ISSN 1006-9895

CN 11-1768/O4

  • Volume 44,Issue 5,2020 Table of Contents
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    • Numerical Simulation Study on the Microphysical Characteristics of Stratiform Clouds with Embedded Convections in Northern China based on Aircraft Measurements

      2020, 44(5):899-912. DOI: 10.3878/j.issn.1006-9895.1908.19114

      Abstract (820) HTML (1225) PDF 7.18 M (1482) Comment (0) Favorites

      Abstract:To characterize the microphysical characteristics and transformation process of stratiform clouds with embedded convections, a study was performed using the WRFV3 model and based on two aircraft measurements taken on May 1, 2009. The aircraft observation results showed that significant differences in the shapes and formation process of ice particles existed between the regions of stratiform cloud and embedded convection. Compared with the embedded convection region, the stratiform cloud featured more complicated shapes of ice crystals, including needle column, capped column, and dendrite types. 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 agreed well with the aircraft observations, including cloud distribution, LWC, and numerical concentration on the flight route. The simulation results showed that in the stratiform cloud, with higher cloud water content and larger W, embedded convection could be developed because of a strong riming process. The hydrometeors of snow, graupel, and rainwater in the clouds accounted for 51.9%, 31.0%, and 16.0%, respectively, while cloud ice and cloud water accounted for very little. In the higher level, snow and graupel grew through deposition process. In the lower level, they grew through the riming process and melted into rain. Stratiform clouds with lower cloud water content and smaller W would remain as stratiform cloud. The hydrometeors of snow, rainwater, and cloud ice accounted for 90.4%, 6.1%, and 3.5%, respectively. The ice and snow grew through deposition process and melted into rain in the lower level.

    • The Causes of Low Predictive Skills of Precipitation in Flood Season in Northeast China

      2020, 44(5):913-934. DOI: 10.3878/j.issn.1006-9895.1911.19132

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      Abstract:The real-time forecast accuracies of summer precipitation in Northeast China (NEC) from 1978 to 2018 were significantly low. Moreover, in the recent four years, when the prediction of the overall distribution of summer precipitation anomaly for the whole country was relatively accurate, the prediction of summer precipitation anomaly in NEC was contrary to the actual situation. Therefore, analyzing the cause for the low forecasting accuracy is necessary. In this paper, the forecasting ability of dynamic models and cognitive defects on forecasting summer rainfall in NEC are discussed. Moreover, by systematically reviewing the climatic characteristics, influencing factors, prediction methods of summer drought and flood in NEC, and the real-time forecasting skills, the causes have been obtained as follows: (1) The precipitation in early summer in NEC is mainly affected by the cold vortex 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 cannot 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 the global sea surface temperature (SST) is weak and unstable. Especially, the influence of El Niño-Southern Oscillation (ENSO) on summer precipitation in NEC is complicated; the relationship between them varies from decade to decade. (3) The summer rainfall in NEC has remarkable multi-timescale variability (inter-seasonal, inter-annual, and inter-decadal timescales), influenced by different circulation systems, which makes accurately predicting summer precipitation in NEC more difficult. Finally, some scientific problems and possible solutions regarding summer rainfall forecasting in NEC are further discussed, which may be helpful for the future summer rainfall predictions in this area.

    • Numerical Investigation of the Effects of Boundary Layer Parameterization Schemes on Typhoon Meranti (1614) Landing Process

