Addressing the rain effects on the ocean scattering: a theoretical model of the ocean surface backscattering coefficient in presence of both wind and rain

Atmospheric rain has a strong impact on spaceborne scatterometer observations at Ku-band and higher frequencies. Several semi-empirical techniques have been developed in the past years to address the rain effects on scatterometer backscattered signal but, a theoretical model is still an open issue [Weissman et al., 2012]. A theoretical model would contribute to establish an accurate approach in deriving winds as well as in studying the sensibility to the rain intensity. It would also ingest the opportunity to develop new techniques to estimate both wind and rain simultaneously. Therefore, in this perspective, the goal of this work is to describe our approach in developing such model. Here, we have focused on modelling the ocean surface backscattering coefficient in presence of rain. The inclusion of rain attenuation and volume backscattering will be the goal of future works. To describe the surface backscattering coefficient, we have used the Two Scale Model, where a new ocean surface roughness model is proposed to account for the ocean surface modification induced by the rain [Contreras and Plant, 2006]. This new spectrum consists on an extension of the ocean wave spectrum model proposed by Elfouhaily et al., (1997) (hereafter E). However, the E spectrum has been firstly tuned to best match the empirical geophysical model functions (GMF). The tuning strategy has been defined such that it does not depend on any instrumental parameters like frequency, incidence angle or polarization as well as on the different available GMFs, in order to allow the model to work in any conditions. Two main rain effects have been included in this spectrum, such as: the short wave damping according to the theory proposed by Nystuen, (1990) and the generation of the ring waves, as shown by Bliven et al., (1997). The results of the modeled Normalized Radar Cross Section (NRCS), in non-rainy conditions, show a very good agreement with the GMF developed for the Ku-band QuikSCAT scatterometer, at vertical polarization. Good results are also obtained at C-band, in vertical polarization, using the CMOD5.N GMF for validation but, in this case, some discrepancies can be seen at wind speed higher than 15 m/s so, further investigation are needed. Moreover, at Ku-band, the horizontal polarization shows a bias which increases with the wind speed. This bias can be ascribed to the presence of the steep breaking waves which are known to introduce an additional scattering to the wind wave backscattering coefficient. Future investigations will focus on modeling such additional scattering, according to the theory proposed by Kudryavtsev et al., (2003). On the other hand, in rainy conditions, the results are consistent to what expected. The Ku-band NRCS increases with the rain rate at low, medium and high winds, demonstrating that the ring waves are the dominant contribution. The rain smooths the directional component of the NRCS at lower winds but does not affects the directional component at moderate and high winds. Moreover, it is shown that the sensibility of the NRCS to rain decreases with the wind speed. Additional tests, using real data, are also planned.