Spaceborne observations of ocean glint reflectance and modeling of wave slope distributions

International audienceMeasurements of wind speed and direction from the NASA Scatterometer and ocean reflectance from the POLDER multi-directional radiometer are used for a quantitative analysis of the ocean glint intensity and shape. These two instruments have been flying on board the ADEOS satellite platform, which assures a very large data set of coincident measurements distributed over all oceans. The glint intensity and pattern is directly related to the wave slope probability function. The slope distribution model developed by Cox and Munk (1954) more than half a century ago, which is a function of wind speed and direction, permits an excellent fit of the observation. In particular, the modeled mean square slope is in near perfect agreement with that derived from the satellite data. The latter permit, however, a novel analysis of the deviations from a pure Gaussian slope distribution. In particular, it is shown that the skewness is a non-linear function of the wind speed. A typical glint reflectance in the specular direction is 0.2, which is therefore significantly smaller than, for instance, a typical cloud reflectance. There are large variations with the Sun zenith angle, however, that are easily accounted for with a simple formula. The glint reflectance allows an estimate of the wind speed that is highly correlated with the independent measurement from the scatterometer, with an estimated error less than 1 m s À1. The multidirectional reflectance measurements from POLDER also permit the retrieval of the wind direction, albeit only in favorable conditions depending on the respective orientation of the view and wind vectors