Revisiting the Cox and Munk wave-slope statistics using IASI observations of the sea surface

Abstract

We use radiances collected from space by the Infrared Atmospheric Sounder Interferometer (IASI) when looking down at ocean surfaces during the day to remotely determine the probability distribution of wave slopes. This is achieved by using about 300 channels between 3.6 and 4.0 $\mu$m and a physically-based approach which properly takes the contribution of the reflected solar radiation into account. Based on about 150 million observations, the same number of wave-slope probabilities are retrieved for wind speeds up to 15 m/s. We revisit and discuss the methodology proposed by Cox and Munk (CM) to derive their celebrated wave-slope probability distribution function (pdf) from photographs of the sun glitter. We propose an original and robust approach for accurate retrievals of the 7 parameters appearing in the Gram-Charlier representation of the pdf. Our results for the mean square slopes are fully compatible with those of CM, and with the more recent results by Br\'eon and Henriot, but our lower uncertainties enable us to point out departures from the linear wind-speed dependencies and a slight overestimation of the upwind MSS described by the linear fit of CM at moderate wind speed. Our skewness and kurtosis coefficients show clear influences of the wind speed, with a steady decrease of the former and the alongwind kurtosis coefficient being maximal at moderate wind speeds, features that CM could not point out due to the limitations of their measurements. We revisit the renormalization procedure employed by CM to obtain the complete variances from truncated pdf and show that it imposes stringent conditions on the kurtosis coefficients that allow us to determine them accurately, with wind-dependent values specific to the local sea state.