低空急流在一次持续性暴雨过程中的作用

本文利用常规高空观测、地面加密自动站和NCEP再分析资料,对2012年8月30日～9月1日发生在四川盆地东部的一次持续性暴雨过程进行了分析.结果表明,过程期间低层西南风风速和雨强具有明显的日变化特征,低层西南风风速变化表明西南低空急流具有日变化特征;低空急流的日变化使得其对暴雨区的水汽、热力和动力条件的输送也具有日变化,从而造成暴雨过程期间雨强在夜间增强白天减弱;低空急流及其输送的水汽、热力和动力条件主要位于700hPa层附近.     

皖北一次特大暴雨过程中边界层急流日变化特征 及其对暴雨形成的影响分析

利用多普勒雷达风廓线产品、ERA5再分析资料和WRF模式,分析了2018年6月27日皖北一次特大暴雨过程中边界层急流的日变化特征及其对特大暴雨形成的作用。结果表明:特大暴雨发生期间存在边界层急流,急流最强达到了18 m·s^-1,强降水主要发生在急流快速增强的时段;急流前部的边界层辐合线是对流的触发因子,强降水落区位于急流核前部。急流为对流系统加强提供水汽和能量,且边界层急流和雷暴高压对峙使对流系统稳定少动,在对流系统西侧激发新的对流单体,有利于特大暴雨的发生;此次过程中天气系统的影响时间主要决定了强降水的落区,而边界层急流的日变化决定了强降水发生的时间段;边界层急流在夜间具有超地转特征,午后具有次地转特征,地转偏差和水平平流作用是导致夜间边界层急流增强的主要原因。     

黑河地区夜间低空急流特征的二维数值模拟

引言低空急流是夜间稳定边界层的一种典型特征,而目前对低空急流的变化特征、生消规律、物理机制等都存在着不同的认识。Ｂｌａｃｋａｄａｒ（１９５７）曾提出了惯性振荡理论,来解释低空急流的夜间特征。Ｔｈｏｒｐ和Ｇｍｙｍｅｒ（１９７７）又发展了全边界层的惯性振荡...     

双低空急流影响下华南初夏降水日变化的时空分布特征

利用2010—2016年5—6月ERA5逐小时再分析数据集和国家气象信息中心逐小时降水量融合产品,对影响华南地区的低空急流事件进行筛选和分类,并分析天气系统相关的低空急流（Synoptic-system-related Low-Level Jet,SLLJ）和边界层急流（Boundary Layer Jet,BLJ）的日变化及其影响下的华南降水日变化的时空分布特征。结果表明,BLJ和SLLJ在白天减弱、夜间增强,并在凌晨达到峰值,其日变化主要与边界层惯性振荡引起的非地转风的顺时针旋转有关。双急流日华南地区降水量显著增加,且降水日变化有明显的区域差异,这与双急流的演变和配置密切相关。广西中北部主要为SLLJ左前方发生的夜间山区降水,且降水量仅有凌晨的单峰。广西沿海和广东地区存在早晨和午后两个峰值,BLJ出口区辐合和SLLJ入口区辐散的维持有利于降水频率的增大,从而导致午后峰值的出现,而早晨的峰值除了受双急流有利配置的影响外,主要归因于早晨降水强度的增加。     

东北冷涡背景下沈阳地区一次强降水过程分析

利用辽宁省区域自动站、探空和多普勒雷达观测资料及ECMWF再分析资料, 对2016年6月30日发生在沈阳地区一次局地短时强降水过程的天气特征及可预报性进行分析。结果表明: 此次天气过程为东北冷涡背景下典型的午后强对流天气, 白天太阳辐射加热使得沈阳城区温度高于周边区域, 配合中低层的高湿环境具有较好的不稳定能量; 暴雨发生前2 h, 近地面至300 hPa高度西南气流不断增强, 低空急流出口区减压, 使得垂直运动增强, 上升运动高度达到对流层顶, 有利于触发和加强对流, 1.5—3.0 km的16 m·s^-1的西南风脉动和急流减弱消失对降水发生和结束有较好的指示意义; 午后低层辐合、高层辐散的不稳定层结在强降水发生前2 h建立, 散度最大时段与强降水时间对应较好; 雷达观测反映这次降水以低质心暖云降水为主导, 具有较高的降水效率, 地面辐合线触发了对流, 并逐渐发展演变成带状对流系统。从数值预报的结果来看, 模式未能报出城市下垫面对气象要素的影响, 对于地面辐合线的预报存在明显的滞后偏弱, 导致强降水预报难度增加。     

