The Study On Spaceborne Spectrometer For Sea Surface Wind Field Retrieval

Sea surface wind field is one of the most important meteorological parameters.Accurate information of sea surface wind field can not only help us to understand the meteorological system on the earth deeply,but can also be used to forecast weather,especially disaster weather.The spectrometer SWIM(Surface Waves Investigation and Monitoring)mounted on CFOSAT(China-France Oceanography Satellite)is a low-incidence radar with rotating scan mode.It is mainly used to measure the wave direction spectrum on a global scale,and its unique working mode can obtain measuring the ocean backscattering coefficient of a certain point under different incidence and azimuth(0?360°).So it can provide basic datasets for sea surface wind field retrieval.In this paper,we retrieve the sea surface wind field successfully based on the datasets of SWIM.Firstly,by using the backscattering coefficient(?~0)datasets from SWIM and DPR(Dual-frequency Precipitation Radar,Ku and Ka band)without rain and wind field from ECMWF(European Centre for Medium-Range Weather Forecasts),we verify that the upwind/crosswind anisotropy and upwind/downwind asymmetry of ?~0 exist with relative wind direction at low-incidence.We also analyzes the dependence of ?~0 after omnidirectional average,upwind/downwind asymmetry and upwind/crosswind anisotropy on wind speed and incidence angle.Then,by using ?~0 datasets from DPR(Ku and Ka band)and wind field datasets from ECMWF with rain,it is verified that the upwind/crosswind anisotropy and upwind/downwind asymmetry of ?~0 still exist at low-incidence under heavy rain.When the incidence is around 10°,?~0 rarely change with the wind speed.So we use ?~0 datasets around 10°incidence to analyze the impact of rain on ?~0.We also use fourth-order polynomial model to fit the rain rate and ?~0 data.The model fits well.Finally,based on the upwind/crosswind anisotropy and upwind/downwind asymmetry of ?~0 and the empirical model of ?~0 varying with wind speed,we use SWIM's data to retrieve the sea surface wind field successfully.Compared with the wind speeds from ECMWF,the root mean square errors of the wind speeds retrieved by the ?~0 of 0°,2°,and 4°beam are 1.22 m/s,1.23 m/s and 1.49 m/s.When the wind speed is greater than 7m/s,the RMSE between wind direction retrieved by the ?~0 of 10°beam and wind direction from ECMWF is 18.87°.