“09.11.10”石家庄特大暴雪中尺度风场分析

利用常规观测、雷达资料以及四维变分方法反演的风场资料,对2009年11月10-12日石家庄特大暴雪过程的形势场和中尺度风场结构进行了详细分析.从影响系统来看,此次特大暴雪过程分为两个阶段:回流降雪和西来槽降雪.回流降雪是特大暴雪的主要时段,此阶段西部山区降雪明显大于东部平原;而西来槽降雪阶段全区降雪比较均匀.由雷达反射率因子和径向速度可见:10日降雪具有对流性质,而且回波不断地自西部山区向石家庄市区移动,产生“列车效应”,造成市区及西部降雪强度较大;11-12日回波强度弱,降雪强度也较弱.分析雷达四维变分反演风场得到如下结论:(1)反演风场能够准确展现东风影响的时间和高度、西来槽影响的始末,对预报员定性外推降雪的强度起到非常好的参考作用.(2)低层水平反演风场上,存在一个比较窄的东风带,回波沿东风带自西向东移动,石家庄市区处于北到东北风的“回流墙”附近,东移回波在此堆积,移速减慢,影响时间较长;同时,市区附近存在风向性和风速性辐合,致使回波在石家庄附近加强或维持.(3)反演风场垂直剖面图上,东风回流降雪阶段,低层东风区上空存在一个“反气旋”,强回波位于“反气旋”所包围的范围.     

LAPS分析场在一次强对流天气过程尺度分析中的应用

利用LAPS（local analysis and prediction system）同化系统融合多种观测资料,对2010年8月25日发生在上海的一次强对流天气过程进行中尺度分析。结果表明,这次强对流天气主要是由中、低空中尺度辐合系统直接触发形成的。强对流形成阶段,地面有分散辐合形成并逐渐加强,成为触发中尺度垂直环流的主要机制,垂直结构上出现低层辐合高层辐散的有利配置,风暴中心附近出现明显的上升气流区,中高层相对湿度显著增加。成熟阶段,强对流云体中心附近的对流层底层开始出现下沉气流,上升气流在其拖曳作用下明显倾斜。衰减阶段,下沉气流加强使中尺度环流动力结构和水汽供应受到破坏,垂直结构上转为底层辐散高层辐合。因此,与天气尺度分析相比,基于LAPS的中尺度分析能更深刻地揭示中小尺度系统的三维结构和时空演变特征。     

渭河流域一次致洪暴雨过程的中尺度滤波分析

利用常规高空观测资料和NCEP/NCAR 1°×1°的再分析资料以及25点低通滤波技术,对2003年8月28—29日发生在渭河流域的一次致洪暴雨进行了中尺度分析和探讨,分析出中尺度环流演变特征,总结了渭河流域致洪暴雨的概念模型,认为这次暴雨过程中在以西南气流为主的平均气流场上隐藏着尺度较中尺度平均流场小得多的涡旋,其移动方向和发展程度决定了强降水落区及其强度;高空西风风速脉动与低层南风风速脉动耦合形成了中尺度次级环流圈,其上升支为强降水提供了足够的动力抬升机制;而850 hPa低空急流、700 hPa中尺度低涡、南风脉动以及高空西风风速脉动等条件的合理配置是中尺度次级环流形成的必备因素;地面中尺度辐合线是本次暴雨的触发机制。     

2005年6月18-12日浙南持续性梅雨锋暴雨过程诊断分析

利用NCEP1°×1°的6h再分析资料、常规观测资料和逐小时加密雨量资料,对2005年6月18-22日浙江省中南部地区出现的一次持续性梅雨锋暴雨过程进行诊断分析。结果表明：此次持续性梅雨锋暴雨过程是在两槽一脊的大环流形势下,由中低层切变、西南涡和冷空气共同影响造成的;过程具有明显的中尺度特征,12个中尺度雨团的持续生成东移导致了暴雨的发生。正涡度大值中心值的强度和中尺度雨团的强弱以及暴雨落区有较好的对应关系;垂直速度以及垂直螺旋度的强弱和中尺度雨团的强度变化一致。急流核的出现导致水汽辐合明显加强．从暴雨区和急流核的位置配置来看,暴雨区出现在急流核的左侧。     

Study on the Dynamic Characteristics of an Eastward-offshore Mesoscale Vortex along the Meiyu-Baiu Front

The eastward-moving Meiyu-Baiu frontal mesoscale vortices (MBFMVs) appear frequently and often cause heavy rainfall events along their tracks. A move-off-shore MBFMV was selected to enhance our understanding of this type of vortex. Synoptic analyses indicate that the MBFMV is a type of meso-α vortex and mainly occurs in the lower troposphere. A short wave trough near the coastline is highly favorable for the formation, sustainment, and displacement of the MBFMV. Vorticity budgets indicate that at lower levels of the MBFMV, convergence is the dominant factor for the increase of positive vorticity, and at high levels of the MBFMV, the vertical transportation associated with convective activities is the most important factor. The hori-zontal transportation was the main factor decreasing the positive vorticity. The land and sea environments are crucial to the evolution of the MBFMV. The characteristics of the Meiyu-Baiu Front (MBF) are also vital to the variation of the vortex.     