      2020, 44(5):935-959. DOI: 10.3878/j.issn.1006-9895.2004.19135

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      Abstract:To study the effects of different boundary layer schemes on the simulation of landing attenuation stage of typhoon Meranti (1614), a series of high-resolution (1.33 km) numerical tests were carried out using seven boundary layer parameterization schemes in the mesoscale numerical model WRF v3.8, namely, YSU, MYJ, QNSE, ACM2, UW, GBM, and Boulac, in terms of movement track, intensity, structure, rainfall, and near-surface physical variables. The results indicate the following. First, boundary layer schemes significantly influenced the simulation of typhoon Meranti’s track, intensity, and rainfall during its landing attenuation stage, and the maximum differences in the 24-h simulated typhoon track, lowest atmospheric pressure, maximum wind velocity, and 24-h cumulative rainfall extremum were 80 km, 11 hPa, 27 m s-1, and 241 mm, respectively. Second, simulation results of the Boulac scheme showed a typhoon track that is closest to real-time results, followed by GBM, YSU, and MYJ schemes, and then by ACM2 and UW schemes, whereas the QNSE scheme displayed the worst simulation. Meanwhile, the UW and QNSE schemes simulated the lowest atmospheric pressure values, and MYJ and QNSE schemes simulated the maximum wind velocity values that are closest to actual observations. All boundary layer schemes simulated the features of the typhoon. For example, the lowest atmospheric pressure increased gradually during the landing stage, and the rate of such increase after landing was greater than that before landing, which agreed with real-time results. However, the increasing rate of the lowest atmospheric pressure before the typhoon landing that was simulated by each scheme is greater than the real-time result, whereas such increasing rate after typhoon landing is less than the real-time result. Third, the Boulac scheme best simulated the 24-h precipitation distribution, heavy precipitation area, structure, intensity, and TS score of precipitation at each level, whereas the MYJ scheme was the second best. As simulated by QNSE, UW, and ACM2 schemes, the rain belt advanced so quickly northwestward that the TS scores of precipitation at various levels were poor. Fourth, in the overall simulation of track, intensity, and precipitation of the typhoon, Boulac and MYJ schemes showed optimal results, in which the Boulac scheme was superior in simulating the typhoon track and precipitation and the MYJ scheme was superior in simulating typhoon intensity. The YSU and GBM schemes had the second best simulation results, whereas QNSE, UW, and ACM2 schemes had worse simulation performance. Moreover, the boundary layer schemes significantly differed in calculating the latent heat flux and sensible heat flux of near-surface layer, thereby affecting the simulation of typhoon track, intensity, and rainfall, leading to significantly different simulation results. The QNSE scheme resulted in an abnormally high latent heat flux, the MYJ and Boulac schemes resulted in the most modest values, and other schemes resulted in slightly smaller values. On the other hand, the QNSE scheme had a slightly higher sensible heat flux, the MYJ scheme showed the most modest one, and other schemes resulted in significantly smaller values. Finally, the boundary layer schemes significantly differed in the simulated thermal and dynamical structure of boundary layer, and Boulac scheme had the obvious advantages, particularly for the structure of boundary layer in daytime.

    • Contribution of Tropical and Subtropical Circulation Anomalies to the Intensity of East Asian Winter Monsoon over Lower-Latitude Region

      2020, 44(5):960-974. DOI: 10.3878/j.issn.1006-9895.1911.19141

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      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 of 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 by using empirical orthogonal function analysis, correlation analysis, and partial correlation analysis. 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 ascensions over the tropical western Pacific and South China Sea are strengthened. This anomalous ascension 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 northerly quasi-geostrophic winds near the jet entrance. Associated with the forced positive secondary circulation anomaly with anomalous descent (ascension) to the north (south) of the jet, the low-level anomalous northerly wind appears under the jet, which 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 the ITCZ seems more important. When the two circulation anomalies simultaneously increase (i.e., more active convective activity of the 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 abovementioned 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 Vertical Second-Order Difference Scheme for Non-uniformly Distributed Layers and Its Application in GRAPES Model

      2020, 44(5):975-983. DOI: 10.3878/j.issn.1006-9895.1906.19145

      Abstract (485) HTML (1142) PDF 2.67 M (1013) Comment (0) Favorites

      Abstract:In the case of non-uniformly distributed layers, the vertical difference scheme used in GRAPES (Global/Regional Assimilation and Prediction System) model can only achieve first-order accuracy. A second-order scheme was designed and introduced into the GRAPES model in order to be used in the process of vertical discretization. An ideal test using the 1-D profile showed that the new scheme can improve the accuracy of difference computation obviously. An ideal density flow test was conducted to verify the correctness and stability of the new scheme in GRAPES model. A statistical evaluation of a medium-range forecast using the second-order scheme showed an improvement of forecast skill in large-scale fields, especially for the forecast after 120 h. Additionally, the second-order scheme was tested with a real-case experiment based on the extreme rainfall at South China, which again showed an improvement in the forecast of precipitation after 48 h.

