Radar determination of winds at sea

This paper reviews the history and the current status of the relationship between the backscattering coefficient σ° and the wind-speed u at microwave frequencies. When σ° is assumed to be proportional to u^γ, measurements have indicated that γ lies in the vicinity of 1.4 to 2.0. Under similar conditions with incidence angle between 30° and 80°, γ for HH polarization is usually larger than that of VV polarization and γ for upwind or downwind is larger than that of the crosswind. Better surface truth and controlled experiments are still needed to obtain a more specific value for γ under a given condition. Theoretical modeling of radar sea scatter indicates that near vertical incidence the physical optics method is applicable while for incidence angles approximately in the range 30° to 80° the return can be explained by Bragg scattering due to capillary waves when the tilting effect of the large-scale waves is also included. The methods used in extracting wind vectors from SEASAT scatterometer measurements are outlined. Areas where further experimental and theoretical studies are needed are indicated.