A Study on the Mesoscale Convective Systems during the Summer Monsoon Onset over the South China Sea in 1998 Part Ⅱ: Effect of Mesoscale Convective Systems on Large-Scale Fields

The apparent heat sources and apparent moisture sinks, and large-scale wind, temperature as well as the surface pressure fields during the summer monsoon onset over the northern South China Sea (SCS) in 1998 were diagnosed. The results suggested that there was a kind of positive feedback mechanism between large-scale circulations and mesoscale convective systems (MCSs). Before the monsoon onset, the largescale background provided favorable synoptic and dynamic conditions for the summer monsoon onset and the formation of mesoscale convective activities, whereas after the summer monsoon onset, occurrence of the persistent and extensive mesoscale convective activities produced obvious feedback effect on large-scale circulations. Because of the release of latent heating produced by enhanced convective activities, the intense atmospheric heating appeared over the northern SCS, which resulted in: (1) the meridional temperature gradient over the SCS reversed from upper-level to low-level and then the large-scale circulations were changed seasonally;(2) correspondingly, the surface pressure over the northern SCS deepened continually and formed a broad monsoon trough and the obvious pressure-fall areas, thus making the subtropical high move out of the SCS eventually;(3) with the development of the low pressure circulations in the middle and low troposphere, the MCSs further enhanced and extended southward, which was conducive to the SCS monsoon onset and maintenance over the middle and southern SCS;and (4) the deepening of monsoon trough facilitated the monsoon flow and moisture transport on its southern side, thus the monsoon onset reaching peak period.     

1998年南海季风爆发时期中尺度对流系统 的研究：Ⅱ中尺度对流系统对大尺度场的作用

通过对1998年南海季风爆发过程中大尺度风场、温度场、厚度场、地面气压场以及视热源与视水汽汇的演变分析研究了对流活动对大尺度场的作用,结果表明:大尺度环流与中尺度对流活动之间可能存在着一种正反馈机制。在季风爆发早期,大尺度背景与中尺度对流活动的关系主要表现为前者为季风爆发以及中尺度对流活动的发生提供有利的天气和动力条件;季风爆发后期持续的大范围中尺度对流活动反过来会对大尺度环流存在明显的反馈作用。由对流活动强烈发展产生的凝结潜热释放在南海北部造成了显著的大气加热,使对流层中上层出现一明显的加热中心,这导致:(1)南海上空经向温度梯度由高层向低层发生反向,形成北高南低的温度梯度,从而使大尺度环流发生季节性改变;(2)相应南海北部地面气压不断加深,形成宽广的季风槽和明显的减压区,促使副热带高压从南海地区最后撤离;(3)随着中低层低压环流的不断发展,对流系统和降水区进一步加强并向南扩展,有利于南海季风在南海中、南部地区爆发和维持;(4)季风槽的加深使其南侧的季风气流与水汽输送进一步加强,促使季风爆发过程达到盛期。     

A Study on the Mesoscale Convective Systems during the Summer Monsoon Onset over the South China Sea in 1998Part I: Analysis of Large-Scale Fields for Occurrence and Development of the Mesoscale Convective Systems

1. Introduction China is located in the East Asian monsoon re- gion. Every year's weather and climate in this region is deeply affected by the monsoon activities. Es- pecially, during flooding season (May to September), the summer monsoon controls large-scale precipitation patterns, the movement of seasonal rain belt and oc- currence of drought/flood disasters. The Asian mon- soon can be divided into two systems (Tao and Chen, 1987). As a major component and its unique location, the South …     

A Study on the Mesoscale Convective Systems during the Summer Monsoon Onset over the South China Sea in 1998 Part Ⅰ: Analysis of Large-Scale Fields for Occurrence and Development of the Mesoscale Convective Systems

