Simultaneous observations of rain cells over the ocean by the synthetic aperture radar aboard the ERS satellites and by surface-based weather radars

Radar images acquired over the ocean by the C band synthetic aperture radar (SAR) aboard the European Remote Sensing satellites ERS 1 and ERS 2 often show sea surface manifestations of rain cells. We have searched the archives of several weather stations for weather radar data acquired concurrently with ERS SAR data and have found four concurrent data pairs: one in the South China Sea, two in the Baltic Sea and one in the North Sea. The comparison of ERS 1/2 SAR images showing radar signatures of rain cells with weather radar images reveals that the radar signatures of rain cells on ERS SAR images vary considerably, which makes it often difficult to distinguish them from radar signatures of other mesoscale or submesoscale atmospheric and oceanic phenomena. The present analysis, together with results obtained from previous analyses of spaceborne multifrequency SAR data and laboratory data as well as results obtained from theoretical models on radar backscattering at the sea surface suggest the following: C band radar signatures of rain cells with rain rates below 50 mm/h are mainly caused by a modification of the sea surface roughness induced by (1) the raindrops impinging on the sea surface and thus modifying the sea surface roughness and by (2) local wind field variations associated with rain cells (spreading downdrafts), Raindrops impinging on the sea surface generate ring waves as well as turbulence in the upper water layer. Depending on rain rate, drop size distribution, wind speed, and temporal evolution of the rain event, the net effect can be an increase or a reduction of the amplitude of the C band Bragg waves and thus of the backscattered radar power. Thus ocean areas struck by rain can show up on ERS SAR images as areas with higher or lower image brightness than the surroundings.