Improving the Extreme Rainfall Forecast of Typhoon Morakot(2009) by Assimilating Radar Data from Taiwan Island and China's mainland

This study examined the impact of an improved initial field through assimilating ground-based radar data from China's mainland and Taiwan Island to simulate the long-lasting and extreme rainfall caused by Morakot(2009). The vortex location and the subsequent track analyzed through the radial velocity data assimilation(VDA) are generally consistent with the best track. The initial humidity within the radar detecting region and Morakot's northward translation speed can be significantly improved by the radar reflectivity data assimilation(ZDA). As a result, the heavy rainfall on both sides of Taiwan Strait can be reproduced with the joint application of VDA and ZDA. Based on sensitivity experiments, it was found that, without ZDA, the simulated storm underwent an unrealistic inward contraction after 12-h integration, due to underestimation of humidity in the global reanalysis, leading to underestimation of rainfall amount and coverage. Without the vortex relocation via VDA, the moister(drier) initial field with(without) ZDA will produce a more southward(northward) track, so that the rainfall location on both sides of Taiwan Strait will be affected. It was further found that the improvement in the humidity field of Morakot is mainly due to assimilation of high-value reflectivity(strong convection) observed by the radars in Taiwan Island, especially at Kenting station. By analysis of parcel trajectories and calculation of water vapor flux divergence, it was also found that the improved typhoon circulation through assimilating radar data can draw more water vapor from the environment during the subsequent simulation, eventually contributing to the extreme rainfall on both sides of Taiwan Strait.     

Formation and Development of a Mountain-induced Secondary Center inside Typhoon Morakot(2009)

The structural evolution of Typhoon Morakot(2009) during its passage across Taiwan was investigated with the WRF model. When Morakot approached eastern Taiwan, the low-level center was gradually filled by the Central Mountain Range(CMR), while the outer wind had flowed around the northern tip of the CMR and met the southwesterly monsoon to result in a strong confluent flow over the southern Taiwan Strait. When the confluent flow was blocked by the southern CMR, a secondary center(SC) without a warm core formed over southwestern Taiwan. During the northward movement of the SC along the west slope of the CMR, the warm air produced within the wake flow over the northwestern CMR was continuously advected into the SC, contributing to the generation of a warm core inside the SC. Consequently, a well-defined SC with a warm core, closed circulation and almost symmetric structure was produced over central western Taiwan, and then it coupled with Morakot's mid-level center after crossing the CMR to reestablish a new and vertically stacked typhoon. Therefore, the SC inside Morakot was initially generated by a dynamic interaction among the TC's cyclonic wind, southwesterly wind and orographic effects of the CMR, while the thermodynamic process associated with the downslope adiabatic warming effect documented by previous studies supported its development to be a well-defined SC. In summary, the evolution of the SC in this study is not in contradiction with previous studies, but just a complement, especially in the initial formation stage.     

2009年“莫拉克”台风登陆过程阵风特征分析

利用上海台风研究所移动观测车获取的“莫拉克”台风登陆过程中超声风、温等观测资料对地面阵风特性进行了诊断分析.结果表明,在风速时间序列中叠加有周期为3-7 min的阵风扰动,显现出明显的相干结构,即沿顺风方向阵风风速峰期有下沉运动,谷期有上升运动;阵风扰动的各向异性特征明显,沿顺风方向的阵风扰动能量最大,其次是沿侧向和垂直方向的扰动能量;沿顺风方向的阵风垂直动量通量向下传播,而沿侧风方向阵风扰动动量垂直通量总体贡献接近于0.阵风扰动沿顺风方向的积分空间尺度和时间尺度最大,沿侧风方向和垂直方向其次,均明显大于湍流的积分空间和时间尺度.此外,阵风扰动的其他特征还包括:感热垂直通量极小;当平均风速较大时阵风风向变化幅度较小,而风速较小时阵风风向变化幅度则较大;动力学分析表明,阵风扰动主要表现出重力内波的一些特性.     

