Thermodynamics and Microphysical Characteristics of an Extreme Rainfall Event Under the Influence of a Low-level Jet over the South China Coast

In this paper,the data of Automatic Weather Stations(AWSs),ERA5 reanalysis,sounding,wind profile radar,and dual-polarization radar are used to study an extreme rainfall event in the south China Coast on 11 to 12 May 2022 from the aspects of thermodynamics and microphysical characteristics under the influence of low-level jets(LLJs).Results show that:(1)The extreme rainfall event can be divided into two stages:the first stage(S1)from 0000 to 0600 LST on May 12 and the second stage(S2)from 0700 to 1700 LST on the same day.During S1,the rainfall is mainly caused by the upper-level shortwave trough and the boundary layer jet(BLJ),characterized by strong upward motion on the windward side of mountains.In S2,the combined influence of the BLJ and synoptic-system-related low-level jet(SLLJ)increases the vertical wind shear and vertical vorticity,strengthening the rainstorm.In combination with the effect of topography,a warm and humid southwest flow continuously transports water vapor to farther north,resulting in a significant increase in rainfall over the study area(on the terrain’s windward slope).From S1 to S2,the altitude of a divergence center in the upper air decreases obviously.(2)The rainfalls in the two stages are both associated with the mesoscale convergence line(MCL)on the surface,and the wind field from the mesoscale outflow boundary(MOB)in S1 is in the same direction as the environmental winds.Due to a small area of convergence that is left behind the MOB,convection moves eastward quickly and causes a short duration of heavy rainfall.In S2,the convergence along the MOB is enhanced,which strengthens the rainfall and leads to strong outflows,further enhancing the surface convergence near the MOB and forming a positive feedback mechanism.It results in a slow motion of convection and a long duration of heavy rainfall.(3)In terms of microphysics,the center of a strong echo in S1 is higher than in S2.The warm-rain process of the oceanic type characterizes both stages,but the convective intensity in S2 is significantly stronger than that in S1,featuring bigger drop sizes and lower concentrations.It is mainly due to the strengthening of LLJs,which makes small cloud droplets lift to melting levels,enhancing the ice phase process(riming process),producing large amounts of graupel particles and enhancing the melting and collision processes as they fall,resulting in the increase of liquid water content(LWC)and the formation of large raindrops near the surface.

Analysis of Rainstorm Process in South China from September 7 to 8, 2022

Based on the NCEP data of the United States, a rainstorm process in South China during September 7 to 8, 2022 was studied. Synoptic method is a qualitative and empirical forecasting method. The results show that: In early September 2022, the cold air behind the trough line from northeast China to North China can directly drive southward and invade South China from the east road. Typhoon Hinnamnor is located in the saddle field between the mainland subtropical high and the offshore subtropical high. It moves northward on the west side of the coastal subtropical high, affecting the Taiwan Island and the coastal areas of East China. During September 7-8, the wave trough of the 925 hPa Easterly wave was located near 110°E. Easterly jet existed in the southeast of South China. The center of the easterly jet was located to the east of Hainan Island, which could transport abundant water vapor from the sea surface to the sky over South China. The precipitable water in the whole layer of the atmosphere decreased from the southern coastal areas to the north, reaching more than 50 mm in southern China, of which most of the South China Sea, Hainan Island and parts of the western part of Guangdong Province exceeded 60 mm, providing sufficient water vapor supply. The circulation field with convergence at low level and divergence at high level is conducive to vertical uplift to form precipitation.

THE IMPACT OF DIFFERENT PHYSICAL PROCESSES AND THEIR PARAMETERIZATIONS ON FORECAST OF A HEAVY RAINFALL IN SOUTH CHINA IN ANNUALLY FIRST RAINING SEASON

