【作者】 慕建利；

【导师】 李泽椿； 杜继稳；

【作者基本信息】 南京信息工程大学 ， 气象学， 2009， 博士

【摘要】 暴雨一直是业务天气预报服务重点,更是预报难点,在陕西汛期日常预报中暴雨预报就有失败或不理想的状况,因为受特定地理环境影响,陕西暴雨具有历时短、强度大、局地性和突发性强的特征,大气环流表现为不同的形势。所以,要做好陕西的暴雨预报,不仅要研究其普遍性,还要研究其特殊性,加强预报的针对性和预见性,有助于做好防灾减灾等预报服务。2007年8月8—9日陕西关中暴雨天气过程历时短、强度大、突发性强为历史罕见,暴雨发生前影响系统不明显,关中处于中纬度高压坝控制之中,南部没有急流存在,没有明显的水汽和能量输送,给暴雨的预报带来极大的困难,加之对该类暴雨研究极少,预报员很难找到暴雨预报的着眼点,从而使暴雨的预报出现了较大的误差。为了揭示该类暴雨的发生发展规律,提高该类暴雨的预报准确率,本文以此暴雨个例做为研究对象。在过去研究和实际业务天气预报中发现,研究中NCEP资料等虽然有较好的预报能力,但在时效和获取方面有时存在问题,难于在实际中及时应用,影响研究的应用效果,T213资料对陕西暴雨是有一定的预报能力,产品及时有效,在实时业务天气预报中使用方便。所以,本文综合利用T213(0.5625°×0.5625°)资料和近年来高时空分辨率观测资料(包括地面区域逐时加密观测资料、分钟降水资料、FY—2C卫星、多普勒雷达资料以及分钟地闪等资料),通过天气学分析、动力和热力诊断、数值模拟等方法,对上述个例进行了细致的分析研究,总结了该类强暴雨发生发展的有利环境背景,分析了其形成的可能原因,揭示了强暴雨中小尺度对流系统的发生发展规律、三维结构,特别是暴雨过程中三个大暴雨中心的MβCS(MγCS)的细微结构特征,为该类强暴雨的短时临近预报预警提供思路和预报着眼点。主要结论有:1、“07.8”关中强暴雨是在高中低空有利的环流配置下,不同纬度天气系统共同作用的结果,对流层中层青藏高原高压和西太平洋副热带高压形成的高压坝在陕西中部断裂对暴雨的形成至关重要,它与低层东西向切变线和高层西风急流入口区右侧发散场的相互配合为强暴雨的形成提供了有利的大尺度环流背景。2、关中周围水汽的集中为暴雨形成提供了水汽和位势不稳定条件,水汽的聚集是通过偏东气流的输送实现的,而水汽的快速变化形成关中暴雨的突发性和历时短而强的特征;强降水的发生和减弱与水汽的局地变化、水汽平流和近地层水汽的增加和减少密切关联。高空反气旋涡度的发展形成强烈的“抽吸作用”和次级环流圈是暴雨形成的动力机制。3、关中强暴雨过程的东西向雨带与秦岭山脉和关中地区喇叭口地形有关,雨带上的降水非均匀分布,强暴雨集中在岐山、礼泉和高陵三个中心,它们是由一个MαCS的发生发展产生的,MαCS又是由2个MβCS合并发展而成,其内部对流单体的发展合并和独立加强形成不同的降水中心,这些对流单体的发展是由地面中尺度辐合系统产生的,强降水落区与地面中尺度辐合系统有很好的对应关系。4、地闪的发生和急剧增加对暴雨发生和发展加强有很好指示意义,初闪的发生提前于强降水发生,地闪急剧增加与降水强度猛增密切关联,负地闪发生密集区是未来强降水发生区。5、非静力中尺度WRF模式能成功地模拟出关中地区的突发性强暴雨过程。模拟结果表明,此次强暴雨与一个中α尺度低涡的生成密切相关,其内部强烈发展的中β(或中γ)尺度对流系统直接产生了岐山、礼泉、高陵强暴雨中心的对流降水。产生这3个强暴雨中心的MβCS有不同的流场、动力、热力垂直结构。在日常天气预报中,把握这些结构特征,为预报提供参考。垂直结构上,中低层不同的方向和不同层次的气流流入中β尺度降水云塔,在不同高度上形成了不同的垂直环流支,云塔中的上升气流一直伸展到200 hPa(或150 hPa)后向东南、东北流出。动力、热力垂直结构上,歧山暴雨中心450 hPa以上为强辐散,450 hPa以下暴雨中心南、北两侧结构相反,南侧为弱辐散、辐合,北侧为辐合、弱辐散。垂直上升运动先向南、后向北倾斜、再直至对流层顶;涡度柱与相当位温的双高能、双重不稳定层结柱和中层两个暖心、上下冷心的温度柱互相耦合。礼泉和高陵暴雨中心:整层强上升运动柱与强散度柱和正涡度柱耦合;礼泉上升运动柱是一个高、低层冷而中上层强暖心的近饱和水汽柱,具有典型“鞍”型场的不稳定层结结构;高陵暴雨中心南缘550 hPa以下是强能量和温度离差锋区,其上空400 hPa以下为近饱和的水汽柱。根据上述研究结果,可以得出关中该类强暴雨短时临近预报预警的着眼点:1)500hPa上空,中纬度地区为青藏高原高压和副热带高压形成的高压坝控制时,高压坝能否断裂形成切变线是暴雨预报的关键;高空南亚高压东北侧发散流场的形成对暴雨的发生有指示意义。2)水汽的局地变化、水汽平流和近地层水汽的增加和减少对该类暴雨的发生发展和减弱消亡有较好的指示意义。3)根据初闪发生的时间、负地闪发生密集区和地闪的急剧增加可估计未来强降水可能发生的时间、地点和强度。4)将卫星、地闪和雷达资料以及NWP与引导气流和地面中尺度系统等结合起来,分析推断未来MCS的发展演变,可以提前短时临近预报预警的时效。今后还应当不断地对产生陕西暴雨的特殊个例进行分析研究,使预报人员对暴雨发生发展的规律有更多的认识,从而提高暴雨预报的准确率。更多还原

【Abstract】 The rainstorm has been the key to operational weather forecast service,which is still difficult to forecast.And rainstorm predicetion of Shaanxi is failing or dissatisfactory.Due to the influence of specific geographical environment,rainstorms of Shaanxi usually happen in small area,and the atmospheric circulation may also present different situation.Therefore,we must study the particularity besides the universality,in order to improve the rainstorm prediction.A process of the heavy rainstorm on 8-9 August 2007 in the Central Shaanxi Plain,with characterictics a short time,heavy rainfall and abrupt occurrence,is rare for the history.Its prediction is very difficult owing to disadvantageous large-scale environment background. In order to research the law of occurrence and development of this kind rainstorm,to enhances the rainstorm forecast the rate of accuracy,this article take this example as the object of study.Founded in the past research and operational weather forecast,although the NCEP data and so on has good ability for rainstorm forecast,but there is still some problem in obtaining the data.While the T213 data has certain forecast ability to the rainstorm,which is easy to operate in real-time operational weather forecast,and its product has been prompt effective.Therefore,the T213(0.5625×0.5625) data and the observed data with high space and time resolution in recent years(including denser surface data for 