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.     

Distribution and Diurnal Variation of Warm-Season Short-Duration Heavy Rainfall in Relation to the MCSs in China

Short-duration heavy rainfall（SDHR） is a type of severe convective weather that often leads to substantial losses of property and life. We derive the spatiotemporal distribution and diurnal variation of SDHR over China during the warm season（April–September） from quality-controlled hourly raingauge data taken at 876 stations for 19 yr（1991–2009）, in comparison with the diurnal features of the mesoscale convective systems（MCSs） derived from satellite data. The results are as follows. 1） Spatial distributions of the frequency of SDHR events with hourly rainfall greater than 10–40 mm are very similar to the distribution of heavy rainfall（daily rainfall 50 mm） over mainland China. 2） SDHR occurs most frequently in South China such as southern Yunnan, Guizhou, and Jiangxi provinces, the Sichuan basin, and the lower reaches of the Yangtze River, among others. Some SDHR events with hourly rainfall 50 mm also occur in northern China, e.g., the western Xinjiang and central-eastern Inner Mongolia. The heaviest hourly rainfall is observed over the Hainan Island with the amount reaching over 180 mm. 3） The frequency of the SDHR events is the highest in July, followed by August. Analysis of pentad variations in SDHR reveals that SDHR events are intermittent, with the fourth pentad of July the most active. The frequency of SDHR over mainland China increases slowly with the advent of the East Asian summer monsoon, but decreases rapidly with its withdrawal. 4） The diurnal peak of the SDHR activity occurs in the later afternoon（1600–1700 Beijing Time（BT））, and the secondary peak occurs after midnight（0100–0200 BT） and in the early morning（0700–0800 BT）; whereas the diurnal minimum occurs around late morning till noon（1000–1300 BT）. 5） The diurnal variation of SDHR exhibits generally consistent features with that of the MCSs in China, but the active periods and propagation of SDHR and MCSs difer in diferent regions. The number and duration of local maxima in the diurnal cycles of SDHR and MCSs also vary by region, with single, double, and even multiple peaks in some cases. These variations may be associated with the diferences in large-scale atmospheric circulation, surface conditions, and land-sea distribution.     

多源探测资料在一次非线状MCS分析中的综合应用

用多种加密观测资料和NCEP日再分析资料分析了2010年7月14日强降水期间咸宁地区一次非线状MCS活动造成短时强降水的发生发展机制.结果表明,14日13-18时非线状MCS回波结构组织性差,强对流单体散乱地分布在大片层状回波中,准静止地维持在湖北咸宁地区大约5h,造成了短时强降水.该MCS发生在梅雨锋锋面附近的地面涡旋环流中,高空冷空气侵入和锋前抬升运动是对流的主要触发机制,切变线南侧不稳定的暖湿气流在长江中游地区辐合集中、局地的地面气流辐合和边界层有利的风切变是该非线状MCS发展维持在成宁地区的有利条件.高时空分辨率探测资料对MCS演变过程有较好的分析能力.     

2013年4月大气环流和天气分析

2013年4月大气环流特征为:北半球极涡呈绕极分布,强度接近常年;中高纬度呈4波型,东亚大槽较常年明显偏强;中低纬度地区南支槽和副热带高压强度都稍偏弱,不利于西南地区旱情的缓解。4月全国平均气温为11.0℃,与常年同期持平,东北地区气温异常偏低。全国平均降水量为43.2 mm,较常年同期偏少1.5 mm。月内我国共出现了2次冷空气过程,并伴有扬沙、浮尘甚至沙尘暴天气;南方地区共有3次大到暴雨过程,以4月29-30日过程强降雨范围最广、强度最强,其中江南、华南地区还伴随有明显的强对流活动。     

用GPS水汽监测资料分析一次强对流性降水过程

用江苏省地基GPS水汽监测系统得到的大气可降水量(PWV)资料,对江苏地区2009年夏季一次强对流性天气产生的降水过程进行了综合分析,分析了各时段GPS-PWV的变化特征和水汽的输送特点,并利用WRF中尺度数值模式对此次过程进行了数值模拟。结果表明:GPS-PWV对于空中水汽变化具有很高的敏感性,能及时地反映大气中水汽的时空变化。通过对数值模式结果进行分析后,发现强盛的水汽输入及辐合上升、中高层弱冷空气的侵入活动、低层西南气流加强、对流不稳定层加剧等多种因素的共同作用是造成此次中小尺度对流性降水的主要原因。GPS-PWV提供的精确水汽变化结合数值模式所模拟出的动力、热力条件对于强对流性暴雨预报和降水区域判定具有较好的参照意义。     

一次发生在α中尺度涡旋东部的特大暴雨过程分析

2010年9月7日苏皖北部出现了暴雨—大暴雨,局部特大暴雨,这次暴雨过程产生在热带风暴（1009号,玛瑙）在东海北上转向日本以后。利用卫星、雷达和NCEP逐日4次1°×1°资料对天气形势和中尺度对流发展的环境场特征等进行诊断分析表明：位于日本海的热带风暴北部的偏东气流与南下深厚的蒙古高压底部的偏东气流汇合,构成了一支强盛的低空偏东风气流,它向西流向太行山东麓,其分支沿太行山南下时,诱生了位于太行山脉南部切变线东部的涡旋系统。当低涡进入黄淮平原后,在越过渤海的边界层东风急流激发下迅速发展为α中尺度涡旋系统。南方的暖湿气流在涡旋系统边界层东风急流的动力强迫下抬升,不稳定能量释放,导致β中尺度系统发生发展。β中尺度系统的影响和γ中尺度系统的合并导致强降水过程的发生。     

