APPLICATION OF GENERALIZED CONVECTIVE VORTICITY VECTOR IN A RAINFALL PROCESS CAUSED BY A LANDFALLING TROPICAL DEPRESSION

A heavy rainfall process, which occurred in Shanghai during 5-6 August, 2001 from a landfalling tropical depression (TD),is examined with a control numerical experiment based on MM5 model. It is found that the contours of generalized equivalent potential temperature (θ*) are almost vertical with respect to horizontal surfaces near the TD center and more densely distributed than those of equivalent potential temperature (θe).Because the atmosphere is non-uniformly saturated in reality, θ* takes the place of θe in the definition of convective vorticity vector (CVV) so that a new vector, namely the generalized convective vorticity vector (CVV*), is applied in this study. Since CVV* can reflect both the secondary circulation and the variation of horizontal moist baroclinicity, the vertical integration of vertical component of CVV* is found, in this study, to represent the rainfall areas in the TD case better than potential vorticity (PV), moist potential vorticity (MPV), generalized moist potential vorticity (Pm), and CVV, with high-value area of CVV* corresponding to heavy-rainfall area. Moreover, the analysis from CVV* implies that the Hangzhou Bay might play an important role in the heavy rain process. A sensitivity experiment without the Hangzhou Bay is then designed and compared with the control run. It is found that the CVV* becomes weaker than that in the control run, implying that the elimination of Hangzhou Bay results in reduced rainfall. Further analyses show that the Hangzhou Bay provides sufficient water vapor and surface heat flux to the TD system, which is very important to the genesis and development of mesoscale cloud clusters around the TD and the associated heavy rainfall.     

AN OBJECTIVE PREDICTION SCHEME FOR TROPICAL CYCLONES MAKING LANDFALLS IN EASTERN CHINA

The landfall of tropical cyclones in the eastern part of China falls in the category of small probability events. Constructing a step function with intervals adequately divided can help reflect the non-linear distribution of conditional probability for a landfall event. For the prediction of landfall event probability, factors applying the step function in transformation are superior to the standardized factors that are linearly related. The prediction scheme discussed in the work uses transformation factors of step function to formulate prediction models for tropical cyclones making landfalls in eastern China, through screening with non-linear correlative ratios and REEP analysis. Classified models for statistic-synoptics, statistic -climatology and statistic-dynamics have been constructed using initial field data and numerical prediction output. Forecasting skills have been improved due to ensemble of predictions using these classified models. As shown in forecasting evaluations and experiments, the scheme is capable of predicting tropical cyclones that make landfalls in eastern China.     

Impacts of Soil Moisture on the Numerical Simulation of a Post-Landfall Storm

Surface heat and moisture fluxes are important to the evolution of a tropical storm after its landfall. Soil moisture is one of the essential components that influence surface heating and moisture fluxes. In this study, the impact of soil moisture on a pre-landfall numerical simulation of Tropical Storm Bill(2015), which had a much longer lifespan over land, is investigated by using the research version of the NCEP Hurricane Weather Research and Forecasting(HWRF) model. It is found that increased soil moisture with SLAB scheme before storm's landfall tends to produce a weaker storm after landfall and has negative impacts on storm track simulation. Further diagnoses with different land surface schemes and sensitivity experiments indicate that the increase in soil moisture inside the storm corresponds to a strengthened vertical mixing within the storm boundary layer, which is conducive to the decay of storm and has negative impacts on storm evolution. In addition, surface diabatic heating effects over the storm environment are also found to be an important positive contribution to the storm evolution over land, but their impacts are not so substantial as boundary layer vertical mixing inside the storm. The overall results highlight the importance and uncertainty of soil moisture in numerical model simulations of landfalling hurricanes and their further evolution over land.     

华南地区热带气旋登陆前强度突变的大尺度环境诊断分析

利用每6小时一次的NCEP再分析资料,对华南地区登陆前突然减弱和突然增强的两类热带气旋（TC）进行大尺度诊断分析,结果表明：（1）突然增强的TC位于副高的西南侧或南侧,低空有明显的西南气流卷入TC内部,而突然减弱的TC基本在副高西侧或西北侧;（2）突然增强TC的低空辐合、高空辐散均较强;（3）充足的水汽输送是TC登陆前突然增强的另一重要原因。     

1957-2004年影响我国的强热带气旋频数和强度变化

Based on tropical cyclone track dataset in the western North Pacific from China Meteorological Administration(CMA),variations in frequency and intensity of tropical cyclones(TCs)in the western North Pacific,affecting-China TCs(ACTCs)and landfall TCs(LTCs)achieving a typhoon intensity during 1957-2004 were studied.Frequencies of strong tropical cyclones showed significant decreasing trends from 1957 to 2004 and the linear trend was much greater when the intensity was stronger.There was no linear trend in the portion of strong tropical cyclones achieving a typhoon(TY)intensity,while those reaching a strong typhoon(STY)and a super typhoon(SuperTY)intensity showed decreasing trends during 1957-2004.The maximum intensities of TCs,ACTCs and LTCs all decreased during the period of 1957-2004.The mean intensities of TCs and ACTCs displayed decreasing trends and the mean intensity of LTCs achieving a TY intensity also showed a decreasing trend.     

