The Surface Effect of Rain on Microwave Backscatter from the Ocean

Abstract

The effect of rain on the ocean surface alters the relationship between the surface wind vector and microwave backscatter, presenting an obstacle to wind retrieval via scat- terometry. To address the effect of rain on surface backscatter we develop a physically based ocean surface wave model modified by rain. Microwave backscatter is then calculated using a multiscale scattering model. Comparisons to observations at Ku-band are used to validate and tune our surface model. Simulations give insight into backscattering surface features: ring waves result primarily from the collapse of the splash-created stalk and the impulse responsible for the generation of ring waves has a radius roughly five times the drop's radius. Comparisons also show that backscatter from stationary splash features is necessary to accurately reproduce the effect of rain at Ku-band. For Ku-band our simulations expand upon prior measure- ments showing that rain increases backscatter and diminishes azimuthal variations. There is however, a wind relative azimuthal signature in the backscatter for most rainfall rates. Simulations at Ka-band, C-band, and L-band without the contribution from stationary splash features show that rain-created ring waves often alter backscatter. The effects are greatest at Ka-band where they mirror changes to the very high-frequency region of the surface wave spectrum. C-band backscatter is increased at moderate and high incidence angles and is sensitive to rain-induced damping. The effect of rain on L-band is to decrease backscatter at high rain rate and it is also dependent on rain-induced damping.