    • Influence of Stochastically Perturbed Parameterization on Ensemble Forecasting of Winter Precipitation in China

      2020, 44(5):984-996. DOI: 10.3878/j.issn.1006-9895.2001.19157

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      Abstract:Precipitation ensemble forecasting is characterized by great uncertainty, and the uncertainty of the parameters in the physical that is closely related to the precipitation forecast is one of the sources of its numerical prediction error. As a frontier research field in international ensemble forecasting, the stochastically perturbed parameterization (SPP) method has been developed to address the uncertainty of representative model precipitation forecasts. To determine whether this method can reflect the uncertainty of numerical predictions of winter precipitation in China and provide a scientific basis for business applications, we used the China Meteorological Administration’s Global/Regional Assimilation and Prediction System (GRAPES) mesoscale regional ensemble prediction model and selected 16 key parameters from four parameterization schemes. These parameters, e.g., cumulus convection, cloud microphysics, boundary layer, and near-surface layer, greatly influence the uncertainty of model precipitation forecasts. In this paper, we introduce the stochastically perturbed parameterization (SPP) method and describe the results of an ensemble prediction experiment conducted from December 12, 2018 to January 12, 2019, a total of 31 days. We compare and analyze the effect of the SPP method on the winter weather situation and precipitation ensemble prediction. The results show that with the addition of a test for the SPP method, the results of probability prediction techniques for precipitation and isobaric elements are better than the control predictions without the SPP method, and the improvement of low-level and near-surface elements is better that of the iso-surface elements in the middle or upper floors. The precipitation prediction results obtained superior scores to those of the control prediction test, but because the improvement did not pass the test of significance, the differences were not statistically significant. The above results indicate that under the influence of the East Asian winter monsoon, the SPP method demonstrates no obvious improvement on the current prediction technique used for winter precipitation in China. The reason for this may be that the SPP method mainly represents the uncertainty of convective precipitation forecasting, whereas the winter precipitation process in China is mainly one characterized by the development of baroclinic instability. Because model precipitation is dominated by large-scale grid precipitation, and less convective precipitation, improvement in the winter precipitation forecast is not obvious. Thus, there is a scientific basis for applying the SPP method to the operation ensemble forecasting model.

    • Spatial Observation of Red Sprites over a Winter Mesoscale Convective System in North America and the Analysis of Its Parent Thunderstorm

      2020, 44(5):997-1012. DOI: 10.3878/j.issn.1006-9895.2002.19169

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      Abstract:Red sprites are large-scale transient luminous events (TLEs) that usually occur between about 40 and 90 km altitudes above thunderstorms, and they are caused by cloud-to-ground (CG) lightning strokes and subsequent continuous current. Compared with studies that focus on sprites that occur in summer, those focusing on winter sprites are fewer due to limited comprehensive synchronous observation data. Influenced by the upper trough and warm, moist airflow at low level, a thunderstorm occurred in Arkansas, North America, on December 27–28, 2008. The Imager for Sprites and Upper Atmospheric Lightning (ISUAL) aboard the FORMOSAT-2 satellite could record two red sprite events. Using the red sprites optical observation data obtained by ISUAL, Doppler weather radar data, National Lightning Location data, ultra-low frequency magnetic field data, and cloud-top brightness temperature data provided by the National Environmental Center/Climate Prediction Center of the United States and the sounding data, this paper presents a detailed study of the characteristics of the winter thunderstorm that produced the red sprites and the related lightning activity. The results show that ISUAL did not record the halo that accompanied the two red sprites. The first was a columnar sprite, and the specific morphology of the second could not be determined because of its dim light. The parent thunderstorm of the red sprites was a mesoscale convective system (MCS), which appeared around 1500 UTC on the 27th near northern Arkansas and moved from west to east. The thunderstorm became stronger at about 2359 UTC, and the area of maximum radar reflectivity (55–60 dBZ) reached 339 km2 and then began to weaken. At 0303 UTC, the thunderstorm intensity increased, then the cloud gradually spread, and the thunderstorm began to weaken and completely dissipated at 1100 UTC. The first recorded sprite occurred at 0446:05 UTC, and the second at 0447:17 UTC. They tended to be produced in the dissipation stage of the MCS, when the frequency of the positive and negative CG lightning was low and the Percentage Of Positive CG to total CG (POP) increased significantly, and they were mostly over the stratiform cloud area with a brightness temperature of -40℃–-50℃. The sprite production was accompanied by an increase in the echo area of 30–35 dBZ. The area of radar reflectivity larger than 40 dBZ decreased, and the area of 10–40 dBZ increased during the sprite time window, suggesting that the sprite production was the decay of the thunderstorm and that the area of the stratiform region developed, which is consistent with the results of previous studies on summer sprites. The parent CG flash of red sprites was positive and with a single return stroke, and it was located in the trailing stratiform region of the MCS, where the radar reflectivity ranged from 25 to 35 dBZ. The corresponding radar echo top heights were 2.5 km and 5 km, and the peak currents were +183 kA and +45 kA, respectively. Based on the ultra-low frequency magnetic field data, the impulse charge moment changes (iCMCs) of two parent lightning discharges were estimated to be +394 C km and +117 C km. The ultra-low frequency magnetic antenna recorded the internal current signal of the first red sprite, indicating that the red sprite was strongly discharged.

