Rain cells over the sea monitored by synthetic aperture and weather radars: a comparison

Rain showers (rain cells) over the sea can be monitored by active microwave sensors because of two processes: (1) the scattering and attenuation of the microwaves by raindrops in the atmosphere and (2) the modification of the sea surface roughness and thus its radar backscatter cross section by the impact of raindrops and by wind gusts associated with rain cells. The first process is used by ground-based weather radars which measure the power backscattered by raindrops in the atmosphere, from which the rain rate can be derived. A combination of both processes causes signatures of rain cells on synthetic aperture radar (SAR) images of the sea. Such signatures consist of irregularly-shaped bright and dark patches with a diameter of several kilometers, often embedded in almost circular structures. In this study, for the first time SAR images of European marginal seas acquired by the C band SAR aboard the European remote sensing satellites ERS-1 and ERS-2 are compared with concurrent C band weather radar images, which allows to unambiguously attribute signatures on the SAR images to rain cells. The study shows even rain cells with relatively low rain rates (<10 mm/h) can cause signatures in ERS SAR images of the sea. These signatures are predominantly caused by the effect of the rain cells on the sea surface roughness.