An ensemble prediction system based on the GRAPES model, using multi-physics, is used to discuss the influence of different physical processes in numerical models on forecast of heavy rainfall in South China in the annually first raining season(AFRS). Pattern, magnitude and area of precipitation, evolution of synoptic situation, as well as apparent heat source and apparent moisture sink between different ensemble members are comparatively analyzed. The choice of parameterization scheme for land-surface processes gives rise to the largest influence on the precipitation prediction. The influences of cumulus-convection and cloud-microphysics processes are mainly focused on heavy rainfall;the use of cumulus-convection parameterization tends to produce large-area and light rainfall. Change in parameterization schemes for land-surface and cumulus-convection processes both will cause prominent change in forecast of both dynamic and thermodynamic variables, while change in cloud-microphysics processes show primary impact on dynamic variables. Comparing simplified Arakawa-Schubert and Kain-Fritsch with Betts-Miller-Janjic schemes, SLAB with NOAH schemes, as well as both WRF single moment 6-class and NCEP 3-class with simplified explicit schemes of phase-mixed cloud and precipitation shows that the former predicts stronger low-level jets and high humidity concentration, more convective rainfall and local heavy rainfall, and have better performance in precipitation forecast. Appropriate parameterization schemes can reasonably describe the physical process related to heavy rainfall in South China in the AFRS, such as low-level convergence, latent heat release, vertical transport of heat and water vapor, thereby depicting the multi-scale interactions of low-level jet and meso-scale convective systems in heavy rainfall suitably, and improving the prediction of heavy rainfall in South China in the AFRS as a result.

THE APPLICATION OF ASSIMILATED AIRCRAFT DATA IN SIMULATING A HEAVY RAIN OVER SOUTH CHINA IN JUNE 2005

Regular and irregular observational data are used to analyze and simulate a torrential rain over the south of China on 18 – 24 June 2005. Since the regular data cannot depict the rainfall system fully, GRAPES model is used to simulate this process. Different data are assimilated for 12 hours by its simulating system and different analysis data are obtained. In order to analyze how well the model forecast has been improved with the addition of assimilated aircraft data, these different analysis data are used as the first-guess data to conduct two control numerical simulation tests. From these tests, it is proved that the model that adds aircraft assimilation data can simulate the main region of precipitation, which is more consistent with the observed precipitation than the model that does not, and that the accuracy rate is also improved. These numerical simulation tests not only show that it is necessary and capable to improve the modeling of this torrential rain process by using aircraft data, but also lays the foundation for forecasting heavy rains in the south of China based on aircraft data.

STRUCTURE OF MESO-β AND –γ-SCALE ON SOUTH CHINA HEAVY RAINFALL ON 12～13 JUNE 2005 USING DUAL-DOPPLER RADAR

The three-dimensional wind fields of the heavy rain on 12-13 June 2005 in Guangdong province are retrieved and studied with the volume scan data of the dual-Doppler radar located in the cities of Meizhou and Shantou. It is shown that the meso-β-scale and meso-γ-scale convergence lines located in the convective system at the low and middle layer play an important role in the heavy rainfall. The convergence line is the initiating and maintaining mechanism of the rain. A three dimensional kinematic structure model is also given.

STUDY ON THE CAUSE OF HEAVY RAIN 200506 (HR200506) IN GUANGDONG

A continuous heavy rain visited Guangdong province during June 18-25, 2005 (named Heavy Rain 200506, HR200506) and had resulted in enormous economic loss. The ageostropic Q vectors, θse, meridional circulation, computed from the NCEP reanalysis, and TBB are used to study the rainfall processes. The results indicated that a convective system moved northwards from the South China Sea (SCS) and stayed in Guangdong for several days, which was a direct cause of HR200506. The process is a result of the activity of the South China Sea summer monsoon. There were two rainbands of HR200506 in Guangdong. One laid in the north of Guangdong that produced frontal rainfall; another situated on the south of Guangdong which produced monsoon rainfall.

The Uncertainty of Mesoscale Numerical Prediction of Heavy Rain in South China and the Ensemble Simulations

