Addressing the rain effects on ocean wind scatterometry at C and Ku band: modification of the ocean surface backscattering coefficient

Ocean surface winds are the main factors affecting the scatterometer backscattered radiation. In clear weather conditions, the surface wind modifies the ocean surface roughness which plays a key role in the regulation of the backscattered radiation and, in turn, in the wind estimates. However, in presence of atmospheric precipitation, the surface roughness is also modified by the impact of the raindrops imping on the surface. Therefore, modeling the ocean surface modification combining both wind and rain offers an opportunity to physically understand the contribution of the rain in the ocean backscattering coefficient. It opens the way towards a complete theoretical forward model to simulate the scatterometer observations in both rainy and non-rainy conditions once the contributions due to rain attenuation and volume backscattering are introduced. Such model allows also possible correction techniques of the wind retrievals and synergistic estimates of rain parameters as well. In this perspective, our work focuses on the development of a physical model of the ocean surface backscattering coefficient accounting for the modification of the surface roughness due to both wind and rain. The ocean surface roughness is described by the equilibrium ocean wind wave spectrum which can be defined as the distribution of the mean waves’ energy considering the spectral sources as balanced, such as wind contributions, breaking dissipation and non-linear interactions. Several representations of the ocean surface spectrum in equilibrium range already exist and, in term of wavenumbers, they describe the ocean surface as composed by two scale of roughness such as large scale gravity waves and small scale capillary waves. Our approach consists in modifying the spectrum in the region of capillary waves in order to include the rain-induced wave damping and the generation of ring waves [Contreras and Plant, 2006]. We have modeled these effects by introducing an attenuation factor accounting for the variation of the water viscosity [Nystuen, 1990] and an additive contribution representing the increase of the surface roughness due to ring waves [Bliven et al., 1997]. The main features of our approach is that the rain model does not depend on the representation of the surface wind wave spectrum so that it can be applied to any equilibrium spectrum as long as it shows a separation between gravity and capillary waves. We have first analyze the spectrum developed by Donelan and Pierson (1987) and the preliminary results are as expected, however analysis using further spectra will follow. To compute the co-polar ocean surface backscattering coefficient, the ocean surface two scale model with the new rain affected wind wave spectrum, has been used. We have first focused on Ku band simulations in non-rainy conditions and comparison to the empirical geophysical model function developed for the NASA scatterometer QuikSCAT show good agreement especially at vertical polarization. Then, we have focused on low-medium wind regime to analyze the ocean surface backscattering coefficient at different rain intensities. As expected, the backscattering coefficient increases when the rain rate becomes higher due to the increasing roughness and this shows that our approach is physically consistent. Analysis at C band will be also carried out. The validation in non-rainy conditions will be performed using the CMOD5.N geophysical model function developed for C band scatterometer such as European Remote Sensing Satellite-2 (ERS-2) and Advanced Scatterometer (ASCAT). Analysis of the ocean response at different rain rates as well as comparisons between C and Ku bands are planned in order to study the impact on the backscattering coefficient, at different frequencies, of the rain induced surface modification.