    • Correction for Cirrus Cloud Top Height of MODIS Based on CALIPSO Dataset in the Beijing–Tianjin–Hebei Region

      2020, 44(5):1013-1022. DOI: 10.3878/j.issn.1006-9895.1911.19181

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      Abstract:Cirrus clouds play an important role in atmospheric radiation and affect weather systems and climate change. Satellite remote sensing has considerable advantage in cirrus cloud detection, relative to traditional observation. As a passive remote sensing instrument, large deviations in the thin cirrus cloud top height data from the Moderate Resolution Imaging Spectroradiometer (MODIS) are detected. Comparatively, the Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) aboard the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), which is an active remote sensing instrument, can acquire more accurate characteristics of thin cirrus cloud. In this study, MODIS cloud products in the Beijing–Tianjin–Hebei region from 2013 to 2017 are selected. Using the CALIPSO cirrus cloud top height data, a linear fitting method based on the cross-validation method is obtained, and the MODIS cirrus cloud top height data are corrected. The difference between MODIS and CALIPSO changes from -3 to 2 km to -2.0 to 2.5 km. Moreover, the maximum difference changes from approximately -0.8 km to approximately 0.2 km. In the context of different vertical levels and cloud optical depths, MODIS cirrus cloud top height data are improved after correction, which is more obvious at the lower cloud top height and optically thinner cirrus clouds.

    • The Properties of Convective Generating Cells Embedded in the Stratiform Cloud on Basis of Airborne Ka-Band Precipitation Cloud Radar and Droplet Measurement Technologies

      2020, 44(5):1023-1038. DOI: 10.3878/j.issn.1006-9895.2004.19185

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      Abstract:On the basis of airborne Ka-band precipitation cloud radar (KPR) and droplet measurement technologies (DMT), the dynamic and microphysical characteristics of convective generating cells (GCs) embedded in stratiform clouds initiated by the Huanghuai cyclone on April 22, 2018 were analyzed. First, a total of 36 GCs were observed by KPR in spring in Shandong Province. The results based on the echo intensity, horizontal scale, and echo top height of these GCs show that the average echo intensity of GCs is concentrated at 20 to 30 dBZ, accounting for 69%. The horizontal scale of GCs is concentrated at 15 to 30 km, accounting for 61%. The echo top height of GCs is concentrated at 6 to 8 km, which is 2 to 4 km higher than the surrounding stratiform clouds. Afterward, the microphysical parameters of GCs in mixed-phase cumulus clouds on April 22, 2018 were counted. The results showed that the inner part of GCs is dominated by updraft with the maximum wind speed of 1.35 m s-1 and average updraft of 0.22 m s-1. GCs have high supercooled water content with the maximum of 0.34 g m-3 and average of 0.15 g m-3. The ice particle concentration in the inner part of GCs is 5.5 times that of its outer part, and the mean diameter of the inner part of GCs is 1.7 times that of its outer part. The images sampled by the cloud image probe showed that the ice particles on the head and tail of GCs were mainly columnar and radial, respectively, whereas the ice particles in the core of GCs were polymers. The growth of ice crystals depended on the accretion and collision processes. The ice crystals formed columns when the supercooled water was insufficient; otherwise, they rapidly formed graupels. The microphysical formation mechanism of precipitation in GCs is different and strongly depends on the supercooled water content. When the supercooled water content of the cloud was sufficient, graupels were rapidly formed, and surface precipitation was formed after they passed through the melting layer. When the supercooled water content of the cloud was insufficient, the formation of precipitation depended on the water vapor deposition and aggregation processes.

