SPATIAL AND TEMPORAL DISTRIBUTION OF MESOSCALE CONVECTIVE VORTICES IN EAST CHINA AND THE WESTERN PACIFIC REGION

Data from high-resolution satellites were used to evaluate the spatial and temporal distribution of mesoscale convective vortices (MCVs) in central and east China and the western Pacific Ocean region. The monthly variation in MCVs was significant. From May to October, MCVs were clearly affected by large-scale environmental conditions, including the South Asian summer monsoon, subtropical high and solar radiation, which resulted in clear changes in MCV spatial distributions from strengthening and weakening processes. Based on the analysis of diurnal MCV variations and the precipitation rate from May to October, MCVs were found to occur more frequently over the ocean than over land. MCVs near the Sea of Japan and northern South China Sea occurred during all types of weather. Ocean occurrences near land, such as the Ryukyu Islands, were categorized as morning-active MCVs. The hilly regions of southeastern China and North China Plain were characterized by afternoon-active MCVs. Limited to topography and the urban heat island effect, the Beijing-Tianjin-Tangshan area had evening-active MCVs, while Changbai Mountain had nocturnal MCVs.     

Diurnal Variation of MCSs over Asia and the Western Pacific Region

Mesoscale convective systems (MCSs) are severe disaster-producing weather systems. Radar data and infrared satellite image are useful tools in MCS surveillance. The previous method of MCS census is to look through the printed infrared imagery manually. This method is not only subjective and inaccurate, but also inefficient. Different from previous studies, a new automatic MCS identification (AMI) method, which overcomes the above disadvantages, is used in the present study. The AMI method takes three steps: searching potential MCS profiles, tracking the MCS, and assessing the MCS, so as to capture MCSs from infrared satellite images. Finally, 47468 MCSs are identified over Asia and the western Pacific region during the warm seasons (May-October) from 1995 to 2008. From this database, the geographical distribution and diurnal variation of MCSs are analyzed. The results show that different types of MCSs have similar geographical distributions. Latitude is the main control factor for MCS distribution. MCSs are most frequent over the central Tibetan Plateau; meanwhile, this area also has the highest hail frequency according to previous studies. Further, it is found that the diurnal variation of MCSs has little to do with MCSs’ size or shape; MCSs in different areas have their own particular diurnal variation patterns. Based on the diurnal variation characteristics, MCSs are classified into four categories: the whole-day occurring MCSs in low latitude, the whole-day occurring MCSs in high latitude, the nocturnal MCSs, and the postmeridian MCSs. MCSs over most places of mainland China are postmeridian; but MCSs over the Sichuan basin and its vicinity are nocturnal. This conclusion is coincidental with the hail climatology of China.     

面向空间数据挖掘的MCSs移动和传播影响因素分析

利用1998年6～8月青藏高原逐时红外遥感云图及青藏高原高分辨率有限区域数值预报值(HLAFS), 运用空间数据挖掘的相关分析技术对青藏高原MCSs的移动和传播与其周围环境场中物理量场之间的关系进行了研究.结果表明:它们东移出高原(105(E)与其东侧在400、500hPa上的高度(H)、涡度(VOR)、散度(DIV)、水汽通量散度(IFVQ)、垂直速度(W)、指数(K)等6个物理量的特征值,以及其自身形状密切相关.这对MCSs的移动和传播这一迄今的难题提供了研究思路和方法,同时对预报高原MCSs东移影响长江中下游地区的强降水也很有帮助.     

与2003年梅雨期西太副高东西向运动有关的热带上空东风带扰动的结构和演变特征

利用NCEP/NCAR 1 000～10 hPa 2.5 (×2.5 (的再分析资料,分析了与西太平洋副热带高压(简称西太副高)东西进退相关系的热带对流层上空东风带扰动(简称EV)的结构和演变特征.结果表明:西太副高与其南侧的东风带扰动存在同时西进的过程,当西太副高南、北两侧的东、西风带上的扰动在相向运动中抵达同一经度上时,西太副高出现异常东退.热带对流层上空东风带扰动为中高层天气系统,它从对流层中层伸展到50 hPa高度附近,在200 hPa上表现得最为明显,在热力场上表现出"上暖强下冷弱"的垂直分布特征;在西太副高东退时,东风带扰动东西侧的辐散效应和垂直运动的性质发生了显著变化,东风带扰动中心附近的垂直速度场出现从上升运动到向下下沉运动转化的过程.     

