Effects of Vertical Wind Shear on Intensity and Rainfall Asymmetries of Strong Tropical Storm Bilis (2006)

The effects of environmental vertical wind shear （VWS） on the intensity and rainfall asymmetries in Tropical Storm （TS） Bilis （2006） have been analyzed based on TRMM/TMI-estimated surface rainfall data, QuikSCAT wind fields, 850- and 200-hPa winds of the NCEP-NCAR reanalysis, precipitation data at 5-min intervals from automatic weather stations over China's Mainland, and the best track data of TS Bilis （2006）. The results show that the simultaneous and 6-hour-lagged correlation coefficients between VWS and storm intensity （the minimum central sea level pressure） are 0.59145 and 0.57438 （P 〈0.01）, respectively. The averaged VWS was found to be about 11 m s-1 and thus suppressed the intensification of Bilis （2006）. Distribution of precipitation in Bilis （2006） was highly asymmetric. The azimuthally-averaged rainfall rate in the partial eyewall, however, was smaller than that in a major outer rainband. As the storm intensified, the major rainband showed an unusual outward propagation. The VWS had a great impact on the asymmetric distribution of precipitation. Consistent with previous modeling studies, heavy rainfall generally occurred downshear to downshear-left of the VWS vector both near and outside the eyewall, showing a strong wavenumber-one asymmetry, which was amplified as the VWS increased.

Assessment of the Impacts of Tropical Cyclone Fantala to Tanzania Coastal Line: Case Study of Zanzibar

The study investigated the impacts of tropical cyclone (TC) Fantala (11th to 27th April, 2016) to the coastal areas of Tanzania, Zanzibar in particular. Daily reanalysis data consisting of wind speed, sea level pressure (SLP), sea surface temperatures (SSTs) anomaly, and relative humidity from the National Centres for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) were used to analyze the variation in strength of Fantala as it was approaching the Tanzania coastal line. In addition observed rainfall from Tanzania Meteorological Authority (TMA) at Zanzibar office, Global Forecasting System (GFS) rainfall estimates and satellite images were used to visualize the impacts of tropical cyclone Fantala to Zanzibar. The results revealed that, TC Fantala was associated with deepening/decreasing in SLP (from 1012 - 1010 mb) around the north-western Madagascar and coastal Tanzania, whereas the mean SSTs was greater than 28°C and an SSTs anomaly ranged from 0 to 2.3°C. The vertical wind shear which ridged at Mozambican Channel and over north-eastern Madagascar was high enough (12 - 15 ms-1) to support the intensifying of Fantala. The thermodynamic and dynamic conditions of Fantala influenced heavy rainfall of greater than 170 mm over most stations in Zanzibar. Moreover, Fantala disrupted the temporal variability of 2016 March to May (MAM) seasonal rainfall. Besides, more than 420 people were homeless, at least 3330 houses were destroyed, and about 2 people died. As for mainland Tanzania Fantala resulted in a death of 12 people in Kilimanjaro and Arusha, more than 315 houses were washed away by flooding leading to 13,933 people being homeless. Conclusively the study calls for an extensive research work based on examining and forecasting the TCs rainfall impacts and their contribution during the two rainfall seasons of OND and MAM in Tanzania.

热带风暴Bilis(2006)登陆期间一波非对称降水分布成因的探讨

利用中尺度数值模式WRF-ARW对Bilis(2006)进行数值模拟,并进行地形和下垫面因素2个敏感性试验,诊断分析Bilis(2006)登陆期间一波非对称降水结构形成的动力因子。综合分析3个试验,结果表明环境垂直风切变对Bilis(2006)一波非对称降水结构形成的影响显著大于地形、登陆过程下垫面属性的改变和风暴移动速度的影响,冷空气的侵入对降水形成也有重要贡献。垂直切变的作用使顺切变左侧形成强降水,冷空气的作用与垂直切变的作用相互配合可能是顺切变方向和顺切变方向右侧强降水的主要成因。