The summer monsoon onset over the northern South China Sea (SCS) in May 16-20, 1998 was characterized by the abrupt onset of mesoscale convective activities and rapid increase of precipitation. The possible mechanism for formation of the mesoscale convective systems (MCSs) and related rain belts were revealed through discussing their forming physical conditions under the large-scale background: (1) The high pseudo-equivalent potential temperature and the convective instability in the lower troposphere, the low-level southwesterly confluence and the high-level divergence over South China and the northern SCS provided the favorable large-scale thermodynamic and dynamic conditions for development of MCSs. The southwesterly flow from the Bay of Bengal (BOB) interacted with that to the western flank of the subtropical high, which constituted the major moisture channels, thus bringing about deep wet layers and strong moisture convergence;(2) triggered by several cold troughs from high and mid latitudes, the convectively unstable energy was released and the convective activities over the northern SCS broke out abruptly;(3)analysis of retrieved precipitation based on the dual-Doppler radar during South China Sea Monsoon Experiment (SCSMEX) indicated that active convection influenced by the monsoon trough and corresponding wind shear line organized and formed continually some mesoscale convective rainbelts. During May 15-19,about 12 precipitation processes with 6-12-hour life span or more were observed;and (4) under the favorable synoptic conditions, establishment of the monsoon trough and shear line in the low levels, as well as production and development of mesoscale low vortex were all necessary conditions for the formation and maintenance of MCSs.     

Overview of the South China sea monsoon experiment

The present paper gives an overview of the key project “ South China Sea Monsoon Experiment(SCSMEX)” operated by the Ministry of Science and Technology of China during the period of 1996-2001. The SCSMEX is a joint atmospheric and oceanic field experiment which aims to better understandthe onset, maintenance, and variability of the summer monsoon over the South China Sea (SCS). It is a large-scale international effort with many participating countries and regions cooperatively involved in this experiment. With the field observation in May-August 1998, a large amount of meteorological and oceanic data was acquired, which provides excellent datasets for the study of the SCS monsoon and the East Asian monsoon and their interaction with the ocean. The preliminary research achievements are as follows. (1) The earliest onset of the Asian monsoon over the SCS and Indo-China Peninsula has been well documented. From the viewpoint of the synoptic process, its onset is closely related to the early rapid development of a twin cyclone to the east of Sri Lanka. The conceptual model of the SCS monsoon onset in 1998 was put forward. The 50-year time series of the SCS monsoon onset date was also made. (2) Two major modes, namely the 30-60-day and 10-20-day oscillations were ascertained. The influences of the abnormal SCS monsoon on the precipitation over eastern China and its modes were identified. A strong(weak) monsoon over the SCS usually leads to less (more) precipitation over the middle and lower reaches of the Yangtze River basin, and more (less) precipitation in North China. (3) During the monsoon onset over the SCS, a wide variety of organized mesoscale convective systems (MCSs) were observed by a Doppler radar array deployed over the northern SCS. The relationship between large-scale circulations and MCSs during the monsoon onset process in 1998 was clearly revealed. It was suggested that there is a kind of positive feedback mechanism between large-scale circulations and MCSs. (4) The SST over the SCS during the early period influences the timing of the monsoon onset date and the monsoon‘s intensity. During the monsoon onset, the ocean undergoes a process of energy release through air-sea interaction. During the break phase of the SCS monsoon, the ocean demonstrates the process of energy re-accumulation. Obvious differences in the air-sea turbulent flux exchange between the southern and northern parts of the SCS due to different characteristic features of the atmosphere and sea structure were observed in those regions.(5) The verification of impact of intensive observations on the predictive performance is made by the use of regional models. The air-sea coupled regional climate model (CRCM) was also developed under the SCSMEX Project . The simulation of the oceanic circulation in 1998 produced with the model was well compared with the observations.     

South China Sea Monsoon Experiment （SCSMEX） and the East Asian Monsoon

The SCSMEX is a joint atmospheric and oceanic experiment by international efforts, aiming at studying the onset, maintenance, and variability of the South China Sea (SCS) summer monsoon, thus improving the monsoon prediction in Southeast and East Asian regions. The field experiment carried out in May-August 1998 was fully successful, with a large amount of meteorological and oceanographic data acquired that have been used in four dimensional data assimilations by several countries, in order to improve their numerical simulations and prediction. These datasets are also widely used in the follow-up SCS and East Asian monsoon study. The present paper has summarized the main research results obtained by Chinese meteorologists which cover six aspects: (1) onset processes and mechanism of the SCS summer monsoon; (2) development of convection and mesoscale convective systems (MCSs) during the onset phase and their interaction with large-scale circulation; (3) low-frequency oscillation and teleconnection effect; (4) measurements of surface fluxes over the SCS and their relationship with the monsoon activity; (5) oceanic thermodynamic structures, circulation, and mesoscale eddies in the SCS during the summer monsoon and their relationship with ENSO events; and (6) numerical simulations of the SCS and East Asian monsoon.     

