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High resolution ocean surface wind measurement by PALSAR
Backscattering intensity of microwave from the ocean has a relationship with incidence angle, wind speed, and wind direction relative to radar azimuth angle (relative wind direction) along with wavelength and polarization. For Ku- and C-bands, empirical relations between surface wind and backscattering intensity have been formulated as Geophysical Model Function (GMF) and they are applied to a scatterometer and a synthetic aperture radar (SAR) to detect ocean surface wind. On the other hand, we did not have, up to this day, an L-band GFM which covers a wide range of incidence angle. After the launch of ALOS, the accumulation of L-band measurements over the ocean enables us to investigate their empirical relationship with ocean surface wind and then develop an L-band GMF. Fig. 3 represents the 3D views of the derived GMF at incidence angles of 30 and 40 degrees. Generally normalized radar cross section (NRCS) decreases with increasing incidence angle and increases with increasing wind speed. In terms of relative wind direction, NRCS at up- (0 degree) and down-winds (180 degree) are strong than those at cross-winds (90 and 270 degrees). For the validation of the GMF, the 220 PALSAR-derived wind speeds are compared with buoy measurements (Fig. 4), resulting in the 0.69 m/s bias and the 1.87 m/s root mean square error (RMSE).
Scatterometer can derive wind direction as well as wind speed from multi-angle measurements based on the wind direction dependency of NRCS shown in Fig.3. On the other hand, SAR, which observes NRCS from only one direction, usually estimates wind speed only by giving wind direction from the other data. In the wind speed estimation in Fig.1, we obtained wind direction from the NCEP/NCAR 6-hourly wind product (http://www.cdc.noaa.gov/).