The Roles of Low-level Jets in “21·7” Henan Extremely Persistent Heavy Rainfall Event

An extremely heavy rainfall event lasting from 17 to 22 July 2021 occurred in Henan Province of China, with accumulated precipitation of more than 1000 mm over a 6-day period that exceeded its mean annual precipitation. The present study examines the roles of persistent low-level jets(LLJs) in maintaining the precipitation using surface station observations and reanalysis datasets. The LLJs triggered strong ascending motions and carried moisture mainly from the outflow of Typhoon In-fa(2021). The varying directions of the LLJs well corresponded to the meridional shifts of the rainfall. The precipitation rate reached a maximum during 20-21 July as the LLJs strengthened and expanded vertically into double LLJs, including synoptic-weather-system-related LLJs(SLLJs) at 850–700 hPa and boundary-layer jets(BLJs)at ~950 hPa. The coupling of the SLLJ and BLJ provided strong mid-and low-level convergence on 20 July, whereas the SLLJ produced mid-level divergence at its entrance that coupled with low-level convergence at the terminus of the BLJ on21 July. The formation mechanisms of the two types of LLJs are further examined. The SLLJs and the low-pressure vortex(or inverted trough) varied synchronously as a whole and were affected by the southwestward movement of the WPSH in the rainiest period. The persistent large total pressure gradient force at low levels also maintained the strength of low-level geostrophic winds, thus sustaining the BLJs on the synoptic scale. The results based on a Du-Rotunno 1D model show that the Blackadar and Holton mechanisms jointly governed the BLJ dynamics on the diurnal scale.

An Extreme Monsoonal Heavy Rainfall Event over Inland South China in June 2022: A Synoptic Causes Analysis

An extreme monsoonal heavy rainfall event lasted for nine days and recurred in the interior of northern south China from June 13 to 21, 2022. Using regional meteorological stations and ERA5 reanalysis data, the causes of this extreme monsoonal rainfall event in south China were analyzed and diagnosed. The results are shown as follows. A dominant South Asian high tended to be stable near the Qinghai-Tibet Plateau, providing favorable upper-level dispersion conditions for the occurrence of heavy rainfall in south China. A western Pacific subtropical high dominated the eastern part of the South China Sea, favoring stronger and more northward transport of water vapor to the northern part of south China at lower latitudes than normal. The continuous heavy precipitation event can be divided into two stages. The first stage(June 13-15) was the frontal heavy rainfall caused by cold air(brought by an East Asian trough)from the mid-latitudes that converged with a monsoonal airflow. The heavy rains occurred mostly in the area near a shear in front of the center of a synoptic-system-related low-level jet(SLLJ), and the jet stream and precipitation were strongest in the daytime. The second stage(June 16-21) was the warm-sector heavy rainfall caused by a South China Sea monsoonal low-level jet penetrating inland. The heavy rainfall occurred on the windward slope of the Nanling Mountains and in the northern part of a boundary layer jet(BLJ). The BLJ experienced five nighttime enhancements, corresponding well with the enhancement of the rainfall center, showing significant nighttime heavy rainfall characteristics. Finally, a conceptual diagram of inland-type warm-sector heavy rainfall in south China is summarized.

Comparison between the Roles of Low-Level Jets in Two Heavy Rainfall Events over South China

Two heavy rainfall events occurred over the Pearl River Delta during 20-22 May 2020:the first was a warm-sector event and the second a frontal event.Based on ERA5 reanalysis data and observations from wind profilers and Doppler weather radars,the structures and roles of low-level jets(LLJs)during these two heavy rainfall events were analyzed.The results show that:(1)South China was affected by a low-level vortex and a low-level shear line during the two processes.The two heavy rainfall events were both associated with a synoptic-system-related low-level jet(SLLJ)and a boundary layer jet(BLJ).The coupling of the convergence at the exit of the BLJ and the divergence at the entrance of the SLLJ produced strong lifting for the warm-sector heavy rainfall,and the strong convergence between the LLJs and northerly winds as the cold front moved southwards was the main lifting reason for the frontal heavy rainfall.(2)The BLJ was the main transport of water vapor during the two processes.The coupling of the BLJ and SLLJ caused the water vapor convergence to be concentrated in the boundary layer during the first process,whereas the strong convergence between the LLJs and northerly winds led to the lower and middle troposphere having strong water vapor convergence during the second process.(3)During the period of these two heavy rainfall events,the lower and middle troposphere remained unstable.Further analysis show that the differences in the intensity,location,and direction between the BLJ and SLLJ resulted in the pseudo-equivalent potential temperature advection in the boundary layer being significantly larger than in the lower and middle troposphere,which compensated for the energy loss caused by heavy rainfall and maintained the convective instability.These findings add to our knowledge on the roles of LLJs in the pre-summer rainfall over South China.

