An algorithm for operational wind field retrieval using RADARSAT-1 ScanSAR images

An algorithm is presented for retrieving mesoscale wind fields from the ScanSAR data acquired aboard the Canadian satellite RADARSAT-1. Its Synthetic aperture radar (SAR) operates at C-band (5.3 GHz) with horizontal polarization in transmit and receive. The algorithm consists of two parts, one for wind direction and another for wind speed retrieval. Wind directions are extracted from wind induced streaks e.g. from boundary layer rolls, Langmuir cells, or wind shadowing, which are approximately in line with the mean wind direction. The algorithm defines wind direction as normal to the gradient of the amplitude image, which is approximated by finite differences over an appropriate length. The resulting wind direction is taken as normal to the best and most frequent values of the retrieved local gradients. Wind speeds are derived from the normalized radar cross section (NRCS) and image geometry of the calibrated ScanSAR images, together with the local wind direction. For this semi empirical C-band models, e.g. CMOD4 or CMOD_IFR2, which describe the dependency of the NRCS on wind and image geometry are inverted. These models were developed for C-band with vertical polarization and have to be modified for horizontal polarization, which is performed by considering several C-band polarization ratios including theoretical and empirical forms. To improve and verify the algorithm, wind fields were computed from several RADARSAT-1 ScanSAR wide swath A images and compared to collocated measurements from the scatterometer aboard ERS-2 and to results of the Danish high resolution limited area model (HIRLAM). Comparison of wind fields retrieved from RADARSAT ScanSAR to results of HIRLAM show the applicability of SAR images especially in coastal regions with high wind variability. To estimate relative errors of wind speed due to uncertainties in wind direction and NRCS, sensitivity studies were performed with respect to various settings, including settings of the future mission of the ENVISAT ASAR. From these studies recommendations are given concerning the best settings for C-band SARs for wind field retrieval. The results are used for operational applications of the Danish Met office like determination and forecasting of sea ice and wind fields around Greenland.