1-hour interval,automatic meteorological observing stations precipitation data for 1-minute interval, FY-2C satellite data,Doppler radar data as well as lightning position observation for 1-minute interval and so on),have been used in this research.Through the synoptic meteorology analysis,dynamic and thermodynamic diagnosis as well as numerical simulation,a process of the heavy rainstorm in a short time on 8-9 August 2007 in the Central Shaanxi Plain was analyzed in detail.We have found the favorable large-scale environment background and the possible reason for formation of the strong rainstorm.Evolution and the three-dimensional structure of MCS,especially the slight structural feature of three cloud burst centers M[JCS(M_γCS) in the rainstorm process were revealed.The findings may provide the scientific basis and the forecast objective point for short-term weather forecast. The main conclusion includes:1.The "07.8" strong rainstorm in the Central Shaanxi Plain was the result of interaction among the different weather system,under a favorable large-scale circulation disposition.In the middle level,the break of high-pressured dam which was resulted from the development merge of Qinghai-Tibet Plain high pressure and west the Pacific Ocean subtropical high in the middle Shaanxi is very important.The mutual coordination of the high-pressured dam’s break in the middle level,right flank divergence’s formation of westerly jet entrance region in the upper level and the shear line’s development in the low level had provided the advantageous large-scale environment background for the strong rainstorm formation.2.The vapor accumulation from around the Central Shaanxi Plain had provided sufficient water vapor and potential instability for occurrence of the heavy rainfall.And the vapor accumulation was achieved through the easterly current.The water vapor increased so fast that rainstorm happened suddenly,and the rapid-decreasing of water vapor made the rainfall last short.The occurrence and weakening of the strong precipitation had close relationship with the local change of water vapor,water vapor advection and the change of vapor in surface layer.The formation of intense "pavement pumping" effect and the secondary circulation circle owing to the development of the upper anticyclone vorticity were the dynamic mechanism of rainstorm formation.3.In this event,the zonal distribution from west to east of the rainfall is related with the Qin ling mountain and trumpet-shaped terrain in the Central Shaanxi Plain.And the rainfall was not distributed uniformly in the space,which was mostly centered in the areas of the Qishan,Liquan and Gaoling.It was resulted from the occurrence and development of MαCS, which was also formed by the mergence and development of 2 MβCS.The convective cells merged and developed,and enhanced individually to form the different precipitation centers. The development of the convective cells was resulted from the surface mesoscale convergence system,and the precipitation areas had a good corresponding relationship with the surface mesoscale convergence system.4.The occurrence and sharp growth of the ground lightning has very good indicative significance to the development and strengthens of the rainstorm.The occurrence of the first flash occurs ahead of the strong precipitation happening.The sharp increase of ground lightning has close connection with the sudden increase of the precipitation intensity.And the concentrated regions of the negative ground lightning may indicate the area where strong precipitation happen.5.The simulation using the Weather Research and Forecasting(WRF) model were successful in