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 southw estward 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 unsym metrical 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 unsym metrical 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.