Extracting Typhoon Environmental Wind Field Using Doppler Radar Radial Velocity

It is always a challeging subject to forecast landing path of tropical cyclone and related wind distribution in operational weather forecast. Therefore, Doppler radar radial velocity is applied to the short term weather forecast as a new attempt. Rankine model is used to simulate the characteristics of the Doppler velocity (CDV) in the pure cyclone, and the impacts of both different environmental wind directions and convergent wind towards typhoon center are considered. Based on the simulation, the method of extracting typhoon environmental wind information is brought up, and then the extracted environmental wind is applied to judge the typhoon path. Comparison of radar observation and numerical simulation typhoon wind mainly focuses on three aspects: Deviation of typhoon center from zero velocity line, curvature difference of zero velocity line and difference between orders of positive and negative extreme velocities. A number of typhoon cases have been analyzed, especially during the key time of the turning of typhoon path, and it indicates that this method is good enough to judge the turning of the environmental wind of the CDV. At the same time, by comparing the Doppler radar velocity image and the simulation of different environmental wind directions, the direction and its change of the environmental air flow can be real time monitored (only 6 minutes for the interval of radar observation), so that the typhoon path can be well monitored, while this can hardly be achieved with large scale meteorological data. In particular, the turning of environmental wind occurs before the turning of typhoon path. For instance, the movement path of typhoon Aere changes from westward to southwestward and the simulation shows that there is an obvious change of the environmental wind more than 3 hours ago. For typhoon Nock Ten, when its movement path changes from northward to northwestward, the environmental wind direction changes from southward to southward by eastward at least 1 hour in advance. These results are important for the short term forecast of typhoon path both theoretically and practically. The stronger a typhoon is, the more obvious cyclonical circulation is, and thus the method performs better in estimating typhoon path, otherwise typhoon environmental wind information may affect the accuracy.