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1.
In this study, a three-dimensional mesoscale model was used to numerically simulate the well-known "98.7" heavy rainfall event that affected the Yangtze Valley in July 1998. Two experiments were conducted to analyze the impact of moist processes on the development of meso-β scale vortices(MβV) and their triggering by mesoscale wind perturbation(MWP). In the experiment in which the latent heat feedback(LHF) scheme was switched off, a stable low-level col field(i.e., saddle field—a region between two lows and two highs in the isobaric surface) formed, and the MWP triggered a weak MβV. However, when the LHF scheme was switched on as the MWP was introduced into the model, the MβV developed quickly and intense rainfall and a mesoscale low-level jet(mLLJ) were generated. The thickness of the air column and average temperature between 400 and 700 hPa decreased without the feedback of latent heat, whereas they increased quickly when the LHF scheme was switched on, with the air pressure falling at low levels but rising at upper levels. A schematic representation of the positive feedbacks among the mesoscale vortex, rainfall, and mLLJ shows that in the initial stage of the MβV, the MWP triggers light rainfall and the latent heat occurs at low levels, which leads to weak convergence and ageostrophic winds. In the mature stage of the MβV, convection extends to the middle-to-upper levels, resulting in an increase in the average temperature and a stretching of the air column. A low-level cyclonic circulation forms under the effect of Coriolis torque, and the m LLJ forms to the southeast of the MβV.  相似文献   

2.
The mesoscale vortex associated with a mesoscale low-level jet(mLLJ) usually causes heavy rainfall in the col field.The col field is defined as a region between two highs and two lows,with the isobaric surface similar to a col.Using a two-dimensional shallow water model,the meso-β scale vortex couplets(MβVCs) induced by eight types of mesoscale wind perturbations in an ideal col field were numerically simulated.With the sizes of ～100 km,the MβVCs induced by northerly perturbation(NP) and southerly perturbation(SP) moved toward the col point.The sizes of MβVCs induced by southwesterly perturbation(SWP),southeasterly perturbation(SEP),northwesterly perturbation(NWP),and northeasterly perturbation(NEP) were relatively small for the perturbations moving toward dilatation axis.The MβVC induced by easterly perturbation(EP) and westerly perturbation(WP) could not develop because they quickly moved away from the col point,before the circulation could form.The size of the circulation was determined by the distance between the vortex and the col point.The closer to the col point the vortex was,the larger the size of vortex.The comparisons of maximum vorticity and vorticity root mean square error(RMSE) of the NP,the SWP,and the WP show that the maximum vorticity and the vorticity RMSE of the NP decreased slower than other perturbations.Therefore,the weak environment of the col field favors the maintenance of vorticity and the formation of vortex.When a mesoscale vortex forms near the col point or moves toward the col point,it may maintain a quasi-stationary state in the stable col field.  相似文献   

3.
Summary  A mesoscale convective system (MCS) case that developed over the Yellow Sea (12–13 July 1993) is studied by using a 23-level, 30 km-mesh Penn State/NCAR mesoscale model MM5. This MCS was generated in northern China, south of the Changma front, in a convectively unstable environment, under the influence of a short-wave trough accompanied by a marked cold vortex aloft. The model with all model physics (refereed to as CNTL) captured the major features of this MCS. A mesoscale low-level jet (mLLJ), with a horizontal scale of a few hundred km, developed within the MCS. Available wind data support the realism of this mLLJ. This mLLJ not only transports convectively unstable air directly toward the MCS but is also responsible for a strong low-level convergence in the MCS. At 200 hPa, an anticyclonic northwesterly flow with a relatively high wind speed core on the east of MCS was simulated. This relatively high-speed flow can be regarded as a mesoscale upper level jet (mULJ), acted as an upper outflow over the MCS. Low-level convergence on the left-front of the mLLJ and upper divergence in the right-rear of the mULJ creates a strong upward motion (≅ 40 cm s−1) in the MCS. Heavy precipitation up to 45 mm between 1800–2100 UTC was observed after this MCS landed on the southern Korean Peninsula. The CNTL run captured this heavy rainfall event. A maximum rainfall of 50 mm 3 h−1 was simulated. In another experiment, with surface sensible and moisture fluxes withheld (NOSF), the 3-h simulated rainfall was decreased to 30 mm. Less latent heat released in the NOSF led to a weaker MCS and mLLJ. The concurrent surface fluxes sustained a high low-level moisture field over the Yellow Sea, which helped the development of the MCS and enhanced its precipitation in this case. Received January 8, 1999  相似文献   

