Evaluation Of Wind And Significant Wave Height Retrieval From Gaofen-3 SAR

Gaofen-3(GF-3)is the first Chinese space-borne satellite to carry the C-band multi-polarization synthetic aperture radar(SAR).Marine applications,i.e.,winds and waves retrieved from GF-3 SAR images,have been operational since January 2017.In the wind retrieval part of this study,we have collected more than 1000 quad-polarization(vertical-vertical(VV);horizontal-horizontal(HH);vertical-horizontal VH);horizontal-vertical(HV))GF-3 SAR images,which were acquired around the China Seas from September 2016 to September 2017.Wind streaks were visible in these images in co-polarization(VV and HH)channel.Geophysical model functions(GMFs),including the CMOD5N together with polarization ratio(PR)model and C-SARMOD,were used to retrieve winds from the collected co-polarization GF-3 SAR images.Wind directions were directly obtained from GF-3 SAR images.Then,the SAR-derived wind speeds were compared with the measurements at a 0.25° grid from the Advanced Scatterometer on board the Metop-A/B and microwave radiometer WindSAT Based on the analysis,empirical corrections are proposed to improve the performance of the two GMFs.Results of this study show that the standard deviation of wind speed is 1.63 m/s with a 0.19 m/s bias and 1.71 m/s with a 0.26 m/s bias for VV-and HH-polarization GF-3 SAR,respectively.Our work not only systematically evaluates wind retrieval by using the two advanced GMFs and PR models but also proposes empirical corrections to improve the accuracy of wind retrievals from GF-3 SAR images around the China Seas and thus enhance the accuracy of near real-time operational S AR-derived wind productsIn the significant wave height(SWH)retrieval part of this study,We evaluated the accuracy of the SAR-derived SWH from 10,514 GF-3 SAR images with visible wave streaks acquired in wave mode by using the existing wave retrieval algorithms,e.g.,the theoretical-based algorithm parameterized first-guess spectrum method(PFSM),the empirical algorithm CS AR_WAVE2 for VV-polarization,and the algorithm for quad-polarization(Q-P).GMF C-SARMOD was employed to retrieve winds for GF-3 SAR at VV-polarization,which was assumed to be prior information in the process of wave retrieval.The retrieved SWHs were compared with the European Centre for Medium-Range Weather Forecasts(ECMWF)reanalysis field with 0.125° grids.The root mean square error(RMSE)of the SWH is 0.57 m,found using CSAR_WAVE2,and this RMSE value was less than the RMSE values for the analysis results achieved with the PFSM and Q-P algorithms.The statistical analysis also indicated that wind speed had little impact on the bias with increasing wind speed.However,the retrieval tended to overestimate when the SWH was smaller than 2.5m and underestimate with an increasing SWH.This behavior provides a perspective of the improvement needed for the SWH retrieval algorithm using the GF-3 SAR acquired in wave mode.