南海季风区冰相相变潜热对中尺度对流云和降水影响作用的数值模拟研究

利用可分辨云模式及中国南海北部试验区加密探空的平均水平风场、位温场和水汽场模拟分析了1998年5月15日至6月11日中国南海北部地区中尺度对流系统(Mesoscal Convective System,简称MCS)中冰相相变潜热对云和降水、辐射传输以及大尺度环境场的影响作用。研究表明,冰相相变潜热总体上不会引起明显的大气辐射通量的变化,但会引起较明显的下垫面热通量的变化。凝华潜热释放显著地增加了大气稳定度,造成对流和下垫面热通量的减弱,从而导致地面降水减小10.11%。碰冻潜热释放也使得大气稳定度增加,不利于中尺度对流系统对流的发展,区域累积降水量减小2.2%。融化潜热的冷却效应,使得融化层以下的大气降温,从而增加了低层大气的不稳定性,有利于海面热通量的输送,导致MCS降水增加4.1%。因此,冰相相变潜热对降水的影响主要是通过影响大气环境稳定,进而影响洋面感热通量和潜热通量的垂直输送和对流的发展,导致区域降水改变。     

Initiation and Evolution of Long-Lived Eastward-Propagating Mesoscale Convective Systems over the Second-Step Terrain along Yangtze-Huaihe River Valley

Based on the previous statistical analysis of mesoscale convective systems(MCSs)over the second-step terrain along Yangtze-Huaihe River Valley,eight representative long-lived eastward-propagating MCSs are selected for model-based sensitivity testing to investigate the initiation and evolution of these types of MCSs as well as their impact on downstream areas.We subject each MCS to a semi-idealized(CNTL)simulation and a sensitivity(NOLH)simulation that neglects condensational heating in the formation region.The CNTL experiment reveals convection forms in the region downstream of a shortwave trough typified by persistent southwesterly winds in the low-to midtroposphere.Upon merging with other convective systems,moist convection develops into an MCS,which propagates eastward under the influence of mid-tropospheric westerlies,and moves out of the second-step terrain.The MCS then merges with pre-existing local convection over the plains;the merged convection reinforces the cyclonic wind perturbation into a mesoscale vortex at 850 hPa.While this vortex moves eastward to regions with local vortex at 850 hPa,another vortex at 925 hPa is also intensified.Finally,the vortices at 850 and 925 hPa merge together and develop into a mesoscale convective vortex(MCV).In contrast,MCSs fail to form and move eastward in the NOLH experiment.In the absence of eastward-propagating MCSs,moist convection and mesoscale vortices still appear in the plains,but the vortex strength and precipitation intensity are significantly weakened.It is suggested the eastward-propagating MCSs over the second-step terrain significantly impact the development and enhancement of moist convection and vortices in the downstream areas.     

用改良的面积重叠法自动跟踪中尺度对流系统的试验研究

中尺度对流系统(Mesoscale Convective Systems, MCSs)的活动和演变,是造成致洪暴雨灾害性天气的主要因素之一.多年来,许多学者加大了对中尺度对流系统研究的力度.但是,中尺度对流系统的活动及其移动和传播的研究进展一直受到计算机自动识别和跟踪问题的制约.首先介绍了一种基于面积重叠法的MCSs自动跟踪技术,然后针对它存在的问题进行了改良,并应用改良后的面积重叠法对1998年6月至8月的青藏高原上空MCSs进行了识别与跟踪实验.实验结果表明,改良后的面积重叠法的跟踪准确率可达80%,比改良前提高了10%以上.     

南海季风爆发期间中尺度对流云带演变特征与持续性加强的机理研究

南海季风爆发与随后爆发的东亚季风,与夏季东亚地区旱涝关系密切,而相伴的南海对流活动与季风爆发的维持和发展存在何种相互关系,是需要探究的.为此,利用热带测雨卫星(Tropical Rainfall Measuring Mission,TRMM)的雷达(Precipitation Radar,PR)、微波成像仪(TRMM ...     