登陆台风莫拉克(2009)的涡度拟能收支分析

利用涡度拟能、散度平方项(简记为散度能量)和总变形平方项(简记为变形能量)来表征涡度、散度和变形的强度,并推导这3个物理量的倾向方程。这些方程中既包含代表这些物理量相互转化的符号相反项,也含有代表共同强迫的符号相同项,其中,涡度拟能局地变化主要由涡度拟能与变形能量的共同强迫项、涡度拟能与变形能量的转化项以及涡度拟能与散度能量的转化项组成。针对2009年第8号台风莫拉克,利用美国全球预报系统分析场资料计算这些方程中的强迫项,分析涡度拟能及其收支特征,以此来探讨影响台风莫拉克涡度拟能局地变化的主要物理因素。结果表明,在莫拉克发展和衰减阶段,涡度拟能与变形能量的共同强迫项(涡-形相同项)和涡度拟能与散度能量的转化项(涡-散转化项)是影响涡度拟能局地变化的重要强迫项。涡度拟能与变形能量转化项(涡-形转化项)的贡献相对来说弱一些。影响位涡拟能局地变化的重要物理因素是涡-形相同项和涡-形转化项中的平流输送项和散度耦合项,以及涡-散转化项中散度耦合项。变形能量一方面通过涡-形转化项向涡度拟能转化,直接促进涡度拟能增长;另一方面通过散-形转化项向散度能量转化,再通过涡-散转化项散度能量向涡度拟能转化,间接促进涡度拟能增长。     

利用位涡趋势法诊断台风“莫拉克”(2009)的移动

位涡趋势(PVT)法可以计算非绝热加热等物理过程对台风移动的直接影响(非引导效应)。在成功模拟2009年第8号台风“莫拉克”移动路径和近地面最大风速的基础上,进一步考虑了台风不对称环流发展的影响,对原有的位涡趋势诊断法进行了改进,并首次将该方法应用于高分辨率数值模式模拟结果中,诊断不同物理过程对“莫拉克”移动的影响。分析表明,虽然非绝热加热等物理过程主要作用于维持台风“莫拉克”的垂直结构,但非绝热加热等物理过程引起的非引导效应也对“莫拉克”移动的短时振荡有重要作用。     

台风“莫拉克”(0908)过台湾岛缓慢西行成因分析

0908号台风"莫拉克"（Morakot）在台湾岛引发了24 h超过1000 mm的强烈降水,与其缓慢西行过岛密切相关。本文利用NCEP-GFS实时分析场资料（0.5°×0.5°）、中央气象局台风资料库关于我国台湾地面气压场资料和中尺度数值模式WRF模拟结果,分析了台风Morakot在台湾岛附近缓慢西行成因。结果表明:（1）西太平洋副热带高压减弱东退、与台风"天鹅"（Goni,0907）的"藤原效应"是Morakot在台湾岛附近移速减慢的环境因素,而台风结构变化也是其缓慢过岛西行的一个主要原因;（2）台风过岛过程中,由于地形作用,Morakot环流内诱生低压活跃,先后或同时出现在岛屿东西两侧,使台风环流出现非对称松散结构;（3）在台湾岛西侧诱生低压中心取代东侧原台风中心形成不连续路径过程中,台风经历了低层环流从分裂到重组,正垂直涡度柱从垂直到倾斜再恢复垂直的变化过程,这是Morakot过台湾岛缓慢西行的一个重要原因;（4）Morakot西行登岛过程中环境引导气流主要是偏南气流,而包含诱生低压的扰动引导气流则为偏东气流。扰动引导气流虽是小量,但其纬向分量占环境引导气流纬向分量的比率从7%增至26%,较好地指示了Morakot的西行趋势,也说明地形诱生低压导致的台风结构变化是其过岛西行和缓慢移动的一个重要原因。     

Effects of surface flux parameterization on the numerically simulated intensity and structure of Typhoon Morakot (2009)

The effects of surface flux parameterizations on tropical cyclone(TC) intensity and structure are investigated using the Advanced Research Weather Research and Forecasting(WRF-ARW) modeling system with high-resolution simulations of Typhoon Morakot(2009).Numerical experiments are designed to simulate Typhoon Morakot(2009) with different formulations of surface exchange coefficients for enthalpy(C_K) and momentum(C_D) transfers,including those from recent observational studies based on in situ aircraft data collected in Atlantic hurricanes.The results show that the simulated intensity and structure are sensitive to C_K and C_D,but the simulated track is not.Consistent with previous studies,the simulated storm intensity is found to be more sensitive to the ratio of C_K/C_D than to C_K or C_D alone.The pressure-wind relationship is also found to be influenced by the exchange coefficients,consistent with recent numerical studies.This paper emphasizes the importance of C_D and C_K on TC structure simulations.The results suggest that C_D and C_K have a large impact on surface wind and flux distributions,boundary layer heights,the warm core,and precipitation.Compared to available observations,the experiment with observed C_D and C_K generally simulated better intensity and structure than the other experiments,especially over the ocean.The reasons for the structural differences among the experiments with different C_D and C_K setups are discussed in the context of TC dynamics and thermodynamics.     

