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1.
Based on the previous statistical analysis of mesoscale convective systems(MCSs)over the second-step terrain along Yangtze-Huaihe River Valley,eight representative long-lived eastward-propagating MCSs are selected for model-based sensitivity testing to investigate the initiation and evolution of these types of MCSs as well as their impact on downstream areas.We subject each MCS to a semi-idealized(CNTL)simulation and a sensitivity(NOLH)simulation that neglects condensational heating in the formation region.The CNTL experiment reveals convection forms in the region downstream of a shortwave trough typified by persistent southwesterly winds in the low-to midtroposphere.Upon merging with other convective systems,moist convection develops into an MCS,which propagates eastward under the influence of mid-tropospheric westerlies,and moves out of the second-step terrain.The MCS then merges with pre-existing local convection over the plains;the merged convection reinforces the cyclonic wind perturbation into a mesoscale vortex at 850 hPa.While this vortex moves eastward to regions with local vortex at 850 hPa,another vortex at 925 hPa is also intensified.Finally,the vortices at 850 and 925 hPa merge together and develop into a mesoscale convective vortex(MCV).In contrast,MCSs fail to form and move eastward in the NOLH experiment.In the absence of eastward-propagating MCSs,moist convection and mesoscale vortices still appear in the plains,but the vortex strength and precipitation intensity are significantly weakened.It is suggested the eastward-propagating MCSs over the second-step terrain significantly impact the development and enhancement of moist convection and vortices in the downstream areas.  相似文献   

2.
王智  高坤 《大气科学进展》2003,20(4):638-649
Whether the initial conditions contain pronounced mesoscale signals is important to the simulation of the southwest vortex. An eastward-moving southwest vortex is simulated using the PSU/NCAR MM5. A modest degree of success is achieved, but the most serious failure is that the formation and displacement of the simulated vortex in its early phase are about fourteen hours later than the observed vortex. Considering the relatively sparse data on the mesoscale vortex and in an attempt to understand the cause of the forecast failure, an adjoint model is used to examine the sensitivity of the southwest vortex to perturbations of initial conditions. The adjoint sensitivity indicates how small perturbations of model variables at the initial time in the model domain can influence the vortex. A large sensitivity for zonal wind is located under 400 hPa, a large sensitivity for meridional wind is located under 500 hPa, a large sensitivity for temperature is located between 500 and 900 hPa, and almost all of the large sensitivity areas are located in the southwestern area. Based on the adjoint sensitivity results, perturbations are added to initial conditions to improve the simulation of the southwest vortex. The results show that the initial conditions with perturbations can successfully simulate the formation and displacement of the vortex; the wind perturbations added to the initial conditions appear to be a cyclone circulation under the middle level of the atmosphere in the southwestern area with an anticyclone circulation to its southwest; a water vapor perturbation added to initial conditions can strengthen the vortex and the speed of its displacement.  相似文献   

3.
A series of idealized model simulations are analyzed to determine the sensitivity of model results to different configurations of the lateral boundary conditions (LBCs) in simulating mesoscale shallow convection over hilly terrain, In the simulations with steady thermal forcing at the model surface, a radiation condition at both boundaries is the best choice under high wind conditions, and the best results are produced when both the normal velocities and the temperature are treated with the radiation scheme in which the phase speed is the same for different variables, When the background wind speed is reasonably small, the LBC configuration with either the radiation or the zero gradient condition at both boundaries tends to make the numerical solution unstable. The choice of a constant condition at the inflow boundary and a radiation outflow boundary condition is appropriate in most cases, In the simulations with diurnal thermal forcing at the model surface, different LBC schemes are combined together to reduce spurious signals induced by the outflow boundary, A specification inflow boundary condition, in which the velocity fields at the inflow boundary are provided using the time-dependent results of a simulation with periodic LBCs over a flat domain, is tested and the results indicate that the specification condition at the inflow boundary makes it possible to use a smaller model domain to obtain reasonable results. The model horizontal domain length should be greater than a critical length, which depends on the domain depth H and the angle between gravity wave phase lines and the vertical, An estimate of minimum domain length is given by [(H-zi)/πU]√N^2L2x-4π^2u^2, where N and U are the background stability and wind speed,respectively, Lx is the typical gravity wavelength scale, and zi is the convective boundary layer (CBL)depth.  相似文献   

