. Atmospheric INP concentration increases exponentially both with lowering temperature and increasing super saturation respect to water. The result also shows atmospheric INP has a good correlation with the concentration of aerosol particles of sizes larger than 0.5 m. With this relation, the relevance of predicted and measured INP is higher than that the relationship built with only temperature. Atmospheric INP in snow days shows a characteristic of increasing firstly and decreasing later. INP concentration increase in the beginning of snow.The strong wind in the later period of snow scavenge the aerosols and lead to reduced concentration of INP. With the first continuous flow diffusion chamber type instrument used in China, the results of this paper will not only help to study the orographic cloud and precipitation in winter of north China, but also improve the support for development of cloud model and study in weather modification.
A Numerical Simulation of Urban Breeze Circulation Structure and its Turbulence Characteristics in Chongqing
Available online:March 18, 2020 DOI: 10.3878/j.issn.1006-9895.2003.19239
Abstract:To investigate the structure and evolution characteristics of urban breeze circulation in the mountain city, WRF V3.9 is used to simulate a typical urban breeze circulation case from August 17 to 18, 2016 in Chongqing. In addition, the characteristics of turbulent kinetic energy and turbulent fluxes during this period are also analyzed. The results show that the rural wind begins to appear at 15:00 and increases as the heat island strengthen. The circulation reaches its maximum at 18:00, and is destroyed at 02:00 in the next day. At 18:00, the horizontal scale of the circulation is about 1.5~2 times that of the urban scale, and the vertical scale is about 1.3 km, the horizontal wind speed is about 2~4 m?s-1, the maximum rising speed is about 0.5 m?s-1. Under the influence of topography, rivers and background wind, the circulation is asymmetrical in structure and weak in intensity. Besides, it is found that the turbulent kinetic energy in the urban area is obviously larger than that in the nonurban area, which results in the stronger transport of heat and water vapor by turbulence in the urban area. When it comes to the relationship between turbulent fluxes and urban breeze circulation, it shows that turbulent water vapor flux is affected by urban breeze circulation obviously, turbulent supply momentum for the dissipation caused by urban breeze circulation.
Available online:March 17, 2020 DOI: 10.3878/j.issn.1006-9895.1912.19223
Abstract:Based on the daily minimum temperature data from 529 stations in China over the period of 1961-2016, the spatio-temporal features of spring extreme low temperature (ELT) events are investigated. The rotated empirical orthogonal function (REOF) analysis indicates that, according to the variability of the ELT frequency, the whole China can be divided into five regions, which are Northeast China-eastern North China, the Jiangnan region, eastern Northwest China-western North China, Southwest China, and northern Xinjiang. The Morlet wavelet analysis shows that the ELT frequencies over the five regions exhibit a 2-4-year quasi-periodicity. Such a quasi-periodicity with 2-4-year is significant over the whole period for the Jiangnan region, eastern Northwest China-western North China, and northern Xinjiang, over the period before the early 1980s for Northeast China-eastern North China, and over the period between early 1980s and mid-1990s for Southwest China. For the long-term variation, the LET frequencies over all five regions exhibit decreasing trends, however their abrupt change years are different. The analysis using the Mann-Kendall (MK) and moving t-test (MTT) suggests that the abrupt change of the LET frequency occurs in 1987/1988 for Northeast China-eastern North China, 1995/1996 for the Jiangnan region, 1990/1991 for eastern Northwest China-western North China, 1987/1988 for Southwest China, and 1997/1998 for northern Xinjiang. Along with the decreasing of the LET frequency, the LET intensity over the five regions are weakened in the past half century. However, in recent 10 years, the frequency and intensity of the LET over eastern China are increased, which should be concerned.
The Relationship between the Early Spring Low Temperature Enhancement in the North China and Sea Surface Temperature in the North Atlantic since the 1990s
Available online:March 17, 2020 DOI: 10.3878/j.issn.1006-9895.1912.19127
Abstract:Based on the reanalyze data from European Centre for Medium-Range Weather Forecasts (ECMWF) and the sea temperature data from Hadley Center(Hadley) ,this paper analyzed the reason for the reason for the extrem low temperature enhancement in spring in the North China since 1990s and verified by numerical simulation results.The results indicated that the North Atlantic “Horse Hoof ”sea surface temperature mode has a significant correlation with the Eurasian Wave Train(EU) affecting North China. At the same time, the North Atlantic “Horse Hoof ”sea surface temperature mode has a close correlation with the vertical wave energy(TNZ2) in the key areas of the North Atlantic after 1997 and the 500hPa North Atlantic Oscillation(NAO) circulation mode in the North Atlantic after 1997 showed a trend of eastward movement and southward withdrawal. After 1997, the surface temperature on the eastern side of Greenland showed an increasing trend in positive region with anomalies thermal forcing. At the same time, the westerly jet stream increased, which could stimulate the Eurasian Wave Train and formed warm ridges on the downstream Eurasian continent. The Localized Multiscale Energy and Vorticity Analysis(MS-EVA) energy analysis method proves that anomalies thermal focing and pressure gradient force of the surfacee temperature of the key area on the eastern side of Greenland do positive work in the whole troposphere, leading to the increase of high-level kinetic energy and divergence to the outside, making the ridge strengthen to the North.Through EU wave train, the cyclonic anomaly in the downstream North China was strengthened, and the Asian polar vortex was strengthened and maintained. The temperature in the North China decreases sharply and the extreme low temperature events increase. Finally, the CAM5.1 model simulation is used to study the effects of the North Atlantic “Horse Hoof ”sea surface temperature mode on atmospheric circulation anomalies and the extreme low temperature in the North China. The simulation results verify the observation results well. It further shows that the North Atlantic “Horse Hoof ”sea surface temperature mode can affect the atmospheric circulation anomalies in the Eurasia by stimulating the Eurasian Wave Train, thus leading to the intensification of cyclone and meridional circulation and the increase of extrem low temperature events in the North China.
Available online:January 15, 2020 DOI: 10.3878/j.issn.1006-9895.1912.19203
Abstract:Aiming at promoting the application of new-type sounding data in NWP (Numerical Weather Prediction) models, this paper presents a basic research work of return radiosonde data. Based on the firstly archived return radiosonde observation datasets in China, a quality control scheme for future operational implementation purpose is established. By comparing and analyzing the statistical characteristics of observation samples before and after the quality control, the rationality of the quality control method is demonstrated. After the quality control procedure, the sampling distribution of the detection variables is more reasonable, and the inner-consistency of variables is also improved. An uncertainty analysis of return radiosonde data is then carried out by referring to the high resolution NWP model forecast field and the conventional sounding observation data of the same site. The results show that the precision of return radiosonde has reached the breakthrough target defined by WMO (World
Contrasting Salinity Interannual Variability in the Tropical Pacific and Its Effects on Recent El Ni?o Events: 1997/1998, 2014/2015, 2015/2016
Available online:January 15, 2020 DOI: 10.3878/j.issn.1006-9895.1912.19172
Abstract:Ocean salinity variation provides a new insight into related ENSO expressed by climate variability. In this study, the comparisons of salinity variability and its related dynamical processes responsible for SSTA have been extensively described and analyzed among two strong ENSO, 1997/1998, 2015/2016 and one special ENSO, 2014/2015. The study shows that the development of ENSO is significantly associated with the occurrence and eastward diffusion of large-scale surface salinity anomaly (SSSA) in the western-tropical Pacific. In April of 1997 and 2015 corresponding to two strong ENSO events, there was a significant negative SSSA in the western-central Pacific, and then it moved eastward to the west of the dateline which induced the mixing layer depth (MLD) shallow, and the barrier layer thickness (BLT) thickened, which enhanced the surface warming in the tropical central Pacific and the early warming in the equatorial eastern-central Pacific. Although negative SSSA occurred in April of 2014/2015 weak event in the equatorial western-central Pacific, it did not develop eastward, resulting in weakened thickening process of BLT and weak modulation effect on surface temperature. For salinity change process corresponding to three ENSO events, surface advection and surface forcing caused by fresh water flux (FWF) were the major contributors to salinity budget. Surface advection influenced the former variability of salinity tendency, inducing the occurrence of ENSO signal. The precipitation in the tropical western Pacific played the most significant negative influence on FWF, which made a decisive role in the occurrence of SSSA and in the development of ENSO. Compared with the two strong ENSO events, the early FWF negative anomaly in 2014/2015 did not develop and moved eastward and weakened rapidly, resulting in a slowing of the negative salinity tendency in the western-central Pacific, deepening of MLD, thinning of BLT, and rapid cooling of the surface layer, which inhibited the early warming in the equatorial-eastern Pacific. The results of study demonstrate that the salinity change is closely related to ENSO, and early SSS in the tropical western-central Pacific could be used as the index of SSTA. In particular, SSSA not only affects the strength of ocean SSTA, but also could be used as a precursor to judge the development and strength of ENSO.
