New features of the sea-surface slope distribution revealed by IASI observations: Directional properties, influence of the wave height, large wave tilts

Abstract

By analysis of the reflected solar contribution to mid-infrared radiance spectra collected by the IASI instrument, we investigate the influences of both the wind and significant wave height on the wave-slope probability distribution function (PDF). We show that, at equal wind speed, smaller wave heights enhance the directional (upwind-downwind and upwind-crosswind) asymmetries of the probabilities. We also point out that the sea-surface mean square slopes slightly decrease as the wave height increases (by about $5\%$ per meter for winds slower than 6 m/s), an original result for which we propose possible causes (short-scale damping by long waves and/or effect of compounding statistical processes). We also investigate the so far practically unstudied case of steep slopes and find large deviations from the quasi Gaussian behavior of the central part of the PDF, with a quite universal exponential decay of the probabilities with increasing tilt. We here observe larger probabilities in the downwind direction, but similar values in the up- and cross-wind ones. The physical mechanisms responsible for these trends are discussed. Finally, we propose refined parametrizations of the mean square slope versus wind-speed, which departs from the Cox and Munk linear relationships at moderate wind speed (5-8 m/s).