引发暴雨天气的中尺度低涡的数值研究

2008年7月17—19日发生在山东的大到暴雨天气是由“海鸥”台风和副热带高压共同向山东输送水汽,与弱冷空气相互作用造成的。对流层低层的中尺度低涡是暴雨天气的直接制造者。利用常规观测资料和中尺度模式WRF（Weather Research and Forecasting）的模拟资料对该中尺度低涡的结构及形成机制进行了分析研究。结果表明,数值模拟可以清楚地捕捉到中尺度低涡东移过程中有新的涡旋中心形成,并与原来的涡旋中心合并的过程,而不是简单的沿切变线东移。中尺度低涡形成在增温增湿明显、上升运动为主的对流区内;中尺度低涡形成后其中心转为下沉运动,对流区东移,降水区位于低涡的东北和东南象限。中尺度低涡上空近地面层的冷池、600～400hPa的弱冷空气堆、900～850hPa的弱风区及高低空急流耦合发展是中尺度低涡形成和发展阶段的重要特征。中尺度低涡减弱阶段,下沉运动变强,低空急流和高空出流都明显减弱。涡度方程的收支表明,对流层低层的散度项、倾侧项及对流层中层的水平平流项和铅直输送项是正涡度的主要贡献者。中低层的水平辐合、涡度由低层向高层的垂直输送都有利于中尺度低涡的形成和发展。倾侧项对中尺度低涡的形成也有重要贡献。中尺度低涡形成后期,低层辐合、高层辐散及垂直输送的减弱导致正涡度制造的减弱,从而使中尺度低涡减弱。     

A three-dimensional numerical sensitivity study of mesoscale circulations induced by circular lakes

Summary A three-dimensional mesoscale planetary boundary layer model with theE- turbulence closure is used to simulate airflow over a lake of circular shape. A series of model sensitivity studies are performed to examine the effects of lake-land temperature difference, ambient wind magnitude and direction, lake size, surface roughness, the Coriolis force and baroclinic ambient wind conditions on mesoscale lake circulations.The lake-land temperature difference is essentially the basic energy source driving the mesoscale circulations over the lake on synoptically undisturbed days. A lake-breeze convergence zone is predicted by the model due to the differential heating between the land and the water. It is found that spatial and temporal variations of this convergence zone and associated convection are strongly controlled by the direction and the magnitude of the ambient wind. Under southeasterly and southwesterly ambient winds, the lake-breeze convergence zone and the associated convection occur primarily along up wind and lateral sides of the lake with reference to the general direction of the ambient flow. In contrast to the southeasterly and southwesterly ambient winds, the lake-breeze convergence zone and the convection are predicted all around the coastline of the lake under calm wind.The model also predicts a cloudless region over the lake in all the case studies due to divergent nature of the lake-breeze circulation. The lake size is found to have a significant effect in intensifying convection. Surface roughness over the land surface is found to be important in determining the intensity of the convection. The combined effect of the Coriolis force and the differential surface roughness between land and water appear to be the responsible mechanism for producing the asymmetric shape of the lake-breeze convergence zone around the symmetric circular lake. Finally, it was found that an initial baroclinic flow has different mesoscale lake-breeze circulation patterns as compared to an initial barotropic flow.With 16 Figures     

Interaction of typhoon and mesoscale vortex

Under two types of initial tropical cyclone structures that are characterized by high and low vorticity zones, four sets of numerical experiments have been performed to investigate the interaction of a tropical cyclone with an adjacent mesoscale vortex (MSV) and its impact on the tropical cyclone intensity change,using a quasi-geostrophic barotropic vorticity equation model with a horizontal resolution of 0.5 km. The results suggest that the interaction of a tropical cyclone characterized by a high vorticity zonal structure and an MSV would result in an intensification of the cyclone. Its central pressure decreases by more than 14 hPa. In the process of tile interaction, the west and middle segments of the high vorticity zone evolve into two peripheral spiral bands of the tropical cyclone, and the merging of the east segment and the inward propagating MSV forms a new vorticity accumulation area, wherein the maximum vorticity is remarkably greater than that in the center of the initial tropical cyclone circulation. It is this process of merging and strengthening that causes a greater pressure decrease in the center of the tropical cyclone. This process is also more complicated than those that have been studied in the past, which indicated that only the inward transfer of vorticity of the MSV can result in the strengthening of the tropical cyclone.     