4.
Mesoscale research conducted by Chinese meteorologists during the past four years is reviewed.Advances in theoretical studies include (a) mesoscale quasi-balanced and semi-balanced dynamics, derived through scale analysis and the perturbation method which are suitable for describing mesoscale vortices;(b) subcritical instability and vortex-sheet instability; (c) frontal adjustment mechanism and the effect of topography on frontgenesis; and (d) slantwise vorticity development theories, the slantwise vortex equation,and moist potential vorticity (MPV) anomalies with precipitation-related heat and mass sinks and MPV impermeability theorem. From the MPV conservation viewpoint, the transformation mechanism between different scale weather systems is analyzed. Based on the data analysis, a new dew-point front near the periphery of the West Pacific subtropical high is identified. In the light of MPV theory and Q-vector theory, some events associated with torrential rain systems and severe storms are analyzed and diagnosed.Progress in mesoscale numerical simulation has been made in the development of meso-α, meso-β vortices,meso-γ-scale downbursts and precipitation produced by deep convective systems with MM5 and other mesoscale models.  相似文献   

5.
王智  高坤 《大气科学进展》2006,23(2):267-281
A relatively independent and small-scale heavy rainfall event occurred to the south of a slow eastwardmoving meso-α-scale vortex. The analysis shows that a meso-β-scale system is heavily responsible for the intense precipitation. An attempt to simulate it met with some failures. In view of its small scale, short lifetime and relatively sparse observations at the initial time, an adjoint model was used to examine the sensitivity of the meso-β-scale vortex simulation with respect to initial conditions. The adjoint sensitivity indicates how small perturbations of initial model variables anywhere in the model domain can influence the central vorticity of the vortex. The largest sensitivity for both the wind and temperature perturbation is located below 700 hPa, especially at the low level. The largest sensitivity for the water vapor perturbation is located below 500 hPa, especially at the middle and low levels. The horizontal adjoint sensitivity for all variables is mainly located toward the upper reaches of the Yangtze River with respect to the simulated meso-β-scale system in Hunan and Jiangxi provinces with strong locality. The sensitivity shows that warm cyclonic perturbations in the upper reaches can have a great effect on the development of the meso-β-scale vortex. Based on adjoint sensitivity, forward sensitivity experiments were conducted to identify factors influencing the development of the meso-β-scale vortex and to explore ways of improving the prediction. A realistic prediction was achieved by using adjoint sensitivity to modify the initial conditions and implanting a warm cyclone at the initial time in the upper reaches of the river with respect to the meso-β-scale vortex, as is commonly done in tropical cyclone prediction.  相似文献   

6.
A relatively independent and small-scale heavy rainfall event occurred to the south of a slow eastward-moving meso-α-scale vortex. The analysis shows that a meso-β-scale system is heavily responsible for the intense precipitation. An attempt to simulate it met with some failures. In view of its small scale, short lifetime and relatively sparse observations at the initial time, an adjoint model was used to examine the sensitivity of the meso-β-scale vortex simulation with respect to initial conditions. The adjoint sensitivity indicates how small perturbations of initial model variables anywhere in the model domain can influence the central vorticity of the vortex. The largest sensitivity for both the wind and temperature perturbation is located below 700 hPa, especially at the low level. The largest sensitivity for the water vapor perturbation is located below 500 hPa, especially at the middle and low levels. The horizontal adjoint sensitivity for all variables is mainly located toward the upper reaches of the Yangtze River with respect to the simulated meso-β-scale system in Hunan and Jiangxi provinces with strong locality. The sensitivity shows that warm cyclonic perturbations in the upper reaches can have a great effect on the development of the meso-β-scale vortex. Based on adjoint sensitivity, forward sensitivity experiments were conducted to identify factors influencing the development of the meso-β-scale vortex and to explore ways of improving the prediction. A realistic prediction was achieved by using adjoint sensitivity to modify the initial conditions and implanting a warm cyclone at the initial time in the upper reaches of the river with respect to the meso-β-scale vortex,as is commonly done in tropical cyclone prediction.  相似文献   

