Rain footprints on C-band synthetic aperture radar images of the ocean - Revisited

Abstract

It is well known that rain leaves footprints on the sea surface that sometimes become visible on synthetic aperture radar (SAR) images. Rain cells can easily be detected on SAR images at all radar frequencies when they are associated with a downdraft pattern. But rain cells are not always associated with downdraft and rain can also occur in other forms, as stratified rain, rain bands, and squall lines. It turns out that radar signatures of rain at C-band are much more complex than at L- or X-band radar and that it is particularly difficult to identify unambiguously rain events on C-band SAR images acquired over the ocean. This is because C-band lies in the transition region where raindrops impinging onto the sea surface can increase (usually) or decrease the backscattered radar power and where volume scattering and attenuation by rain drops in the atmosphere are not always negligible (at very high rain rates). In order to get an insight into the physical mechanisms causing the C-band radar signatures of rain, we first revisit results obtained from historic laboratory and field experiments and multi-frequency/multi-polarization SAR data acquired during the SIR-C/X-SAR spaceshuttle mission in 1994. Then we analyze several C-band SAR images acquired by the European satellites Envisat and Sentinel-1A, and the Canadian satellite Radarsat-2 and compare them, whenever possible, with quasi-coincident and collocated weather radar images. The observational data show that, at low to medium rain rates, the main physical mechanism causing C-band radar signatures of rain is Bragg scattering at ring waves generated by the rain drops impinging onto the sea surface, which increase the radar backscatter. However, areas of increased radar backscatter are often accompanied by adjacent areas of decreased radar backscatter, which is due to attenuation of the Bragg waves by turbulence also generated by the impinging rain drops. Furthermore, we present a full-polarimetric Radarsat-2 SAR image of a rain cell together with a polarimetric decomposition analysis, which shows that the C-band radar signature of a rain cell is caused by surface scattering. The observation show that radar signatures of rain cells often contain segments, where the co-polarized as well the cross-polarized radar backscatter are strongly enhanced, which indicates non-Bragg scattering contributions to the scattering process. Furthermore, the polarimetric decomposition analysis shows that the C-band radar signature of a rain cell is dominated by surface scattering. Possible mechanisms, like scattering at splash products, are discussed. Whether the normalized radar cross section (NRCS) due to rain is increased or decreased depends on rain rate, wind speed, incidence angle, and history of the rain event. At low to moderate wind speeds (< 10 ms− 1) and low to medium high rain rates (< 50 mm h− 1), the NRCS is usually increased by up to 8 dB, and at high wind speeds (> 10 m s− 1) and low to high rain rates (but < 50 mm h− 1), the NRCS is usually decreased by up to 3 dB.