一次西南涡东移引起的长江上游流域强降水过程分析

利用MICAPS常规观测资料、FY2E卫星云图资料、fnl 1°×1°资料和三峡梯级调度自动化降水资料对2016年6月30日长江上游流域的暴雨过程进行分析。结果表明:在有利的大尺度环流背景配合下,产生此次强降水过程的主要影响系统是西南涡和低空急流,副热带高压和大陆高压的稳定维持使得西南涡移动缓慢,低空急流的加强使得西南涡加强,从而造成大范围的暴雨;本次过程地面没有冷空气触发,是一次在西南低空急流中出现的暖区强降水过程;高空分流区增强了高层辐散抽吸作用,使得西南涡不断发展加强。      

夜间低空急流的分析研究

本文利用北京325米气象塔取得的连续观测资料,分析了1979年9月5日夜间低空急流风温场的时空演变过程,这种低空急流的极值在夜间达到最大,其极值位置与逆温层顶的生长密切配合。低空急流是超地转的,超地转气流有一个惯性振荡周期。 低空急流的发展是起伏式的。急流的消散较快,当Ri ≤0.25达到切变不稳定的临界条件时加速崩溃。     

台风艾云尼非对称降水及动热力结构演变特征分析

台风艾云尼（1804号）第2次登陆广东过程中降水表现出显著的非对称分布,强降水主要位于其路径前进方向的右侧（简称台风右侧）。利用欧洲中期天气预报中心ERA5再分析资料、广东风廓线雷达观测资料以及降水观测资料,对造成非对称降水的环流背景和动力、热力结构演变特征进行了分析。结果表明:艾云尼左右两侧水汽输送及动力、热力条件差异是造成降水非对称的主要原因。加强的低空急流以及台风马力斯（1805号）水汽的输送为台风右侧强降水的产生提供了更好的水汽背景,而低空急流的加强配合高空强的辐散抽吸使得右侧垂直上升运动也明显大于左侧。边界层内强盛的低空急流以及珠江三角洲地区下垫面强摩擦辐合作用导致艾云尼右前侧径向入流强度更强、强入流层厚度更厚、边界层高度更高,且由于距离台风眼墙越近风速越大,上述现象越明显,为强降水的产生提供的动力和水汽条件越好。强降水期间艾云尼右侧低层大气维持不稳定状态,分析表明强低空急流携带的θ_se平流及其随高度的减弱弥补了强降水造成的能量损耗,是不稳定能量维持的重要原因。      

一次西南涡特大暴雨过程中MCS的演变特征

采用常规观测、地面自动观测、风廓线雷达及多普勒雷达资料,结合NCEP/NCAR 1°×1°再分析资料对2013年6月30日川渝特大暴雨过程中MCS的演变特征进行了分析,结果表明:(1)强降水及MCS均呈现增强—减弱—再次增强的演变特征,在MCS的增强阶段有中气旋和强降水超级单体的活动。(2)MCS生成并维持于西南涡前部次级环流的上升支内,西南涡缓慢东移并持续增强,为MCS的长时间维持提供了有利的背景动力条件。(3)MCS的波动性演变特征与西南涡的持续增强特征并不一致,其波动性演变与中低层西南风或南风急流的变化、低空及地面暖湿入流的变化有关。中低层西南风或南风急流的增强、低空及地面暖湿入流的增强均有利于MCS的增强。     

黑河地区夜间低空急流和冷世流特征分析

本文根据在ＨＥＩＦＥ实验期间大气边界层的探测资料，分析了夜间冷泄流和低空急流的发展过程。在一般情况下，黑河地区夜间低空急流的特征与稳定边界层在惯性振荡作用下所形成的低空急流特征一致。由于祁连山的作用，在戈壁地带会发生夜间冷泄流，并带有干旱地区的明显特征。当午夜前有冷泄流发生时，会影响到低空急流的发展，使急流层高度下降到近地层顶附近。这些对黑河地区边界层结构与地气交换过程必将产生重要的影响。     

