COMPARISON OF TOPEX/POSEIDON SIGMA(0) AND SIGNIFICANT WAVE HEIGHT DISTRIBUTIONS TO GEOSAT

Monthly Ku band sigma(0) and significant wave height (SWH) histograms from the NASA altimeter on the TOPEX/POSEIDON satellite are presented for January through June 1993 for three latitude bands between +/-60 degrees. The data are compared to distributions from the Geosat mission for the same months in 1987-1989. Generally, the distributions agree quite well, although there are some seasonal/hemispherical differences. The sigma(0) comparison reveals an overall bias between the two altimeters with the TOPEX sigma(0) higher by about 0.7 dB, which is consistent with algorithm improvements for TOPEX. The SWH distributions show strong hemispherical/seasonal changes. The seasonal/hemispherical. differences between TOPEX and Geosat are consistent for SWH and sigma(0). The joint distribution of sigma(0) and SWH is extremely stable from month to month. The typical SWH is independent of sigma(0) for sigma(0) greater than 11.3 dB. The minimum SWH grows exponentially with wind speed. This joint distribution may be useful for understanding electromagnetic bias in altimeter measurements. Finally, altimeter data are compared to buoy values from 21 overflights of the NASA verification site near Pt. Conception, California. Wave heights agree well with an RMS difference of only 0.2 m. Altimeter sigma(0) values are compared to buoy wind speeds. The results are consistent with the -0.7 dB sigma(0) offset from the histogram comparisons.