7.
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.  相似文献   

8.
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   

9.
The cumulus merging processes in generating the mesoscale convective system （MCS） on 23 August 2001 in the Beijing region are studied by using a cloud-resolving mesoscale model of MM5. The results suggest that the merger processes occurred among isolated convective cells formed in high mountain region during southerly moving process play critical role in forming MCS and severe precipitating weather events such as hailfall, heavy rain, downburst and high-frequency lightning in the region. The formation of the MCS experiences multi-scale merging processes from single-cell scale merging to cloud cluster-scale merging, and high core merging. The merger process can apparently alter cloud dynamical and microphysical properties through enhancing both low- and middle-level forcing. Also, lightning flash rates are enhanced by the production of more intense and deeper convective cells by the merger process, especially by which, the more graupel-like ice particles are formed in clouds. The explosive convective development and the late peak lightning flash rate can be found during merging process.  相似文献   

10.
The CSU-RAMS-2A was used to simulate the meteorological fields over the Zhujiang River Delta in South China.Initialized from a horizontally homogeneous atmosphere,real topography and inhomogeneous surface boundary condi-tions,the model was run with thermal and terrain forcing.The modeling results of winter and summer cases are com-pared with those observed.The similarity of the predicted distributions of winds,temperatures and humidities to the ob-served patterns permits us to conclude that the mesoscale distribution of meteorological elements for the two study datesis the result of the thermal and dynamical forcing by the underlying surface and topography.  相似文献   

11.
青藏高原上中尺度对流系统(MCSs)的个例分析及其比较   总被引：3，自引：0，他引：3  
对1995年7月25—28日高原上连续数日出现MCSs的现象进行了红外云图特征及其演变、大尺度环境背景场和对流有效位能的分析。可以发现，所有这些MCSs有着相似的日变化演变过程；它们的初始对流在中午由于日射加热开始活跃，之后迅速发展，这些MCSs在后下午形成，在傍晚达到最强，之后逐渐减弱。其中26日MCS最为强大，它是在单一的强大的近于圆形的高原反气旋高压背景下受强的低层热力强迫和条件不稳定的驱动而发生的。这些发生条件都与高原本身的热力作用紧密相关，所以它的发生发展主要与高原特有的较为纯粹的热力因子相联系。28日MCS是另一个很强的MCS，它明显地受到中纬度西风槽的斜压区的影响，这二个很强的MCS有着不同的发展机制和显著不同的表现特征。  相似文献   

12.
王欢  倪允琪 《气象学报》2006,64(6):734-742
2003年7月4—5日淮河流域发生了一次中尺度强暴雨过程,致使淮河洪水泛滥。这次暴雨过程由中尺度对流系统(MCS)以及因其发展而产生的低涡造成。通过对此次过程的诊断分析和新一代细网格WRF中尺度预报模式的数值模拟,研究了这次过程发生发展的机制。模拟结果较好地描述了本次暴雨及中尺度系统发生、发展的时空演变过程。分析结果表明:此次移动性暴雨过程的前期由不断向东移动发展的MCS造成,后期降水则由低涡切变线产生的中尺度低涡引起。同时,副热带高压明显偏西偏北,并维持较长时间,造成雨带一直维持在淮河流域。高层辐合中心的加强使低空急流不断增强,低空急流的增强进而引起低层辐合的加强,而低层辐合的加强以及上升运动的潜热释放导致低涡的发生,低涡形成及形成后移动缓慢,造成了淮河流域的大暴雨。高层中尺度辐散区的抽吸对低层中尺度涡旋的发生发展起到了促进和加强的作用。低层的中尺度辐合场和高层的中尺度辐散场的发展与耦合对中尺度系统的发展有很好的预示作用。低层中尺度辐合区的减弱预示着系统的衰减,西南偏西的中层相对干冷空气侵入并在梅雨锋前缘下沉促进了系统的衰减。  相似文献   

