Research Of Retrieving Typhoon Wind Field Based On Sentinel-1 SAR Images

Typhoon is a strong process of sea air interaction,which is accompanied by the process of wind and rain.It will cause serious meteorological disasters and loss of lives and property.Therefore,typhoon monitoring and forecasting is particularly important.The traditional monitoring methods of sea surface wind field include buoy,ocean station and drilling platform.These observation methods are difficult to realize large scale synchronous observation while the spaceborne microwave loads,such as scatterometer,radiometer,altimeter and synthetic aperture radar(SAR),are the important methods of typhoon monitoring with the advantages of all-time,all-weather and wide coverage.However,the spatial resolution of the scatterometer and the radiometer is low,and it is difficult for them to obtain the wind field within the range of tens of kilometers near the coast.The altimeter can only observe the ground point and lack the wind direction information.Compared with the above instruments,the synthetic aperture radar has high spatial resolution,and it can obtain the fine structure characteristics of typhoon.The Sentinel-1 satellite series consists of two C band SAR satellites,sentinel-1A and sentinel-1B.The interferometric wide imaging mode(IW)of the satellite series has the width of 250km and the spatial resolution of 20m.In this paper,the inversion of typhoon wind field is studied by using the SAR data of sentinel-1 satellite.The wind direction inversion LG method and FFT method respectively.The inversion of wind speed is carried out by CMOD4 and CMOD5 which are the empirical functions.The inversion results are combined with WindSat,ASCAT and HY-2A data to analyze and compare the inversion results,and verify the accuracy of SAR for typhoon wind field inversion.After comparing the results of wind field inversion with the matched ASCATdata of typhoon Koppu,it is found that the RMSE of the wind direction inversion results by the FFT method is 31.26 degrees RMSE of the wind direction inversion results from the LG distribution is 23.83 degrees.The FFT method is suitable for the open ocean area according to the previous research results.Most of the SAR images used in this paper are located in the near shore area.Therefore,the wind direction inversion method should be LG method.The wind speed results of typhoon Koppu are compared with the ASCAT data.The maximum wind speed in the ASCAT data is 36.2m/s,the maximum wind speed of the CMOD4 function inversion result is 28.07m/s,and the maximum wind speed of the CMOD5 function inversion result is 44.87m/s.The RMSE of CMOD4 is 5.86m/s while the RMSE of CMOD5 is 4.09m/s.The inversion results of the CMOD5 function of typhoon Koppu are better than CMOD4 especially in the high wind speed region.The wind field was also retrieved from typhoon Phanfone in 2014,typhoon Megi in 2016 and typhoon Saola in 2017.The inversion of these three typhoons is based on the LG method and the CMOD5 function.The inversion results of typhoon Phanfone are compared with those of WindSat,and the RMSE of wind speed is 5.05m/s.The inversion results of typhoon Saola are compared with the HY-2A data,and the RMSE of wind speed is 5.45m/s.The inversion results of these typhoon show that the wind field inversion of SAR data based on LG method and CMOD5 function can obtain the typhoon wind field information with fine accuracy.The inversion results can also describe the detailed structure of typhoon with the high resolution of SAR.