    • Advances in Atmospheric Predictability of Heavy Rain and Severe Convection

      2020, 44(5):1039-1056. DOI: 10.3878/j.issn.1006-9895.2003.19186

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      Abstract:Atmospheric predictability research is the basis for weather and climate prediction. Under the background of global warming, meso/micro-scale extreme weather events such as heavy rain and severe convection have occurred more frequently in recent years, and their predictability has attracted wide attention. After briefly reviewing the history of atmospheric predictability research, this paper systematically reviews the latest advances in the predictability of heavy rain and strong convection over the last 20 years (1999–2018). The main research methods for meso/micro-scale predictability and their differences with traditional large-scale weather predictability methods are first discussed. Then, the primary initial error growth mechanism (error upscaling under deep moist convection) is elaborated in detail, and some arguments (error downscaling, error upscaling, and downscaling coexisting) are discussed. The effects of errors in NWP (Numerical Weather Prediction) models and convective environments on the practical predictability are also highlighted, and some recent mesoscale predictability experiments are reviewed. Finally, this paper briefly discusses the current problems, challenges, and future directions of the predictability research of heavy rain and severe convection.

    • Contrasting Salinity Interannual Variations in the Tropical Pacific and Their Effects on Recent El Niño Events: 1997/1998, 2014/2015, and 2015/2016

      2020, 44(5):1057-1075. DOI: 10.3878/j.issn.1006-9895.1912.19172

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      Abstract:Ocean salinity variation provides a new insight into related ENSO (El Niño–Southern Oscillation) expressed by climate variability. In this study, salinity variations and their related dynamic processes responsible for SSTA (sea surface temperature anomaly) were extensively compared and analyzed considering two strong El Niño events, 1997/1998 and 2015/2016, and one special El Niño, 2014/2015. The study shows that the development of ENSO is significantly associated with the occurrence and eastward diffusion of large-scale SSSA (sea surface salinity anomaly) in the western tropical Pacific. In April 1997 and 2015, corresponding to two strong El Niño events, there was a significant negative SSSA in the western–central Pacific. The anomaly moved eastward to the west of the dateline, which induced a shallower MLD (mixing layer depth), and a thicker BLT (barrier layer thickness), which enhanced the surface warming in the tropical central Pacific and the early warming in the equatorial eastern–central Pacific. Although a negative SSSA occurred in the April 2014/2015 weak event in the equatorial western–central Pacific, it did not develop eastward, resulting in a weakened thickening process of the BLT and a weak modulation effect on surface temperature. For the salinity change process corresponding to three El Niño events, surface advection and surface forcing caused by FWF (freshwater flux) were the major contributors to the salinity budget. Surface advection influenced the former variability of salinity tendency, inducing the occurrence of an ENSO signal. The precipitation in the tropical western Pacific had the most significant negative influence on FWF, which played a decisive role in the SSSA occurrence and ENSO development. Compared with the two strong El Niño events, the early FWF negative anomaly in 2014/2015 did not develop, did not move eastward, and weakened rapidly; this resulted in the slowing down of the negative salinity tendency in the western–central Pacific, deepening of the MLD, thinning of the BLT, and rapid cooling of the surface layer, which inhibited early warming in the equatorial eastern Pacific. The results of this study demonstrate that the salinity change was closely related to ENSO, and early SSS in the tropical western–central Pacific could be used as an index of SSTA. In particular, SSSA not only affects the strength of SSTA in oceans, it can also be used as a precursor to judge the development and strength of ENSO.