A numerical study of a TOGA-COARE squall-line using a coupled mesoscale atmosphere-ocean model

1. Introduction Air-sea interaction plays an important role in theglobal seasonal to inter-annual climate variability,most notably, the El Ni?no and Southern Oscillation(ENSO) phenomenon (Webster and Lukas, 1992). Be-cause of its widespread impacts on …     

中国台风特大暴雨综述

围绕国内外近年来中尺度涡的动力学研究进展,聚焦中尺度对流涡的涡度方程和热量方程,对中尺度对流涡的动力学进展进行了梳理总结,指出环境风切变、垂直运动、平流的动力辐合、风场变形、热力场变化及地形等对中尺度涡的涡度发展均有作用。此外,中尺度涡动力学的研究进展,还体现在能量方程和中尺度垂直运动方程的推导和诊断方面。强对流系统中,垂直运动的变化与不稳定能量及对流层中层水平运动的风速有关。

     

中尺度地形对柳州一次大暴雨过程影响的数值试验

利用中尺度模式WRF对2009年7月2—3日柳州大暴雨过程进行数值模拟,得到与实况相吻合结果。通过地形敏感性试验,研究了中尺度地形对这次暴雨过程的影响。结果表明:地形对这次大暴雨过程的雨带分布未起到决定性的作用,但对强降水的落区和强度有着重要影响。地形作用使西南暖湿气流所带来的水汽和热量在迎风坡堆积,融安融水一带中低层位温增加,导致其上空对流不稳定性增强,当与低层冷空气绕过山脉从西北路侵入时产生的垂直扰动叠加后,激发垂直上升运动强烈发展,从而触发了对流不稳定发展。而地形降低为"平台"后,山脉附近降水中心减弱,物理量场分析表明,由于缺乏地形的抬升作用,山脉附近垂直上升运动及正涡度强度均较有地形时减弱。     

冷涡外围一次强冰雹的中尺度天气过程分析

本文通过个例分析,看到两次不同系统的云系合并,大大增强了中尺度系统,以及降雹和雷暴大风天气的事实;看到降雹初期,中尺度系统的存在,在雷达回波上的反映比地面资料分析的早而明显;看到地面上的露点锋可能是开始降雹的一个条件。     

台风榴莲（2001）生成初期中尺度涡旋合并过程研究

由于热带海洋上观测资料的稀缺和热带气旋系统本身发生、发展的复杂性,热带气旋生成机制研究领域至今仍然存在很多未解之谜。已有的观测和模拟研究证明,中尺度涡旋合并过程对于热带气旋的生成可能有触发作用,但尚未见到南海季风槽内热带气旋生成过程中中尺度涡旋合并现象的实例模拟研究。利用新一代中尺度天气研究与预报模式WRF对南海热带气旋榴莲(2001)生成过程中的中尺度涡旋合并过程进行了高分辨率(4 km)数值模拟,并与观测资料进行对比,利用模式输出结果重点分析两个中尺度涡旋合并过程中的主要动力学和热力学特征,并在此基础上进一步分析了合并过程中系统中心附近涡度方程中各项涡度收支的演变情况,最后通过两个敏感性试验与控制试验结果的对比,初步探讨中尺度涡旋合并过程对于热带气旋榴莲生成的作用。结果表明,南海季风槽中的新生中层中尺度涡旋V2,是榴莲生成过程中的主导涡旋,预先存在的东部低层的中尺度涡旋V1对于台风榴莲的生成则起到了辅助作用,两个不同高度的涡旋合并叠加促使涡度的辐合、辐散项率先在低层引起涡度的快速增长,随后垂直输送项在对流层中层对涡度的增长起主要作用。两个涡旋的最终合并,使热带气旋系统正绝对涡度在垂直方向上从低层到中层得以贯通,进而触发榴莲的生成。     

基于西南涡加密探空资料同化的一次奇异路径耦合低涡大暴雨数值模拟研究

利用WRF模式及WRFDA同化系统,引入业务探空资料和西南涡加密探空资料,对一次四川盆地奇异路径低涡耦合大暴雨过程进行了数值试验,对比检验不同同化试验对本次过程降水和低涡移动路径的模拟能力,分析了加密探空资料同化对西南涡结构及其降水演变的影响。结果表明:在同化业务探空资料的基础上,引入西南涡加密探空资料能改善模式对本次降水和低涡移动路径的模拟,而仅同化业务探空资料对模拟结果的改善作用有限;引入西南涡加密探空资料,一方面能在初始风场上产生气旋式扰动,增加初始高原涡和西南涡的强度,另一方面通过调整初始四川盆地上空大气温、湿度结构,使模式在积分初期就能产生出实况量级的降水;西南涡加密探空资料的同化试验揭示了仅靠高层的高位涡不足以激发和维持700 hPa的西南涡,需要通过低层水平辐合引起正涡度增加并向上输送来增强700 hPa的气旋式环流,进而促进西南涡的移动和发展,而模拟初期降水的潜热释放也起重要作用,加深了对西南涡及其降水成因的认识。      

气溶胶对北京中尺度对流系统影响的数值试验

利用可分辨云模式(WRF),模拟研究了不同气溶胶浓度对北京地区2001年8月23日一次产生强降水和冰雹的对流天气的影响。结果表明,气溶胶浓度的增加不利于对流云的发展,导致地面降水减小,但是对降水结构没有明显影响。气溶胶浓度增加导致云中水成物数浓度和质量浓度均发生变化,其中云水、冰晶和雪含量增加,而雨水、霰和雹含量减小。从云微物理学分析发现,气溶胶浓度减小有利于高层云的形成,云滴有效半径随着气溶胶浓度增加而减小。     

Roles of Mesoscale Terrain and Latent Heat Release in Typhoon Precipitation： A Numerical Case Study

The mesoscale orographic effects on typhoon Aere＇s precipitation are simulated using an Advanced Regional Eta-coordinate Model （AREM） version 3.0. In particular, the effects of the latent heat release are studied by two comparable experiments： with and without condensational heating. The results show that the typhoon rainfall is tripled by the southeastern China mesoscale terrain, and the condensational heating is responsible for at least half of the increase. One role of the latent heat release is to warm the atmosphere, leading to a depression of the surface pressure, which then causes a larger pressure difference in the zonal direction. This pressure gradient guides the water vapour to flow into the foothills, which in turn amplifies the water vapour flux divergence amplified, causing the typhoon rainfall to increase eventually. The other role of the latent heat release is to make the convection more organized, resulting in a relatively smaller rain area and stronger precipitation.     