In the context of non-hydrostatic MM5 version we have explored the impact ofconvective parameterization schemes on uncertainty in mesoscale numerical prediction of South Chinaheavy rain and mesoscale heavy rainfall short-range ensemble simulation by using two kinds ofphysics perturbation methods through a heavy rain case occurring on June 8, 1998 in Guangdong andFujian Provinces. The results show the physical process of impacts of convective schemes on heavyrainfall is that different latent heat of convective condensation produced by different convectiveschemes can make local temperature perturbation, leading to the difference of local vertical speedby the intrinsic dynamic and thermodynamic processes of atmosphere, and therefore, making differenceof the timing, locations and strength of mesh scale and subgrid scale precipitation later. Newprecipitations become the new source of latent heat and temperature perturbation, which finally makethe dynamic and thermodynamic structures different in the simulations. Two kinds of methods areused to construct different model version stochastically. The first one is using differentconvective parameterization and planetary boundary layer schemes, the second is adjusting differentparameters of convective trigger functions in Grell scheme. The results indicate that the firstensemble simulations can provide more uncertainty information of location and strength of heavyrainfall than the second. The single determinate predictions of heavy rain are unstable; physicsensemble predictions can reflect the uncertainty of heavy rain, provide more useful guidance andhave higher application value. Physics ensembles suggest that model errors should be taken intoconsideration in the heavy rainfall ensembles. Although the method of using different parameters inGrell scheme could not produce good results, how to construct the perturbation model or adjust theparameter in one scheme according to the physical meaning of the parameter still needs furtherinvestigation. The limitation of the current study is that it is based on a single case and morecases will be addressed in the future researches.

Relationship Between the Western Pacific Subtropical High and the Subtropical East Asian Diabatic Heating During South China Heavy Rains in June 2005

Based on the daily NCEP/NCAR reanalysis data,the position variation of the western Pacific subtropical high（WPSH） in June 2005 and its relation to the diabatic heating in the subtropical East Asia are analyzed using the complete vertical vorticity equation.The results show that the position variation of the WPSH is indeed associated with the diabatic heating in the subtropical East Asian areas.In comparison with June climatology,stronger heating on the north side of the WPSH and relatively weak ITCZ（intertropical convergence zone） convection on the south side of the WPSH occurred in June 2005.Along with the northward movement of the WPSH,the convective latent heating extended northward from the south side of the WPSH.The heating to the west of the WPSH was generally greater than that inside the WPSH,and each significant enhancement of the heating field corresponded to a subsequent westward extension of the WPSH.In the mid troposphere,the vertical variation of heating on the north of the WPSH was greater than the climatology,which is unfavorable for the northward movement of the WPSH.On the other hand,the vertical variation of heating south of the WPSH was largely smaller than the climatology,which is favorable for the anomalous increase of anticyclonic vorticity,leading to the southward retreat of the WPSH.Before the westward extension of the WPSH in late June 2005,the vertical variation of heating rates to（in） the west（east） of the WPSH was largely higher（lower） than the climatology,which is in favor of the increase of anticyclonic（cyclonic） vorticity to（in） the west（east） of the WPSH,inducing the subsequent westward extension of the WPSH.Similar features appeared in the lower troposphere.In a word,the heating on the north-south,east-west of the WPSH worked together,resulting in the WPSH extending more southward and westward in June 2005,which is favorable to the maintenance of the rainbelt in South China.

Intraseasonal Oscillation of the South China Sea Summer Monsoon and Its Influence on Regionally Persistent Heavy Rain over Southern China

The intraseasonal oscillation（ISO） in the South China Sea summer monsoon（SCSSM） and its influence on regionally persistent heavy rain（RPHR） over southern China are examined by using satelhte outgoing long wave radiation,NCEP/NCAR reanalysis,and gridded rainfall station data in China from 1981 to 2010.The most important feature of the ISO in SCSSM,contributing to the modulation of RPHR,is found to be the fluctuation in the western Pacific subtropical high（WPSH）,along with a close link to the Madden-Julian oscillation（MJO）.Southern China is divided into three regions by using rotated empirical orthogonal functions（REOFs）for intraseasonal rainfall,where the incidence rate of RPHR is closely linked to the intraseasonal variation in rainfall.It is found that SCSSM ISOs are the key systems controlling the intraseasonal variability in rainfall and can be described by the leading pair of empirical orthogonal functions（EOFs） for the 850-hPa zonal wind over the SCS and southern China.Composite analyses based on the principal components（PCs） of the EOFs indicate that the ISO process in SCSSM exhibits as the east-west oscillation of the WPSH,which is coupled with the northward-propagating MJO,creating alternating dry and wet phases over southern China with a period of 40 days.The wet phases provide stable and lasting circulation conditions that promote RPHR.However,differences in the ISO structures can be found when RPHR occurs in regions where the WPSH assumes different meridional positions.Further examination of the meridional-phase structure suggests an important role of northward-propagating ISO and regional air-sea interaction in the ISO process in SCSSM.

