引起“碧利斯”强降水的MCS数值模拟研究

利用多种观测资料和数值模拟,对0604号强热带风暴碧利斯登陆后在湖南、广东等地引发强降水的中尺度对流系统活动特征进行了分析.结果表明,在"碧利斯"登陆后西行减弱过程中,由于西南季风的持续维持,"碧利斯"减弱后的低压环流中仍保持有强降水所需的充足水汽供应,造成局地强降水的MCS十分活跃.ARPS模式较好地模拟了7月15日发生在湖南南部的中尺度降雨过程,并揭示出"碧利斯"变性过程中,环境风场垂直切变结构强迫的次级环流决定了MCS活动特点,同时利用湿Q矢量诊断了低压次级环流的垂直运动特征.造成这次强降水过程的MCS在台风低压切变线以北的偏北潮湿气流中生成发展,低层偏北急流造成的动力辐合效应、对流不稳定性层结的建立是MCS在湖南南部迅速发展的重要原因.     

基于静止卫星红外云图的MCS自动识别与追踪

由于缺乏成熟的中尺度对流系统 (MCS) 自动追踪工具,使基于静止卫星红外云图的MCS研究的深度和广度受到制约。手工目测法识别追踪MCS,工作量大且容易造成误差,无法用于长时间序列卫星云图的MCS普查。该文结合我国中尺度对流天气的特点,在重新定义MCS云团识别标准的基础上,给出了一种基于图像处理和时间序列分析技术的MCS自动识别、存储、追踪和时间序列特征统计方法。通过对2003年淮河大水期间连续时次卫星云图的MCS自动识别、追踪和特征统计,对该方法的应用效果进行了检验,结果表明:该方法不仅能对每个时次的MCS进行快速识别,同时也能对多时次的MCS进行有效追踪。     

NUMERICAL SIMULATION OF A MESOSCALE CONVECTIVE SYSTEM (MCS) DURING THE FIRST RAINY SEASON OVER SOUTH CHINA*

Based on the NCEP/NCAR reanalysis data and observations collected during the SCSMEX,a mesoscale convective system (MCS) occurring over South China during 23-24 May 1998 has been studied with a numerical simulation using the Fifth Generation Penn-State/NCAR Mesoseale Modeling System (MMS).The successful simulations present us some interesting findings.The simulated MCS was a kind of meso-β scale system with a life cycle of about 11 hours.It generated within a small vortex along a cold front shear line.The MCS was characterized by severe convection.The simulated maximum vertical velocity was greater than 90 cm s^-1,and the maximum divergence at about 400 hPa.The rainfall rate of MCS exceeded 20 mm h^-1.To the right of the simulated MCS,a mesoscale low-level jet (mLLJ) was found.A strong southwesterly current could also be seen to the right of MCS above the mLLJ.This strong southwesterly current might extend up to 400 hPa.A column of cyclonic vorticity extended through most part of the MCS in the vertical direction.Additionally,the simulated MCS was compared favorably with the observational data in terms of location,precipitation intensity and evolution.     

MESOSCALE CONVECTIVE SYSTEMS DURING 20-23 JUNE 2002 REVEALED BY SATELLITE OBSERVATION

The 20-23 June 2002 mesoscale convective systems,which produced heavy rainfalls over the middle and lower reaches of the Changjiang River Basin,are studied using satellite imagery, satellite products and conventional sounding data. Results demonstrate that the torrential rain was caused by three MαCSs (Meso-a scale Convective System) and some MβCS (Meso-β-scale Convective System) activities in succession. The TBB (black-body temperature) analysis depicts that several meso-β-scale cloud clusters and convective cells were embedded and alternately developed within an MαCS.As the strongest convection gradually decreased,the cold TBB area expanded quickly in hours before dissipation. However,the heavy rainfall occurred in MαCS developing and maturing stages.And the minimum TBB fluctuation matched well to the precipitation trend with the lower TBB for the heavier rainfall. A kind of favorable synoptic environment for MαCS genesis and activities could be described as follows.The West Pacific subtropical high is stable with its western ridge reaching to the west region of South China.To its south and west sides,there is the robust ITCZ (Intertropical Convergence Zone),the active Indian-Bengal monsoon cloud surges carry warm and moist air by low-level jet (LLJ) to mid-latitudes where a frontal zone existed for days,meanwhile a 500 hPa short-wave trough moved eastward.At the upper troposphere,the southeasterly divergent flow dominates the environment due to the South-Asian high enhancing and moving eastward,and a monsoon water vapor plume (WVP) is stretched from the Bay of Bengal to the Changjiang River Basin.With a certain favorable configuration including a monsoon WVP,a frontal cloud system,a shortwave trough cloud system,and monsoon cloud clusters,MαCSs could initiate and develop successively in the same region.     