前汛期福建地区GMS-5红外云图的亮温度与地面雨强的关系研究

运用1998～2000年夏季青藏高原上182天的极轨气象卫星资料,研究了由计算机客观自动追踪中尺度对流系统(MCS)方法和由此得到的28685个MCSs及提取的相关信息,较为全面地揭示了夏季高原上MCS初生阶段的日频数、强度、形状、日变化、地理分布等诸多特征。得到了高原上每日平均有163个MCSs生成,其中约有76 %的面积仅为数百km2,>104 km2的极少(占4 %);它们的形状多为带状、逗点状以及其他不规则形状,圆形和椭圆形只占近30 %,云顶平均亮温为-51 ℃左右;午后MCS数是上午前期的2倍以上,并且云顶亮温≤-54 ℃的多达5倍以上;它们主要活动在33 oN以南的高原南部,其中在雅鲁藏布江中上游地区和川西至横断山一带各有一个高频中心,这充分展示出南亚季风活动对MCSs初生的主导作用。     

1998年7月南亚高压影响西太平洋副热带高压短期变异的过程和机制

利用逐日NCEP/NCAR再分析资料,对1998年7月二度梅期间南亚高压影响西太平洋副热带高压短期变异的过程和作用机制进行了诊断.结果发现,梅雨间歇期副高单体异常向西、向北发展期间,高层南亚高压曾离开高原上空东移(伸)至120°E以东;南亚高压返回高原上空时,中层副热带高压减弱南落,两者有相向然后相背移动的趋势.进一步研究这两个高压系统密切关联的原因又发现,南亚高压通过两种作用机制影响中层副高的短期变异南亚高压在东移过程中,高空负涡度平流动力强迫的下沉运动在中层副高区域产生辐散,从动力上影响副高内的负涡度发展;另一方面,强烈下沉运动伴随的绝热加热效应又有利于纬向温度梯度维持,有利于南北风发展,从热力上间接影响西太副高的发展.最后,利用R42L9/LASG大气环流模式进行敏感性数值试验,通过改变高空南亚高压东移产生的负涡度效应,发现中层副热带高压区确有强迫产生的动力辐散和绝热加热出现,并对应副高的异常加强,成功地验证了诊断所揭示的1998年夏季副高短期变异过程的两种机制.     

初夏热带西太平洋对流云团周期性西北传播与梅雨带的变动

为了了解来自低纬度的气象因素对梅雨带季节内变化的影响,文中对1985年和1986年6～7月份的个例进行了细致诊断,并针对有关问题用历史资料做了统计和合成.发现(1)梅雨雨带的向北位移与10～15°N/140°E附近的热带对流云团向西移动有连带关系,当热带对流云团向西北移动时副热带高压也向西北扩张,顺次由东向西影响梅雨带北抬.(2)低纬度向西北方向移动的对流云团具有准8,18以及30 d周期.(3)中国江淮梅雨的入梅也与6月份第一次出现这种对流云团周期性(8,18以及30 d)西北移动有密切关系.     

西太平洋热带海域强对流天气过程湍流通量输送的某些特征

本根据“实验３号”科学考察船在ＴＯＧＡ－ＣＯＡＲＥ（１９９２年１１月－１９９３年２月）定点（２°１５＇Ｓ，１５８°００＇Ｅ）连续观测的大气，海洋资料，利用考虑风速和大气层结影响的整体输送动力学公式，计算给出了在西太平洋热带海域强对流天气过程中动量、感热和潜热等湍流通量垂直交换和水平输送的一些特性，并与该海域其它天气过程湍流通量交换和输送的特性作了比较，此外，中还讨论了更接近实际的曳力系数，感热     

西北太平洋双热带气旋相互作用统计分类及其特征分析

采用中国气象局与上海台风研究所提供的1949-2007年西北太平洋热带气旋最佳路径资料,统计分析了西北太平洋上的双(多)热带气旋相互作用现象.针对统计出的双热带气旋样本,根据相互作用两气旋的单独移动路径和它们相互作用的路径特征,归纳得到7种相互作用类型( A～G),并用聚类分析方法拟合得到每种类型的平均回归路径.分析讨...     

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.     

