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

Abstract

A model for the ocean surface normalized radar cross section σ o is derived from 3 months of NASA scatterometer (NSCAT) observations (September 15 to December 18, 1996). The model expresses σ o as a function of wind speed, relative wind direction, incidence angle, and polarization. The dependence of σ 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 σ o (i.e., the second harmonic) is found from histograms of the σ o difference between the forward and aft antennas. The σ 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° and 18.5° (7.9° and 15.7°), respectively. The difference between the NSCAT and the ECMWF (buoy) direction exceeds 90° only 1.1% (1.2%) of the time. We have no explanation for why the buoy wind directions are biased 8° relative to both NSCAT and ECMWF.