2016年吉林省“7•26”暴雨天气综合分析

利用NCEP 1°×1°再分析资料、FY-2G卫星云图、多普勒C波段天气雷达以及吉林省区域自动站和加密自动站资料,运用统计分析和天气动力学诊断方法,综合分析2016年7月25—26日吉林省暴雨天气过程,此次暴雨和大暴雨落区位于吉林省四平东部、辽源、吉林和通化北部,此次暴雨过程出现东北冷涡天气背景下,25日14—22时的短时强降水由东北冷涡前部局地对流活动的加强触发中尺度对流系统(MCS)生成。500 hPa为两脊一槽形势,受西太平洋副热带高压阻挡,东北冷涡较为稳定,吉林省位于东北冷涡前沿的西南气流中,850 hPa西太平洋副热带高压西侧的西南急流直达吉林省中南部,在其北端产生西南—东南向暖锋式切变,并与地面黄河气旋暖锋区相对应。在地面中尺度辐合线、地形抬升触发下,造成中尺度对流,形成短时强降水。通过FY-2G卫星相当黑体温度(TBB)产品分析,发现吉林省上空TBB低于-52℃时,就会产生短时强降水,而TBB高于-48℃则MCS趋于消失;此次强降水的新一代C波段天气雷达回波具有较明显的强回波低质心结构特征,降水效率较高,持续时间短,但达到短时强降水标准。卫星TBB产品和雷达监测可为以后吉林省东北冷涡暴雨定时、定点暴雨预警提供依据。     

浙江中部地区初夏两次暴雨天气过程对比分析

文中利用NCEP再分析资料、FY-2E红外云图TBB资料和加密观测资料,对浙江中部地区2018年5月7—8日（简称“5.7”暴雨）、5月18—19日（简称“5.18”暴雨）两次暴雨天气过程进行对比分析,研究暴雨发生发展的环流特征与中尺度条件差异。结果表明：地面辐合线加强触发暖区暴雨发生,且地面辐合线的强度、移向与新生单体的发展密切相关,强回波在地面辐合线附近合并加强形成“列车效应”。金衢盆地内地形辐合与阻挡产生强迫上升运动对MCS的发展起到促进和加强作用。“5.7”暴雨对流发展高度较低,暖云层深厚,降水回波在暴雨区稳定维持,属于积状和层状云混合降水;“5.18”暴雨对流云发展旺盛、具有混合相层与暖云层剖面结构,属于积状云为主的混合降水。“5.7”暴雨属冷锋前部型暴雨,“5.18”暴雨为暖切变型暖区暴雨。暖区暴雨预报可着眼于浙中地区低空急流发展,中层的干侵入和地面辐合线的加强维持作用。     

云南省一次切变冷锋型暴雨过程的中尺度对流系统分析

利用NCEP 1°×1°再分析资料和多种加密观测资料,对2013年初夏云南省一次典型冷锋切变型暴雨天气过程进行诊断和中尺度分析,结果表明：青藏高原和川西高原西北气流引导冷空气和切变线南下影响云南省,地面冷锋与切变线位置基本一致,冷暖空气交汇于切变线和冷锋附近,产生强降水。天气尺度系统的有效合理配置及相互作用,为中尺度对流系统的发展提供了有利的环流背景。CAPE高能区和等Δθse(500-800)线密集区的分布与对流系统的发生、发展有一定对应关系。暴雨发生的局地性和突发性等中小尺度特征与地面中尺度辐合系统密切相关。地面锋面及叠加在其上的加密地面风场辐合区的位置和移动可以作为短时强降水短临预报的重要参考依据。地面强降水强度和落区与对流云团的TBB等值线梯度大小以及梯度大值区的位置相关。地闪频数的发生发展,可以作为对流云团发生发展的判据之一。受多方面因素影响,低纬高原不同暴雨点的地闪频数峰值出现时间与强降水峰值时间的关系复杂。大风区、第二类γ中尺度辐合区的存在和“列车效应”是造成局地短时强降水的直接原因。边界层急流为此次强降水过程提供了重要的动力强迫和水汽输送。     