对东海热带气旋登陆后路径趋势的若干探讨

应用相关分析与合成分析方法分析了影响东海热带气旋登陆后路径趋势的若干因素,结果表明：登陆后路径趋势随时间和登陆地点的变化分布说明热带气旋登陆后路径仍受基本气流的引导和制约;热带气旋登陆时的惯性和地转力的变化对东海热带气旋登陆后的路径趋势有一定影响;环境场及其变化对东海热带气旋登陆后路径有较大影响,尤其对流层中层流场对登陆后的热带气旋的移动仍有明显的引导作用;中国东部至黄海区域是环境场影响东海热带气旋登陆路径趋势的关键区,当区域内的西北太平洋副热带高压加强西伸,西风槽北撤时,东海热带气旋登陆后在副高南侧东风气流引导下向内陆西行至消亡,当区域内副高减弱东退,西风槽南压时,东海热带气旋登陆后受副高西侧偏南气流与西风槽前西南气流引导转向后入海;东海热带气旋登陆前的环境场对登陆后路径趋势影响相对较小,登陆后12～24小时是登陆后路径趋势受环境场影响的敏感时段,环境场的变化对登陆后路径趋势的影响要比当前环境场的影响超前6小时。     

两类登陆热带气旋的大尺度环流特征分析

采用动态合成分析方法,对1970-2006年登陆后北上类TC（tropicalcyclone）和西行类TC各7个样本做动态合成分析和诊断,结果表明：（1）北上类TC在背景场长波槽前北移靠近中纬度斜压锋区,通过吸附运动使TC低压并入西风槽,而西行类TC背景场没有长波槽,离中纬度斜压锋区较远;（2）北上类TC登陆时存在西南低空急流水汽输送带,当其强度减弱后,TC东南侧存在东南暖湿气流作为补充,而西行类TC减弱后逐渐与之分离,且不存在东南暖湿气流作为补充;（3）北上类TC高层辐散区与高空急流边界靠近,因此增强了其向东北方向的辐散,低层由于高层动量下传,加强了低空西风,从而使TC低压环流维持,而西行类TC离高空急流边界较远;（4）北上类TC从中纬度斜压锋区获取斜压能量,其环流垂直切变增强,相对涡度差负值增大,在高空TC中心散度由大变小后又由小变大的过程中,TC发生了变性,而西行类TC没有环境能量补给,逐渐填塞消亡。因此,当一个TC登陆后,其预报移动方向、水汽输送状况、与斜压锋区的关系以及高空辐散气流等特征,可以作为初步判定登陆TC将减弱消亡还是将变性加强的可能原因。     

卫星资料在登陆热带气旋降水研究中的应用

本文全面、系统地归纳总结了国内外利用卫星资料对登陆热带气旋降水的研究工作,指出利用卫星资料对登陆热带气旋降水估计及预报研究所存在的不足,并对未来研究进行了展望。     

A COMPOSITE STUDY OF RAINFALL ASYMMETRY OF TROPICAL CYCLONES AFTER MAKING LANDFALL IN GUANGDONG PROVINCE

Based on observed rainfall data, this study makes a composite analysis of rainfall asymmetry in tropical cyclones(TCs) after making landfall in Guangdong province(GD) during 1998—2015. There are 3.0 TCs per year on average making landfall in GD and west of GD(WGD) has the most landfall TCs. Most of TCs make landfall in June,July, August, and September at the intensities of TY, STS, and TS. On average, there is more rainfall in the southwest quadrant of TC in CGD(center of GD), WGD, and GD as a whole, and the maximum rainfall is located in the southwest near the TC center. The mean TC rainfall in the east of GD(EGD) leans to the eastern side of TC. The TC rainfall distributions in June, July, August, and September all lean to the southwest quadrant and the maximum rainfall is located in the southwest near the TC center. The same features are found in the mean rainfall of TD, TS, STS, TY,and STY. The maximum rainfall is mainly in the downshear of vertical wind shear. Vertical wind shear is probably the dominate factor that determines asymmetric rainfall distribution of TCs in GD. Storm motion has little connection with TC rainfall asymmetry in GD.     

台风Haitang和Matsa引发浙江暴雨强度和分布的对比分析

以2005年两个严重影响浙江台风Haitang和Matsa为研究对象,首先对两次台风暴雨过程在强度和落区上的相似和差异进行了比较,指出两者都给浙江带来了大范围的暴雨天气,并且最大降水中心都出现在台风登陆点北侧;虽然Haitang先在福建登陆、且登陆时强度不及Matsa,但其雨量明显大于后者。然后从台风的强度、移速、结构、环境场、地形等多个方面对两次台风暴雨过程进行了比较分析,结果表明:台风暴雨的非对称分布与台风结构、水汽输送、地形密切相关。高温高湿在台风的北侧表现得比南侧更为强烈,水汽含量也更为丰富。Haitang降水量大于Matsa则是多种因素共同作用的结果,弱冷空气的侵入、动力场上较Matsa减弱得更为缓慢,均有利于降水的增加和持续;Haitang影响前控制浙江气团的“冷场”性质和影响期间移速缓慢也对暴雨增幅起到了促进作用。