2023年麦收期河南省连阴雨的气候特征和可能成因

2023年麦收期河南省出现了罕见的连阴雨天气,降水和阴雨日数异常偏多,分别为自1961年以来的历史第五位和第二位。分析连阴雨的可能气候成因,主要结论如下:(1)造成连阴雨的环流形势为欧亚中高纬度西高东低,乌拉尔山高压脊持续发展并出现阶段性阻塞,东北冷涡活跃,冷空气能持续南下,西太平洋副热带高压(以下简称副高)偏北、偏强且异常偏西,来自副高西北侧边缘的暖湿气流和冷空气在河南省交汇。(2)副高偏强、偏北、偏西是由偏强、偏北、偏大、偏东的南亚高压和偏强、偏北的东亚副热带西风急流(以下简称西风急流)的引导所致,而东南亚对流层中高层的暖中心偏大、偏北和偏东且呈东西向带状分布,使南亚高压形成了上述的特征。暖中心和东北冷涡的共同作用,使东亚中高层的气温梯度增大,进而加强了西风急流。(3)2023年前期的La Ni1a事件有利于麦收期副高偏北,但不利于副高偏强和明显偏西。麦收期由于台风扰动由泰国湾至菲律宾群岛以东的热带西太平洋对流活跃释放的凝结潜热较多,以及麦收期前期的春季和麦收期伊朗高原至青藏高原的地面感热和潜热较强,使暖中心形成了上述的特征,进而决定了南亚高压和副高的特征。(4)2023年麦收期北大西洋海温偏高,以及北大西洋三极子为负位相,导致了乌拉尔山高压脊持续发展并出现阶段性阻塞,麦收期前期的春季和麦收期伏尔加河至贝加尔湖西北侧地面感热和潜热偏强,而下游贝加尔湖东南侧至鄂霍次克海地面感热和潜热偏弱一定程度上导致上述两区域分别出现了大范围的位势高度正距平和负距平。

“5·22”广州西北气流控制下特大暴雨过程的中尺度特征分析

为提高对西北气流控制下暴雨过程的认识,以期为此类暴雨预报提供支撑,利用国家级自动站与区域加密自动站逐小时降水资料,FY-2G卫星和广州多普勒天气雷达探测等资料,采用中尺度分析技术与重要物理量参数计算等方法,分析“5·22”广州特大暴雨过程的中尺度特征。结果表明:降水主要发生在夜间至凌晨,短时强降水占日雨量的比均达75%,强对流性明显。暴雨过程由在西北气流控制下的下滑槽配合对流层低层的西南低空急流及其左侧南移进入广东的深厚低涡切变系统产生,中高层西北气流与低层暖平流形成上干冷、下暖湿的不稳定层结。在高温高湿环境下东北风与西南风形成的地面辐合线触发对流,夜间随着低空急流增强,地面辐合增幅,上升运动加剧。β中尺度对流云团合并加强,后向传播的对流单体在西北气流引导下形成列车效应,伴随着高降水效率的热带低质心暖云降水,导致珠三角强降水产生并持续,从而形成特大暴雨。

非地转强迫对Fitow(0114)暴雨的影响

利用非静力中尺度模式MM5对 0 114号台风Fitow从 2 0 0 1年 8月 31日 0 0时～ 9月 2日 0 0时 (UTC ,下同 )的降水过程进行了模拟研究。结果表明 ,MM5对Fitow登陆过程中暴雨落区和强度的模拟与实况比较一致。模拟结果较好地再现了暴雨的中尺度特征。正是维持少动的台风倒槽和嵌入其中的中小尺度系统相互作用造成了暴雨的发生、发展 ,而高、低空中尺度散度场的配置对暴雨有很好的指示意义。在华南台风暴雨区无论是高层还是低层 ,都存在很强的非地转作用 ,非地转涡度项对散度倾向项是重要的强迫因素 ;但非地转作用的实现与中高纬度地区有本质的区别 ,在低层非地转作用是由于强的位势场气旋涡度 (- 2  <0 )与弱的流场气旋涡度 (fζ >0 )不平衡产生的 ;而高层非地转作用是由于强的位势场反气旋涡度 (- 2  >0 )与弱的流场反气旋涡度 (fζ <0 )不平衡产生的。非地转作用是暴雨中尺度系统上升运动发展的触发机制。从动力学角度解释了用非地转 Q矢量散度场来判断暴雨落区要比用准地转 Q矢量散度场好的原因。