The Mesoscale Predictability of a Heavy Precipitation Event

Using the mesoscale model MM5, the development of initial condition uncertainties at different scales and amplitudes and their influences on the mesoscale predictability of the ＂0185＂ Shanghai heavy precipitation event are investigated. It is found that different initial conditions obtained from different globe model analyses lead to large variations in the simulated location and strength of the heavy precipitation, and the scales and amplitudes of the initial condition perturbations significantly influence the model error growth. The power spectrum evolution of the difference total energy （DTE） between a control simulation and a sensitivity experiment indicates that the error growth saturates after 12 h, which is the predictable time limit of the heavy precipitation event. The power spectrum evolution of the accumulated precipitation difference between the control and sensitivity simulations suggests a loss of the mesoscale predictability for precipitation systems of scales smaller than 300 kin, i.e., the predictable space for the heavy precipitation event is beyond 300 km. The results also show that the initial uncertainties at larger scales and amplitudes generally result in larger forecast divergence than the uncertainties at smaller scales and amplitudes. The predictable forecasting time and space can be expanded （e.g., from 12 to 15 h, and from beyond 300 kin to beyond 200 km） under properly prescribed initial perturbations at smaller scales and amplitudes.     

低涡过程对流云和降水结构特征及其环境条件分析

利用遥感及加密观测资料详细分析了2005年6月25～26日一次低涡过程对流云和降水的结构特征,并利用NCEP再分析资料以及实况风场资料分析了其形成的有利条件。分析表明：此次低涡过程中出现的中尺度对流云团（MCC）经历了十几小时的发展演变,云顶亮温低达-90℃,导致了局地暴雨出现,对流云和降水的分布有很好的对应但同时具有显著的时空分布不均匀性。对流云内部高空存在冰晶的大值区,云水含量也比周围环境丰富,冰晶下落过程中通过碰粘其它冰晶、碰冻云水导致了云内的可降水增多,播撒-供应机制起主要作用。而对流降水的“双峰“廓线结构表明暖雨过程非常强。此次过程有形成强对流非常有利的环境：强西南低空急流水汽输送和水汽辐合与高空偏西风急流的相互作用为对流云团和强降水的形成提供水汽和不稳定条件;低层不稳定区域上强垂直上升运动则提供了不可缺少的动力条件;中尺度辐合系统的形成是此次强对流产生的直接原因。     

Mesoscale Analysis of a Heavy Rainfall Event Along the Huaihe River Valley During 8-9 July 2007

During 8-9 July 2007,several successively developed rainstorms along the Meiyu front produced heavy rainfall in the Huaihe River Valley,which led to the most catastrophic flooding in this region since 1954.Through mesoscale analysis of both conventional and intensive observations from upper air and surface stations,automatic weather stations,Doppler radars,and the FY-2C satellite,the current study examines the developing style and environmental conditions of the mesoscale convective systems(MCSs)that led to the development of the rainstorms.Our analysis showed that this event went through three phases.The first phase of the heavy rainfall(Phase Ⅰ)was caused by a meso-α-scale wind shear in the lower troposphere during 0200-1700 BT(Beijing Time)8 July.Phase Ⅱ was characterized by a reduction in rain rate and the formation of a low-level vortex between 1700 BT 8 and 0200 BT 9 July.In Phase Ⅲ,the well-organized mature meso-α-scale low-level vortex brought about intensified rains during 0200-0800 BT 9 July.Satellite and raclar observations showed a backward development of MCSs(new convective cells were generated at the back of the system)in PhaseⅡ,a forward development in Phase Ⅲ,and a spiral organization of the convective lines in Phase Ⅱ.The heavy rainstorm systems were initiated continuously along a surface mesoscale dew-point front with a horizontal scale of～300 km(as part of the Meiyu front)in the upper reaches of the Huaihe River Valley near Fuyang City,Anhui Province and then gradually decayed in the middle and lower reaches.It is hypothesized that lifting by strong low-level convergence is sufficient to trigger convection in the high CAPE(convective available potential energy)environment.     

Mesoscale Vortex Generation and Merging Process： A Case Study Associated with a Post-Landfall Tropical Depression

An observational analysis of satellite blackbody temperature (TBB) data and radar images suggests that the mesoscale vortex generation and merging process appeared to be essential for a tropical-depression-related heavy rain event in Shanghai, China. A numerical simulation reproduced the observed mesoscale vortex generation and merging process and the corresponding rain pattern, and then the model outputs were used to study the related dynamics through diagnosing the potential vorticity (PV) equation. The t...     

初始场中尺度信息对暴雨预报的影响

由于观测资料分辨率与模式分辨率的不同,利用高分辨率模式对暴雨进行预报时,常规观测资料形成的初始场不能直接分辨出中尺度系统,这种中尺度系统特征的缺少可以认为是初始场的一种信息误差。利用中尺度天气分析的尺度分离方法可以提取这种中尺度信息。通过分析初始场中尺度信息的结构、演变特征及其对暴雨预报影响的机理,发现初始场中尺度信息的结构在主要雨带的对流敏感区具有明确的天气学意义,包含了有利暴雨产生的信息;其能量随时间也是增长的,特别是在积分12小时以后,能量迅速增长然后趋于稳定,超过了初始随机扰动的能量增长。利用减弱和增强初始场中中尺度信息的两种初始场作暴雨预报,其结果反映了初始场中尺度信息对暴雨预报的重要性,特别是对雨团位置和强度的预报,这些信息会直接影响暴雨的精细预报。