Microwave Remote Sensing of Ocean Surface Roughness, Whitecap Coverage, and Surface Wind Stress: Global Coverage and Extreme Conditions

Whitecaps manifest surface wave breaking that affects many ocean processes, of which surface wind stress is the driving force. For close to a half century of quantitative whitecap reporting, only a small number of observations are obtained under conditions with wind speed exceeding 25 m/s. In contrast, spaceborne remote sensing provides global reach and measurements in extreme conditions. Surface roughness and whitecaps are critical components of microwave ocean responses. In the forward computation of microwave thermal emission, the input forcing parameter is wind speed, which generates the modeled surface wind stress, surface wave spectrum, and whitecap coverage necessary for the subsequent electromagnetic (EM) computation. In this respect, microwave radiometer data are effectual for evaluating various formulations of the drag coefficient, whitecap coverage, and surface wave spectrum. In reverse, surface wave spectrum, whitecap coverage, and surface wind stress are retrievable in principle from microwave radiometer data by employing pre-calculated solutions of an analytical microwave thermal emission model that yields good agreement with field measurements. Active microwave scatterings provide independent evaluation of the ocean surface roughness through the Bragg resonance scattering mechanism and tilt modulation. There are many published active and passive microwave datasets collected in tropical cyclones from global oceans. They cover a wide range of frequency, incidence angle, both vertical and horizontal polarizations, and with maximum wind exceeds 90 m/s. Information of surface wave spectrum, whitecap coverage, and surface wind stress extracted from these datasets will be presented. Results from this analysis enhance considerably the oceanographic databases of surface roughness, surface wind stress, and whitecap coverage. They are also useful for deducing other wave breaking properties such as the energy dissipation rate per unit surface area.