影响华南持续性强降水的西南涡分析和数值模拟

基于AREM模式对发生在华南地区的3次西南涡暴雨过程进行了数值模拟,并利用模拟结果分析了暴雨过程中西南涡的演变特征,结果表明:高层200 h Pa西风急流入口区、中层500 h Pa西太平洋副热带高压位置、中纬度短波槽、东北亚强冷涡的适当配置,中低层孟加拉湾和南海暖湿气流的持续输送,是有利于西南涡东移发展,从而造成华南地区持续性强降水的典型环流形势;降水落区与低涡位置密切相关,一般集中在西南涡中心南侧,雨带延伸方向与低涡移动路径一致;而其强度则与低涡中心区域位势高度等值线梯度及低层大气风场强度息息相关。西南涡中心低层为东风和弱北风,中层以南风为主,高层为强西风和弱北风,低层辐合、高层辐散及正涡度结构特征显著。涡度平流项和辐合辐散项的作用集中体现在中低层大气,涡度对流项、扭转项的作用则在中高层更为明显,而涡度辐合辐散项对西南涡的发展加强起最主要的作用。

华南前汛期持续暴雨环流分型初步研究

采用1961—2010年NCEP/NCAR逐日再分析资料和台站观测降水量资料,按一定标准选取了华南前汛期24个持续暴雨过程;并且按基本判据确定逐年华南夏季风降水开始日期。然后依据南亚高压环流型和相对于该年夏季风降水开始的早晚,将这些暴雨过程划分为夏季风降水前、后南亚高压东部型,夏季风降水后南亚高压带状、西部型共4个类型;其中,夏季风后南亚高压西部型次数最多、平均持续时间最长。所有类型持续暴雨的相同点是:广东东北部附近均为暴雨频率和雨量高值区;暴雨期间华南150 h Pa位势高度增加、500 h Pa位势高度减少;华南处在150 h Pa偏西风急流南侧辐散区中;850 h Pa华南沿海有明显的西南气流,低层辐合在华南东北部最明显;两广沿海为可降水量大值区;华南的整层水汽输送主要呈现西南向。不同点是:夏季风后南亚高压西部型平均雨量较小,夏季风后南亚高压带状型与西部型在印度洋上存在明显的偏东风高空急流;夏季风后南亚高压类型在两广沿海的可降水量数值较大。

飞机报资料在0506华南致灾暴雨过程模拟中的应用

利用常规和非常规观测资料,对2005年6月18～24日华南地区的一次致灾暴雨过程进行了分析,并对18～19日的过程进行了模拟研究。由于常规观测资料的分析对于描述暴雨的系统结构等方面尚显不足,因此利用GRAPES模式对此次暴雨过程进行了模拟研究。采用不同资料,利用模式同化系统经过12小时的同化,将得出的不同的同化分析场作为模式的初始场,对此次过程进行24小时的数值模拟做对比模拟试验,来对比分析飞机报同化场对于模式预报的改进作用。试验证明,利用有飞机报加入的同化分析场做初始场的模拟结果不仅能够模拟出主要的降水区域,而且降水强度与实况更加吻合,比未加入飞机报的控制试验结果的准确率提高了。飞机报资料的使用对于提高此次暴雨过程模拟效果是可行的、必要的,而且也为该资料用于华南暴雨的预报奠定了基础。

影响华南后汛期季风持续性暴雨和热带气旋持续性暴雨的大尺度环流背景分析

利用1961—2008年NCEP逐日、逐月再分析资料和全国和华南各省台站逐日降水资料,得到华南后汛期持续性暴雨74例,其中热带气旋(TC)引起的持续性暴雨(TCR)有54例,季风引起的持续性暴雨(MSR)有20例。TCR主要发生在8月,占TCR总数的52%,MSR主要发生在7月,占MSR总数的70%。两类持续性暴雨的出现次数具有明显的年代际变化特征,自1980年代以来发生的次数明显增加。通过对比分析得到,MSR主要由前期和同期热带中东太平洋异常海温持续偏暖所致,其一方面加强了南海夏季风环流、水汽辐合异常增强;另一方面增强了菲律宾海的对流,使得高空西风急流位置偏北;在两者的共同作用下,大气环流激发出"-、+、-"的EAP遥相关型波列分布,为7月持续性暴雨的发生提供有利条件。相比之下,TCR主要由于8月局地海温-黑潮区海温异常偏冷致使高空西风急流位置偏北所致。此外,叠加在这种尺度背景下,导致MSR和TCR发生的关键是10~20天季节内振荡导致系统由东南向西北传播。

