C. Melsheimer et al., Simultaneous observations of rain cells over the ocean by the synthetic aperture radar aboard the ERS satellites and by surface-based weather radars, J GEO RES-O, 106(C3), 2001, pp. 4665-4677
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 archive
s of several weather stations for weather radar data acquired concurrently
with ERS SAR data and have found four concurrent data pairs: one in the Sou
th China Sea, two in the Baltic Sea and one in the North Sea. The compariso
n 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 im
ages vary considerably, which makes it often difficult to distinguish them
from radar signatures of other mesoscale or submesoscale atmospheric and oc
eanic 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 backscatterin
g 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 se
a surface and thus modifying the sea surface roughness and by (2) local win
d field variations associated with rain cells (spreading downdrafts), Raind
rops 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 th
an the surroundings.