A study on the structure and precipitation of Morakot (2009) induced by the Central Mountain Range of Taiwan

The three-dimensional structures and ingredients leading to extremely heavy precipitation associated with the passage of Typhoon Morakot (2009) over the Central Mountain Range (CMR) of Taiwan are investigated. Using a numerical model, the track, track deflection, characteristic rainbands, and precipitation patterns and maxima are successfully reproduced after verification against observational data. The high-level outward flow of the secondary circulation around the eyewall is not very clear even during Morakot’s strongest stage. In the control case, the eyewall collapses within 5 h after landfall that is closely associated with limited precipitation along the track after landfall. During the early stage of landfall, the deep convection on the windward (west) side of the CMR helps strengthening the secondary circulation. A quantitative comparison of total precipitable water, translation speed, and orographic lifting among 12 typhoons in recent years causing large accumulated rainfall in Taiwan shows that the abundant water vapor around Taiwan outweighs translation speed and orographic lifting in resulting in the record-breaking precipitation. It is found that the major processes leading to strong upward motion in the extremely heavy precipitation during 0000 UTC 8 August–0000 UTC 9 August are initiated by orographic lifting by CMR.     

Impact of assimilating FY-3C MWHS2 data in the RMAPS-ST forecast system on its rainfall forecasts

中层水汽初值的误差是数值预报,尤其是短期预报不确定性的重要来源.星载微波湿度计资料能很好地弥补常规资料的不足,同化微波湿度计资料对改善对流层中高层的水汽初始场精度有着重要意义.本研究基于新一代快速更新多尺度资料分析和预报系统短期数值预报系统(RMAPS-ST),选取2019年6月4-6日中国华中地区降水过程开展了 FY...     

0908号台风“莫拉克”诱发浙闽暴雨的螺旋度分析

利用气象常规资料和NCEP同化资料,对0908号台风“莫拉克”登陆浙闽诱发的暴雨过程进行螺旋度诊断分析。结果表明：925hPa螺旋度与浙闽暴雨中心强降水演变一致;中低层水平螺旋度由负转正时,浙闽降水急剧增加,减少时则降水趋于结束;垂直螺旋度正极值区与强降水中心对应,预报台风强降水落区,垂直螺旋度比水平螺旋度更具有优势;垂直方向上,用中低层垂直螺旋度“由极大值减小”来判别“莫托克”暴雨结束时间,比水平螺旋度有更多有效预报时效;若能结合水平和垂直螺旋度预报强降水,将有助于提高台风强降水预报准确率。     

台风\

利用地面观测资料、多普勒雷达资料、中央气象台热带气旋资料、数值模式预报资料以及全球再分析资料等,对北上热带气旋（TC）“巴威”引发的辽宁不同阶段降水特征和强降水形成机制进行分析。结果表明：（1）北上热带气旋“巴威”影响下,辽宁强降水过程分为TC远距离影响、外围螺旋雨带影响和TC本体影响三个阶段;前两个阶段辽宁均产生了对流性强降水,第二阶段对流强度弱于第一阶段,而第三阶段辽宁产生稳定性降水,降水强度偏弱。（2）TC北上过程中为辽宁降水提供持续的水汽和能量输送,前两个阶段均存在干冷空气作用和锋生强迫动力机制：TC输送的暖湿气流与辽宁境内干冷空气相互作用产生较强锋生,通过锋生强迫作用,第一阶段在对流不稳定下激发垂直对流,而第二阶段在湿对称不稳定下形成倾斜对流。TC本体影响阶段,TC强度快速减弱,辽宁位于TC热动力均减弱明显的偏西侧,同时缺乏适当的冷空气而锋生较弱,导致降水偏弱。（3）TC外围螺旋雨带影响时,强降水与850 hPa强锋生区域基本吻合,对TC降水预报具有一定指示作用。

     