东亚盛夏北上转向台风年代际变化及与PDO的关系

利用1951—2008年台风及海温资料,分析了东亚盛夏(7—8月)北上转向台风个数的年代际变化特征及其与PDO的关系。结果表明东亚盛夏北上转向台风个数存在明显年代际变化特征,并在1978年前后发生突变。东亚盛夏北上转向台风个数与PDO关系密切,两者呈反位相关系。PDO位相转变后影响北上台风的海温关键区由赤道中东太平洋和西风漂流带区域变为黑潮区域,并且北上台风个数与各海温关键区存在时滞相关。在PDO不同位相的年代际背景下,西太副高位置的变化是北上台风个数年代际变化的原因之一。      

华南沿海一次暴雨中尺度对流系统的形成和发展过程

应用观测分析和数值模拟方法研究了2004年5月13～14日发生在华南西部沿海的一次暴雨中尺度对流系统（MCS）。结果发现：MCS开始发生在低层切变线南侧的偏南气流中,强盛时其水平范围达中口尺度,生命史近10个小时。在MCS的形成和发展过程中,低层的涡旋环流不明显,但其上空对流层中层500hPa上有流场的扰动与之对应。观测分析及数值模拟结果均表明沿海地形引起的辐合在对流的启动中有重要的影响。而对流发展起来后,由于凝结加热的作用,中层的扰动得到加强和发展,主要表现为涡度的增大,气旋性环流在500hPa高度附近表现最为明显,并在MCS东移发展的过程中起着重要的组织作用。MCS在有利地形触发作用下发生。并通过对流层中层扰动组织发展的过程,有别于华南其它一些暴雨过程常伴有低层涡旋系统向上发展的形式,有其独特的特征。还对中层扰动的增强过程和物理机制进行了分析,并提出了一个用于解释中层扰动对MCS组织发展影响的观点。     

EFFECTS OF CONDENSATION HEATING AND SURFACE FLUXES ON THE DEVELOPMENT OF A SOUTH CHINA MESOSCALE CONVECTIVE SYSTEM (MCS)

A sensitive numerical simulation study is carried out to investigate the effects of condensation heating and surface fluxes on the development of a South China MCS that occurred during 23 – 24 May 1998. The results reveal the following: (1) Condensation heating plays an important role in the development of MCS. In every different stage, without condensation heating, MCS precipitation is significantly reduced, and quickly dissipates. (2) Condensation heating demonstrates most importantly during the early development stages of MCS vortex; as the vortex develops stronger, the condensation heating effects reduces. (3) By affecting the MCS development processes, condensation heating also influences the formation of MCS mesoscale environment structure features such as low-level jet (mLLJ), upper-level divergence. (4) By changing the antecedent environmental circulation, the surface fluxes also play an important role in the development of MCS. Because of the surface heating, pressure declines over the heavy rainfall and MCS happening regions, which results in the intensification of southerly flows from the ocean along the South China coastline areas, and leads to the enhancement of horizontal convergence and increase of vapor amount in the lower layer. All of these make the atmosphere more unstable and more favorable for the convection.     

CAPABILITIES AND LIMITATIONS OF GRAPES SIMULATIONS OF EXTREME PRECIPITATION IN THE WARM SECTOR OVER A COMPLEX OROGRAPHY

A comprehensive observational study on a warm sector torrential rain (WSTR) on 20 May 2016 over south China is presented with a pioneering examination of simulation capabilities based on the Global/Regional Assimilation and Prediction System for Tropical Mesoscale Model (GRAPES_TMM). The results show that the meso-scale convective boundary formed between north wind from mountainous areas and the south wind from plain regions as well as the cold outflows boundary both contribute to the convections over Xinyi, and the convections were blocked and stayed stagnant in the trumpet-shaped topography for about 8 hours which eventually caused the torrential rains. Comparative verifications of the observational studies by simulations showed that GRAPES_TMM had better estimations of large-scale frontal precipitation than the local warm-sector torrential rains. The simulations of local torrential rains in the warm sector showed strong biases in precipitation location and amount. GRAPES_TMM also showed overestimated surface winds and a faster moving speed bias, as well as an overall underestimation of the nocturnal surface temperature during the WSTR. This work may lead to several prospective researches of its model improvements on model physics such as land surface process and PBL parameterization.     