Available online:January 15, 2020 DOI: 10.3878/j.issn.1006-9895.1912.19168
Abstract:This study projects the change in the environmental fields and genesis potential index of typhoon (GPI) over the western North Pacific (0°–40°N and 100°–180°E) at the end of the 21st century (2080–2099) using outputs from the Coupled Model Intercomparison Project Phase 5 (CMIP5) RCP4.5 and RCP8.5 experiments undertaken by 19 climate models. These models are capable of reproducing modern background fields of typhoon reasonably and are selected out for the analysis. Compared to the reference period of 1986–2005, there appears increased sea surface temperature (SST) over the western North Pacific, weakened vertical wind shear (VWS) and decreased outgoing longwave radiation (OLR) over the key regions where there are significantly negative correlations between these factors and the typhoon frequency, beneficial to the formation and development of typhoon; in contrast, the low pressure system extending from the mainland to the South China Sea is weakened, suppressing typhoon activities. Overall, the changes of environmental fields of typhoon under the RCP8.5 scenario are larger than under RCP4.5 scenario. Further, the signal to noise ratio is examined for measuring consistency across individual models. It is shown that such a ratio is higher than 3.0 for the change of SST and larger than 1.0 for sea level pressure over regions under the domination of low pressure system; for changes of VWS and OLR, the ratio of less than 0.6 denotes a degree of disagreement across models; nevertheless, models agree well on the sign of the change of OLR in regions associated with typhoon activities. The aforementioned changes of environmental fields of typhoon are in line with increased GPI in the future.
Correction for the cloud top height of cirrus with MODIS and CALIPSO dataset in Beijing-Tianjin-Hebei region
Available online:December 23, 2019 DOI: 10.3878/j.issn.1006-9895.1911.19181
Abstract:Cirrus plays an important role in atmospheric radiation. It affects weather system and climate change. Satellite remote sensing has great advantage in cirrus detection, relative to traditional observation. As a passive remote sensing instrument, large deviations are found at thin cirrus cloud top height from MODIS (Moderate Resolution Imaging Spectroradiometer). Comparatively, CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) aboard CALIPSO (Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation) which is an active remote sensing instrument, can acquire more accurate characteristics of thin cirrus cloud. In this study, MODIS cloud products in Beijing-Tianjin-Hebei region from 2013 to 2017 are selected. Using CALIPSO cirrus cloud top height data, a linear fitting method is obtained based on cross-validation method, and MODIS cirrus cloud top height is corrected. The difference between MODIS and CALIPSO changes from -3~2 km to -2.0~2.5 km, and the maximum difference changes from about -0.8 km to about 0.2 km. In the context of different vertical levels and cloud optical depth, MODIS cirrus cloud top height is improved after correcting, which is more obvious at lower cloud top height and optical thinner cirrus.
Available online:December 20, 2019 DOI: 10.3878/j.issn.1006-9895.1911.19183
Abstract:In the TRAMS_RUC_1km (Tropical Region Assimilation Model for South China Sea _ The Rapid Update Cycle _ 1km) model, the initial field and lateral boundary condition scheme was revised (including its vertical resolution, time update frequency and completeness) and tested with a typical squall line case. Preliminary analysis revealed: (1)The meso-scale character of precipitation in squall line was better simulated after increasing vertical resolution of initial field and lateral boundary, while increasing vertical resolution of middle and low layers played the most critical role in it. (2)The diurnal change of lateral boundary forcing got more accurate after increasing its time update frequency. The squall line moved south after using high frequency updated boundary condition, because of the convergence of moist flux alone coastline was strengthened. (3)With high vertical resolution and high update frequency of lateral boundary condition, lateral forcing of vertical speed and cloud particle had no obvious impact on forecast of squall line. In general, it was important to increasing the vertical resolution of initial field and lateral boundary field, the time update frequency was also needed to be increased, while the lateral forcing of vertical speed and cloud particle could be omitted. The impact of new scheme was verified against observation in April of 2019 (last for a month). The distribution pattern and diurnal cycle of mean hourly precipitation were improved significantly with the revised initial and lateral condition.
Available online:December 19, 2019 DOI: 10.3878/j.issn.1006-9895.1912.19217
Abstract:Based on summer precipitation data from North China and MJO Index, NCEP/NCAR reanalysis circulation data, the paper uses a variety of statistical methods to analyze the relationship between MJO and summer precipitation in North China and its influence mechanism. The results show that: Summer, MJO has a good relationship with summer precipitation in North China. When MJO enters the 5th and 6th phases, there will be a clear precipitation process in summer in North China. In terms of the influence mechanism, MJO induced the occurrence of favorable water vapor transport anomaly in the summer of North China through the propagation of high and low disturbances to medium and high latitudes, resulting in the occurrence of heavy precipitation in North China during the summer. Specifically, in terms of water vapor conditions, MJO can enhance the transmission of south wind vapor (corresponding to RMM1) or southeast wind vapor(corresponding to RMM2) in North China by affecting the 850 hPa wind field, thus contributing to the occurrence of summer precipitation in North China; In terms of dynamic rising conditions, MJO has contributed to the summer precipitation process in North China by affecting the deepening of the 500 hPa Baikal trough and the strengthening of the subtropical high near the Korean peninsula, as well as the enhancement of the tropospheric upward movement over North China. MJO can be used to make extended period prediction of summer precipitation in North China.
Contribution of Tropical and Subtropical Circulation Anomalies to the Intensity of East Asian Winter Monsoon over Lower-latitude Region
Available online:December 09, 2019 DOI: 10.3878/j.issn.1006-9895.1911.19141
Abstract:The East Asian winter monsoon (EAWM) has two dominant modes: the in-phase and out-of-phase variations of wind anomalies over northern and southern China. Different from the first mode reflecting a uniform strong/weak situation of the EAWM throughout eastern China, the second mode indicates a situation that the intensity of low-latitude EAWM over southern China is independent from and even opposite to that of mid-high-latitude EAWM over northern China. The present study focuses on the characteristics of tropical and subtropical circulation anomalies associated with the variability of low-latitude EAWM under the background of the second mode. The results reveal that the inter-tropical convergence zone (ITCZ) can be considered as an important circulation system that contributes to the variation of low-latitude EAWM. Corresponding to a stronger and northward-extended ITCZ, convective ascending over the tropical western Pacific and South China Sea is strengthened. This anomalous ascending may induce low-level anomalous northerly wind, thus resulting in a stronger low-latitude EAWM. In addition, the subtropical upper-level jet can be regarded as another important circulation system affecting the low-latitude EAWM. An increase in wind speed along the axis of the jet may cause anomalous quasi-geostrophic northerly near the entrance of the jet. Associated with the forced positive secondary circulation anomaly with anomalous descending (ascending) to the north (south) of the jet, the low-level anomalous northerly appears under the jet, in turn, facilitates a stronger low-latitude EAWM. Finally, both the individual and joint effects of tropical convective activities and upper-level subtropical jet on the low-latitude EAWM are further investigated. Relatively, the influence of ITCZ seems more important. When the two circulation anomalies simultaneously enhance (i.e., more active convective of ITCZ and stronger wind speed along the subtropical upper-level jet), their joint effect can significantly reinforce northerly winds to the south of 35°N, over southern China, and vice versa. The above-mentioned results imply that, the variability of low-latitude EAWM is not only affected by cold air surges from northern China, but also modulated by the joint effects of tropical and subtropical circulation anomalies.
A second-order vertical difference scheme for non-uniform distributed layers and its application in GRAPES model
Available online:November 20, 2019 DOI: 10.3878/j.issn.1006-9895.1906.19145
Abstract:Under non-uniform distributed layers, the vertical difference used in GRAPES (Global/Regional Assimilation and Prediction System) model can only achieved one order accuracy. A second order scheme was designed and introduced into GRAPES model for the process of vertical discretization. Ideal test with 1-D profile showed the new scheme could improve the accuracy of difference computation, while the improvement was not so markedly compared to the error caused by difference process itself. Density flow ideal test was conducted to verify the correctness and stability of new scheme in GRAPES model. A statistical evaluation of medium-range with second order scheme showed an improvement of forecast skill in large-scale fields, especially for the forecast after 120 hours. In addition, the second order scheme was tested with a real-case experiment for the extreme rainfall at South China, which again indicated improvement for the forecast of precipitation after 48 hours.
Contributions of Local and Remote water vapor transport to Precipitation variations over Songhua River BasinYongsheng Li1, Lixia Zhang2 and Bo Wang1
Available online:November 05, 2019 DOI: 10.3878/j.issn.1006-9895.1909.19177
Abstract:The relative contributions of the local and remote atmospheric moisture fluxes to summer precipitation over Songhua River Basin and its interannual variability are investigated using Brubaker model in this study. Climatologically, due to the prevailing westerly wind in early summer (May-June), the remote atmospheric moisture is the dominant contributor to early summer precipitation, which accounts for 78.9%. Accordingly, the local evaporation contribution is 21.1%. In late summer (July-August), the East Asian summer monsoon brings more moisture via the south boundary, the contribution of the remote moisture increases to 86%, and the contribution of the local evaporation reduces to 14%. JRA-55 can well capture the interannual variation of summer precipitation over Songhua River basin, with the correlation coefficients of precipitation in early summer and late summer for 1961-2016 at 0.73 and 0.83, respectively. It shows that the moisture flux via the southern boundary caused by stronger southwest monsoon plays the dominant role in early summer, and the moisture fluxes via the western and northern boundary are significantly negatively correlated with the early summer precipitation. The contribution of local evaporation is not statistically significant. The moisture fluxes anomalies tend to occur in the early summer with El Ni?o decaying. In late summer, the significantly positive contribution is from the moisture flux via the southern boundary and negative contribution from the local evaporation. The oceanic forcing on late summer precipitation anomaly is not significant, and internal atmospheric variability dominates. The significant negative contribution of local evaporation is due to significant negative correlation between surface temperature and precipitation. When precipitation is less than normal, surface temperature is warmer than normal, thus more evaporation and more contribution from local evaporation to precipitation.
Beijing Lightning Network and ime-space evolution characteristics of lightning during a thunderstorm
Available online:November 04, 2019 DOI: 10.3878/j.issn.1006-9895.1910.19161
Abstract:Beijing Lightning NETwork (BLNET) is a regional full-flash three-dimensional（3D） location network combining research and business. In 2015, the BLNET hardware, station network layout and location algorithm were updated and upgraded to improve the sensitivity of the sensor and improve the computational efficiency and detection performance. BLNET not only has the functions of intra-cloud（IC） flash, cloud to ground（CG） flash pulse type identification and current peak estimation, but also realizes the 3D real-time location of lightning radiation pulse and the fine location of the channel-resolvable lightning discharge process. The analysis of the real-time 3D location results of the lightning radiation source pulse during a thunderstorm on July 7, 2017 shows that a total of 11,902 lightning flashes were observed during the thunderstorm process. Most of which were dominated by IC flash, and the CG flash accounted for 28% of the total flash. The positive cloud to ground （PCG）flash only accounts for 5% of the total CG flash. During the mature period of the thunderstorm, the maximum lightning frequency is 927 times/6 minutes. By comparing and analyzing the location of the lightning source and the radar echo at the corresponding time, it is found that the radiation source is basically concentrated in the strong echo range. The fine location of a PCG flash indicates that the initial stage shows a clear pre-breakdown process. The origin of the lightning source is about 5.4 km above sea level, and then the channel develops upwards. At about 10 km, the channel begins to follow. Level development. The fine location of a negative cloud to ground（NCG） flash indicates that the discharge first originates from a height of about 7.1 km, the channel develops to the south, and some negative pilot branches develop downward. After about 38 ms, the channel stops developing for a short time. After 17 ms, the channel begins. The hair is re-energized. The above results show that BLNET can not only locate and monitor the 3D real-time lightning activity of the whole thunderstorm life history, but also realize the fine location of the lightning three-dimensional discharge channel.
Available online:November 04, 2019 DOI: 10.3878/j.issn.1006-9895.1910.19134
Abstract:Influenced by the Northeast China cold vortex and warm-moist airflow in low level, a damaging thunderstorm with 5 hail-fall stages took place in Beijing on 10 June 2016. Based on the 3D-location results of total lightning from Beijing Lightning Network (BLNET) and Doppler radar data during the STORM 973 2016 campaign, the characteristics of lightning activity and radar reflectivity structure during this thunderstorm were analyzed. The thunderstorm consisted of three isolated cells triggered in sequence and merged together finally, total lightning frequency increased significantly during the 4 hail-falling stages analyzed, up to 365 fl/min. IC (intra-cloud) lightning flashes accounted for more than 80% of total lightning. The ratio of PCG/CG (positive CG to CG) increased sharply before 3 hail fall stages, up to 56%. During the developments of hailstorm, the area of radar echo greater than 45dBZ increased and echo top exceeded 13km. Lightning radiation sources mainly distributed in the altitude layer from 6 to 10 km throughout the hailstorm process, which was consistent with strong radar echo region. In addition, the total lightning flashes increased dramatically before the 4 analyzed processes, and passed 2б-threshold test. Three of them were 8-18 minutes ahead of the hail-fallings, which shows that the total lightning frequency has a certain early warning ability for hail fall process.
Distribution of Different Cloud Types and their effects on Near-surface Air Temperature during Summer Daytime in Central Eastern China
Available online:October 31, 2019 DOI: 10.3878/j.issn.1006-9895.1909.19160
Abstract:The spatial-temporal variation characteristics of the daytime cloud fraction and cloud optical thickness of various cloud types over central and eastern China in summer are explored based on ERA5 reanalysis data and CERES satellite data during the period of 2001-2017. Then, the effects of various cloud types on near-surface air temperature are analyzed quantitatively using a Radiative-Convective Model. The observations show that the annual mean daytime total cloud fraction and its optical thickness decrease gradually from south to north, while upper-middle cloud fraction dominates the total cloud fraction over central and eastern China in summer. For annual mean changing rates, the total cloud fraction shows a significant decrease of 0.3% a-1 with the largest contribution from low cloud (-0.27% a-1).The increasing trend of total cloud optical thickness ranges from 0 to 0.1 a-1, with low and lower-middle cloud optical thickness show an increasing trend of 0.06 a-1 and 0.03 a-1, respectively, while the upper-middle and high cloud optical thickness show a decreasing trend of 0.08 a-1 and 0.03 a-1, respectively. The model results show that the annual mean CETs of the four different cloud types are negative, with values of 2.9 ℃, 2.7 ℃, 2.2 ℃, and 1.7 ℃ for low, lower-middle, upper-middle and high cloud, respectively, indicating the cooling effects of various cloud types. The low cloud CET in the North China Plain can be up to -5 ℃, while the lower-middle and upper-middle cloud can both come up to -7.8 ℃ in Sichuan Basin and Yunnan-Guizhou Plateau. The interannual variations of CETs of different cloud types and near-surface air temperature have a good consistency. The near-surface air temperature decreases (increases) before (after) 2004, while the CETs of different cloud types decrease (increase) during this period, which indicates a good correspondence between the strengthening (weakening) of the cloud cooling effect and the decrease (increase) of near-surface temperature. It means, in other words, a positive correlation relationship appearing in four cloud types and near-surface air temperature over central and eastern China during daytime in summer. In particular, the annual mean daytime upper-middle cloud fraction plays an important role in all types of clouds over central and eastern China in summer, and the correlation coefficient between the CET and near-surface air temperature is as high as 0.63. In summary, the effects of different cloud types on near-surface air temperature over central and eastern China in summer are different, but all of them show positive correlations. The quantitative analysis of the influence of different cloud types on near-surface air temperature can provide scientific references for the accurate measurement of global warming, the role of cloud feedback in regional warming, and the accurate prediction of regional warming scenarios, respectively.
The Differences and Possible Mechanisms of Inter-decadal Trend Shifts of Latent Heat Flux in Western Boundary Currents on the Warming Hiatus Background
Available online:October 31, 2019 DOI: 10.3878/j.issn.1006-9895.1907.19137
Abstract:The authors researched the inter-decadal trend shifts of latent heat flux (LHF) over the Kuroshio extension (KE) region and Gulf Stream (GS) region during the warming period and the warming hiatus period by using the latent heat flux data and relevant variables produced by the Objectively Analyzed Air-Sea Fluxes project (OAFlux) of the Woods Hole Oceanographic Institution, and the Ishii subsurface temperature and salinity data from Japan Agency for Marine-Earth Science and Technology. Small perturbation method, EOF analyzation, and International Thermodynamic Equation Of Seawater – 2010 (TEOS?10) are applied in this research. Contrast inter-decadal trend shifts of latent heat flux exist in KE and GS region. The inter-decadal LHF trend of KE region shifts from positive to negative around 2001, while the GS region shifts from negative to positive around 1993. The variation of KE region is primarily resulted from sea surface temperature change (ocean-induced) , while in GS region it is resulted from wind speed (1979~1992) (atmosphere-induced) and sea surface temperature (1993~2013).The inter-decadal variations of seawater heat content in KE and GS regions are also different: the inter-decadal variation of surface heat content in KE region is consistent with mixed layer; the inter-decadal variation of surface heat content in GS region is different from that in deep layer, while the changes below surface layer are more consistent; the deep heat content changes in both regions reflect the phenomenon of warming hiatus, and KE region may have the influence of the lower layer to the upper layer; while GS region may have the influence of the upper layer to the lower layer. The difference between the surface of KE and GS can be attributed to the difference between ocean and atmospheric factors, and the vertical difference of inter-decadal variation of internal heat content may be attributed to the structural difference between the two regions. All of those variations are associate with the warming hiatus, and may affect the warming hiatus conversely.
A Record-breaking Heavy Rainfall Event over Southern China During August 2018: Analysis of Observation and Mechanism
Available online:October 31, 2019 DOI: 10.3878/j.issn.1006-9895.1906.18265
Abstract:An extreme daily rainfall (maximum of 1056.7mm) induced by a long-lived linear-shaped mesoscale convective system (β-MCS) occurred over Gaotan town (GT) of Guangdong province during 30-31 August 2018, which broke the historical record in Guangdong province, caused severe flash flood and aroused social concern. Based on multiple observation data and NCEP/NCER_FNL reanalysis, we performed properties of the precipitation, convective systems evolution, synoptic and meso scale environmental conditions, initiation and maintenance of the β-MCS. It is shown that tropical cloud clusters move northward and induce large-scale heavy rainfall, with background of monsoon depression as well as mesoscale favorable environment of high-thermal, high-humidity. Extreme rainfall over GT is caused by a linear-shaped β-MCS, characterized by back-building, over-long-lived, quasi-stationary, low echo-top-height and low echo-convective-centroid. Organization of β-MCS is closely related to near-surface wind field, which is affected by multi-scale systems that we qualitatively analyze using rotation rate equation of sea and land breezes. The southerly flow is able to sustain for a long period that is determined by the forcing of multiple interaction, that monsoon depression and local terrain induce southerly flow to rotate counterclockwise, but barometric gradient induces southerly flow rotate clockwise, and take a balance finally. Southerly flow on the side of river valley HJ over slope terrain strengthening helps the warm-ridge development of temperature field, blocking cold pool outflow boundary move southeastward on the side of mountain over slope terrain, leading to sharp temperature gradient over that region. Quantitative diagnosis using mesoscale atmospheric dynamics equation shows the dynamic mechanism to sustain convection maintenance and β-MCS organization stems from local vertical wind shear, causing by the sharp temperature gradient.
Influence of tropical cyclone activities on WPSH meridional movement and its possible mechanism: a case study
Available online:October 31, 2019 DOI: 10.3878/j.issn.1006-9895.1907.18257
Abstract:Characteristics of meridional movement of western Pacific subtropical high (WPSH) ridge lines at different heights under the influence of a single tropical cyclone (TC) ——Megi and its possible mechanism are analyzed by conducting sensitivity experiment with WRF. The results show that the WPSH ridge lines shift southward under the influence of TC Megi. Furthermore, the higher the ridge lines are, the more southward they move. The possible mechanisms are that the shift of WPSH ridge is directly affected by the zonal wind anomalies in its vicinity regions, and the zonal wind anomalies and temperature anomalies near the ridge line caused by TC Megi activities generally satisfy the thermal wind relationship. Therefore, when the temperature gradient anomalies near the ridge line are positive under the influence of TC Megi, the westerly anomalies will strengthen with altitude, which will lead to the southward movement of ridge increasing with altitude, too. In addition, the diagnostic analysis results of temperature tendency equation show that the horizontal advection anomalies and non-adiabatic heating anomalies caused by TC Megi mainly lead to the abnormal increase of atmospheric temperature, while the vertical transport anomalies are always beneficial to the abnormal decrease of atmospheric temperature. It is concluded that thermal effect of TC plays an important role in the process of changing the vertical distribution of WPSH ridge lines.
Available online:October 31, 2019 DOI: 10.3878/j.issn.1006-9895.1907.18254
Abstract:As the latest generation of geostationary meteorological satellite in our country, a great development has been made for Fengyun-4A(FY-4A). Compared with the previous generation(Fengyun-2),FY-4A has higher observation accuracy and shorter scanning time. Taking full advantage of AGRI data, the level of weather and meteorological disasters forecasting in countries along the " The Belt and Road Initiatives" will be effectively improved. The interface for the FY-4A AGRI (Advanced Geosynchronous Radiation Imager) data assimilation is complemented in WRFDA v3.9.1 (Weather Research and Forecasting model’s Data Assimilation v3.9.1) before investigating the bias’s characteristics of FY-4A AGRI data based on RTTOV v11.3 and GFS analysis. Bias-correction experiments of FY-4A AGRI data in infrared channel 8-14 are further conducted. The results show that: 1) Channel 8-10 and 14 have warm biases. There are cold biases in channel 11-13. Because the biases and standard deviation of water vapor channels 9 and 10 are small. The characteristics of biases show obvious differences between land and ocean in channels 11-14. The land’s biases are more complex than ocean’s. For these channels, observations on land can be eliminated in quality control. 2) The slope of linear regression equation between bias and satellite zenith angle is less than 0.035. There is no obvious dependence of biases on the satellite zenith angle.3) The bias in channels 8 and 11-14 show more obvious dependence on scene temperature than those in channels 9-10. 4) The variational bias correction experiment during 1800 UTC 13-15 May 2018 shows that the systematic bias has been corrected effectively.
Available online:September 29, 2019 DOI: 10.3878/j.issn.1006-9895.1908.19144
Abstract:In order to obtain more accurate winter precipitation data, this paper aims at the correction and error calculation of the influence of near-surface horizontal wind during snowfall measurement using PARSIVEL2 (Particle Size and Velocity). The revised results show that under certain wind speeds, the influence of ignoring wind will cause significant underestimation of the diameter of large particles, while for particles in the same size, larger wind speed means the underestimation of particle diameter during calculation is more obvious. When the wind speed does not exceed 2m·s-1, the calculation error of the falling speed of the snowfall particles is about 3%, and the calculation error of the diameter is within 7%. In the analysis of the real snowflake spectrum obtained during a snowfall in Nanjing on January 4th, 2018, it can be seen that ignoring the influence of wind will lead to the shift of the peak of the snowflake spectrum and the narrowing of the spectrum, which will result in the overestimation of concentration of small particles and underestimation of the concentration of large particles, which in turn affect the calculation of microphysical quantities. Specifically, the radar reflectivity factor Z and the snowfall intensity I are underestimated, and the actual value of the Z-I relationship fitting coefficient a is greater than the calculated value, and b is smaller. However, when the wind speed is larger, the flow near the ground is more complicated, and the vertical turbulent motion cannot be ignored. This correction method is likely to be no longer applicable.It is recommended to add a windbreaker in future observations or to add corrections in subsequent data processing to eliminate the impact of wind on snowfall measurements.
Available online:July 09, 2019 DOI: 10.3878/j.issn.1006-9895.1907.19123
Abstract:In this study, the CORE-IAF (Coordinated Ocean-ice Reference Experiments - Interannual Forcing ) dataset is used to force two ocean models: LICOM3 (LASG/IAP Climate System Ocean Model Version 3) and POP2 (Parallel Ocean Program version 2). The North Equatorial Countercurrent (NECC) simulated by these two models has been found weaker than the observation. These results are consistent with the findings of Sun et al (2019), which further suggests that the surface wind stress and its curl is the most important forcing term for correctly simulating the NECC in ocean models. At the same time, the differences in NECC dynamical mechanisms between LICOM3 and POP2 are also analyzed, including the wind stress, advection and other terms. In spite of the same CORE-IAF dataset is used to force these two ocean models, the influences of dynamical forcing terms (the wind stress, advection and other terms) are not exactly same.
Three-Dimensional Circulation Structure of Summer Heavy Rainfall in the middle-lower reaches of the Yangtze River Valley
Available online:July 09, 2019 DOI: 10.3878/j.issn.1006-9895.1905.19119
Abstract:By using the daily station data and Japanese 55-year Reanalysis Project (JRA-55) data in the course of 1959-2013, a three-dimensional background circulation structure of summer heavy rainfall in the middle-lower reaches of the Yangtze River Valley (MLYRV) is analyzed. Composite analyses of circulation in advance of 373 heavy rainfall days reveal that a prominent warm anomaly with a center at 300hPa emerges in the upper troposphere over MLYRV. Because of hydrostatic and quasi-geostrophic equilibriums, an anticyclonic (cyclonic) anomaly forms above (below) the warm center. On one hand, the warm anomaly makes the westerly winds to the north of the warm center strengthened by high-level anticyclonic circulation anomaly, which results in the jet stream in the upper-level over East Asia shifting southward and eastward to the north side of MLYRV. So the upper-level divergent anomaly field over MLYRV is enhanced. On the other hand, the cyclonic anomaly below the warm anomaly reinforces the low-level southwesterly winds to MLYRV, which makes more water vapor transporting to MLYRV and convergence stronger. The favorable configuration of high and low altitude circulation anomalies caused by warm anomalies plays an important role in the formation of strong precipitation in MLYRV. The 300 hPa warm anomaly exists at 400-300 hPa in the eastern part of the Qinghai-Tibet Plateau 48 hours before the precipitation in MLYRV, and the 700-hPa cyclonic circulation appears in the middle and lower layers over the Sichuan Basin 24 hours ahead of schedule. The high and low circulation elements cooperate with each other and move eastward with time. The warm anomaly first reaches MLYRV, and cooperates with the low-level cyclonic circulation and water vapor convergence area, resulting in strong precipitation in MLYRV.
Available online:July 09, 2019 DOI: 10.3878/j.issn.1006-9895.1905.19115
Abstract:The spectral width of cloud droplet spectra is crucial to the parameterization of the cloud optical depth, the assessment of the indirect aerosol effect and the formation of precipitation in numerical simulation. Based on the aircraft observations of cloud microphysics in stratocumulus on July 19, 2008 during the Physics of Stratocumulus Top (POST) field campaign, the vertical distribution of microphysical properties and cloud droplet spectra are analyzed, as well as the cloud microphysical processes. The results show that the spectral width of cloud droplet spectra is basically larger near cloud base which is caused by aerosol nucleation. The spectral width decreases as height increases in the middle layer of the cloud, which is caused by condensation growth. The increase of the spectral width near the cloud top is caused by the entrainment-mixing processes. The increase of vertical velocity in the adiabatic cloud increases the cloud droplet number concentration by promoting the activation of cloud condensation nuclei (CCN), and increases the cloud droplet size and leads to the decrease of the spectral width by promoting the condensation growth, and ultimately leads to the negative correlation between the spectral width and the cloud droplet number concentration as well as the cloud droplet size. The decrease of vertical velocity caused by entrainment-mixing processes in cloud holes decreases the cloud droplet number concentration and the cloud droplet size and increases the spectral width by promoting the evaporation of cloud droplets, which will be enhanced by the decrease of adiabatic fraction. Based on these observations, it is recommended that the parameterization of the spectral width of cloud droplet spectra takes into account vertical velocity, cloud droplet number concentration, cloud droplet size, adiabatic fraction, etc.
Study on Temporal and Spatial Distribution Characteristics of Temperature Increasing Trend Caused by Unnatural Factors
Available online:July 09, 2019 DOI: 10.3878/j.issn.1006-9895.1906.19106
Abstract:Based on the long-term correlation of data, the relative change trend is used to construct the probability density function and the exceedance probability. The confidence limit of temperature relative change trend belonged to natural variability category is studied and calculated under a certain confidence level in China from 1951 to 2017. Determine whether the relative change trend is caused by unnatural factors (whether the temperature increase is significant), and explore the threshold value of temperature changes caused by unnatural factors in different regions, the corresponding transition time period and the evolution trend. The results show that: (1) 10% of the site temperature in Chinese 160 stations are overestimated when using traditional linear regression methods to calculate trends significance . These sites are mainly located in the northwest, southwest and east coastal areas of China. (2) From the perspective of the spatial distribution of temperature trends in the country, except for the cooling trend in the central and western regions of Xinjiang, the other regions are all warming trends. The relative temperature changes in Shanxi, Inner Mongolia, parts of Ningxia, southwestern Xinjiang, Yangtze River Delta and southwestern Yunnan are relatively large. The unnatural trends in Northeast China, Inner Mongolia and the northern Shanxi Province are large, and the temperature increase is significant. (3) From the spatial evolution of the significant inter-decadal warming areas, the North China and Northeast China regions took the lead in increasing temperature, and then gradually expanded to the south and west. During the period of 1966-2001, most of Chinese regions showed an increase in unnatural factors; for 1971-2006, the northeastern region and the northeastern Inner Mongolia region began to gradually decrease in temperature, while the significant warming area in southwestern China began to increase gradually; the number of significant warming sites was the largest during 1976-2011; from 1981 to 2016, the significant warming sites was mainly concentrated in the Yellow River, the Yangtze River Basin and the two major river basins and the southern part of China. In summary, there are prominent inter-decadal transitions in both time and space in Chinese significant warming areas caused by unnatural factors. This paper maybe provide new perspectives and new ways for the attribution and prediction research of temperature change in China , and strengthen the transformation and linkage of climate change research results to short-term climate prediction.
The analysis of the influence of dynamic and thermal effects on the sea temperature in the tropical Pacific Ocean during the spring
Available online:July 09, 2019 DOI: 10.3878/j.issn.1006-9895.1904.18270
Abstract:The spring predictability barrier (SPB) of El Ni?o-Southern Oscillation (ENSO) is a difficult problem in the ENSO prediction. To understand how the dynamic and thermal factors affect the variability of sea surface temperature over the tropical Pacific Ocean during the spring is very important to understand the changes of SST in key area and resolve the SPB problem. In this work, a set of monthly data including sea surface wind stresses, sensible heat flux, latent heat flux, longwave radiation, shortwave radiation, and ocean current fields which are coordinated with each other during 1986 -2017 are generated by numerical model simulation. Based upon these data, we analyzed and diagnosed the dynamic and thermal influences and their contributions to the variability of the sea surface temperature (hereafter, ). The main results show as follows: (1) Compared with other seasons, the sea surface temperature presents a unique asymmetric seasonal shift that is from warming to cooling in the Ni?o3.4 area during the spring. It is due to a similar shift in the wind stress, net energy fluxes, and ocean current which have a robust relationship with . Further analyses demonstrate that the thermal effect plays an important role in the variability of local . Differently, the meridional advection always shows a negative contribution to the seasonal variability of . Meanwhile, the zonal advection terms turn into a cooling effect from a warming effect to during the spring, and the vertical advection effect is quite weak. (2) The interannual correlation between the tendency of anomaly and the dynamic/thermal effect shows that the thermal heating is positively associated with the Ni?o3.4 anomaly in the spring, as well as the zonal advection. However, the correlation between the meridional advection and the anomaly changes from negative to positive during the spring. (3) The quantitative analysis of the dynamic and thermal variance contributions in the Ni?o3.4 region also suggests that, the contribution rate of thermal effect is more than 50%, and the corresponding correlation coefficient is over 0.7. The contribution of zonal and meridional advection is about 10- 20%, respectively, but they are opposite to each other. And the other items provide less contribution.
The Stratiform cloud over the Eastern China Simulated by Two Versions of LASG/IAP Climate System models
Available online:July 04, 2019 DOI: 10.3878/j.issn.1006-9895.1903.18249
Abstract:The Eastern China, which is located in the leeside of Tibetan Plateau, is covered by stratiform cloud in winter. This kind of stratiform cloud is unique along the same latitudes of the global subtropical continent. A reasonable simulation of the stratiform cloud over the leeside of the Tibetan Plateau remains to be challenge for the current state of the art climate models. In this study, we compared the cloud properties and their radiative forcing at the top-of-the atmosphere from 2 IAP/LASG climate system models, FGOALS-s2 and FGOALS-g2, to observations from International Satellite Cloud Climatology Project (ISCCP). It is found that both two models underestimate the strength of short-wave cloud forcing, cloud water path and cloud fraction over Eastern China. FGOALS-s2 can reproduce the domination of low stratiform cloud over Eastern China, while FGOALS-g2 overestimates the proportion of high cloud. Such kind of bias in the simulation of stratiform cloud is related to the weak low tropospheric stability and insufficient moisture in low level. In addition, the overestimated average cloud top height in FGOALS-g2 is explained by the unrealistic updraft, which produces stronger vertical moisture transports over Eastern China.
Available online:June 20, 2019 DOI: 10.3878/j.issn.1006-9895.1904.19105
Abstract:The process of making landfall of tropical cyclone Mujigae (1522) are simulated at high resolution by using WRF (Weather Research and Forecasting) Model. The verification demonstrates that the simulation agrees well with a variety of observations, especially for the track, intensity and precipitation distribution. Then the kinematic and thermodynamic structure and the wind distribution during making landfall are analyzed and the wind field is diagnosed from an azimuthal momentum budget based on the simulation data. The analysis of the budget on representative height of near-surface layer and the top of PBL (planetary boundary layer) reveal that the main contributors to the wind tendency are the radial absolute vorticity flux (VVOR) and the azimuthal pressure gradient force(VPGF) near the surface at an altitude of 0.4km. Near the top of the PBL at an altitude of 1.3km, the azimuthal pressure gradient force (VPGF) and radial advection (VVA) contributed to the acceleration of the azimuthal flow, while the inward (outward) advection of VVOR is responsible for the strengthening (weakening) of the azimuthal wind. The analysis of the azimuthally averaged fields of the budget reveal that the VVA and VVOR are main contributors to the wind tendency before making landfall of Mujigae.
Available online:June 20, 2019 DOI: 10.3878/j.issn.1006-9895.1906.19102
Abstract:Himawari-8 is a new generation of geostationary meteorological satellite launched by Japan Meteorological Administration. The sensor AHI (Advanced Himawari Imager) carried by Himawari-8 can achieve high temporal resolution observation for 10 minutes/time.. Level 2 Aerosol Optical Depth (AOD) dataset from Himawari-8 satellite was compared to AERONET AOD at 70 sites from September 2015 to December 2017. The results show that the precision of AOD products retrieved by Himawari-8 satellite has great spatial difference. Among them, there are 48 sites where there is a good correlation between Himawari-8 AOD and AERONET AOD (R>0.5). The Himawari-8 AOD clearly underestimated ground-based AOD at 22 sites. At the sites of American_Samoa, Bandung, Birdsville, Bukit_Kototabang, Canberra, Fowlers_Gap, Jabiru, QOMS_CAS, etc., ground-based AOD are small and Himawari-8 satellite retrived AOD is large. Analysis of the absolute error (difference between Himawari-8 AOD and AERONET AOD) found that there is a good correlation between absolute error and AERONET AOD when the Himawari-8 AOD underestimates ground-based observations. In areas where ground-based observations are small and satellite inversion datas is large, there is a good linear relationship between absolute error and Himawari-8 AOD. This provides useful basic information for the improvement and perfection of Himawari-8 AOD inversion algorithm.
Potential Vorticity diagnosis on the formation, development and eastward movement of a Tibetan Plateau Vortex and it’s influence on the downstream precipitation
Available online:June 20, 2019 DOI: 10.3878/j.issn.1006-9895.1904.18275
Abstract:During the period of 28 June to 1 July, 2016, a Tibetan Plateau(TP) Vortex was generated, which then developed and moved eastward to subtropical region of China, resulting in precipitation in the lower reaches of the Yangtze River. The potential vorticity (PV) is used to diagnose the process by using the second Modern-Era Retrospective analysis for Research and Applications (MERRA-2) data and the Tropical Rainfall Measuring Mission (TRMM) precipitation data. Results indicate that surface heating over the TP pocesses obvious diurnal variation. It changes from heat source in the daytime to cold source in the night and directly influences the vertical gradient of diabatic heating. Negative PV is generated near the surface in daytime and positive PV is generated in night, demonstrating a prominent diurnal cycle. When the night-time positive PV generation becomes very strong and cannot be compensated for by the daytime negative PV generation, the TP low vortex is formed. By the time the low vortex system moves to the eastern TP, diabatic heating associated with strong precipitation reinforces the vortex. As the low vortex system continues to propagate eastward, the PV advection increasing with height acts as the large circulation background over the middle and lower reaches of the Yangtze River, which is in fever of the development of air ascent, resulting in occurrence of precipitation.
Interannual variations of the upper tropospheric water vapor and its transport into the stratosphere over the Tibetan Plateau area in summer, Part Ⅱ: adiabatic and diabatic transport into the stratosphere
Available online:June 20, 2019 DOI: 10.3878/j.issn.1006-9895.1905.18268
Abstract:The interannually geographic pattern of the upper tropospheric water vapor mass anomaly is dominated by the uniform mode and the east-west dipole mode over the Tibetan Plateau (TP) re-gions in July–August. In this paper, the relationship between the two leading modes and the adi-abatic and diabatic water vapor mass transport from the troposphere to the stratosphere are ana-lyzed based on the European Centre for Medium-Range Weather Forecasts Interim Re-Analysis (ERA-Interim) datasets and the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) trajectory model. Results show when the water vapor mass is dominated by the posi-tive (negative) phase of the uniform mode, namely more (less) water vapor mass over the entire TP area, the intensity of the South Asian High (SAH) and the upward diabatic water vapor mass transport are enhanced (weakened), thus both adiabatic and diabatic water vapor mass transport from the troposphere to the stratosphere are stronger (weaker). The regions and layers of the oc-currence of the adiabatic and diabatic water vapor mass transport from the troposphere to the stratosphere change little from positive to negative phase of the uniform mode, though the layers where diabatic water vapor mass transport from the troposphere to the stratosphere is slightly higher for the positive phase. When the water vapor mass is dominated by the positive (negative) phase of the west-east dipole mode, namely more (less) water vapor mass in the west (east) of the TP, the SAH center shifts westward, enhancing the adiabatic water vapor mass transport from the troposphere to the mid-latitude stratosphere in the northwest and northeast flank of the TP, and the meridional adiabatic water vapor mass transport from the troposphere to the tropical strato-sphere at upper layers in the south flank of the TP, but weakening the meridional adiabatic water vapor mass transport from the troposphere to the mid-latitude stratosphere in the north flank of the TP. Meanwhile, the diabatic water vapor mass transport from the troposphere to the stratosphere is enhanced over the TP, though weakened at upper layers in the south flank of the TP and at lower layers in the north flank of TP. Vice versa, when less (more) water vapor mass in the west (east) of the TP. The trajectory model simulation experiments for the positive phase of the uni-form mode confirms that higher frequency of the trajectory enters the stratosphere adiabatically over the TP regions. And trajectory model simulation experiments for the positive phase of the west-east dipole mode is also in agreement with the analyzed results, showing higher (lower) frequency of trajectory entering the stratosphere adiabatically in the northwest, south and north-east flank (north flank) of the TP.
1992,16(4):482-493, DOI: 10.3878/j.issn.1006-9895.1992.04.11
Analyses of the Causes of Severe Drought Occurring in Southwest China from the Fall of 2009 to the Spring of 2010
2012,36(3):443-457, DOI: 10.3878/j.issn.1006-9895.2011.11101
Abstract:A severe drought occurred in Southwest China from the fall of 2009 to the spring of 2010. Either its persistent time and area or decreased amount of rainfall were less observed during the last 50 years. Thus, in this paper, the occurring causes of this drought are analyzed by using the NCEP/NCAR reanalysis data and SST data from the impacts of thermal anomalies in the tropical western Pacific (TWP) and the tropical Indian Ocean (TIO) on the atmospheric circulation over the TWP and South Asia. The results show that during the period from the fall of 2009 to the spring of 2010, both the TWP and the TIO were in a warming state. Under the common thermal effect of both oceans, a strong anticyclonic anomalous circulation appeared in the lower troposphere over the TWP and the South China Sea, which caused not only the strengthening of the southwest flow anomaly, but also the appearance of a low trough anomaly over South China and Central China. In this case, the northwest flow anomaly and descending flow anomaly behind the trough controlled the eastern part of the Tibetan Plateau, and water vapor was difficultly transported from the Bay of Bengal into the Yunnan-Guizhou Plateau (Yun-Gui Plateau in short). Thus, less rainfall for a long time was caused in this region. Moreover, the analysis results also show that the circulation anomaly over the mid- and high latitudes had an important impact on the severe drought. Since the polar wave guide of quasi-stationary planetary wave propagations over the high latitudes was stronger, but the low-latitude wave guide was weaker from the fall of 2009 to the spring of 2010, which led to convergence and divergence of the wave E-P fluxes for quasi-stationary planetary waves in the upper troposphere and the stratosphere over the region about 60°N and in the middle and upper troposphere over the region about 35°N, respectively. Thus, the zonal mean wind was weakened in the upper troposphere and the stratosphere over the region about 60°N, but it was strengthened in the upper troposphere over the region about 35°N. This caused an obviously negative phase of the AO (Arctic Oscillation) and brought strong winter monsoon and eastward tracks of cold waves into East Asia, which led to a weakening of cold air arriving in Southwest China. Thereby, the persistent severe drought occurred in Southwest China.
Assessing the Quality of APHRODITE High-Resolution Daily Precipitation Dataset over Contiguous China
2012,36(2):361-373, DOI: 10.3878/j.issn.1006-9895.2011.11043
Abstract:A daily gridded precipitation dataset (APHRO) for Asia was created by the Asian Precipitation-Highly Resolved Observational Data Integration Towards the Evaluation of Water Resources (APHRODITE) project in Japan. The resolution of this dataset is 0.25°×0.25°, and the length is from 1951 to 2007. This study aims to assess the quality of the APHRO rainfall in contiguous China from the perspective of climatological mean, rainfall classes, and long-term trend. Daily rainfall records observed by 559 rain gauges are used for the comparison. The results are as follows: (1) For the mean states, the APHRO shows similar distribution of precipitation amount to station data, and can accurately characterize the seasonal migration of rain-belt. However, compared with station data, this dataset underestimates precipitation intensity, but overestimates precipitation frequency.(2) For the distribution of different classes of precipitation, annual mean precipitation amount for heavy rainfall derived from the APHRO data is lower, while the amounts for light and moderate rainfall are higher than the station data.(3) The trends of precipitation amount in China during 1956－2005 derived from two dataset are identical, the trends of precipitation frequency are also highly consistent, and both show an “increase in the west but decrease in the east” pattern. A large difference between the two datasets is found in the spatial pattern of precipitation intensity trends. The precipitation intensity derived from station data shows an increasing trend throughout the Chinese mainland in the past 50 years; the APHRO data exhibit a similar trend in the southeastern coastal region and northwestern China, but a different pattern in northern China, northeastern China, and Jianghuai region. In addition, analysis on the seasonality of interdecadal variability indicates that the characteristics of “southern China food and northern China drought” and “Jiangnan late spring drought” derived from the APHRO data are different from those derived from station data. The APHRO data tend to underestimate the trends.
An Overview of Recent Studies on Atmospheric Boundary Layer Physics and Atmospheric Environment in LAPC
2008,32(4):923-934, DOI: 10.3878/j.issn.1006-9895.2008.04.18
Interdecadal Variation of the Leading Modes of Summertime Precipitation Anomalies over Eastern China and Its Association with Water Vapor Transport over East Asia
2011,35(4):589-606, DOI: 10.3878/j.issn.1006-9895.2011.04.01
Abstract:Interdecadal variation of the leading modes of summertime precipitation anomalies in the monsoon regions of eastern China and its association with the spatio-temporal variations of summertime water vapor transport fluxes over East Asia are analyzed by using the daily data of the ERA-40 reanalysis and precipitation data at 516 observational stations of China for 1958－2000 and the EOF analysis method. The analysis results show that there are two leading modes in the spatio-temporal variations of summertime precipitation anomalies over the monsoon region of eastern China: The first leading mode exhibits not only a characteristic of obvious interannual variation with a quasi-biennial oscillation, but also a feature of interdecadal variability, and its spatial distribution is of a meridional tripole pattern. And the second leading mode exhibits a characteristic of obvious interdecadal variability, and its spatial distribution is of a meridional dipole pattern. This shows that these two leading modes have a significant interdecadal variability. During the period of 1958－1977, the distribution of summertime precipitation anomalies in eastern China exhibited a “＋－＋” meridional tripole pattern from the south to the north, and the distribution of precipitation anomalies for 1978－1992 showed a “－＋－” meridional tripole pattern in the region, which was opposite to that for 1958－1977, but during the period of 1993－1998, since the role of the second leading mode in summertime precipitation anomalies in eastern China was intensified, the distribution of summertime precipitation in this region showed a combination of “＋－＋” meridional tripole pattern and “＋－” meridional pattern, which caused the increase of summertime precipitation in South China. Moreover, the analysis results also show that the interdecadal variation of these two leading modes is closely associated with the spatio-temporal variations of summertime water vapor transport fluxes over East Asia, which is associated not only with the interdecadal variation of the EAP pattern teleconnection-like wave-train distribution of summertime water vapor transport flux anomalies over East Asia and the western North Pacific, but also with the interdecadal variation of the EU pattern teleconnection-like wave-train distribution of summertime waver vapor transport flux anomalies in the westerly zone over middle and high latitudes of Eurasia.
2003,27(4):712-728, DOI: 10.3878/j.issn.1006-9895.2003.04.18
Projection and Evaluation of the Precipitation Extremes Indices over China Based on Seven IPCC AR4 Coupled Climate Models
2009,33(1):109-120, DOI: 10.3878/j.issn.1006-9895.2009.01.10
Abstract:Climatology of the observed daily precipitation extreme indices (SDII, simple daily intensity index; CDD, the maximum number of consecutive dry days; R10, number of days with precipitation greater than 10 mm; R5d, maximum 5-day precipitation total; R95t, fraction of total precipitation due to events exceeding the 95th percentile of the climatological distribution for wet day amounts) at 550 stations in China during 1961－2000 is used to evaluate the simulation ability of 7 IPCC AR4 Coupled Climate Models, the projected change of the precipitation extreme indices over China under IPCC SRES A2、A1B and B1 is also studied. The results show that the state-of-the-art IPCC AR4 models can simulate the spatial distributions and the linear trends of precipitation extremes well. The multi-model ensemble (MME) shows the best skill, but both the MME and single model fail to simulate the interannual variability and have large biases, such as there are excessive extreme precipitation over the eastern side of the Tibetan Plateau while in the monsoon regions the modeled intensity of precipitation extremes is lower than the observation. In the 21st century the precipitation will become more “extreme”， there would be an overall increasing trend in extreme precipitation events over most of China under a warming environment, and the change scope is scaled to the emissions scenarios.
2008,32(4):653-690, DOI: 10.3878/j.issn.1006-9895.2008.04.01
Characteristics of the Water Vapor Transport in East Asian Monsoon Region and Its Difference from that in South Asian Monsoon Region in Summer
1998,22(4):460-469, DOI: 10.3878/j.issn.1006-9895.1998.04.08
The Seasonal March of the North Pacific Oscillation and Its Association with the Interannual Variations of China's Climate in Boreal Winter and Spring
2011,35(3):393-402, DOI: 10.3878/j.issn.1006-9895.2011.03.01
Abstract:Based on the monthly mean NCEP/NCAR reanalysis dataset and the observed surface air temperature and precipitation from 160 China stations, the seasonal march of the North Pacific Oscillation (NPO) and its association with the interannual variations of China's climate in boreal winter and spring are investigated in this paper. By employing the Empirical Orthogonal Function (EOF) analysis method, the NPO is identified as the second EOF mode of the monthly mean Sea Level Pressure (SLP) field over the North Pacific. The seasonal mean NPO indices are then defined as the average of the monthly mean indices for each season. Wavelet analysis reveals that the significant periods of the NPO indices are below 8 years for all the four seasons, indicating strong interannual variability and weak interdecadal variability of the NPO. Besides, the winter mean NPO index experiences significant linear trend towards its positive polarity. For all the seasons, NPO is featured with a large-scale meridional dipole in the SLP field over the North Pacific region and resembles the western Pacific pattern in the middle troposphere. The two surface centers are located around Aleutian Islands and the northwest of Hawaii, respectively. Their positions vary a little with season. Comparatively the south center experiences more zonal movement and the north center bears more meridional movement. Vertically the NPO is featured with an equivalent barotropic structure in summer and tilts a little westward with height for the rest three seasons. To put the NPO jet fluctuation in perspective, the positive phase of NPO is characterized by a northward shift and downstream extension of the East Asian jet stream especially in the jet exit region. The NPO variability is influential for the China's climate. Regression analysis indicate that during boreal winter the positive phase of NPO favors significant southerly anomalies along the coasts of East Asia, which may bring warm and moisture air from the south. Consequently, significant warming is observed over most areas of eastern China as well as the eastern part of the Tibetan Plateau. Meanwhile, more precipitation is observed over southeastern China, the Huaihe River valley, and several stations of northwestern China. During boreal spring, the influence of NPO is mainly confined to North China. More precipitation and weak cooling can be observed over this region in the positive phase of NPO. Such changes may be accounted for by the NPO-associated anomalous low-level wind convergence and the secondary circulation around the entrance of East Asian jet stream. In addition to the simultaneous influences, it is further found that the wintertime NPO is closely related to the temperature and precipitation of the following spring. If the NPO phase is positive in the preceding winter, significant warming will be observed over northern China and southwestern China in the following spring. Meanwhile, about 20% more-than-normal precipitation will be observed over southwestern China. Therefore, the wintertime NPO may act as a potential predictor for the climate of the following spring in China.