强热带风暴碧利斯(0604)引发的特大暴雨中尺度结构多普勒雷达资料分析

强热带风暴 “碧利斯” (Bilis) 于2006年7月14日12:50在福建省霞浦县北壁镇再次登陆, 与南海季风相互作用, 在福建省引发特大暴雨。作者利用双多普勒雷达三维风场反演技术对厦门和龙岩新一代多普勒雷达时间同步探测资料进行了风场反演, 综合利用雷达回波强度资料, 对造成长泰、 漳州特大暴雨的中尺度对流系统的三维结构及其演变特征进行了详细分析。结果表明: (1) 此次特大暴雨主要是由中低层西南[CD*2]东北走向的β中尺度辐合线引发的, 辐合线对于水汽输送以及暴雨的形成、 触发、 维持具有重要作用, 辐合线在暴雨的整个生命史过程中经历了由弱变强、 由强变弱的演变过程, 变化过程与降水的强弱演变过程基本同步。 (2) 由于丰富的水汽供应和中低层辐合线的动力抬升作用, 西南[CD*2]东北走向的β中尺度回波带的西南不断有新的γ中尺度对流单体生成, 在沿着辐合线向东北移动过程中进一步发展、 合并形成β中尺度对流线, 造成持续的强降水。最后, 还给出了此次特大暴雨的三维云系结构模型。     

Impact of Coastal Radar Observability on the Forecast of the Track and Rainfall of Typhoon Morakot(2009)Using WRF-based Ensemble Kalman Filter Data Assimilation

This study explored the impact of coastal radar observability on the forecast of the track and rainfall of Typhoon Morakot(2009)using a WRF-based ensemble Kalman filter(EnKF)data assimilation(DA)system.The results showed that the performance of radar EnKF DA was quite sensitive to the number of radars being assimilated and the DA timing relative to the landfall of the tropical cyclone(TC).It was found that assimilating radial velocity(Vr)data from all the four operational radars during the 6 h immediately before TC landfall was quite important for the track and rainfall forecasts after the TC made landfall.The TC track forecast error could be decreased by about 43% and the 24-h rainfall forecast skill could be almost tripled.Assimilating Vr data from a single radar outperformed the experiment without DA, though with less improvement compared to the multiple-radar DA experiment.Different forecast performances were obtained by assimilating different radars, which was closely related to the first-time wind analysis increment, the location of moisture transport, the quasi-stationary rainband, and the local convergence line.However, only assimilating Vr data when the TC was farther away from making landfall might worsen TC track and rainfall forecasts.Besides, this work also demonstrated that Vr data from multiple radars, instead of a single radar, should be used for verification to obtain a more reliable assessment of the EnKF performance.     

地面和卫星降水产品对台风莫拉克 降水监测能力的对比分析

本文以2009年台风莫拉克为例,利用分布密集的自动站观测降水资料比较分析了国家气象信息中心研发的地面降水分析产品CPAP和卫星反演降水产品CMORPH、FY2C等对强天气过程降水的监测能力.结果表明:CPAP准确地把握了这次台风登陆过程中降水的空间分布和强度,但由于海洋区域无站点分布,CPAP无法展现一个完整的台风降水空间结构;CMORPH能够合理地描述完整的台风降水空间结构及其演变特征,但与地面观测相比,其降水中心的位置差异较大,且量值普遍偏低;FY2C的降水在陆地与海洋出现了严重的结构不连续的现象,对台风降水空间结构的描述不如CMORPH合理.由此可见,选择对降水空间结构描述完整且合理的卫星降水产品与精度较高的地面观测降水融合,结合两者的优势,是发展高质量降水分析产品的一种有效途径.     

Computation of extreme ice drift velocities on the northeastern shelf of Sakhalin Island from radar measurements

The technology ofthe method of joint probabilities ofthe tidal and residual (de-tided) components developed to estimate the possible sea level fluctuations [1, 10] and modified to calculate rare extreme total sea current velocities [8], is applied to compute ice drift velocity on the northeastern shelf of Sakhalin Island. The initial data are the hourly series of the drift velocity vector obtained from radar observations at Odoptu coastal station (1986-1996, during the whole ice season) and at the Molikpaq drilling platform (in May in 1999, 2000, 2003, 2005, and 2006). The distribution of the tidal component was determined by the prediction of the corresponding series for 19 years that is possible due to the stability of characteristics of the tidal drift. The distribution of the residual component was estimated by combining all de-tided series for the entire time period. The obtained estimates of total ice drift velocities of rare occurrence are in good agreement with those presented in [8] and can be used for designing facilities for the extraction and transportation of hydrocarbons on the northeastern shelf of Sakhalin Island.     

