Sea Surface Wind Field by TerraSAR-X and Tandem-X Data
Li, XiaoMing; Lehner, Susanne
German Aerospace Center (DLR), GERMANY

A new Geophysical Model Function (GMF), denoted XMOD2, was developed to retrieve the sea surface wind field from X-band TerraSAR-X/Tandem-X (TS-X/TD-X) data. In contrast to the previous GMF XMOD1, XMOD2 is based on a nonlinear GMF, and moreover, it depicts the difference between upwind and downwind of the sea surface backscatter in X-band SAR. By exploiting 371 collocations, the retrieved TS-X/TD-X sea surface wind speed by XMOD2 agrees well with in situ buoy measurements with a bias of -0.32 m/s, an RMSE of 1.47 m/s and a scatter index (SI) of 16.0%. To apply XMOD2 to TS-X/TD-X data acquired at HH polarization, we proposed three models which consider the effect of incidence angle on Polarization Ratio. By comparing the retrieved sea surface wind speed to in situ buoy measurements using 60 collocated pairs, it is found that by using the exponential type PR model X-PR and XMOD2 yields the best retrieval with a bias of -0.88 m/s, an RMSE of 2.20 m/s and a SI of 22.4%.

Several TerraSAR-X and TanDEM-X ScanSAR images were acquired in October, 2012 to track the Hurricane Sandy. Three of the images were acquired in the open sea, which were presented in this study to demonstrate observations of sea surface wind and wave extracted from X-band ScanSAR image with high spatial resolution of 17 m in the hurricane. In the case of the TerraSAR-X image acquired on October 26, 2012, we analyze the peak wave direction and length of swell generated by Hurricane Sandy, as well as interaction of swell with the Abaco Island, Bahamas. For the other two cases, sea surface wind field derived from the TerraSAR-X and TanDEM-X acquired on October 27 and 28 are presented. The sea surface wind speed retrieved by the X-band Geophysical Model Function (GMF) XMOD2 using wind direction derived from SAR images and the NOAA Hurricane Research Division (HRD) wind analyses are both presented for comparisons. We also compare the retrieved sea surface wind speed with Stepped Frequency Microwave Radiometer (SFMR) to quantify effect of rainfall on X-band SAR images.