华东地区夏季中尺度对流系统的活动特征及成因研究

采用FY-2E卫星云图TBB(Black Body Temperature)资料,统计分析2010-2014年夏季(6-8月)华东地区的α中尺度对流系统(MαCS)和β中尺度对流系统(MβCS),发现两类MCS(Mesoscale Convective System)均具有夜发性,且发生主要集中于安徽、江苏、江西和浙江地区,形成后自西向东移动。进一步利用NCEP-CFSR和NCEP-CFSV2每6 h的再分析数据,通过主成分分析(Principal Component Analysis,PCA)和K-means聚类分析对两种尺度MCS成因进行分析,结果显示:850 hPa切变线和低空急流、500 hPa副高和中纬度短波槽以及200 hPa的高空急流是影响MCS形成主要的天气系统,对流层中层以下的水汽供应、低层大气不稳定性和低层辐合、高层辐散的动力结构是MCS形成的必要条件。MαCS发生前的天气形势可以分为两类:①生成位置位于850 hPa低空急流的西侧、气旋性环流的南侧,500 hPa、200 hPa分别受槽前西南气流、反气旋性环流的影响;②850 hPa切变线南部的偏西气流、500 hPa的偏西气流和200 hPa的高空急流配合。MβCS发生前的两类环流形势中,850 hPa切变线南部的偏西气流控制的为第一类,切变线南部的西南气流和生成位置东部的低空急流影响的为第二类,500 hPa生成位置位于短波槽东部,200 hPa均有西风急流与中低层配合。     

A cloud-resolving study on the role of cumulus merger in MCS with heavy precipitation

The cumulus merging processes in generating the mesoscale convective system （MCS） on 23 August 2001 in the Beijing region are studied by using a cloud-resolving mesoscale model of MM5. The results suggest that the merger processes occurred among isolated convective cells formed in high mountain region during southerly moving process play critical role in forming MCS and severe precipitating weather events such as hailfall, heavy rain, downburst and high-frequency lightning in the region. The formation of the MCS experiences multi-scale merging processes from single-cell scale merging to cloud cluster-scale merging, and high core merging. The merger process can apparently alter cloud dynamical and microphysical properties through enhancing both low- and middle-level forcing. Also, lightning flash rates are enhanced by the production of more intense and deeper convective cells by the merger process, especially by which, the more graupel-like ice particles are formed in clouds. The explosive convective development and the late peak lightning flash rate can be found during merging process.     

2016年7月18—20日湖北省特大暴雨过程的中尺度特征分析

利用多种常规和非常规观测资料诊断分析了湖北2016年7月18—20日特大暴雨过程的降水特点以及中尺度对流系统(MCS)的发生发展特征和环境场条件,结果表明：此次特大暴雨过程具有很强的极端性,分为梅雨锋南侧暖区降水和梅雨锋面降水两个阶段,都具有较为极端的水汽条件。第一阶段在地面风场辐合的作用下触发了初生对流单体,西南低空急流出现脉动,湿层的增厚促进了强雷暴的发展。新生单体在雷暴上游生成,迅速并入强雷暴,后向传播是其稳定少动的重要原因之一。强盛阶段,强雷暴具有暖云低质心的特征,低层垂直风切变与雷暴偏北风出流的方向配置、中气旋的出现都促进了上升运动,促使强雷暴长时间维持,造成了马良站连续数小时出现高强度降水。第二阶段,环境风平行分量远大于垂直分量,促进了东北—西南向平行层状MCS（PS型MCS）的形成。具有多单体依次排列的特征,新生单体在系统西南侧地面辐合线的作用下不断生成发展,使得PS型MCS增强维持,移动缓慢。马良站受到PS型MCS对流线和西北侧层状回波的影响,小时强降水明显较第一阶段偏弱。