Probability distribution of surface wave slope derived from spaceborne remote sensing data

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The probability distribution of the sea surface slope has been estimated using sun glitter images derived from the visible wavelength radiometer on the Geostationary Meteorological Satellite (GMS) and surface vector winds observed by spaceborne scatterometers. The brightness of the visible images is converted to the probability of wave surfaces which reflect the sunlight toward GMS in grids of 0.25° x 0.25° (latitude x longitude). The slope and azimuth angle required for the reflection of the sun's rays toward GMS are calculated for each grid from the geometry of GMS observation and location of the sun. The GMS images are then collocated with surface wind data observed by three scatterometers. Using the collocated data set of about 30 million points obtained in a period of 4 years from 1995 to 1999, the probability distribution function of the surface slope is estimated as a function of wind speed and azimuth angle relative to the wind direction. The results are compared with those of Cox and Munk (1954). The surface slope estimated by the present method shows a narrower distribution and much less directivity relative to the wind direction than that reported by Cox and Munk. It is expected that their data were obtained under conditions of growing wind waves. In general, wind waves are not always developing, and the slope distribution might differ from the results of Cox and Munk. Most of our data are obtained in the subtropical seas under clear-sky conditions. This difference in the conditions may be the reason for the difference of slope distribution.