西北太平洋环境风垂直切变和热带气旋移动对涡旋内对流非对称分布影响的特征分析

热带气旋(TC)预报特别是强度预报是当今大气科学研究和业务预报的重点、难点问题,TC环流内部的对流系统对气旋的结构和强度变化有着十分重要的影响。利用FY-2C/2E黑体亮温(TBB)资料和NCEP分析资料,研究了2005—2012年西北太平洋热带气旋外雨带区的对流非对称分布特征,及其与环境风垂直切变和TC移动的关系。分析发现,整层风垂直切变的方向与TBB一波非对称大值区关于方位角的分布有很好的对应关系。在弱整层风垂直切变条件下(<5 m/s),TC移动引起的非对称摩擦效应会使对流易出现在移动方向的右前象限。在中强整层风垂直切变条件下(>5 m/s),风切变成为影响对流非对称分布的主要因子,TC外螺旋雨带区的对流集中于顺风切方向及其左侧,对流偏离顺切变左侧的程度一方面受到TC内逆时针环流的影响,另一方面与风垂直切变的强度有关:对于发展阶段的TC,当风垂直切变增强时,一波非对称分布更加显著,切变越强,TC强度越大,外雨带区的对流越偏离顺风切左侧;对于消亡阶段的TC,风垂直切变的影响作用并不明显。

强热带风暴“莲花”(0903)非对称降水结构分析

利用雷达回波和NCEP分析资料,本文从水汽条件、环境风垂直切变和风暴移动状态等方面诊断分析了0903号强热带风暴"莲花"非对称降水结构形成的可能机制。结果表明:"莲花"南侧充足的水汽输送为强降水的发生提供了基本的水汽条件,同时水汽通量在水平空间上的非对称分布也在一定程度上导致了降水的非对称分布。环境风垂直切变是导致"莲花"降水结构改变并最终形成一波非对称降水结构的主要动力因子。随着垂直切变的增强,同时配合风暴南侧充足的水汽条件,一波非对称降水结构逐渐形成,在较强垂直切变长时间的作用下,强降水最终集中于顺切变方向左侧。在较强垂直切变的作用下,逆切变一侧的下沉运动抑制了陆地摩擦和地形抬升所形成的对流的发展。相对于较强的垂直切变而言,"莲花"相对稳定的移速和移向条件难以主导强热带风暴降水的空间分布。

2013年相似路径台风“苏力”与“潭美”造成福建暴雨落区差异的成因

利用中国气象局(CMA)最佳台风路径资料、地面加密自动站资料、SRTM30数据与ERA5再分析资料,对2013年相似路径台风“苏力”与“潭美”造成的福建暴雨落区差异的成因进行了分析。结果表明:(1)两次台风暴雨过程中福建强降水落区相对于台风移动路径方向的位置在其登陆前后有所不同,台风登陆前强降水均位于其路径北侧(闽东北),台风登陆后“苏力”强降水转至其路径南侧,而“潭美”强降水则位于路径附近。(2)台风登陆前“潭美”相较于“苏力”低纬存在强水汽输送带、闽东北上空高能高湿、低层辐合和高层辐散形成的抽吸作用,配合垂直正涡度场及地形抬升作用,该区域上升运动强且伸展高度高,导致其路径北侧降水强度更大、范围更大。(3)台风登陆后,“苏力”路径的南侧为强水汽辐合中心、气流汇合区及能量锋区,垂直正涡度柱南倾,上升运动强烈且强对流不稳定;“潭美”移动路径附近为强水汽辐合中心,西北气流与偏西气流汇合,维持暖湿结构,正涡度柱范围位于台风中心附近,该区域配合低层辐合、高层辐散,强垂直上升运动。受地形的引导、阻挡作用,低层气流加速辐合与抬升,有利于强降水的维持和加强。(4)台风强降水落区与环境风垂直切变有较好的对应关系,强降水区往往位于环境风垂直切变矢量下游和左侧;“苏力”和“潭美”登陆前后环境风垂直切变强弱和方向不同,可能是造成福建暴雨落区差异的重要原因之一。

海气热通量和垂直风切变对强热带风暴“温比亚”(2018)影响研究

利用地面加密自动气象站观测的1h降水、NCEP提供的海温、海气界面热通量以及风场等资料,分析了海温、海气界面热量交换以及环境垂直风切变对强热带风暴"温比亚"(2018)强度和降水结构的影响。结果表明:在"温比亚"生命史过程中,垂直风切变对其强度发展起到了一定的抑制作用。但是,垂直风切变不是影响其强度变化的唯一因素,"温比亚"登陆前27~29℃的高海温海域为其强度的增强提供了大量潜热。"温比亚"深入内陆期间,其降水分布具有显著的非对称结构。在环境垂直风切变的作用下,"温比亚"强降水位于顺切变方向及其左侧。