| Rainfall intensity estimates by passive microwave (PMW) measurements from
space perform generally better over the sea surface with respect to land,
due to the problems in separating true rain signatures from those produced
by surfaces having similar spectral behaviour (e.g. snow, ice, desert and
semiarid grounds). The screening procedure aimed at recognizing the various
surface types and delimit precipitation is based on tests that rely on PMW
measurements only and global thresholds. The shortcoming is that the
approach tries to discard spurious precipitating features (often detected
over the land-sea border) thus leading to no-rain conservative tests and
thresholds. The TRMM mission, with its long record of simultaneous data from
the Visible and Infrared Radiometer System (VIRS), the TRMM Microwave Imager
(TMI) and rain profiles from the Precipitation Radar (PR) allows for
unambiguous testing of the usefulness of cloud top characterization in rain
detection.
An intense precipitation event over the North Africa is analysed exploiting
a night microphysical RGB scheme applied to VIRS measurements to classify
and characterize the components of the observed scenario and to discriminate
the various types of clouds. This classification is compared to the rain
intensity maps derived from TMI by means of the Goddard profiling algorithm
and to the near-surface rain intensities derived from PR. The comparison
allows to quantify the difference between the two rain retrievals and to
assess the usefulness of RGB analysis in identifying areas of precipitation. |