Effects Of Large Precipitation To The Wind Retrieval From Synthetic Aperture Radar

Typhoon,as one of the most powerful weather conditions,always accompanying with a wide range of heavy rainfall,often causes serious property damage and loss of life.Thus,it is a very important research direction to better monitor and forecast typhoon track with its intensity.Synthetic Aperture Radar(SAR)plays an important role in monitoring typhoon and typhoon wind field retrieval because of its high spatial resolution and wide observational range barely influenced by the weather.However,due to the impact of heavy rainfall occurring during the typhoon period,the SAR signal will change,and then influence the precision of wind inversion.This paper mainly explores the common method that can retrieve the SAR wind field,qualitatively and quantitatively evaluates the effect of precipitation on the precision of SAR typhoon wind field retrieval.Geophysical Model Function(GMF)is the common method retrieving the SAR image wind speeds.In this paper,we firstly use reanalysis data of wind direction from National Centers for Environmental Prediction(NCEP)and The European Center for Medium-Range Weather Forecasts(ECMWF)and correct the wind direction in six SAR typhoon images.Then we use CMOD5 to retrieve wind speeds.SAR images we used in this study include RADATSAT SAR and ENVISAT ASAR.Because of precipitation,the inversion precision of wind field from SAR image will reduce.The influence mainly includes three aspects,i.e.rain-induced attenuation in the atmosphere,raindrop volumetric scattering,and rain-induced sea surface damping or perturbation.We evaluated these three aspects qualitatively and found that the signal's attenuation increases with the increases of rainfall and incident angle,raindrop volumetric scattering increases with the increases of rainfall.The damping or perturbation of sea surface caused by raindrops is much more obvious than the other two factors.The interaction between sea surface and raindrop is very complex.In order to effectively evaluate the influence,we use synchronous data to quantitatively investigate three effects and the perturbation or damping of water surface is simulated by rainfall and incident angles.Combined with physical-based rainfall model,the rain effect model of SAR wind field retrieval has been established.To validate the performance of the present rain effect model,we select co-located scatterometer data with SAR image cases.By applying our rain model to scatterometer data and comparing it with SAR measured data to evaluate model performance,the results prove that the present model can effectively consider the rain effect on SAR wind field inversion precision.After using the rain effect model,the error decrease varying from 39%to 89%.In summary,this paper firstly corrected the wind direction of reanalysis data according to the wind stripe,and discussed the common method of SAR wind field retrieval by external wind direction + GMF,and then introduced three impact factors of SAR signal change caused by heavy rainfall,and further qualitatively evaluated the relationship between rainfall influence with rain rate and incident angle.Rain effect model was proposed by a curve fitting method and its capability was proved by several study cases.This work provided a fundamental solution about rain effect to the wind field retrieval of SAR image for the future research.