超强台风“桑美”(2006)近海急剧增强特征及机理分析

The Features and Mechanism Analysis on Rapid Intensity Change of Super Typhoon Saomai(2006) over the Offshore of China

应用NCEP/NCAR再分析资料,对超强台风“桑美”（2006）在中国近海急剧增强的特征及机理进行分析。结果表明,“桑美”台风强度变化与南亚高压、副热带高压的强度变化呈反相变化关系;介于-4～4 m/s弱的200 hPa和850 hPa高低层环境风垂直切变是“桑美”台风急剧增强的必要条件;台风中心附近对流层高层辐散的增强、中心附近正涡度的增大和正涡度柱向对流层中上层伸展导致“桑美”台风急剧增强,对流层中层辐散和涡度的增大与台风的减弱密切相关;“桑美”台风急剧增强过程中,对流层高层动能的下传是对流层低层动能补充的重要途径之一;“桑美”台风近海急剧增强具有前兆性,急剧增强对风垂直切变、850 hPa角动量和动能区域平均值变化的响应时间大约为18 h,这些可为提前预测我国近海台风的强度急剧变化提供参考。

Most tropical cyclones weaken or their intensity remains unchanged over the offshore of China,while some tropical cyclones intensify rapidly.The extreme weather events would arise from those rapid intensification tropical cyclones,even much more serious damage would happen.Super typhoon Saomai（2006）,which struck China in August 2006 as the most intense typhoon that China had experienced in 50 years,is a typical rapid intensification tropical cyclone over the offshore of China,whose intensity is even stronger than hurricane Katrina（2005） which is the costliest and one of the deadliest hurricanes in the history of the United States.Saomai is estimated to have been responsible for nearly RMB 10 billion yuan in damage.Saomai can be traced back to a tropical storm on 5 August.The system strengthened into typhoon 2 days later and rapidly intensified into super typhoon over the offshore of China on 9 August as the central pressure fell to 920 hPa.It then maintained its peak intensity for about 18 h before beginning to weaken.Saomai tracked into Zhejiang Province on the afternoon of 10 August,making landfall in Cangnan county and then rapidly weakened.Taking super typhoon Saomai as an example,detailed analyses are made on the dynamic and kinetic features and mechanisms of the typhoon for different developing stages in terms of quantitative and temporal evolution,including structures of vorticity,divergence,vertical motion,angular momentum,kinetic energy,etc.by the use of NCEP-NCAR reanalysis data with horizontal resolution of 1°×1° and vertical resolution of 11 layers.The study shows that the intensity change of Saomai is in inverse proportion to that of the South Asia high and the subtropical high,the South Asia high and the subtropical high decay in the typhoon rapid intensification stage.The intensity changes of Saomai in environmental vertical wind shear are investigated in this study using the NCEP-NCAR reanalysis data.The shear values averaged over the 6°,10°,20° and 40° latitude-longitude squares,centered at the location of typhoon center,are calculated to analyse the impact of wind shear on the typhoon intensity change.Typically a critical vertical shear,850-200 hPa shear with magnitude of 1-4 m/s,favors typhoon rapid intensification over the offshore of China.There appears to be a time lag between the change of vertical wind shear and the typhoon intensity rapid change,typically about 18 h.There is a favorable environment for typhoon intensification,such as strong divergence at the upper level and remarkable vorticity near the center of Saomai.And the addition of divergence or vorticity at the middle layer is associated with typhoon weakening.Saomai is also investigated by means of energy and angular momentum analysis.The losses of kinetic energy are compensated by the downward transport of the kinetic energy from the upper troposphere.Typhoon intensification is mainly caused by the enhancement of the kinetic energy and angular momentum in the lower troposphere.And there also appears to be an 18 h time lag between the change of the kinetic energy and angular momentum and the typhoon intensity rapid change.Some interesting results in this paper are useful for the operational forecasting and some of them are valuable for the theoretical study.The authors hope to extend the study to numerical simulations of intensity change based on idealized and real data initial conditions.