MESOSCALE WIND MEASUREMENTS USING RECALIBRATED ERS SAR IMAGES

The precision images (PRI) of the synthetic aperture radars (SAR) on b oard the European Remote Sensing Satellites ERS-1 and ERS-2 are used t o derive mesoscale wind fields over the ocean. For calculation of the wind speed the C-band model (CMOD4) is used, which was originally deve loped by Stoffelen and Anderson [1993] for the European Space Agency ( ESA) to derive wind fields from measurements of the wind scatterometer (SCAT). In the case of the ERS-1/2 SAR the CMOD4 is used to compute t he wind speed from the normalized radar backscatter cross section (NRC S) and the incidence angle of the radar beam, both computed from the S AR.PRI data. The third input variable is the wind direction, which is estimated from the wind streaks in the images or from ground truth mea surements. The SAR data are affected by a power loss, caused by satura tion of the analog to digital converter (ADC) of the SAR. Therefore th e images have to be recalibrated. Errors in the derived wind speed are mainly due to ADC saturation and uncertainties of the input wind dire ction. These errors are estimated for various wind conditions. Mesosca le wind fields computed from ERS-1/2 SAR.PRI images taken between the Shetland Islands and the west coast of Norway are compared to ground t ruth measurements and modeled wind fields from the German weather serv ice (DWD). Wind fields of the nonhydrostatic mesoscale model Geesthach t simulation model of the atmosphere (GESIMA) are compared to the deri ved wind field of the ERS-1 SAR.PRI image at the island Rugen in the B altic Sea.