地形重力波举力效应对台风路径偏转的影响

举力效应是指方向性切变气流经过地形激发的重力波能够使平均气流发生水平旋转.本文首先根据日本气象厅台风数据集统计了2006—2015年登陆台湾的台风个例,分析台风路径的偏转情况.然后利用二维傅里叶变换对欧洲中间尺度天气预报中心的再分析数据进行滤波,得到台风的环境大气层结和水平风场.最后根据台风环境风场以及台湾地形计算地形重力波举力,分析举力对台风路径偏转的可能贡献.结果表明,近十年共有20个台风登陆台湾,其中2006—2008年有10个.除去5个路径较为复杂的台风,本文对15个台风的地形重力波举力进行分析,发现有14个台风的路径偏转方向与举力方向一致.对于路径偏转最为明显的6个台风(偏转角度大于30°),地形重力波举力对台风路径偏转的贡献可达10%,部分解释了台风经过台湾地形时发生的路径偏转.

Influence of the mountain-wave lifting effect on the deflection of typhoon track

Mountain waves generated in directionally sheared wind are able to induce horizontal rotation of the mean flow, which is designated as the lifting effect. In this work, we first investigate the typhoons land-falling in Taiwan between 2006 and 2015 from the Japan Meteorological Agency (JMA) typhoon dataset and analyze their track deflection. Then the environmental winds and stratification of land-falling typhoons are derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data using the two-dimensional Fourier transform. Lastly, the lift force is computed according to the environmental wind and Taiwan orography. The results show that there are 20 typhoons land-falling in Taiwan in recent 10 years, half of which occurred during 2006-2008. The effect of orographic gravity wave lift (OGWL) is examined for 15 typhoon cases except 5 typhoons with complex tracks. In 14 of these 15 typhoons, the orientation of OGWL is consistent with that of typhoon track deflection. As for the 6 typhoons with most notable track deflection (greater than 30 degrees), OGWL is found to contribute up to 10% of the track deflection, which partly explains the track deflection of typhoons passing over Taiwan.