“98.5”华南前汛期暴雨的非静力数值模拟和中尺度系统分析

为了对华南暴雨进行深入的数值模拟研究,在对1998年5月23～24日(简称'98.5')华南暴雨进行天气分析的基础上,利用非静力中尺度数值模式MM5对该次暴雨过程进行了数值模拟.数值模拟结果和客观分析结果的比较表明,模拟结果可以再现造成暴雨的大、中尺度环流条件.造成此次暴雨的中尺度系统具有暖心高湿结构,高空辐散,低空辐合及对应的强上升运动和气旋性涡柱是造成这次暴雨的动力学机制,低空偏南气流对这次暴雨的产生和发展起着重要的作用.模拟的降水中心与观测的较接近,位置略偏南、偏西,雨量略小,但降水时段和雨区模拟较好.降水发生在喇叭口等有利地形;高低分辨率的地形资料对本次降水的模拟结果影响不大.

华南持续性暴雨的大尺度降水条件分析

利用1958—2004年NCEP/NCAR全球逐日再分析格点资料和同期我国华南地区45站的逐日降水资料,从水汽条件、不稳定能量及抬升条件对发生在华南地区的157个连续性暴雨进行分析,并结合各自的爆发时间,对暴雨进行分类,讨论了不同类型华南持续性暴雨的形成原因。

我国南方洪涝暴雨期西太平洋副高短期位置变异的特点及成因

利用NCEP/NCAR多年逐日再分析资料、美国环境预报中心CMAP(NOAA NCEP Climate Prediction Center Merged Analysis of Precipitation)候平均降雨量资料以及全国740站逐日降水资料,对华南前汛期和江淮梅雨期大范围持续性暴雨过程中西太平洋副高短期位置变异的异同及其可能成因进行了分析。结果表明:华南和江淮大范围持续性暴雨期间,西太平洋副高位置均比同期气候平均值异常偏南偏西,且强度偏强。华南暴雨期间,副高西北侧华南地区以及西侧孟加拉湾地区存在异常强烈的视热源和视水汽汇;江淮暴雨期间,副高北侧江淮流域及西侧孟加拉湾地区也存在异常强烈的视热源和视水汽汇。运用全型垂直涡度倾向方程理论,研究非绝热加热对西太平洋副高短期位置变异的影响,结果表明:副高位置的短期变异与非绝热加热场及其配置有密切联系。华南暴雨期间,副高西北侧边缘的华南地区加热场可在短期内迫使副高东撤南退;江淮暴雨期间,副高北侧江淮流域加热场的存在不利于副高北进,而西侧较远处孟加拉湾热源会诱导副高西伸,两者的共同作用导致副高在江淮以南维持,且会明显西伸。

2008年1月中国南部低温雨雪冰冻天气特征及其与东亚大气环流异常探讨

根据NCEP/NCAR提供的1968—1996年全球逐月、2008年1月全球逐日再分析资料以及中国气象局提供的降水资料,分析了2008年1月中国南部持续低温雨雪冰冻天气期间东亚地区中低空大气环流异常特征。结果表明,乌拉尔山阻塞高压和地面蒙古冷高压是这次灾害性天气重要的冷空气源地;700 hPa异常偏强的低空西南风急流以及低空急流大风速中心随时间沿急流轴的传播为此次低温雨雪冰冻天气提供了充足的动量、热量和水汽;850 hPa流场西太平洋上空异常东南风、印度洋上空异常西南风以及850 hPa垂直速度场中国南部大陆异常上升气流,在很大程度上影响着2008年1月中国南部的天气;赤道辐合带、西太平洋副热带高压以及南支槽的异常,致使东亚上空不仅存在异常的南支槽前西风带水汽输送和西太平洋副热带高压西南侧东风带水汽输送,还存在异常的由印度洋经孟加拉湾向中国南方大陆的水汽输送。