大气对流层重力波研究进展

重力波对暴雨等强对流天气有着触发机制的作用,甚至影响着大气环流、大气结构和演变.综合近50年来国内外有关大气对流层重力波的研究成果,为重力波的进一步研究提供参考.多年研究表明：暴雨等强对流系统、山脉地形等为对流层重力波波源;重力波强度与风垂直切变、背景绝对涡度成正比,传播速度与波的振幅、水平波宽成正比,重力波在稳定大气中得到了加强,潜热加热有利于重力波的形成;重力波的探测手段为微压器、卫星、雷达观测等,通过重力波探测目前基本掌握了重力波的日、月、季节的活动规律.     

庐山地区层状云和对流云降水特征对比分析

根据Parsivel激光雨滴谱仪在庐山高海拔观测场获取的2011年降水资料,结合宏观特征量、雨滴谱资料和雷达图像资料,将降水划分为对流云降水和层状云降水,选取了12次典型降水过程。对两类云降水的6种特征直径、各档雨滴对降水参量的贡献、降水微物理参量的演变等进行了分析,并利用M-P分布和Gamma分布对两类云降水雨滴谱进行拟合,对拟合参数以及拟合效果进行了分析。结果表明：两类云降水微物理特征有着本质的区别,层状云降水谱宽相对较窄,参量随时间变化比较平缓,直径不超过1 mm的小滴对降水贡献最大;对流云降水谱宽相对较宽,出现了直径接近10 mm的大滴,参量起伏较大,对数密度贡献很小的大滴对雨强、含水量贡献却比较大。从拟合效果检验来看,层状云降水拟合时的M-P曲线在大部分区段比Gamma曲线更接近实测雨滴谱曲线;对流云降水拟合时的Gamma分布曲线与实际雨滴谱分布曲线整体吻合程度较高。M-P分布和Gamma分布两种拟合方法都适用于层状云降水,对流云降水雨滴谱拟合时Gamma拟合效果优于M-P拟合效果。     

Simulation of a mesoscale convective system over Northern India: Sensitivity to convection partitioning in a regional NWP model

Simulations of a mesoscale convective system (MCS), which propagated across Northern India on 2nd May 2018 - leading to many fatalities when the gust front knocked down homes and tore apart building roofs - have been performed using the National Centre for Medium Range Weather Forecasting (NCMRWF) Unified Model – Regional (4 km horizontal grid spacing), to evaluate the model’s convective treatments. Though the model captures many of the qualitative and quantitative features, it slightly lags behind the observed MCS organisation and movement, produces lesser precipitation, and lacks the spatial separation between two adjacent organised convective systems in the satellite observations – leading to a faintly offset MCS track. Sensitivity simulations are then performed, for this non-equilibrium MCS case, with different partitioning between parametrized and explicit convection to assess the reliance of the convective treatments on the large-scale environment, as well as to test the notion of a breakdown of convective parametrization at the mesoscale model resolution. Fully parametrized (FP) convection produces even lesser rainfall and are dominated by orographic precipitations along the foot hills of Himalayas with no any trace of the MCS. Fully explicit (FE) convection realistically simulates most of the prominent convective cells and enhance precipitation along the MCS track that agree better with the observations, though the ‘two lobes’ of intense precipitation are not resolved; instead it produces a squall line of precipitation. The FE configuration generates the most vigorous convective updraft, along with a vertical shear that is tilted westward. The simulation with partially parametrized and partially explicit convection resembles the fashion in the FP and FE scenarios, with a transition over the duration of the run from parametrized to explicit precipitation. The results are in line with the notion from previous studies; that the majority of successful explicit simulations of mesoscale organisation are those associated with strong large-scale forcing for convection, wherein resolved vertical motions are sufficient to minimise delays in onset.     

华北两类产生极端强天气的线状对流系统分布特征与环境条件

华北线状对流系统精细气候分布及其所产生的极端天气特征尚不清楚,本研究利用雷达拼图资料和客观识别方法普查2013—2018年华北171例线状对流系统的时、空分布特征,根据其所致强对流天气的统计结果,发现华北地区至少有2类线状对流系统,分别产生极端强雷暴大风和极端强降水。分析了这2类线状对流系统的环流形势、环境条件、地形作用和关键中尺度系统地面冷池等的特征。主要结论如下:华北线状对流系统的空间分布尤其是初始形成位置与大地形关系密切,京津冀的太行山和燕山山脚区域为其中的一个高发区;2类线状对流系统发生月份、空间尺度、移动速度、形成时刻和维持时间等都具有显著差异;2类线状对流系统的环流背景、环境条件和冷池也差别明显。强雷暴大风型线状对流系统的环境大气斜压性强,中层干和大的垂直减温率造成的最优对流有效位能、下沉对流有效位能大值区是产生极端大风的重要环境条件,地面强冷池以及0—3 km风垂直切变对前向传播起到了重要作用。强降水型线状对流系统产生的降水极端性较前一类型更为凸出,天气尺度强迫相对较弱,水汽条件极其充沛,地面弱冷池或地形与低层南风气流相互作用维持的后向传播是其发展和缓慢移动的主要机制,也是产生极端强降水的直接原因。