Wednesday, 19 June 2002
A COMPARISON OF PRECIPITATION ESTIMATES OVER THE HIMALAYAS AND ANDES
Estimating precipitation over complex terrain is especially difficult due to the
small spatial and temporal scales of orographic precipitation and the lack of ground truth
in steep terrain. Two areas where the various precipitation estimates
diverge are the Andes and the Himalayas. Both areas have
precipitation from systems varying from small showers to large
Mesoscale Convective Systems, which provide differing estimation
problems from a remote sensing point of view. This study will compare
rain gauge data from the Global Precipitation Climatology Project
(GPCP) in these areas with remote-sensing data from infrared estimates
and estimates from the Tropical Rainfall Measuring Mission (TRMM)
Satellite. Infrared estimates rely on a cloud-top
temperature-precipitation relationship which contain large
uncertainties in instantaneous rainfall estimates. Passive microwave
estimates (such as those from the TRMM Microwave Imager at 85 and 37
GHz) over land correlate the optical depth of frozen hydrometeors
within a cloud system to near surface rainfall, a relationship which
is more directly related to rainfall than infrared estimates.
However, these relationships still contain scatter due to the varying
macro- and microphysical structure of precipitating systems and the
presence of artifacts of elevated terrain in the data. Estimates from
the world's first spaceborne radar designed for rainfall estimation,
the TRMM Precipitation Radar, provide the most physically direct
precipitation estimator, however issues such as sampling biases and
surface echo contamination reduce the accuracy of the retrievals.
Estimates from the 3-year University of Utah TRMM Precipitation
Feature Database are also compared to examine the dependence of
system frequency by type in the various estimates.
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