Wind and Sea State Signatures in Wind Scatterometry: An analysis

A better understanding of how wind vectors are obtained in scatterometer retrievals may be useful to further develop wind scatterometry. Scatterometer ocean surface wind estimation involves air–sea and wind–wave interactions in rather diverse conditions. Curiously, scatterometer geophysical model functions (GMFs) depending only on the wind vector prove very successful operationally, and sea state (ocean wave) parameters are not considered. It has been suggested in theories that, on the one hand, wind–wave features are modulated by fetch or wave age and affected by longer waves, or swells, in terms of nonlinear interactions and, on the other hand, by direct wind-induced wave movements in the short ocean wave spectrum, modulated by gusty winds. Before scatterometers came into play, field campaigns and laboratory experiments, often with limited sampling, suggested wind stress would be a function of sea state. However, generally, scatterometer wind-wave-induced ocean roughness back-scattered signals can be associated with local winds quite accurately, without considering the wave parameters. In this article, sea state and wind variability parameters are specifically investigated for their effect on scatterometer measurements and retrieved winds. The results confirm that scatterometer backscatter values are generally not affected by the nonlocal sea state, and this is further discussed. Finally, an extreme wind case is provided for a better demonstration. The conclusions provide good references for high-wind model modification and the next generation of scatterometers designed for simultaneous measurements of ocean surface wind, wave motion, and ocean currents.