夏季青藏高原上的对流云和中尺度对流系统

运用1998年6～8月逐日逐时日本地球静止气象卫星(GMS)红外辐射亮温资料,计算和分析了青藏高原及周边地区对流云和中尺度对流系统的活动,揭示了它们形成和发展的月际变化和地理分布、强度、日变化、移动和传播等诸多特征,以及与长江流域暴雨过程的关系.     

南海暖季天气系统与中尺度对流过程研究进展

本文总结了近几十年来暖季(5～10月)南海热带天气系统及中尺度对流过程的相关研究进展.聚焦暖季南海中尺度对流过程,概述性回顾了与南海中尺度对流过程相关的热带大气环流和夏季风的基本特征,影响南海中尺度对流发生、发展的重要天气系统,并着重归纳了南海中尺度对流系统的活动规律、结构特征与形成机理.在此基础上,探讨了当前及未来南...     

南海夏季风对强热带风暴Bilis(0604)引发暴雨的影响

Bilis(0604)是一个登陆后长久维持并造成了特大暴雨的强热带风暴.通过对常规观测资料、雷达资料和红外卫星云图的分析,发现暴雨过程中不断有中尺度对流系统(MCSs)产生和发展,这些MCSs是造成强暴雨的主要中尺度系统.在此基础上用中尺度模式MM5对Bilis登陆后66 h的过程进行了数值模拟,模拟的雨带和雨量与实况较符合.利用模拟结果,对MCSs的发展过程和特征作了分析,发现Bilis的低压环流和南海夏季风在华南一带交汇,使得华南一带多中尺度涡旋、辐合中心和中尺度辐合线发展,这些系统为中尺度对流系统的生成和维持提供了有利的环境.敏感性试验表明,南海夏季风输送暖湿气流为暴雨区补充不稳定能量和水汽,对暴雨的产生具有十分重要的作用.     

2011年南海夏季风爆发过程及其与长江中下游梅雨的联系

采用NCEP/NCAR再分析资料、FY2E-TBB及台站降水资料,对2011年南海夏季风爆发前后的环流特征进行分析。结果表明:2011年强对流活动由孟加拉湾扩展到南海地区,同时伴随着南亚高压移至中南半岛北部,西太平洋副热带高压向东撤出南海地区,南海夏季风于5月第4候(第28候)爆发;季风爆发后,印度-孟加拉湾季风槽形成,南海地区低空开始盛行西南气流,并伴有对流降水的发展和温、湿等要素的突变。随着季风活动的推进,我国雨带北抬,长江中下游一带进入梅雨期,出现降水大值区。通过分析发现长江中下游梅雨与南海夏季风均受副热带高压影响,且两者的强度为显著的负相关关系,梅雨开始时间与南海夏季风爆发时间呈显著的正相关关系。2011年南海夏季风偏弱,爆发时间偏早,长江中下游梅雨强度偏强,入梅时间异常偏早。     

Effect of condensational heating over the Bay of Bengal on the onset of the South China Sea monsoon in 1998

Summary In this study, the authors analyse the observational features of the onset of the South China Sea (SCS) monsoon in 1998 shown in reanalysis data and use a numerical model to understand the mechanisms responsible for these features.The onset of SCS summer monsoon in 1998 occurred around 21 May. Prior to this period, monsoon depression activity was strong over the Bay of Bengal (BOB) and warm temperature anomalies appeared at the mid-upper troposphere over the northern BOB. In the meantime, warm horizontal thermal advection occurred over the northern Indo-China peninsula and South China. This warm advection seemed to play an important role in the winter-to-summer transition of the patterns of mean meridional temperature gradient in the 500–200-mb layer over South Asia.The PSU/NCAR Mesoscale Model Version 5 (MM5) is used to understand the physical link between the latent heating associated with the monsoon depression over BOB and the establishment of SCS monsoon. Full-physics simulations, for a 6-day period coinciding with the onset of the observed monsoon, reproduce realistically the evolution of the monsoon depression and monsoon onset process. It is found that the condensational heating over BOB is important for the formation of large-scale circulation pattern that favors the establishment of SCS monsoon. In an experiment without latent heating, the winter-to-summer reversal of meridional temperature gradient over South Asia was delayed and the onset of SCS summer monsoon did not occur during the simulation period.     