青藏高原上中尺度对流系统(MCS)的数值模拟

A mesoscale convective system (MCS) developing over the Qinghai-Xizang Plateau on 26 July 1995 issimulated using the fifth version of the Penn State-NCAR nonhydrostatic mesoscale model (MM5). Theresults obtained are inspiring and are as follows. (1) The model simulates well the largescale conditionsin which the MCS concerned is embedded, which are the well-known anticyclonic Qinghai-Xizang PlateauHigh in the upper layers and the strong thermal forcing in the lower layers. In particular, the modelcaptures the meso-α scale cyclonic vortex associated with the MCS, which can be analyzed in the 500 hPaobservational winds; and to some degree, the model reproduces even its meso-β scale substructure similarto satellite images, reflected in the model-simulated 400 hPa rainwater. On the other hand, there aresome distinct deficiencies in the simulation; for example, the simulated MCS occurs with a lag of 3 hoursand a westward deviation of 3-5° longitude. (2) The structure and evolution of the meso-α scale vortexassociated with the MCS are undescribable for upper-air sounding data. The vortex is confined to thelower troposphere under 450 hPa over the plateau and shrinks its extent with height, with a diameter of4° longitude at 500 hPa. It is within the updraft area, but with an upper-level anticyclone and downdraftover it. The vortex originates over the plateau, and does not form until the mature stage of the MCS. Itlasts for 3-6 hours. In its processes of both formation and decay, the change in geopotential height fieldis prior to that in the wind field. It follows that the vortex is closely associated with the thermal effectsover the plateau. (3) A series of sensitivity experiments are conducted to investigate the impact of varioussurface thermal forcings and other physical processes on the MCS over the plateau. The results indicatethat under the background conditions of the upper-level Qinghai-Xizang High, the MCS involved is mainlydominated by the low-level thermal forcing. The simulation described here is a good indication that itmay be possible to reproduce the MCS over the plateau under certain large-scale conditions and with theincorporation of proper thermal physics in the lower layers.     

应用NASA MERRA再分析资料对一次高原切变线的诊断分析

在引发中国东部夏季降水的天气系统中,高原低值系统扮演着十分重要的角色,其中高原低涡与高原切变线对强降水的协同作用是高原天气影响的一种常见样式,预报员将其称为低涡切变暴雨。本文回顾了高原低涡、切变线及其暴雨的研究历史和当前研究所取得的最新成果,重点探讨了人工智能应用、诊断计算、动力理论以及数值模式等多方法研究高原切变线与高原低涡的关系、相互作用过程以及诱发暴雨机理等科学问题,并基于低涡、切变线暴雨的最新研究成果和相关理论方法、技术手段的发展应用趋势,提出这一研究领域值得关注的一些新方向。由于目前对这两类几何形状迥异但物理属性相近的高原低值天气系统关系的认知仍存较大分歧,两者相互作用进而引发高影响天气过程的物理机理尚不十分清楚,高原低涡、切变线气候学统计结果的差异还相当明显。因此,对这一研究领域的深入探索与交叉拓展,不仅对推动青藏高原天气、气候影响的理论发展有重要科学意义,对高原及下游灾害性天气、气候业务能力的提升亦有较大应用推广价值。

     

A budget analysis of a long-lived tropical mesoscale vortex over Hainan in October 2010

The vorticity, eddy kinetic energy, and helicity budgets were calculated to study the variations of a long-lived tropical mesoscale vortex that occurred over Hainan during the period 05?C09 October 2010. The main results are as follows: the vortex was mainly located at middle to lower levels of the troposphere, and among different levels, the dominant factors responsible for the variations of the vortex were different. Intense convergence at the lower troposphere dominated the formation and longevity of the vortex. The vertical transport of positive vorticity which was closely related to the convective activities was conducive to the formation and maintenance of the vortex. The barotropic energy conversion was favorable for the formation of the vortex, while the baroclinic energy conversion accelerated its attenuation. Background circulations were favorable for the longevity of the vortex, and interactions with other synoptic systems were important to its variations. The variation of helicity was closely related to the vortex, and the maintenance of positive helicity was another favorable factor for the longevity of the vortex.     

涡旋自组织现象在上海一次暴雨预报中的应用

根据涡旋自组织理论的研究成果,通过对天气背景以及卫星、雷达等资料的分析发现,在2008年8月25日早晨上海地区发生的一场雨强为百年未遇的强雷电和大暴雨天气过程中,确实存在着涡旋自组织现象。地面中低压的发展与中小尺度涡旋的组织、合并关系密切,而中低压的发展导致了剧烈而集中的降水发生,进而造成了上海地区的这场特大暴雨。此外,详细阐述了在业务预报中预报员如何应用涡旋自组织理论的相关成果来改进和提高暴雨预报的时效和准确度,显示该理论在业务应用中具有广阔前景。