13.
Summary There exists a common observational phenomenon over the offshore areas of the northwest Pacific, that is, when several mesoscale vortices evolve suddenly into a larger scale typhoon-like vortex within one day, often with serious consequences. In this paper a series of numerical experiments has been designed and performed to emulate this evolution. The model is based on the Charney-Hasegawa-Mima equation, where there are around 40 initial meso-β vortices with parabolic profiles whose central positions, dimensions and intensities are all set stochastically. The self-organization process of these stochastically-distributed multiple meso-β vortices can be divided into two phases. During the first phase, a larger scale vortex similar to a typhoon-like vortex forms near the computational center through the gradual stretching and merging of adjacent meso-β vortices while there are more than 10 isolated vortices surrounding this typhoon-vortex. During the second phase, the isolated vortices are stretched and drawn into the typhoon-vortex circulation and become its spiral arms which are gradually incorporated into the inner area of the typhoon. This is then repeated as new isolated vortices are stretched and become new spiral arms until all the isolated vortices are drawn into the typhoon-vortex. The center of the self-organized typhoon-vortex rotates counterclockwise around the computational center when no topography is involved and is thus a transient vortex. When topography is present the vortex remain in the NE quadrant of the model domain, locked by the topography, and this quasi-steady vortex is thus capable of causing local disasters. Correspondence: Chongjian Liu, Chinese Academy of Meteorological Sciences (CAMS), State Key Labaratory of Severe Weather, 46 Zhongguancum South Avenue, 100081 Beijing, P.R. China  相似文献   

14.
内蒙古夏季典型短时强降水中尺度特征   总被引：2，自引：1，他引：1       下载免费PDF全文
利用常规观测资料、NCEP FNL分析资料、FY-2D逐时云顶亮温(TBB)资料、内蒙古地区自动气象站资料和闪电定位资料, 对2012—2015年内蒙古夏季37例典型短时强降水事件进行分析。结果表明:冷锋云系尾部、涡旋云系和暖湿切变云系中发展的中尺度对流系统(MCS)是造成内蒙古短时强降水的直接影响系统, 短时强降水发生在MCS发展或成熟阶段, 而且位于TBB梯度密集区MCS移出区域靠近干冷空气侵入一侧。自动气象站观测到的中气旋、中低压以及中小尺度气旋式辐合风场和切变线诱发MCS发展, MCS发展到成熟阶段地闪密度达到最大值, 地闪密度值较高对应的MCS面积扩展率也较大。内蒙古西部和中部偏北地区短时强降水发生前3 h相对湿度达到60%~80%, 但其余地区相对湿度基本为80%~90%, 温度锋区浅薄冷空气是触发MCS发生发展的关键因素。  相似文献   

15.
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   

16.
青藏高原低涡活动对降水影响的统计分析   总被引：6，自引：0，他引：6  
郁淑华  高文良  彭骏 《高原气象》2012,31(3):592-604
利用1998—2004年逐日08:00(北京时,下同)和20:00 500hPa高空图、日雨量和青藏高原低涡(下称高原低涡)切变线年鉴资料,统计分析了冬、夏半年不同生命史的高原低涡对我国和四川盆地东、西部降水的影响。结果表明,冬、夏半年高原低涡以东部涡占多数,6-10月有三分之一的东部涡能移出高原。冬半年高原低涡出现次数少,约占全年的五分之一,但也可造成高原及其周边地区的雨雪天气,特别是生命史超过36h以上的高原低涡有近半数可移出高原,造成高原区域暴雨雪,四川盆地中雨,半数可造成云南大雨雪或暴雨雪。夏半年,随着低涡生命史的增长,高原低涡影响高原及其周边地区和我国其他地区的降水范围和强度在增大,生命史超过60h以上的高原低涡可造成高原暴雨、甘肃中雨以上、四川盆地暴雨或大暴雨及云南大部分地区大雨以上的降水,每年都有1～5次可影响到华中、华东地区产生大雨以上的降水。100°E以东的高原低涡,不论是否移出,均可造成四川盆地中雨以上的降水。影响四川盆地降水的高原低涡以偏东路径为主,但东南路径影响更强。  相似文献   

