Rain cells off the US Coast

Spaceborne Synthetic Aperture Radar (SAR) is able to detect storm footprints on the sea. The figure shows an ERS-1 SAR image off the Atlantic coast of the US taken on 18 July 1992.

The dashed lines in the figure include the zones where the precipitation rate is greater than 12 mm/h as measured with a coastal weather radar 3 minutes after the SAR overpass.

The direction of the satellite track, the radar beam, and the true north are shown in this expanded view which shows a plume-like structure expanding out largely to the northeast with its axis oriented towards 46º.

The coastal radar provides evidence that the echo free hole at the footprint core is due to wave damping by rain. The increased backscattering from the sea surrounding the echo free hole results from the divergence of the precipitation-forced downdraft upon impacting the sea.

The footprint boundary is the gust front; its orientation accords with the direction of the wind aloft, and its length implies that the downdraft impact started one hour earlier supposing a quasi-stationary phenomena.
Within the echo free hole we found a small bright spot of about 1 by 1.5 km in size which is most likely due to scattering from the splash products of the rain upon impacting on the sea surface.

Some of the key surface features and the process by which the effects arose are shown in this figure which sketches and clarifies the phenomena.

D. Atlas, Goddard Space Flight Center, USA

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry