ISSN 1006-9895

CN 11-1768/O4

  • Volume 0,Issue 3,2020 Table of Contents
    Select All
    Display Type: |
    • Validation Analysis of Himawari-8 Aerosol Optical Depth Products

      2020, 44(3):455-471. DOI: 10.3878/j.issn.1006-9895.1906.19102

      Abstract (261) HTML (75) PDF 8.90 M (362) Comment (0) Favorites

      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 (361) HTML (59) PDF 6.81 M (392) Comment (0) Favorites

      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 (112) HTML (43) PDF 14.25 M (233) Comment (0) Favorites

      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 (137) HTML (33) PDF 5.54 M (184) Comment (0) Favorites

      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 (105) HTML (31) PDF 3.81 M (290) Comment (0) Favorites

      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 (160) HTML (36) PDF 7.52 M (189) Comment (0) Favorites

      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.

    • Projection of Typhoon-Related Environmental Fields in the Western North Pacific

      2020, 44(3):552-564. DOI: 10.3878/j.issn.1006-9895.1912.19168

      Abstract (127) HTML (43) PDF 4.24 M (174) Comment (0) Favorites

      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 (121) HTML (27) PDF 4.99 M (175) Comment (0) Favorites

      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.

    • Spectral Width of Cloud Droplet Spectra and Its Impact Factors in Marine Stratocumulus

      2020, 44(3):575-590. DOI: 10.3878/j.issn.1006-9895.1905.19115

      Abstract (134) HTML (28) PDF 6.00 M (277) Comment (0) Favorites

      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.

    • Simulation Bias Analysis of Pacific North Equatorial Countercurrent in LICOM3.0 Ocean Model

      2020, 44(3):591-600. DOI: 10.3878/j.issn.1006-9895.1907.19123

      Abstract (128) HTML (25) PDF 13.70 M (211) Comment (0) Favorites

      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 (175) HTML (48) PDF 3.43 M (220) Comment (0) Favorites

      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 (123) HTML (44) PDF 4.92 M (253) Comment (0) Favorites

      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.

    • Revised Initial Field and Lateral Boundary Condition Scheme for the TRAMS_RUC_1 km Model

      2020, 44(3):625-638. DOI: 10.3878/j.issn.1006-9895.1911.19183

      Abstract (107) HTML (48) PDF 6.95 M (169) Comment (0) Favorites

      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.

    • Influence of MJO on Summer Precipitation in North China in 2018

      2020, 44(3):639-656. DOI: 10.3878/j.issn.1006-9895.1912.19217

      Abstract (192) HTML (45) PDF 9.03 M (255) Comment (0) Favorites

      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.

    • A Numerical Simulation of Urban Breeze Circulation Structure and Turbulence in Chongqing

      2020, 44(3):657-678. DOI: 10.3878/j.issn.1006-9895.2003.19239

      Abstract (171) HTML (49) PDF 10.21 M (396) Comment (0) Favorites

      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.