17.
长江下游梅汛期中尺度涡旋特征分析   总被引：6，自引：3，他引：3  
利用2006～2009 年日本再分析资料对长江下游地区梅汛期间(5～7 月)边界层内中尺度涡旋进行普查,并分类统计分析了边界层内中尺度涡旋与暴雨、低空急流的关系。研究结果表明:每年的5～7 月该地区经常在对流层低层或(和)边界层内出现中尺度扰动涡旋,根据中尺度涡旋最初生成的高度不同,可划分为边界层中尺度涡旋、对流层低层中尺度涡旋和对流层低层—边界层中尺度涡旋三类。边界层中尺度涡旋中与暴雨有密切关系的中尺度涡旋称为边界层中尺度扰动涡旋(PMDV),根据涡旋前或后6 小时累积雨量,可以进一步将其分成两类:第一类是暴雨的直接制造者中尺度对流系统(MCS)先于边界层中尺度扰动涡旋发生(MCS-PMDV);第二类是边界层中尺度涡旋产生后,激发了中尺度对流,造成了暴雨过程(PMDV-MCS)。PMDV-MCS 类涡旋暴雨的特点是在对流层低层850 hPa 是一条切变线,其南侧有一支西南低空急流,边界层925 hPa 则是一个闭合的涡旋,暴雨区主要落在涡旋的东北面和东南面。  相似文献   

18.
为了进一步研究高原涡、西南涡对西南地区暴雨的影响,本文用中国气象局自动站与CMORPH降水数据融合的逐时降水资料、国家卫星气象中心的逐时FY-2E卫星的云顶亮温（TBB）资料、欧洲气象资料中心（ERA-interim）的再分析资料,通过天气学诊断分析方法以及拉格朗日轨迹模式HYSPLITv4.9,对发生在四川盆地的有高原涡东移影响西南涡发展引发暴雨的两次过程进行对比分析,发现:（1）两次暴雨过程的降水强度和分布有明显区别,并且TBB活动特征显示在过程一中有MCC（Mesoscale Convective Complex）的产生和发展,过程二则没有。（2）对于过程一,500 hPa上,高原涡逐渐减弱为高原槽并伸展到四川盆地上空,850 hPa上,在鞍型场附近有MCC的产生和发展,200 hPa上,高原涡在南亚高压北部偏西风急流下方的强辐散区内,位于南亚高压东南侧急流区下方稳定少动,偏东风急流北部有辐散中心,有利于西南涡的加强。对于过程二,500 hPa高原涡东移在四川盆地上空与西南涡耦合,形成一个稳定且深厚的系统,这也是过程二的暴雨强度比过程一强的最主要原因。200 hPa上,四川盆地始终位于南亚高压东侧的西北气流中,“抽吸作用”明显。（3）在过程一中,位涡逐渐东传且位涡增加的地方对应强降水区与MCC发展区,反映了暴雨和位涡的发展基本一致。在过程二中,中层位涡高值区从高原上东移并下传至盆地上空,两涡耦合使得上下层打通,位涡值比耦合之前单独的两涡强度更强。 MCC产生的必要条件是中层大气要有强正涡度、强辐合和强上升运动,在未产生MCC前,过程一与过程二在盆地上空的动力条件甚至是相反的;从热力条件看,过程一中有明显的干冷空气入侵,增强不稳定条件,有利于MCC的产生并引发强降水;另一方面,本文也应证了二阶位涡的水平分布与暴雨落区有较好的对应关系。（4）通过拉格朗日方法的水汽轨迹追踪模式和聚类分析方法分析可得两次暴雨过程的水汽输送源地和通道也有明显区别,过程一主要有两条水汽通道,通道一来自阿拉伯海和孟加拉湾洋面的底层,通道二来自四川南部750 m以下高度;而过程二的主要水汽输送通道有三条,通道一来自西方地中海、黑海和里海上空1500～2500 m高度附近,通道二来自阿拉伯海和印度洋的底层,通道三的水汽从孟加拉湾低层绕过云贵高原直接输送到四川盆地。  相似文献   

19.
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   

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