2007年南海夏季风爆发前后大气环流的非典型性突变特征

南海夏季风爆发的一般特征是南亚高压移至中南半岛北部;西太平洋副热带高压连续向东撤出南海地区,移到120°E以东的热带洋面上;高（低）空东北（西南）气流占据南海大部分地区,相应的105°E附近的越赤道气流建立,南海季风槽形成并同时伴有对流降水的发展和温、湿等要素的突变。国家气候中心的监测表明,2007年南海夏季风于5月第5候爆发。该年季风爆发后,虽然源自热带地区的低空西南气流迅速占据南海上空,高空盛行东北气流,且南亚高压西移至中南半岛上空,但对流、高度场以及降水场的突变特征均很不明显,表现为季风爆发后南海上空的对流依然偏弱,副高没有马上撤离南海,同时华南地区的降水量也没有迅速增强。因此,2007年南海夏季风爆发前后大气环流的变化特征具有非典型性。     

南海季风试验与东亚夏季风

南海季风试验是一次国际性大气与海洋的联合试验 ,旨在更好地了解南海季风的爆发、维持与变化 ,以改进东亚和东南亚地区的季风预报。 1998年 5～ 8月进行的外场试验取得了圆满成功 ,获得了大量气象与海洋资料。不少国家对这些资料进行四维资料同化 ,并改进数值模拟和预报 ;同时也为东亚与南海地区季风的研究提供了必要的资料集。文中总结了中国科学家在这方面的主要研究成果 ,共包括 6个方面 :(1)南海夏季风的爆发过程与机理 ;(2 )南海季风爆发过程中对流与中尺度系统的发展及其与大尺度环流的相互作用 ;(3)低频振荡与遥相关作用 ;(4 )南海海 气通量的测量及其与季风活动的关系 ;(5 )夏季风时期南海海洋的热力结构、环流和中尺度涡旋及其与ENSO事件的关系 ;(6 )南海与东亚季风的数值模拟。     

孟加拉湾季风爆发对南海季风爆发的影响Ⅱ:数值试验

通过数值试验研究了孟加拉湾季风爆发期间该地区旺盛的对流凝结加热对南海季风爆发和副热带高压“撤出”南海的影响,结果证明在孟加拉湾地区引入模拟的对流凝结潜热使该地区出现了强烈的上升运动,引起了孟加拉湾季风的爆发。同时由于对凝结加热的非对称Rossby响应,在南海北部导致西风出现和增强及垂直上升运动。因低层水汽平流的共同作用下,在南海北部引起了对流的发展。而正是南海北部的凝结加热促使南海地区温度经向梯度逆转,使副热带高压脊面的倾斜从冬季型转为夏季型,即低层的副热带高压减弱南移。最后当对流在南海地区发展起来时,副热带高压移出南海地区。     

关于南海夏季风建立的大尺度特征及其机制的讨论

使用1998年南海季风试验期间高质量资料和NCEP/NCAR40年再分析资料分析了南海季风建立前后的大尺度环流特征和要素的突变及爆发过程。发现南亚高压迅速地从菲律宾以东移到中南半岛北部,印缅槽加强,赤道印度洋西风加强并向东向北迅速扩展和传播,以及相伴随的中低纬相互作用和西太平洋副高连续东撤是南海夏季风建立的大尺度特征,与此同时,亚洲低纬地区的南北温差和纬向风切变也发生相应的突变。数值实验结果指出,印度半岛地形的陆面加热作用在其东侧激发的气旋性环流对于印缅槽的加强有重要作用,并进而有利于南海夏季风先于印度夏季风爆发。     

基于动态模板函数的线状中尺度对流系统自动识别

在中尺度对流系统(mesoscale convective systems,MCSs)自动识别、跟踪基础上,根据拟合椭圆长轴设计动态模板和得分函数,完成了雷达拼图资料上的线状MCSs自动识别。并利用不同类型中尺度天气过程对算法进行检验,分析结果表明:(1)算法能够实现对线状MCSs的自动识别,由于对流系统分裂与合并造成的回波形态"突变",得分值结果能得到较好的反映;(2)算法能够实现对线状MCSs天气的跟踪,一般情况下跟踪效果较好,在分裂或者合并发生的时刻跟踪效果较差。