Global ERS 1 and 2 and NSCAT observations: Upwind crosswind and upwind downwind measurements

This paper presents an analysis of the wind speed dependence of upwind/down wind asymmetry (UDA) and upwind-crosswind anisotropy (UCA) as derived from global C band VV-polarized ERS 1 and 2 and Ku band VV- and HH-polarized NAS A scatterometer (NSCAT) data. Interpretation of the results relies on ident ifying relationships between the differing frequencies and incidence angles that are consistent with Bragg scattering theory from gravity-capillary wa ves. It is shown that globally derived parameters characterizing UDA and UC A hold information on the wind dependence of short gravity and gravity-capi llary wave growth and dissipation. In particular, the UCA behavior is found quadratic for both the C and Ku band, peaking at moderate wind speeds. In addition, the dual-frequency results appear to map out the expected, more r apid adjustment of centimeter-scale (Ku band) waves to the wind direction a t light winds. However, as wind increases, the directionality associated wi th these shorter waves saturates at a lower speed than for the slightly lon ger waves inferred at C band. It is suggested that this observed phenomenon may be related to increasing wave-drift interactions that can potentially inhibit short-scale surface wave growth along the wind direction. Concernin g UDA properties, our present analysis reveals that the NSCAT and ERS 1 and 2 scatterometers give quite different results. Our preliminary interpretat ion is that C band measurements may be easier to interpret using composite Bragg scattering theory and that upwind/downwind contrasts are mainly suppo rted by short gravity waves.