Analysis of the Characteristics of the Low-level Jets in the Middle Reaches of the Yangtze River during the Mei-yu Season

Here,we analyze the characteristics and the formation mechanisms of low-level jets(LLJs)in the middle reaches of the Yangtze River during the 2010 mei-yu season using Wuhan station radiosonde data and the fifth generation of the European Centre for Medium-Range Weather Forecasts(ERA5)reanalysis dataset.Our results show that the vertical structure of LLJs is characterized by a predominance of boundary layer jets(BLJs)concentrated at heights of 900-1200 m.The BLJs occur most frequently at 2300 LST(LST=UTC+8 hours)but are strongest at 0200 LST,with composite wind velocities>14 m s^(-1).Synoptic-system-related LLJs(SLLJs)occur most frequently at 0800 LST but are strongest at 1100LST,with composite wind velocities>12 m s^(-1).Both BLJs and SLLJs are characterized by a southwesterly wind direction,although the wind direction of SLLJs is more westerly,and northeasterly SLLJs occur more frequently than northeasterly BLJs.When Wuhan is south of the mei-yu front,the westward extension of the northwest Pacific subtropical high intensifies,and the low-pressure system in the eastern Tibetan Plateau strengthens,favoring the formation of LLJs,which are closely related to precipitation.The wind speeds on rainstorm days are greater than those on LLJ days.Our analysis of four typical heavy precipitation events shows the presence of LLJs at the center of the precipitation and on its southern side before the onset of heavy precipitation.BLJs were shown to develop earlier than SLLJs.

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.

The Monsoon Low-Level Jet:Climatology and Impact on Monsoon Rainfall over the West Coast and Central Peninsular India

The monsoon low-level jet(MLLJ)originates at Mascarene high and after traveling thousands of kilometers enters India from the western boundary causing deep convection,cloudiness,and rainfall.Although its core lies at 850 hPa,it has a large vertical extent;therefore,different meteorological parameters at different levels have a large influence on the Indian summer monsoon rainfall.This study aims to examine the upper-air climatology of 9 stations on the west coast and central Peninsular India and to find out the effects of various parameters at different standard pressure levels on the Indian summer monsoon rainfall variability.We used the 34-yr(1971–2004)actual upper-air radiosonde/radio wind and standard synoptic surface observations data from these 9 stations and reported some new aspects of the MLLJ.The NCEP/NCAR and ECMWF reanalysis wind data have also been used to holistically study the features of MLLJ over sea and land areas.This study,as opposed to some recent studies,confirms the splitting of MLLJ into two branches,which can be seen on a few days during the monsoon season.Further analyses show that the change in geopotential height between 800 and 900 hPa has a strong bearing on the strength of MLLJ.The change in the upper-air pressure gradient force over the Indian landmass can cause a change in the wind speed of MLLJ during the monsoon season.

Case studies on mesoscale structures of heavy rainfall system in the Yangtze River generated by Meiyu front

A heavy rainfall system was observed over the Yangtze River during Meiyu season by using dual-Doppler radar systems in the field experiment conducted by the project of National Fundamental Research Planning “Research on formation mechanism and the prediction theory of hazardous weather over China”. The three-dimensional mesoscale kinematic structure and process of a heavy rain on 22 and 23 July, 2002 are investigated by using the radial velocity and dual-Doppler radar technique. The results show that a southwest-northeast oriented rain band with the length of about 1000 km involves numbers of meso-β or meso-γ-scale convective cells with the size of 20―50 km. The heavy rain band in the middle reaches of the Yangtze River ex-ists in a low-level wind shear. The interaction between southwest low-level jet on the southern side of wind shear and east wind on the northern side formed the updraft. The wind disturbance, wind shear and convergence generate the convective action. The new cell developed in right rear flank of rain band and moved to the southwest wind area, where the vapor is abundant. This kind of echo can last a long time and developed well. The strong convective echoes are often ac-companied by the meso-γ-scale vortex and convergence.