4.
一次冷涡暴雨的中尺度对流云团分析及数值模拟研究   总被引：2，自引：10，他引：2  
利用MM5模式对2000年8月发生在河北省的一次暴雨过程进行了模拟,发现本次过程是由蒙古东部的低涡与副热带高压共同作用形成的,而局地大暴雨的直接原因是中尺度对流云团的作用。中尺度对流云团形成前期有一个能量急剧积累的过程。通过改变地形、低层东南风大小的敏感性试验,发现这次暴雨虽然发生在河北东部,但华北西部、北部的地形对暴雨影响很大:当降低地形高度后,雨区的位置和强度都发生变化;减弱低层东南风后,蒙古东部冷涡的强度和移动速度都有不同程度的改变,并且层次越低影响越明显。  相似文献   

5.
单多普勒天气雷达的中尺度风场探测技术研究   总被引：16，自引：11，他引：16       下载免费PDF全文
利用1998年6月海峡两岸暴雨试验中收集的香港天文台的多普勒雷达资料、GMS—5红外卫星云图等资料对1998年6月9日00～11UTC的暴雨过程进行综合对比分析。分析结果表明：UW方法反演的低空风场中的中尺度流场系统的位置、移动和生消变化与雷达回波的分布及时间演变完全配合，并且与红外卫星云图有良好的对应，因此可以认为，利用UW方法能够从多普勒速度场反演出风场中的中—β以上尺度的切变线系统，为强对流天气的临近预报提供高效、实用的预警产品。  相似文献   

6.
Summary  Two organized mesoscale convective systems (MCSs) developed sequentially along the Meiyu front over the Yangzi-Huai River basin and caused severe flooding over eastern China during 12–13 June 1991. In this paper, the structure and evolution of these MCSs are studied with a high-resolution (18 km) numerical simulation using the Fifth Generation Penn-State/NCAR Mesocale Model (MM5). The model reproduced the successive development of these two MCSs along the Meiyu front. The evolution of these MCSs was recorded clearly on satellite-derived cloud-top black body temperature (T bb ) maps. A mesoscale low-level jet (mLLJ) and a mesoscale upper-level jet (mULJ) were simulated, respectively, to the south and east of each of these two MCSs. Our analyses shows that the mLLJ and mULJ were formed as a responses to the intense convection associated with the MCS. The mLLJs transported warm, moist air with equivalent potential temperature greater than 352 K into the MCSs, and strong low-level convergence can be identified on the left-front end of the mLLJ. This strong convergence was associated with intense upward motion in the MCS with speed up to 80 cm s−1. Much of inflow into the MCSs extends up to the middle and upper troposphere, and ventilated through the mULJ. The development of the MCSs was also associated with substantial increase in potential vorticity (PV). The build up of PV in the lower-level along the Meiyu front was in turn related to a local intensification of the frontal equivalent potential temperature gradient, suggesting a relationship between the MCSs and the local enhancement and cyclogenesis of the front. In a sensitivity experiment without the effect of latent heating, a series of ascent centers with average separation of about 300 km were simulated. This result suggests that the initial formation of the MCSs along the Meiyu front could occur in absence of moist-diabatic process. Since the horizontal velocity gradient across the Meiyu front near the synoptic-scale low-level jet (LLJ) was quite large while the corresponding temperature gradient across the frontal zone was rather weak, we speculate that barotropic process may be responsible for triggering these MCSs along the Meiyu front. Received December 28, 1999 Revised May 11, 2000  相似文献   

7.
The evolution of a mesoscale convective system (MCS) that caused strong precipitation in the northern area of Dabie Mountain during 21-22 June 2008 is analyzed, along with the evolution of the associated meso-β-scale convective vortex (MCV). The mesoscale reanalysis data generated by the Local Analysis and Prediction System (LAPS) at a 3-km horizontal resolution and a 1-h time resolution during the South China Heavy Rainfall Experiment (SCHeREX) were utilized. The results show that two processes played key roles in the enhancement of convective instability. First, the mesoscale low-level jet strengthened and shifted eastward, leading to the convergence of warm-wet airflow and increasing convective instability at middle and low levels. Second, the warm-wet airflow interacted with the cold airflow from the north, causing increased vertical vorticity in the vicinity of steeply sloping moist isentropic surfaces. The combined action of these two processes caused the MCS to shift progressively eastward. Condensation associated with the MCS released latent heat and formed a layer of large diabatic heating in the middle troposphere, increasing the potential vorticity below this layer. This increase in potential vorticity created favorable conditions for the development of a low-level vortex circulation. The vertical motion associated with this low-level vortex further promoted the development of convection, creating a positive feedback between the deep convection and the low-level vortex circulation. This feedback mechanism not only promoted the maturation of the MCS, but also played the primary role in the evolution of the MCV. The MCV formed and developed due to the enhancement of the positive feedback that accompanied the coming together of the center of the vortex and the center of the convection. The positive feedback peaked and the MCV matured when these two centers converged. The positive feedback weakened and the MCV began to decay as the two centers separated and diverged.  相似文献   

8.
An extremely heavy rainfall event occurred in Zhengzhou, China, on 20 July 2021 and produced an hourly rainfall rate of 201.9 mm, which broke the station record for mainland China. Based on radar observations and a convection-permitting simulation using the WRF-ARW model, this paper investigates the multiscale processes, especially those at the mesoscale,that support the extreme observed hourly rainfall. Results show that the extreme rainfall occurred in an environment characteristic of warm-sec...  相似文献   

9.
The conventional and intensive observational data of the China Heavy Rain Experiment and Study (CHeRES) are used to specially analyze the heavy rainfall process in the mei-yu front that occurred during 20-21 June 2002, focusing on the meso-β system. A mesoscale convective system (MCS) formed in the warm-moist southwesterly to the south of the shear line over the Dabie Mountains and over the gorge between the Dabie and Jiuhua Mountains. The mei-yu front and shear line provide a favorable synoptic condition for the development of convection. The GPS observation indicates that the precipitable water increased obviously about 2-3h earlier than the occurrence of rainfall and decreased after that. The abundant moisture transportation by southwesterly wind was favorable to the maintenance of convective instability and the accumulation of convective available potential energy (CAPE). Radar detection reveals that meso-β and -γ systems were very active in the MαCS. Several convection lines developed during the evolution of the MαCS, and these are associated with surface convergence lines. The boundary outflow of the convection line may have triggered another convection line. The convection line moved with the mesoscale surface convergence line, but the convective cells embedded in the convergence line propagated along the line. On the basis of the analyses of the intensive observation data, a multi-scale conceptual model of heavy rainfall in the mei-yu front for this particular case is proposed.  相似文献   

10.
06·6福建大暴雨的数值模拟及复杂地形影响试验   总被引：2，自引：0，他引：2  
利用WRF（Weather Research Forecast）中尺度模式对2006年6月5-7日福建地区出现的一次大暴雨过程进行了数值模拟,根据模式输出的物理量进行了诊断分析,并通过地形敏感性试验讨论福建地形对此次暴雨的影响。结果表明：中尺度WRF模式成功模拟出了这次暴雨的雨况及高低空流场分布。这是建立在静止锋、低空切变线和低空急流等系统基础上的一次典型的华南准静止锋降水。冷暖气流在底层交汇并产生强烈的垂直上升运动,不稳定能量的释放是暴雨发生和维持的机制之一,位温的垂直分布有利于低层涡度的发展。福建北部的喇叭口地形和武夷山迎风坡共同作用,导致西南气流的转向辐合,触发了中尺度切变线和中尺度涡旋的形成,加速了上升运动和中层对流发展,有利于位于迎风坡的建瓯、邵武、蒲城等地区降水的增强。  相似文献   

11.
青藏高原上中尺度对流系统(MCS)的数值模拟   总被引：4，自引：0，他引：4  
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.  相似文献   

12.
A strong cyclonic wind perturbation generated in the northern South China Sea （SCS） moved northward quickly and developed into a mesoscale vortex in southwest Guangdong Province, and then merged with a southward-moving shear line from mid latitudes in the period of 21-22 May 2006, during which three strong mesoscale convective systems （MCSs） formed and brought about torrential rain or even cloudburst in South China. With the 1° ×1° NCEP （National Centers for Environment Prediction） reanalysis data and the Weather and Research Forecast （WRF） mesoscale model, a numerical simulation, a potential vorticity inversion analysis, and some sensitivity experiments are carried out to reveal the formation mechanism of this rainfall event. In the meantime, conventional observations, satellite images, and the WRF model outputs are also utilized to perform a preliminary dynamic and thermodynamic diagnostic analysis of the rainstorm systems. It is found that the torrential rain occurred in favorable synoptic conditions such as warm and moist environment, low lifting condensation level, and high convective instability. The moisture transport by strong southerly winds associated with the rapid northward advance of the cyclonic wind perturbation over the northern SCS provided the warm and moist condition for the formation of the excessive rain. Under the dynamic steering of a southwesterly flow ahead of a north trough and that on the southwest side of the West Pacific subtropical high, the mesoscale vortex （or the cyclonic wind perturbation）, after its genesis, moved northward and brought about enormous rain in most parts of Guangdong Province through providing certain lifting forcing for the triggering of mesoscale convection. During the development of the mesoscale vortex, heavy rainfall was to a certain extent enhanced by the mesoscale topography of the Yunwu Mountain in Guangdong. The effect of the Yunwu Mountain is found to vary under different prevailing wind directions and intensities. The location o  相似文献   

13.
2013年广西一次前汛期暴雨过程分析   总被引：1，自引：0，他引：1  
利用地面观测资料、探空资料和卫星观测资料对2013年4月29日至5月1日广西东部地区出现的一次暴雨天气过程进行分析,结果表明：（1）本次强降雨过程前期具有典型的暖区暴雨天气过程,主要受高原槽、低层急流系统影响,低层风向辐合高层辐散的配置触发了暴雨天气的产生,后期冷空气南下影响,触发强对流云团发展,造成强降雨天气的持续;（2）过程期间涡度场、散度场低层辐合高层辐散配置较好,水汽辐合上升强烈;（3）中尺度雨团在广西东部连续的生成并维持,造成桂东地区大范围的暴雨天气.  相似文献   

14.
针对2020年8月11—12日四川盆地西部特大暴雨过程中尺度系统演变特征和维持机制,利用欧洲中心ERA5逐小时再分析资料以及FY-4A的云顶相当黑体温度TBB资料进行诊断分析。(1) 本次过程发生在500 hPa巴湖长波槽分裂短波和高原低槽东移发展在四川盆地停滞,副高加强西伸形成阻挡的形势下,同时200 hPa有南亚高压和高空分流区配合。(2) 在上述有利的背景条件下,中尺度系统活动经历了中尺度辐合扰动-西南涡生成发展-低空急流影响-西南涡再次发展增强等4个阶段,西南涡两个阶段的发展对降水影响最大,初生发展阶段雨强最强,再次发展阶段强降雨范围最大。(3) 西南涡在暖区内初生发展,对流不稳定性强,地面潜热和感热加热以及500 hPa层以下水汽凝结潜热加热均十分显著,在较强暖湿平流作用下,配合低层涡度拉伸项和扭转项的动力作用加强,西南涡迅速发展,但低层辐合相对较弱,正涡度柱高度仅发展至500 hPa。(4) 西南涡再次发展阶段冷平流入侵,大气斜压性增强,中高层感热和凝结潜热加热作用加大,“低层辐合-中高层辐散”的动力机制显著加强,配合垂直向上输送正涡度和涡度拉伸项的动力发展作用,西南涡发展旺盛,正涡度柱中心强度和发展高度较初始发展阶段均明显增强。   相似文献   

15.
对惯性重力内波方程组分别通过线性和非线性求解探讨造成2010年10月海南岛一次特大暴雨中一类热带中尺度涡旋生成发展的动力、热力机制,研究发现:（1）在副热带高压和大陆冷高压南侧反气旋性纬向水平风切变大值区、静力不稳定大气层结、积云对流潜热释放、低空急流、适当强度的冷空气有利于热带中尺度涡旋的形成和发展;（2）非线性惯性重力内波的孤立波解与这类热带中尺度涡旋有很好的联系,在静力不稳定的大气层结下,热带中尺度涡旋的形态主要由对流凝结潜热加热所决定,即潜热加热下的孤立波解要求热带中尺度涡旋在垂直方向是一个浅薄的涡旋系统;另外强盛的对流凝结潜热对热带中尺度涡旋垂直运动振幅的增强起主要作用,更有利于涡旋的发展和维持。基于天气事实分析的理论研究为深化影响海南的热带中尺度涡旋乃至南海中尺度对流系统的机理认识进行了探索。   相似文献   