On the Diurnal Cycle of Heavy Rainfall over the Sichuan Basin during 10–18 August 2020※

A sustained heavy rainfall event occurred over the Sichuan basin in southwest China during 10–18 August 2020, showing pronounced diurnal rainfall variations with nighttime peak and afternoon minimum values, except on the first day. Results show that the westward extension of the anomalously strong western Pacific subtropical high was conducive to the maintenance of a southerly low-level jet (LLJ) in and to the southeast of the basin, which favored continuous water vapor transport and abnormally high precipitable water in the basin. The diurnal cycle of rainfall over the basin was closely related to the periodic oscillation of the LLJ in both wind speed and direction that was caused by the combination of inertial oscillation and terrain thermal forcing. The nocturnally enhanced rainfall was produced by moist convection mostly initiated during the evening hours over the southwest part of the basin where high convective available potential energy with moister near-surface moist air was present. The convective initiation took place as cold air from either previous precipitating clouds from the western Sichuan Plateau or a larger-scale northerly flow met a warm and humid current from the south. It was the slantwise lifting of the warm, moist airflow above the cold air, often facilitated by southwest vortices and quasi-geostrophic ascent, that released the convective instability and produced heavy rainfall.     

EFFECTS OF SEA SURFACE TEMPERATURE AND ITS DIURNAL VARIATION ON DIURNAL VARIATION OF RAINFALL: A PARTITIONING ANALYSIS BASED ON SURFACE RAINFALL BUDGET

The effects of sea surface temperature(SST) and its diurnal variation on diurnal variation of rainfall are examined in this study by analyzing a series of equilibrium cloud-resolving model experiments which are imposed with zero large-scale vertical velocity.The grid rainfall simulation data are categorized into eight rainfall types based on rainfall processes including water vapor convergence/divergence,local atmospheric drying/moistening,and hydrometeor loss/convergence or gain/divergence.The rainfall contributions of the rainfall types with water vapor convergence are insensitive to the increase in SST from 27°C to 29°C during the nighttime,whereas they are decreased during the daytime.The rainfall contributions of the rainfall types with water vapor convergence are decreased as the SST increases from 29°C to 31°C but the decreases are larger during the nighttime than during the daytime.The rainfall contributions of the rainfall types with water vapor convergence are decreased by the inclusion of diurnal variation of SST with diurnal difference of 1°C during the nighttime,but the decreases are significantly slowed down as the diurnal difference of SST increases from 1°C to 2°C.The rainfall contributions of the rainfall types with water vapor convergence are insensitive to the inclusion of diurnal variation of SST during the daytime.     

三套再分析资料的中国夏季降水日变化特征

利用台站观测降水,评估分析了三套再分析(NCEP,ERA40和JRA25)降水资料对中国夏季降水日变化的再现能力.结果表明,三套再分析资料呈现的中国夏季降水日变化特征较观测存在明显偏差.对比台站观测的白天(08-20,北京时)和夜间(20-08时)降水比例.再分析降水在大部分区域都表现为白天较夜间偏多,NCEP和ERA...     

Numerical Simulation of the Heavy Rainfall in the Yangtze-Huai River Basin during Summer 2003 Using the WRF Model

In this study, a 47-day regional climate simulation of the heavy rainfall in the Yangtze-Huai River Basin during the summer of 2003 was conducted using the Weather Research and Forecast (WRF) model. The simulation reproduces reasonably well the evolution of the rainfall during the study period’s three successive rainy phases, especially the frequent heavy rainfall events occurring in the Huai River Basin. The model captures the major rainfall peak observed by the monitoring stations in the morning. Another peak appears later than that shown by the observations. In addition, the simulation realistically captures not only the evolution of the low-level winds but also the characteristics of their diurnal variation. The strong southwesterly (low-level jet, LLJ) wind speed increases beginning in the early evening and reaches a peak in the morning; it then gradually decreases until the afternoon. The intense LLJ forms a strong convergent circulation pattern in the early morning along the Yangtze-Huai River Basin. This pattern partly explains the rainfall peak observed at this time. This study furnishes a basis for the further analysis of the mechanisms of evolution of the LLJ and for the further study of the interactions between the LLJ and rainfall.     

