多普勒雷达探测“艾利”台风风场不对称结构

2004年8月25日, “艾利”台风在靠近我国台湾东北部海域后不同于正常路径, 先后发生两次左折, 先是西北转偏西, 再偏西转西南, 这种路径在历史上极为罕见。通过“艾利”台风的雷达观测事实, 从单多普勒雷达和双多普勒雷达均发现在台风前进方向的右前方, 即西北象限风速极值大于其他象限, 进一步证明了“艾利”台风的风场分布遵循波数为1的非对称性, 从位于台风不同象限的福州长乐和厦门两站雷达资料, 以及常规的地面观测、高空探测资料和NCEP数值预报再分析资料都得到证实。研究表明, “艾利”台风处在大陆高压东南侧, 台风外围存在一支7 m/s左右的环境风场引导气流。因此, 西北象限风速极值大于其他象限的不对称分布可能是由环境风场和台风本身两方面共同造成的, 这种不对称分布有利于维持“艾利”台风向西南方向移动。在我国东南沿海地区, 由于单部多普勒雷达不可能同时探测到台风的西北、东北和西南3个象限的风速极值, 而双多普勒雷达或多多普勒雷达联合探测台风, 则可以同时探测到3个象限的风速极值, 根据风速极值可能存在的不对称分布情况来预测台风的路径转折趋势, 因此重视双多普勒雷达或多多普勒雷达联合探测对于提高监测预报水平具有重要的意义。

The Asymmetric Wind Structure of Typhoon Aere Detected by Doppler Radar

On August 25,2004,after typhoon Aere approaches the ocean regions northeast of Taiwan,China,its track path turns left twice consecutively,i.e.,it turns firstly from northwest to west,and then from west to southwest,which differs from the normal cases and seldom occurs in history.According to the observations of Aere both by single Doppler radar and double Doppler radars,the maximum wind speed always appears in the front right side of Aere forward direction,which means the wind speed in the northwest quadrant is the maximum among all the quadrants.Furthermore,the above results further prove that the wind distributions of Aere follow the one wave number asymmetry.And it is also confirmed by other dataset analyses,i.e.,the radar data from two radars situated in Changle of Fuzhou and Xiamen which are distributed in different quadrants,regular data with both ground-based observation and radiosonde observation,and NCEP reanalysis data.Among these dataset,it is revealed by the detected results with Doppler radars in both Changle of Fuzhou and Xiamen that the fundamental causes for the phenomenon that the variation of extreme radial velocity accompanies with that of the relative positions of radar and typhoon,are the unsymmetrical structure of typhoon wind.And because of that,the wind speed in the northwest quadrant is the maximum compared with the others.When the relative position of radar to that of typhoon center changes,different extreme radial velocity in different quadrants can be detected by radar.This research indicates that when Aere locates to the southeast of the continent high,there exists an environmental guide-airflow around it with velocity of about 7 m/s.Therefore,the unsymmetrical distribution with the largest maximum wind speed occurring in the northwest quadrant may be caused by both the environmental wind and Aere itself,and such asymmetry is in favor of maintaining the southwestward movement of Aere.In the coastal regions of the southeast of China,the maximum wind speed of typhoon in the northwest, northeast and southwest quadrants can not be detected simultaneously by single Doppler radar,it can be done by the double Doppler radars and the multiple Doppler radars.Therefore,when forecasting the turning tendency of typhoon track path based on the possible unsymmetrical distributions of maximum wind speed,the significance for the enhancements of capability in monitoring and forecasting typhoon is great by using the double Doppler radars or multiple Doppler radars.At the same time,methods of calculating unsymmetrical Rankine wind speed with the observations of Doppler radar are discussed.The calculated results show that the variation characteristics of the direction and distance of Aere unsymmetrical wind speed are very clear,and they are identical to the observations of Doppler radars.Additionally,the calculation process is simple,and to some extent,it can work as a reference to the operational forecast.The final aim is that with the detected wind velocity structure of typhoon by Doppler radars,especially the unsymmetrical structure,the possible mechanism for the sudden turn of typhoon track path can be presented,and the beneficial clues can be the further supplied for its forecast.