    • Experiments on Simulation of Typhoon Soudelor with GRAPES Hybrid-3DVar System

      2020, 44(5):1076-1092. DOI: 10.3878/j.issn.1006-9895.1911.19193

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      Abstract:To improve the analysis quality by incorporating the flow-dependent ensemble covariance into the variational data assimilation system, the new GRAPES (global/regional assimilation and prediction system) hybrid-3Dvar system was built. The new system is based on the GRAPES regional 3DVar system, which uses the statistic covariance, and was built by augmenting the state vectors with another set of control variables preconditioned upon the ensemble dynamic covariance. The new hybrid-3DVar system and the localization method were verified through a single-observation assimilation experiment with ensemble samples produced by the 3D-Var’s control variable perturbation method. The real observation assimilation and forecast experiment for Typhoon Soudelor yielded the following conclusions: (1) The background covariance, which is represented by ensemble samples, is flow-dependent, and the root mean square spread in the ensemble of momentum field and mass field is largest near the typhoon center. (2) The analysis increments of the new hybrid-3DVar have a more detailed structure and more medium- and small-scale information. (3) The analysis and 24 h prediction qualities of model variables in the new hybrid-3DVar are significantly improved compared with the 3DVar system, and the precipitation position predictions are more accurate. (4) The 24 h forecast track of Typhoon Soudelor is closer to the observational one, and the 48 h-predicted intensity also approaches the real observation.

    • Numerical Simulation of the Effect of Urbanization on a Single Extreme-High-Temperature Event in Beijing

      2020, 44(5):1093-1108. DOI: 10.3878/j.issn.1006-9895.2004.19229

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      Abstract:Urbanization has a significant influence on the frequency and intensity of heat waves, but the mechanism of the effect of urbanization on the high-temperature process is not fully understood. In this study, the authors used the Weather Research and Forecasting (WRF) model to simulate a summer high-temperature process on 2–6 July 2010 in Beijing. This paper reports the main results obtained regarding the urbanization effect on the surface air temperature of urban areas during the heat-wave process. The optimized WRF model was able to simulate the temporal characteristics of the five consecutive days of high temperature and the variation in the urban-heat-island intensity (IUHI) in Beijing. The impermeability of the underlying urban surface lowers the 2-m relative humidity of urban areas with respect to that of rural areas, which weakens the ability of urban areas to regulate the surface air temperature via latent heat. After sunset, the urban-sensible-heat flux decreases slowly, and the cooling rate in urban areas is slower than that in rural areas. At night, the structure of the boundary layer is stable, and its height is low, as is the wind speed. In this case, the energy transmitted between urban and rural areas is constrained, and the strong urban heat island is formed, resulting in the temperature in urban area is significantly higher than that in rural area at night. After sunrise, both the sensible and latent heat fluxes of urban and rural land surfaces increase rapidly, and the stability of the boundary layer decreases. In the afternoon, the underlying urban surface favors high and low value centers in the sensible and latent heat fluxes, respectively, with a weakened ability to regulate temperature via latent heat. This is conducive to vertical exchange of energy, which decreases the stability of the boundary layer. The IUHI is lower in the afternoon than in the evening. Therefore, the obvious urban-heat-island effect created by the underlying urban surface in Beijing increases the strength of extreme-high-temperature events. Furthermore, in this heat-wave process, most of the eastern part of China is controlled by continental warm high pressure with clear skies and few clouds, and the northwesterly winds flowing over the Taihang Mountains generate a Fohn effect, which is the synoptic situations of the heat-wave formation in Beijing.

    • The Causes of Variation in the Zonal Asymmetry of the Asian Westerly Jet and Its Impacts on East Asian Climate in Boreal Summer

      2020, 44(5):1109-1124. DOI: 10.3878/j.issn.1006-9895.2005.19232

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      Abstract:Based on CMAP (Climate Prediction Center (CPC) Merged Analysis of Precipitation) monthly mean rainfall data and NCEP/DOE (National Centers for Environmental Prediction/Design of Experiments) II Reanalysis data from NOAA (National Oceanic and Atmospheric Administration) and the monthly precipitation and average temperature data from NMIC (National Meteorological Information Center), the authors defined an index (IAja) that describes the zonal asymmetric variation of the Asian westerly jet in the upper troposphere, from which the authors investigated the characteristics of the intensity difference between the eastern and western parts of the Asian westerly jet and its impacts on the climate of East Asia from 1979 to 2019. The conclusions are as follows: There are prominent interannual variations in the zonal asymmetry of the Asian summer westerly jet, with significant quasi-periods of 6–8 years and 2 years. When the zonal asymmetry of the Asian summer westerly jet is typically strong (weak), the wave-like anomalous rainfall pattern generates positive (negative)—negative (positive)—positive (negative) signs in the lower to higher latitudes in the East Asia sector along with negative (positive) temperature anomalies in the Lake Baikal area, and simultaneously significant positive (negative) anomalies in regions in West China and North Japan. The divergent and convergent wind components by the anomalous diabatic heating as a potential vorticity source directly induce the circulation anomalies in the mid-latitudes. The anomalous anticyclonic circulation causes the intensity of the west Asian jet to increase and the eastern segment to decrease, which strengthens the zonal asymmetry of the Asian summer westerly jet. The formation of and support provided by the zonal asymmetric anomalies of the Asian jet are affected by their convergence and divergence in the tropics and mid-latitudes, as well as the eastward propagation of wave energy in the westerlies. This eastward propagation of wave energy may be related to the sea surface temperature anomaly of the Northern Atlantic. These results facilitate a better understanding of the formation mechanisms of the zonal asymmetry of the Asian summer westerly jet.

