Observations of sea surface mean square slope under light wind during the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment

Observations of sea surface mean square slope (mss) were made with a scanning radar altimeter (SRA) from a low‐flying research aircraft on a light‐wind day during the Tropical Ocean‐Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment. Wind speed (measured at 60‐ to 90‐m height and extrapolated to the 10‐m level) and mss were found to be positively correlated, with correlation coefficients in some cases as high as those obtained for wind speed measurements on adjacent aircraft flying wingtip to wingtip with a 100‐m lateral separation of their fuselages. The SRA measurements suggest a much larger azimuthal asymmetry in mss at 36 GHz than was observed in the seminal Cox and Munk optical experiment; the SRA measurements showed a crosswind‐to‐upwind ratio as low as of 0.26 at 1.8 m s−1 compared to the typical value of 0.8 for Cox and Munk. For higher wind speed the mss becomes more isotropic. The data suggest that waves generated by a wind speed less than 2 m s−1 are restricted to the downwind direction. The crosswind‐to‐upwind ratio would probably have been even lower had it not been for an isotropic background mss caused by an assortment of swell.