Comparison of Seasat scatterometer winds with tropical Pacific observations

The Seasat‐A satellite scatterometer (SASS) observed near‐surface vector winds over the world ocean from an 800‐km orbit by measuring radar backscatter from the wind‐roughened surface. The early end of the mission in 1978 forestalled some of the planned ground‐truth validation experiments; questions remain about the instrument's performance, in particular away from mid‐latitudes. To increase the geographical range of SASS and in situ comparison experiments, we have compared data from nine islands in the tropical Pacific and contemporaneous scatterometer winds. We used satellite winds derived from two different wind vector algorithms (the signal processing that reduces the radar backscatter to wind speed and direction). The SASS‐2 algorithm due to Wentz provides clearly better agreement in wind speed than the earlier SASS‐1 algorithm. SASS‐1 speeds tend to be higher than the island measurements by about 1 m s−1, while daily mean SASS‐2 minus island wind speed differences average −0.07 m s−1. The rms differences between SASS‐2 and the daily mean island data average 1.7 m s; the SASS‐1 rms differences average 2.2 m s−1. Because the Seasat wind algorithms yield solutions with more than one possible direction, a single direction can only be selected from the others using ancillary information. We compared direction results from the objective selection method developed at the Goddard Laboratory for Atmospheres, as well as results from the somewhat artificial method of selecting the SASS direction closest in agreement with the comparison island direction. When averaged over tens of days the Goddard directions agree closely with the island observations. However, Goddard daily mean directions exhibit considerably higher variance than those measured at the islands. The closest direction technique gives daily mean SASS minus island rms direction differences averaging 20°; the Goddard rms differences average 52°.