Scatterometer Ocean Surface Wind Results From the Gulf of Alaska Seasat Experiment

The SeaSat-A Satellite Scatterometer (SASS) was a 14.6 GHz microwave scattermeter which was used to infer the 19.5 m neutral stability wind vector over the world's oceans. This paper describes an evaluation of the SASS derived wind vectors during the Gulf of Alaska SeaSat Experiment (GOASEX) in September 1978. The SASS data, processed using three candidate wind vector algorithms were compared to "surface truth." The surface truth consisted of wind fields generated from a man/machine meteorological analysis as well as spot observations from well-calibrated meteorological buoys and oceanographic research vessels. Statistics for the scalar differences between the SASS and surface truth wind speed and direction were compiled for various categories of radar parameters (polarization, incidence angle) and surface conditions (wind speed, latitude and longitude location). Further these statistics were weighted by the quality of the surface truth (estimated wind speed and direction accuracy) and the atmospheric transmissibility (derived from satellite infrared and visible cloud imagery). Results of this comparison indicated that all SASS algorithms (based on prelaunch aircraft scatterometer data) were biased high by approximately 0.5 to 3 m/S compared to surface truth wind speeds and that the standard deviations about this bias were of order 2 m/s. For wind direction, biases for SASS were less than
10\deg
with standard deviations about this mean of approximately
20\deg
. Results are presented which describe the contribution of instrument, geophysical algorithm and surface truth errors to these standard deviations.