reproducing the strong rainfall in the Central Shaanxi Plain.The simulation result indicated that,the strong rainfall was related to occurrence of a mesoscale-αvortex,in which the rapid developing MβCS(M_γCS) formed the three strong rainstorm centers.MβCS had different stream field,vertical dynamical and thermal structure.In the daily weather forecast,the numerical forecasting product(NWP) can be used as a forecast reference,by which we may analyze the mesoscale structural feature under the large-scale circulation background.The vertical structure of the stream field:there were different current in different levels in the troposphere that flowed into the ascendant current of theβ-scale precipitation cloud tower,forming different vertical circulations in different levels.And the ascendant current had extend to 200 hPa(or 150 hPa),and flowed out to the southeast and northeast above the level. The vertical dynamical and thermal structure:in Qishan,there was a strong divergence center above 450 hPa,while below 450 hPa the structure over the rainstorm center is opposite from north side and the south.There was weak divergence or convergence on the south side, and convergence or divergence on the north side;The vertical ascent movement inclined to the south,then to the north,and finally to the tropopause;The vorticity column was mutually coupled with double energy center of the potential equivalent temperature,dual unstable stratification columns and two warm centers in middle level,and temperature column with high-low cold center;In the other two precipitation centers,Liquan and Gaoling:the entire column of ascent movement was coupled with thestrong divergence column,as well as the positive vorticity column;In Liquan,the ascent movement column was a saturated vapor column with one strong warm center in the mid-high levels while cold in the high and low levels,which has the unstable stratification structure as "the saddle" field.In Gaoling:there is the strong energy center and frontal zone of temperature deviation below 550 hPa,and below 400 hPa there was a saturated water vapor column.According to the research results above,we may obtain some vantage points for the short-term forecast of this kind of strong rainstorm:when 1) above 500 hPa in the troposphere, when the middle latitude region was controlled by a high-pressure dam,which was formed by the connection between Tibetan Plateau High and the subtropical high,whether does the high-pressure dam break to form the shear line is the key to the rainstorm forecast;the formation of divergence stream field in the northeast of the upper South Asia High has a indicative significance to the occurrence of rainstorm.2) the local change of water vapor, water vapor advection and the change of water vapor in surface layer have a indicative significance to the occurrence and development of the rainstorm,as well as its weakening and extinction.3) according to the time that first fresh occurred,the concentrated regions of the negative ground lightning and the sharp increase of ground lightning,we may predict the occurrence,location and intensity of the strong precipitation in the future.4) comprehensive analysis of the satellite data,radar data and NWP,as well as the steering current and the ground mesoscale system and so on,we may predict the development evolution of MCS and have more time to make short-term forecast of this kind of rainstorm to avoid disaster.Must to investigate a lot of special example of Shaanxi rainstorm and understand law of occurrence and development of heavy rainfall unceasingly in the future,we are able to enhances the rainstorm forecast the rate of accuracy.更多还原