Study On The Method Of Acquiring The Enviromental Wind Field From The Doppler Radar Velocity Data

The error of tropical cyclone track prediction has been decreased but the prediction of the sudden typhoon track changes is still difficult. This article introduces a method of acquiring the middle and lower enviromental wind field from the Doppler radar radial velocity data. Based on this method, the typhoon track near the coast will be predicted.First, the 3D axisymmetric and nun-axisymmetric Rankine vortex wind field structure is introduced. Second, the factors that have impacts on the horizontal and vertical structure of Rankine vortex are analyzed. Finally, the characters of vertical tangential wind profile between axisymmetric and nun-axisymmetric Rankine vortex are compared. Based on these, the 3D Rankine vortex model that comprises horizontal uniform wind, the enviromental wind field whose direction and speed change is vertical linear and the vortex radial inflow is built. This model simulates an axisymmetric Rankine vortex Doppler radial velocity field that is composed of enviromental wind with different horizontal speed and direction which vary vertically. It is found that the axisymmetric Rankine vortex Doppler zero radial velocity curve is a line, which just goes through the typhoon centre. The zero velocity curve of the nun-axisymmetric Rankine vortex is also a line. The typhoon centre is at the zero velocity line. However, the positive and negative radial velocity zones are no more symmetry to the zero velocity line. By adding the enviromental wind that has the different horizontal wind speeds and directions and their vertical changes, the Doppler radial velocity of axisymmetrical Rankine vortex will change. If the enviromental wind direction is perpendicular to the line that linked between the centre of radar and the centre of vortex, the zero velocity curve will be a line and the centre of vortex will be at the zero velocity line as well. Simply, the positive or negative speed will change. If the enviromental wind direction is not perpendicular to the line that linked between the centre of radar and the centre of vortex, the zero velocity line will bend at different degrees and directions towards the positive or negative velocity zone with the changing of environmental wind speed and direction. At the same time, the zero velocity line deviates from the centre of vortex at different degrees. The absolute values of the extreme values of positive velocity and negative velocity are no more equal and they are not symmetry to the zero velocity line. The asymmetrical structure of the axisymmetric Rankine vortex Doppler radial velocity caused by the added enviromental wind is different from the asymmetrical structure of the nun-axisymmetric Rankine vortex. Based on this, the article introduces a method to acquire the real enviromental wind field by comparing the simulated Doppler radial velocity image of axisymmetric Rankine vortex and observed Doppler radial velocity image of the typhoon. The simulated image is produced by adding different horizontal wind speeds and directions, vertical changes of wind field. This method can identify the simulation radar image that is the most similar to the observed Doppler radar radial velocity image.Using this method, the enviromental wind field of typhoon'Aere'was obtained from the image of radial velocity of Radars located in Changle and Xiamen. It is found that the enviromental wind field caused the asymmetrical Doppler velocity of 'Aere'. Before'Aere'changed the direction near the coast, the enviromental wind field of the middle troposphere has changed direction from east to north-east. The change of enviromental wind field was followed by the change direction of'Aere'. Using high resolution temporal and spatial Doppler radar velocity data, this method can provide the enviromental wind field to monitor the changes of the enviromental wind real-time. Furthermore, the changes of typhoon track can be predicted. These are vital for forecasting the typhoon track.