Wave and Wind Direction Effects on Ocean Surface Emissivity Measurements in High Wind Conditions

Wave and wind direction effects on remote sensing measurements of ocean surface emissivity are investigated using a microwave radiometer in high wind conditions with a focus on tropical cyclones. Surface wind speed, which drives many atmospheric and oceanic phenomena, can be inferred from the ocean surface emissivity measurements through the use of a radiative transfer model and inversion algorithm. The accuracy of the ocean surface emissivity to wind speed calibration relies on accurate knowledge of the surface variables that are influencing the ocean surface emissivity. This study will identify an asymmetry in ocean surface emissivity measurements at off-nadir incidence angles that is related to the surface wind direction modifying the distribution of whitewater coverage, which is composed of active whitecaps and residual foam that persists after wave breaking, on the ocean surface in high wind conditions viewed by the radiometer. It will also be shown that asymmetries are present in ocean surface emissivity measurements from a nadir pointing instrument in hurricanes. This asymmetry can be related to swell and wind wave propagation directions with respect to the wind direction modifying the stress on the ocean surface, which presumably impacts the wave breaking and thus the whitewater coverage characteristics on the ocean surface. These results help achieve the study goals: 1) improving the understanding of how wave and wind direction modify ocean surface emissivity in high wind conditions and 2) identifying conditions, particularly in tropical cyclones, where wind direction and sea state modify the ocean surface emissivity and should be considered in order to further improve algorithms for the remote sensing of the surface wind.