A NEW RELATIONSHIP BETWEEN RADAR CROSS-SECTION AND OCEAN SURFACE WIND-SPEED USING ERS-1 SCATTEROMETER AND BUOY MEASUREMENTS

The ERS-1 spacecraft scatterometer, C-band VV polarization, acquired r adar cross-section measurements over the global oceans during 1992 and 1993. We investigate the cross-section dependence on mean wind speed U using collocated buoys within +/-25 km of the scatterometer cells. T hese collocated measurements result in over 75000 matches in two diffe rent oceanic regions. The buoys measure hourly mean wind speeds from 0 .2-10 m s(-1) and 0.2-18 m s(-1) in the equatorial Pacific Ocean and a t mid-latitudes off the North American coasts, respectively. We presen t experimental evidence for a new and compact exponential model depend ence on wind speed. The previously used power-law form inadequately ch aracterizes the cross-section measurements based on a single index ove r a large wind speed range. The cross-sectional slope varies from abou t zero dB/m s(-1) at high wind speeds U = 18 m s(-1) and small inciden ce angles theta = 20 degrees to about 1.3 dB/m s(-1) at low wind speed s U = 3 m s(-1) and large incidence angles, theta = 55 degrees. The CM OD4 model significantly underestimates the radar cross section measure ments for U less than or similar to 3 m s(-1) whereas the exponential model exhibits less bias.