A Study Of The Influence Of Non-Linear Wind-Wave-Growth Processes On Scatterometry

From satellite altitudes, the only feasible way at present to globally monitor wind stress over the oceans is by the use of scatterometers. The principle of wind measurement by the scatterometer is simple. It has been established that for the scatterometer, Bragg scattering is the predominant scattering mechanism; the backscattering power is produced only from those water waves that satisfy the Bragg resonant condition. It has been assumed in the past that the resonant water waves grow monotonically with the wind stress; a simple empirical relationship between the wind stress and the backscattering power has been used until now as the algorithm for scatterometer data reduction. Recent results from laboratory studies indicate that the growth of wind waves does not follow such a simple relationship. Furthermore, the relationship is not even monotonic. These complications can be attributed to the nonlinear nature of the wind-wave interaction processes. Specifically, the nonmonotonic relationship between the wind stress and the wave energy density is the consequence of the overshoot phenomenon. Although over-shoot as a function of fetch has been widely accepted, overshoot as a function of wind stress has never been emphasized. Published satellite scatterometer data from Skylab and Seasat and also aircraft results are examined to show that the radar backscattering does not vary smoothly with wind stress either as modeled by the empirical scatterometer algorithms. It is asserted that the nonlinearities exhibited in the satellite and aircraft radar results are the manifestation of the nonlinear overshoot wave growth phenomenon. To further supply proof of this hypothesis, preliminary results from a laboratory radar set are also presented which reinforce the satellite results. These findings raise questions about the scatterometer data reduction method employed at the present.