台湾地形对台风Meranti(1010)经过海峡地区时迅速增强的影响研究

台风在趋近大陆过程中强度一般衰减, 但Meranti(1010)北上进入台湾海峡过程中却迅速加强, 且在登陆福建时达到最强。采用中国气象局台风资料、NCEP GFS 0.5°×0.5°再分析资料及台湾雷达资料, 结合中尺度数值模式WRF(The Weather Research and Forecasting Model)开展台湾地形敏感性试验, 研究Meranti进入台湾海峡过程中的结构变化及迅速加强机理。结果表明:台湾地形是Meranti迅速加强的一个重要影响因子。Meranti北上过程中, 一方面通过台湾岛地形分流作用及其背风坡效应在台湾海峡内诱生中尺度涡旋, 形成正负相间的涡度分布, 激发出与台风相关的扰动波列。地形强迫抬升及扰动波列可加强垂直运动和积云对流, 有利于台风对流发展。另一方面, 台湾地形还通过改变环境气流使台风高空辐散场加强, 环境风垂直切变减小, 形成有利于台风发展的环流背景。比较不同高度台湾地形试验中台风动能收支发现, 台湾地形激发的扰动波列和积云对流增强了次网格尺度系统与台风间能量的交换, 成为Meranti登陆前迅速加强的主要动能源。     

The effects of orography in indochina on wind, cloud, and rainfall patterns during Typhoon Ketsana (2009)

The objective of this study is to i) investigate the effects of orography on the rainfall, wind, and cloud systems of the Typhoon Ketsana (2009) in Indochina, ii) determine rainfall distribution patterns and which parts of Indochina were most affected during Typhoon Ketsana, iii) identify trends in the cloud and rainfall distribution patterns and wind flow patterns in the synoptic scale on orographic effects during Typhoon Ketsana. Remote sensing techniques have been used to study the impacts of TCs. Using data from the remote sensing data such as Fengyun 2D (FY-2D) satellite, Global Digital Elevation Model (GDEM) from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite, wind information from the Navy Operational Global Atmospheric Prediction System (NOGAPS), and radiosonde data were applied in this study to determine the relationship of the typhoon with the orographic effect. This study provides examples of how the orographic effect is important to weather forecasters, as high mountain ranges were able to influence the distribution of the cloud, rainfall and even wind flow patterns during the typhoon season. This remote sensing technique allows tropical cyclones to be forecasted and their impacts to be defined, and it allows disaster zones to be determined.     

中国大陆地区小时极端降水阈值的计算与分析

利用广义极值分布和百分位两种阈值确定方法,对我国465个气象站点不同极端程度的小时降水强度阈值进行了分析。广义极值分布结果表明,重现期为2、5、10、50a的降水强度阈值具有一致的空间分布特征：华南沿海阈值最高,长江中下游地区北部、四川盆地西部、华北地区东部次之,云南中西部、华北西部和东北西部阈值相对更低,最小值出现在我国西部地区。百分位法得到的阈值空间分布呈现出与广义极值分布结果较为一致的东南大、西北小的整体特征。考察465站中位数发现,第99．9百分位的强度阈值与二年一遇降水的阈值接近。具体分析各站第99．9百分位降水阈值对应的重现期发现,长江流域及其以南地区重现期大多低于2a;35°N一带重现期长于4a;我国北方和西北部分地区重现期长于8a。     

ECMWF降水极端天气指数在浙江的应用评估

利用2016年6月—2017年5月ECMWF降水极端天气指数(EFI)预报资料,分析了降水EFI与不同量级强降水、降水气候百分位在浙江的关系。结果表明:总体而言,浙江省降水EFI与实况降水存在明显的正相关关系。随着EFI阈值的增加,暴雨发生频次先增加后减少,而且暴雨发生的概率随着EFI阈值的增加而增大。综合考虑TS、BS评分,EFI阈值随着预报时效的延长而减小;随着降水量级的增加而增大。降水EFI值与降水气候百分位存在明显的正相关关系,当EFI值较高时,预示着较大的几率出现极端降水,此时可参考当地相对应的气候百分位的降水量来估计降水。