2002,26(6):721-730, DOI: 10.3878/j.issn.1006-9895.2002.06.01
2011,35(3):403-410, DOI: 10.3878/j.issn.1006-9895.2011.03.02
Abstract:The authors propose a method to separate the truncation error and the round-off error from the numerical solution. The analytical truncation error formulas of a partial differential equation are given for the upstream scheme and the centered difference scheme, respectively. The reference solution method is then introduced to separate these two types of errors for more general equations. A scheme based on the reference solution is used to obtain the approximate truncation error. Comparing the results for the upstream scheme and the centered difference scheme, the authors find that：1) the approximate truncation error is highly consistent with the analytical one. 2) The truncation errors of 1-D wave equations for the two schemes both show wavy periodicities with amplitudes being related to the parameters of computation. 3) The analytical error is suitable for the analysis of any slice of t, while the approximate one is only suitable for the analysis of a certain time range. However, the approximate error can be more easily obtained for general differential equations without a complex theoretical deduction.
A Study on the Relationships between ENSO Cycle and Rainfalls during Summer and Winter in Eastern China
1999,23(6):663-672, DOI: 10.3878/j.issn.1006-9895.1999.06.03
The Impact of the Tibetan Plateau Warming on the East Asian Summer Monsoon—A Study of Numerical Simulation
2008,32(5):997-1005, DOI: 10.3878/j.issn.1006-9895.2008.05.01
Abstract:East Asian monsoon precipitation and circulation have experienced an interdecadal change in the last 50 years that was concurrent with the changes in global SST and recent global warming. What gives rise to this long term change has been an outstanding issue. One possible cause is the change over the Tibetan Plateau. To understand the possible impact of the Tibetan Plateau surface conditions on the downstream East Asian monsoon, a suite of sensitivity experiments were performed with Hamburgs atmospheric general circulation model (ECHAM). The land surface albedo was changed in two sets of sensitivity tests so that the Tibetan Plateau land surface temperature was changed accordingly. The results show that a warmer condition over the Tibetan Plateau tends to enhance the upper tropospheric South Asia high and the westerly jet stream to its north and the Indian monsoon to its south, meanwhile the moisture transport toward East Asia increases. The changes in the precipitation pattern are featured by increasing rainfall over northwestern India and Meiyu and decreasing rainfall in the regions under the control of Pacific subtropical high and the Bay of Bengal. Preliminary diagnostic analysis suggests that the plateau warming has initially increased sensible heating and convective heating locally, which then enhanced the low-level southwest monsoon transport toward down stream of the plateau, resulting in increased precipitation and latent heat release in eastern China. The changes in the overall diabatic heating strengthen the upper-tropospheric South Asia high and downstream subtropical trough. This result may have meaningful implications for understanding the interdecadal change and future change of the East Asian summer monsoon.
Recent Progresses in Studies of the Temporal-Spatial Variations of the East Asian Monsoon System and Their Impacts on Climate Anomalies in China
2008,32(4):691-719, DOI: 10.3878/j.issn.1006-9895.2008.04.02
Atmospheric Water Vapor Transport and Corresponding Typical Anomalous Spring Rainfall Patterns in China
2009,33(1):121-134, DOI: 10.3878/j.issn.1006-9895.2009.01.11
Abstract:The atmospheric water vapor transports associated with typical anomalous spring rainfall patterns have been investigated using NCEP/NCAR, ERA40 monthly mean reanalysis data and precipitation data of 160 stations in China during 1951－1999. Results show that origins of water vapor supply related to anomalous rainfall patterns are different from the climate mean situation. In anomalous pattern 1, with a heavier rainbelt along the South China coast, the main moisture comes from the Philippine Sea and the adjacent South China Sea. The background large-scale circulation changes include the intensification of the western Pacific subtropical high (WPSH) and the southwest shift of the East Asian jet stream (EAJS). In anomalous pattern 2, with a main rainbelt along the middle and lower reaches of the Yangtze River, the origins of water vapor supply contain the western tropical Pacific and the tropical Indian Ocean. Both the WPSH and the EAJS move to the north of their normal position. In anomalous pattern 3, a rainy region is located in the Huaihe River valley. The moisture originates from the northwestern Pacific. The 500-hPa anticyclone anomaly moves to northeastern China and the EAJS is weaker than its normal condition. Although water vapor transport along the southern edge of the Tibetan Plateau is one of the main branches in the climate mean pattern, none of the typical water vapor transport related to typical anomalous rainfall reflects this water vapor flow.
2004,28(6):979-992, DOI: 10.3878/j.issn.1006-9895.2004.06.15
Interdecadal Variation of Summer Precipitation Pattern over Eastern China and Its Relationship with the North Pacific SST
2009,33(4):835-846, DOI: 10.3878/j.issn.1006-9895.2009.04.16
Abstract:By use of monthly precipitation at 160 stations over China, ERA-40 reanalysis data and NOAA ERSST extend-reconstructed global sea surface temperature data, the authors find that the precipitation pattern over eastern China experiences two interdecadal variation processes in recent 50 years, and has close relation to interdecadal variation of the North Pacific SST. In the interdecadal variation process of the mid- and late-1970s, the precipitation pattern form north to south over eastern China changes from “＋－＋” to “－＋－” tripole distributions, and has close relation to the PDO (Pacific decadal oscillation) phase, with SST anomaly from positive to negative in the mid-latitude area of the North Pacific. East Asian summer monsoon circulation is a main bridge of PDO influencing the precipitation pattern. In the interdecadal variation process of the late 1980s and early 1990s, the precipitation pattern changes from “－＋－” tripole to “－＋” dipole distributions, and has close relation to higher SST from the northwestern Pacific to the south of Japan and round the Philippines. The first interdecadal variation shows that the same mode changes from one phase to another, and the second one exhibits the changes between different modes.
Transient Response of the Atlantic Meridional Overturning Circulation to the Enhanced Freshwater Forcing and Its Mechanism
2009,33(1):179-197, DOI: 10.3878/j.issn.1006-9895.2009.01.16
Abstract:The mechanism for the transient response of the Atlantic Meridional Overturning Circulation (AMOC) to the enhanced and continuous freshwater input into the Arctic Ocean and the Nordic Seas for 150 years is investigated using a fully coupled climate model (Bergen Climate Model, BCM for short). The responses of the Sea Surface Temperature (SST), Salinity (SSS), Potential Density (SPD), the North Atlantic Deep Water (NADW) formation, Diapycnal Mixing (DM) and the wind stress are analyzed. The transient response of AMOC follows a quick dropping down during the first 50 years, with a gradual recovery for the later 100 years in the freshwater perturbation experiment (FW1). The authors find that the initial weakening of AMOC in the FW1 is mainly caused by decreasing of SSS and SPD which leads to a stable vertical stratification and then to the weakening of the NADW formation; however, AMOC recovers though the enhanced freshwater input is continuous and constant during the following 100-year integration by means of a series of feedbacks, which can be summarized as follows: 1) With the reduction in the NADW formation, the vertical density stratification in the mid-to-deep ocean at the mid-low latitudes of the North Atlantic is getting weaker, as a result, the strength of DM, parameterized by stratification-dependent method, increases and then leads to the increased upwelling; 2) the strengthened westerly and weakened easterly over the Southern Ocean and the strengthened easterly over the North Atlantic Ocean contribute together to the recovery of the northward Ekman transport; 3) the increased northward salt transport and the reduced precipitation over the North Atlantic subpolar region cause the recoveries of the SSS and SPD at northern high latitudes, and then the recovery of NADW formation, mainly in the Irminger Sea.
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