Comparison of ERS Tandem Phase Scatterometer and SAR derived wind speed

A validation of wind retrieval by synthetic aperture radar (SAR) has been performed by comparing wind speed estimated from European Remote Sensing Satellite (ERS)-2 SAR with ERS-1 Wind Scatterometer (WSC) measurements, both using the CMODIF2 algorithm. This was possible by using data north of 63°N during the Tandem Phase of the ERS satellites. The WSC and SAR coverage overlapped with a time difference in the data acquisition of only 30 minutes. The WSC has a grid size of 25 km×25 km. 145 sub-images of 25 km×25 km collocated at the WSC wind vectors were extracted from the SAR images and used in the study. The SAR derived wind speed values from these sub-images are within ±2 m/s from the WSC wind speed values and the root-mean-square error is less than 0.5. This is a good correlation considering that ±2 m/s is also the accuracy of the WSC retrieved wind speed. The SAR is observed to underestimate the WSC wind speed with 0.5 m/s. This suggests that (i) the SAR backscatter measurements is not directly comparable to the WSC measurements due to differences in calibration, or (ii) the CMODIF2 needs a correction in its dependence on incidence angle. Their results confirm the usefulness of the SAR derived wind field down to a spatial resolution of 200 m×200 m, provided that external information on wind direction is available