    • Interdecadal Variation Characteristics of Extreme Low Temperature Index in Winter in China

      2020, 44(5):1125-1140. DOI: 10.3878/j.issn.1006-9895.2003.19242

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      Abstract:Although the long-term trend of extreme temperatures has been extensively explored in previous studies, few studies have addressed the interdecadal variation of extreme temperatures. Based on the daily maximum temperature, minimum temperature, and daily temperature at 839 stations in China from 1961 to 2016, the authors analyzed the interdecadal variations in the winter extreme-low-temperature index in China. The first four wave components of the extreme temperature at each station were extracted by harmonic decomposition, which is regarded as the interdecadal component. A station is regarded as having undergone an obvious interdecadal variation if the cumulative variance explained by the interdecadal component is greater than 25%. The results show that the stations with obvious interdecadal variation in their winter extreme-low-temperature index are mainly located north of the Yangtze River, in northern Xinjiang, and in eastern Qinghai–Tibet Plateau. The interdecadal variations north of the Yangtze River and in northern Xinjiang are basically consistent after 1979. The years after 1979 can be divided into three periods: previously cold period (1979–1986), warm period (1987–2007), and later-cold period (2008–2016). The interdecadal variation in the extreme temperature indices of the stations located in the abovementioned two areas might be modulated by the interdecadal variation in the East Atlantic/West Russia (EAWR) teleconnection pattern, which corresponds to the interdecadal variation in both the frequency of the blocking-like circulation over the Ural Mountains and the amplitude of the planetary trough over East Asia.

    • Evaluation of Simulated Tropical Cyclones over the Western North Pacific with IAP AGCM4.1 Based on K-Means Method

      2020, 44(5):1141-1154. DOI: 10.3878/j.issn.1006-9895.2002.19252

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      Abstract:As the atmospheric component of CAS-ESM1 (Chinese Academy of Sciences Earth System Model, version 1), IAP-AGCM4.1 (Institute of Atmospheric Physics Atmospheric General Circulation Model, version 4.1) is being developed independently by Institute of Atmospheric Physics. In this study, the authors used TECA (Toolkit for Extreme Climate Analysis) to identify and evaluate tropical cyclones (TC) over the western North Pacific simulated by IAP AGCM4.1 from 1979 to 2012. The results show that IAP AGCM4.1 can reproduce the spatial distribution, track, and source of TC reasonably well compared to observation data, but it underestimates the number of TC, with only 36% of the observed tropical cyclones over the western North Pacific simulated. Further analysis using K-means clustering revealed that this underestimation is mostly due to the model’s inability to reproduce northwestward-turning and westward TC. For TC with westward–northwestward, westward-turning, and eastward-turning tracks, the numbers simulated are approximately 39%, 48%, and 85% of those observed, respectively. Moreover, the correlation coefficients of the seasonal variations between simulated and observed TC can reach 0.91, with duration biases of roughly 1–2 d. IAP AGCM4.1 performs well in simulating the tracks of the westward–northwestward and eastward-turning TC, with relative biases ranging between 1%–5% for the longitude of the centroid, 4%–16% for the latitude of the centroid, and 5%–15% for the latitudinal and meridional standard deviations. In addition, IAP AGCM4.1 reproduces the evolutions of environmental circulation and subtropical highs quite well during the lifetime of eastward-turning TC, with the simulated strength and area indexes of the subtropical highs highly correlated with the observations (the correlation coefficient is 0.89). The poor simulations of northwestward-turning and westward TC are likely due to simulated biases in the subtropical high.