川北两次特大暴雨天气过程成因的对比分析

采用天气动力学和动力诊断相结合的分析方法 ,通过对四川盆地北部两次小范围特大暴雨 (2 0 0 1年 9月 18日绵阳特大暴雨与 2 0 0 2年 6月 7日遂宁特大暴雨 )发生时的环流背景、主要影响系统、要素场特征等进行对比分析 ,找出了两次过程的相似和不同之处 。

关于暴雨和湿急流的讨论

湿急流是20世纪70年代末谢义炳在长期研究降水问题的基础上、在雷雨顺能量学方法对暴雨和强对流天气研究成果的启发下提出的一个科学猜想。近年来在多个暴雨实例的中尺度数值模拟结果都与湿急流猜想相符,即暴雨区中是存在将低空急流和高空急流在垂直方向上连接起来的湿急流,利用可视化技术还可将湿空气块在暴雨云团中的上升、加速和转向的轨迹清楚地展示出来。但是,湿空气块的加速并不仅仅是湿绝热上升过程中浮力的作用,动量收支的计算表明,中尺度水平气压梯度力对气块动量的增加也有贡献。文中还讨论了与暴雨和湿急流相关联的湿斜压平衡方程(湿热成风方程)等问题,指出从总能量收支平衡出发是不能解决暴雨过程中不同形式能量之间的转换过程是如何实现的。由于暴雨过程是高度非地转平衡的,也是非静力平衡的,同时还存在复杂的多尺度之间的相互作用,因此具有完善物理过程的高分辨率的数值模式是暴雨机理研究最有效的方法。

“8.19”华北区域暴雨的数值模拟与诊断分析

利用中尺度数值预报模式WRF3.2和NCEP 1°×1°再分析资料,对2010年8月18—19日发生在华北地区的暴雨天气过程进行了数值模拟,基于模拟结果,着重分析产生此次暴雨的动力、热力条件以及中尺度天气系统之间的相互作用。结果表明,此次暴雨过程是在有利的高、中、低层系统配置下,由高空冷涡带动干冷空气南下,与副热带高压外围的暖湿气流在华北地区上空交汇而形成;高低空急流的适宜配置,产生了动力场的耦合作用,形成深厚、强烈的上升运动,是大暴雨发生发展的主要动力条件;低空急流是主要的水汽输送通道,水汽主要来自南海和孟加拉湾;暴雨强盛时期,650h Pa以下大气表现为对流不稳定,此时华北地区上空的K指数>35℃,强降水时段出现在K指数梯度明显增大的过程中;对流层高层的干冷空气不断侵入触发对流层低层不稳定能量释放,强降水发生在低层暖平流向上抬升、高空冷平流向下侵入的时段。

2007年7月川东北连续3场大暴雨过程的诊断分析

利用常规观测资料及NCEP 1°×1°6 h再分析资料,对2007年7月上旬四川东北部连续出现的3场大暴雨过程的环流形势及动力结构、水汽输送和热力不稳定条件进行了诊断分析。结果表明:(1)前2场区域性大暴雨出现在副热带高压和巴尔喀什湖冷涡两个长波系统稳定少动的阻塞环流形势下,第3场局地性大暴雨发生在环流调整过程中,副热带高压快速东撤导致对流云团在东移过程中迅速减弱消亡;(2)暴雨的水汽主要来自南海,低空偏南风急流的维持为连续暴雨提供了源源不断的水汽输送和持续的能量供应,3场暴雨的中心均出现在位于低空急流出口区左侧水汽辐合中心的巴中地区;(3)造成严重洪涝灾害的前2场区域性大暴雨过程期间,从地面到高层形成了"辐合—辐散—辐合—辐散"接力式上下大气运动的动力结构,大气层结处于高能和对流不稳定状态,且有冷空气触发,大暴雨发生在能量锋区偏向暖区一侧。

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.