A model function for the ocean-normalized radar cross section at 14 GHz derived from NSCAT observations

A model for the ocean surface normalized radar cross section sigma(o) is derived from 3 months of NASA scatterometer (NSCAT) observations (September 15 to December 18, 1996). The model expresses sigma(o) as a function of wind speed, relative wind direction, incidence angle, and polarization. The dependence of sigma(o) on wind speed is based on collocated special sensor microwave/imager (SSM/I) satellite wind retrievals and European Centre for Medium-Range Weather Forecasts (ECMWF) model winds. We find that at low winds (< 5 ms(-1)), the SSM/I winds are more reliable than ECWMF, probably owing to small location errors in the ECMWF wind features. The primary wind direction dependence of sigma(o) (i.e., the second harmonic) is found from histograms of the sigma(o) difference between the forward and aft antennas. The sigma(o) versus wind speed relationship is adjusted for cross-swath incidence angle differences and is then incorporated into the NSCAT 1 model used to process the 10-month (September 15, 1996, to June 29, 1997) NSCAT data set. The resulting NSCAT 1 wind vectors are compared to ECMWF wind fields and buoys. The mean and standard deviation of the NSCAT minus ECMWF (buoy) wind speed difference are 0.05 and 1.78 ms(-1) (-0.29 and 1.26 ms(-1)), respectively. The wind direction mean and standard deviation differences are 0.8 degrees and 18.5 degrees (7.9 degrees and 15.7 degrees), respectively. The difference between the NSCAT and the ECMWF (buoy) direction exceeds 90 degrees only 1.1% (1.2%) of the time. We have no explanation for why the buoy wind directions are biased 8 degrees relative to both NSCAT and ECMWF.