Research on impact of upwelling on SAR-derived wind speeds along the Zhejiang coast

Abstract

Spaceborne synthetic aperture radar (SAR) plays an important role in sea surface wind field. Upwelling is a common oceanic phenomenon which influences the wind retrieving from SAR imagery. It smoothes the sea surface and its backscatter cross section in SAR images is low. The sea surface wind retrieved directly from SAR imagery in upwelling region will be underestimated. In this article, using sea surface temperature (SST) and chlorophyll2-a derived from EOS MODIS, the low backscatter features in ENVISAT ASAR image are analyzed along the Zhejiang Coast in the East China Sea. And CMOD4 algorithm is adopted to retrieve sea surface wind speed, using wind directions from interpolated NCEP / NCAR reanalysis data. Results show that wind speed is negatively biased due to the low Normalized Radar Cross Section (NRCS) associated with the Zhejiang Coastal Upwelling. Based on statistical method, the quantitative relationship between SAR-derived wind speeds and high resolution numerical meteorological model results is established. Then using this quantitative relationship, the underestimated wind speeds in upwelling region are corrected and the impact of coastal upwelling on SAR wind retrieval is quantitatively evaluated.