2004年西北太平洋热带气旋的活动特点与成因研究

对2004年台风汛期西北太平洋热带气旋的活动特点进行研究总结,进而对其成因做了分析。结果指出：2004年西北太平洋热带气旋主要特点是生成总数较常年略偏多,热带气旋生命史普遍较长、：生成时间相对集中、生成源地偏东,转向路径偏多,登陆气旋数多于常年平均值,登陆地段集中且偏北,集中在华东到日本一带。其成因在于：（1）副高面积偏大、强度偏强、位置偏北且西伸脊偏西,同时副高分布形态多呈块状,使得2004年台风的高频活动区向北偏移,直接对华东到日本一带造成威胁,乃至正面登陆影响;（2）澳大利亚高压强大,越赤道气流强,赤道辐合带辐合偏强、位置偏东,造成台风活动比较活跃且源地偏东;（3）西风带环流经向度较常年偏大而且副高多呈块状分布,东西两环副高之间形成的鞍型场有利于台风过此通道北上转向,导致转向台风偏多。     

VARIATIONS OF SEA SURFACE TEMPERATURE AND SEA LEVEL PRESSURE OVER PACIFIC AND INDIAN OCEANS AND THEIR INFLUENCE ON COLD SUMMER IN EAST ASIA

An EOF analysis was performed to investigate the variations of sea surface temperature (SST) of Pacific and Indian Oceans.Result shows that the distribution of SST anomaly exhibits a distinct anticorrelation pattern between Northwest and Southeast Pacific,as well as between Northwest Pacific and the Arabian Sea.It also shows that the sea level pressure (SLP) anomaly between North Pacific and North Indian Oceans is of a seesaw pattern,which we named the North Paci-fic and North Indian Ocean Oscillation (PIO).Such a phenomenon is closely correlated with the cold summer in East Asia.     

ANALYSIS OF CLOUD-TO-GROUND LIGHTNING CHARACTERISTICS IN MESOSCALE STORM IN BELIING AREA

The research indicates that there is a positive correlation between the negative CG flash rate and the area of radarecho with its reflectivity being equal to or greater than 30 dBz in the mesoscale convective system in Beijing area.A max-imum of the positive CG flash rate exists at both the initial and the dissipating stages.The CG flashes are usually locatednear but not within the high reflectivity center.The negative flashes are associated with.the positions of the updraft re-gion,and with the regions of wind convergence and wind shear.Generally,the negative CG flashes are concentrative andthe positive ones are dispersive.     

江淮梅雨特征及其与北太平洋海温的SVD分析

利用江淮地区1954～2001年48年梅雨量资料,采用谐波分析、EOF、合成分析和SVD分解等方法讨论了江淮梅雨的时空变化特征以及与北太平洋海温的关系.结果表明:江淮梅雨量时空分布不均,存在显著的年际和年代际变化特征.影响江淮梅雨的海温关键区是北太平洋西风飘流区,关键影响时段是当年的1～3月;当年1～3月北太平洋西风飘流区海温异常偏高,同年江淮大部地区梅雨量异常偏多,反之亦然.SVD分解结果与合成分析的结果相吻合,通过0.05的Monte-Carlo显著性检验.     

CHARACTERISTICS OF MEI-YU PRECIPITATION AND SVD ANALYSIS OF PRECIPITATION OVER THE YANGTZE-HUAIHE RIVERS VALLEYS AND THE SEA SURFACE TEMPERATURE IN THE NORTHERN PACIFIC OCEAN

Based on the precipitation data of Meiyu at 37 stations in the valleys of Yangtze and Huaihe Rivers from 1954 to 2001, the temporal-spatial characteristics of Meiyu precipitation and their relationships with the sea surface temperature in northern Pacific are investigated using such methods as harmonic analysis, empirical orthogonal function (EOF), composite analysis and singular value decomposition (SVD). The results show that the temporal evolution and spatial distribution of Meiyu precipitation are not homogeneous in the Yangtze-Huaihe Rivers basins but with prominent inter-annual and inter-decadal variabilities. The key region between the anomalies of Meiyu precipitation and the monthly sea surface temperature anomalies (SSTA) lies in the west wind drift of North Pacific, which influences the precipitation anomaly of Meiyu precipitation over a key period of time from January to March in the same year. When the SST in the North Pacific west wind drift is warmer (colder) than average during these months, Meiyu precipitation anomalously increases (decreases) in the concurrent year. Results of SVD are consistent with those of composite analysis which pass the significance test of Monte-Carlo at 0.05.     

ANALYSIS OF CLOUD-TO-GROUND LIGHTNING CHARACTERISTICS IN MESOSCALE STORM IN BELIING AREA*

The research indicates that there is a positive correlation between the negative CG flash rate and the area of radar echo with its reflectivity being equal to or greater than 30 dBz in the mesoscale convective system in Beijing area.A maximum of the positive CG flash rate exists at both the initial and the dissipating stages.The CG flashes are usually located near but not within the high reflectivity center.The negative flashes are associated with.the positions of the updraft region,and with the regions of wind convergence and wind shear.Generally,the negative CG flashes are concentrative and the positive ones are dispersive.