东风和西风切变环境下西北太平洋热带气旋快速增强特征的对比

Characteristic comparison between the rapid intensification of tropical cyclones in easterly and westerly wind shear over the Northwest Pacific

摘要:

利用1980-2009年美国联合台风警报中心(Joint Typhoon Warning Center,JTWC)整编的热带气旋(tropical cyclone,TC)最佳路径资料,定义西北太平洋TC 24 h强度变化达到总体样本96%累积概率的变化值,即35 kn作为TC快速增强的阈值。根据NCEP/NCAR资料将200~850 hPa之间 TC所处的环境纬向风切变(wind shear,WS)划分为东风切变(east wind shear,EWS)和西风切变(west wind shear,WWS)。对比了EWS和WWS环境下快速增强热带气旋(rapid intensification tropical cyclones,RITC)的统计和大尺度环境合成场特征,结果表明,近70%的TC快速增强发生在东风切变环境下。TC快速增强概率最高的月份在9月,初始强度区间为[65,75) kn。大的EWS下,850 hPa有来自南海地区的西南气流为RITC输送充沛水汽,500 hPa、200 hPa高压势力强但脊线位置偏北,RITC流出层温度低于-79 ℃,垂直结构上底层的辐合与高层的辐散也相对较强。大WWS下,850 hPa的水汽主要为来自西北太平洋的东南气流,500 hPa副热带高压断裂为几个分散的中心,200 hPa辐散相对较弱,RITC合成位置位于副热带高压西北侧的西风气流,流出层温度约-76 ℃。

Abstract:

Based on the tropical cyclone best track data from the Joint Typhoon Warning Center (JTWC) and the NCEP/NCAR reanalysis data from 1980 to 2009,we set a threshold for rapid intensification tropical cyclones (RITC) as 35 kn,which was the magnitude of intensity change of tropical cyclones in a period of 24 h with the accumulative probability of 96% over the western North Pacific.According to zonal wind shear between 200 hPa and 850 hPa,the paper divided the vertical wind shear (WS) into the westerly vertical wind shear (WWS) and easterly vertical wind shear (EWS) and compared the characteristics of rapid intensification of tropical cyclones in EWSwith those inWWS.The results show that about 70% of RITCs occur inEWS.There are most RITCs in September,whose initial intensity are between 65 and 70 kn.In largeEWS,abundant vapor comes from southwest at 850 hPa.At 500 hPa and 200 hPa,the Pacific subtropical anticyclone is strong and the ridge line is situated in north area.The temperature in outflow layer is below -79 ℃.In large WWS,the water vapor is from southeast at 850 hPa.At 500 hPa,subtropical high is split into several centers.Compared with that of the former,the divergence field of RITCs at 200 hPa is weaker.And the temperature in outflow layer is about -76 ℃.