Observation and Numerical Simulations with Radar and Surface Data Assimilation for Heavy Rainfall over Central Korea

This study investigated the impact of multiple-Doppler radar data and surface data assimilation on forecasts of heavy rainfall over the central Korean Peninsula;the Weather Research and Forecasting(WRF) model and its three-dimensional variational data assimilation system(3DVAR) were used for this purpose. During data assimilation,the WRF 3DVAR cycling mode with incremental analysis updates(IAU) was used. A maximum rainfall of 335.0 mm occurred during a 12-h period from 2100 UTC 11 July 2006 to 0900 UTC 12 July 2006.Doppler radar data showed that the heavy rainfall was due to the back-building formation of mesoscale convective systems(MCSs).New convective cells were continuously formed in the upstream region,which was characterized by a strong southwesterly low-level jet(LLJ).The LLJ also facilitated strong convergence due to horizontal wind shear,which resulted in maintenance of the storms.The assimilation of both multiple-Doppler radar and surface data improved the accuracy of precipitation forecasts and had a more positive impact on quantitative forecasting(QPF) than the assimilation of either radar data or surface data only.The back-building characteristic was successfully forecasted when the multiple-Doppler radar data and surface data were assimilated.In data assimilation experiments,the radar data helped forecast the development of convective storms responsible for heavy rainfall,and the surface data contributed to the occurrence of intensified low-level winds.The surface data played a significant role in enhancing the thermal gradient and modulating the planetary boundary layer of the model,which resulted in favorable conditions for convection.     

Climatic features of the south-westerly low-level jet over southeast china and its association with precipitation over east China

The major features of the south-westerly low-level jet (LLJ) in the lower troposphere over Southeast China and its climatic impacts are investigated by using FNL reanalysis data and observational precipitation data. Results show that LLJ mainly occurs in spring and summer and the occurrence frequency of LLJ over southeast China has significant diurnal cycle, most LLJ occur in the nighttime (0200 LST and 0800 LST). The high nocturnal occurrence frequency of LLJ is mainly resulting from increased nocturnal ageostrophic wind. Research on the climatic impacts of large-scale conditions depicts that, the occurrence of LLJ in April mainly results from the northward shifting of western pacific subtropical high (WPSH), and the occurrence of LLJ in July results from the strengthening of detouring flow around Tibetan Plateau. Analysis of the climatic effects of LLJ on precipitation distribution in 3 rainy seasons over Southeast China indicates that the rainfall events with strong intensity correspond to strong LLJs. The LLJ affects the precipitation over Southeast China by transporting water vapor and triggering upward motion. Rainfall regions well corresponds to the regions of the moisture convergence and strong upward motion triggered by LLJ. Negative wind divergence anomalies at 850 hPa and positive wind divergence anomalies at 200 hPa over the Yangtze-Huaihe River Valley strengthen the upward motion over this region, which are conductive to produce more precipitation over the Yangtze-Huaihe River Valley.     

Impacts of the diurnal cycle of radiation on tropical cyclone intensification and structure

To investigate the impacts of the diurnal cycle on tropical cyclones （TCs）,a set of idealized simulations were conducted by specifying different radiation （i.e.,nighttime-only,daytime-only,full diurnal cycle）.It was found that,for an initially weak storm,it developed faster during nighttime than daytime.The impacts of radiation were not only on TC intensification,but also on TC structure and size.The nighttime storm tended to have a larger size than its daytime counterparts.During nighttime,the radiative cooling steepened the lapse rate and thus reduced the static stability in cloudy regions,enhancing convection.Diabatic heating associated with outer convection induced boundary layer inflows,which led to outward expansion of tangential winds and thus increased the storm size.     