16.
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.  相似文献   

17.
利用2008年6月广西致洪暴雨过程高分辨率数值模式资料,分析引起暴雨的中尺度涡旋在移动和持续性发展过程中其内部结构的演变以及系统的动力学特征。研究结果表明,中尺度涡旋是导致此次广西暴雨的主要系统,其发展移动过程中伴随有强烈上升运动的深厚湿对流,并在中高层持续形成明显的暖心结构。此次涡旋系统移动过程可分为涡旋南移和涡旋东移两个阶段,由于受到环境场的影响,涡旋中心区域动量、热量和水汽的再分配为其持续性发展提供有利条件。同时诊断分析表明,涡旋内部存在具有继发特征的长时间维持的组织化深厚湿对流系统,以强非地转分量为特征的超地转流,并表现出涡散运动共存且同量级的特征,具有典型的准平衡特征。  相似文献   

18.
局地强对流暴雨的多普勒天气雷达个例分析   总被引：3，自引：6，他引：3  
从天气背景的分析出发,利用多普勒天气雷达的强度、速度资料以及由速度资料反演的垂直风廓线和二维风场对一次北京局地暴雨从开始到强降水结束时段做了详细的分析。分析发现,这次局地强对流暴雨是大尺度系统和中尺度系统共同作用的结果,大尺度环流为暴雨提供了充沛的水汽;本站层结有很大的不稳定度,为强对流风暴的产生提供了必要条件;低层中尺度辐合线起到了触发对流和加强对流的作用。  相似文献   

19.
An analysis was conducted on the evolutional process of a mesoscale convective vortex (MCV) and associated heavy rainfall in the Dabie Mountain area on 21-22 June 2008,as well as their structural characteristics in different stages,by using the mesoscale reanalysis data with 3 km and 1 h resolution generated by the Local Analysis and Prediction System (LAPS) in the Southern China Heavy Rainfall Experiment.The results showed that the latent heat released by convection in the midtroposphere was the main energy source for the development of a low-level vortex.There was a positive feedback interaction between the convection and the vortex,and the evolution of the MCV was closely related to the strength of the positive interaction.The most typical characteristics of the thermal structure in different stages were that,there was a relatively thin diabatic heating layer in the midtroposphere in the formative stage;the thickness of diabatic heating layer significantly increased in the mature stage;and it almost disappeared in the decay stage.The characteristics of the dynamic structure were that,in the formative stage,there was no anticyclonic circulation at the high level;in the mature stage,an anticyclonic circulation with strong divergence was formed at the high level;in the decay stage,the anticyclonic circulation was damaged and the high-level atmosphere was in a disordered state of turbulence.Finally,the structural schematics of the MCV in the formative and mature stage were established respectively.  相似文献   

20.
"99.8"山东特大暴雨形成机制的数值模拟分析   总被引：20，自引：9，他引：11  
利用双重嵌套的非静力数值模式MM5V3,成功地模拟了1999年8月11~12日山东诸城发生的特大暴雨。利用模式输出的高时空分辨率资料,对这次暴雨天气及中尺度低涡的形成机制进行了诊断分析。结果表明,这次特大暴雨是在弱冷空气侵入台风低压环流,热带辐合带北伸的形势下形成的;倾斜涡度发展是特大暴雨及中尺度低涡产生、发展的重要机制,鲁东南有利的地形对中尺度低涡的形成具有重要作用;台风低压倒槽顶部的强辐合作用首先触发上升运动,产生强降水,强降水位于低层气旋性暖式切变最明显的地方;强降水释放的凝结潜热使高层气层增暖,高层辐散加强,引起低层中尺度低涡强烈发展,导致降水增幅。可见,CISK机制是特大暴雨和中尺度低涡加强的另一种机制。  相似文献   

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