KINEMATIC ANALYSIS OF TYPHOON POLLY(No.9216)*

We partition the observed wind field into rotational and divergent wind fields to analyze the stream field of Typhoon Polly(No.9216),which landed on 31 August 1992 and caused severe weather and large damage in the eastern China.The results indicate that the preservation of typhoon intensity after landing and the heavy rainfall took place onthe northern periphery of typho on are due to the strong divergent winds on the regions of low level jet(LLJ) and high level jet(HLJ) around the typhoon.The direction of divergent winds in the LLJ is perpendicular to the observed wind.But,the direction of divergent winds around the HLJ axis is parallel to the observed winds.The stream function and the rotational wind corresponding to the horizontal vorticity display the vertical circulation associated with the heavy rain,which is stronger than the vertical circulation around the typhoon center.The three-dimensional trajectories exhibit the warm and moist air parcels of LLJ traveling northward into the heavy rainfall region and ascending,then turning eastward in the HLJ.     

Simulation of urban climate with high-resolution WRF model: A case study in Nanjing, China

In this study, urban climate in Nanjing of eastern China is simulated using 1-km resolution Weather Research and Forecasting (WRF) model coupled with a single-layer Urban Canopy Model. Based on the 10-summer simulation results from 2000 to 2009 we find that the WRF model is capable of capturing the high-resolution features of urban climate over Nanjing area. Although WRF underestimates the total precipitation amount, the model performs well in simulating the surface air temperature, relative humidity, and precipitation frequency and inter-annual variability. We find that extremely hot events occur most frequently in urban area, with daily maximum (minimum) temperature exceeding 36°C (28°C) in around 40% (32%) of days. Urban Heat Island (UHI) effect at surface is more evident during nighttime than daytime, with 20% of cases the UHI intensity above 2.5°C at night. However, The UHI affects the vertical structure of Planet Boundary Layer (PBL) more deeply during daytime than nighttime. Net gain for latent heat and net radiation is larger over urban than rural surface during daytime. Correspondingly, net loss of sensible heat and ground heat are larger over urban surface resulting from warmer urban skin. Because of different diurnal characteristics of urban-rural differences in the latent heat, ground heat and other energy fluxes, the near surface UHI intensity exhibits a very complex diurnal feature. UHI effect is stronger in days with less cloud or lower wind speed. Model results reveal a larger precipitation frequency over urban area, mainly contributed by the light rain events (< 10 mm d^?1). Consistent with satellite dataset, around 10?C20% more precipitation occurs in urban than rural area at afternoon induced by more unstable urban PBL, which induces a strong vertical atmospheric mixing and upward moisture transport. A significant enhancement of precipitation is found in the downwind region of urban in our simulations in the afternoon.     

Evaluation of MM5 mesoscale model at local scale for air quality applications over the Swedish west coast: Influence of PBL and LSM parameterizations

Summary The performance of MM5 mesoscale model (Version 3.6.3) using different planetary boundary layer (PBL) and land surface model (LSM) parameterizations is evaluated and compared using high temporal and spatial resolution G?TE2001 campaign data at local scale (a few kilometers) over the Greater G?teborg area along the Swedish west coast during 7–20 May 2001. The focus is on impact of PBL and LSM parameterizations on simulated meteorological variables important for air quality applications such as global radiation, diurnal cycle of near-surface air temperature and wind, diurnal cycle intensity, near-surface vertical temperature gradient, nocturnal temperature inversion, boundary layer height, and low-level jet (LLJ). The model performance for daytime and nighttime and under different weather conditions is also discussed. The purpose is to examine the performance of the model using different PBL and LSM parameterizations at local scale in this area for its potential applications in air quality modeling. The results indicate that the influence of PBL and LSM parameterizations on simulated global radiation, diurnal cycle of near-surface air temperature and wind speed, diurnal cycle intensity, vertical temperature gradient, nocturnal temperature inversion and PBL heights, which are critical parameters for air quality applications, is evident. Moreover, the intensity and location of LLJ are simulated well by all schemes, but there also exist some differences between simulated results by using different PBL and LSM schemes. Therefore, the choice of PBL and LSM parameterizations is important for MM5 applications to air quality studies. Correspondence: Junfeng Miao, Department of Earth Sciences, G?teborg University, P.O. Box 460, 405 30 G?teborg, Sweden