Geoscience Reference
In-Depth Information
products. One microwave-based data product is the Goddard Profiling Algorithm
(GPROF)
fractional occurrence of precipitation
which gives the fraction of area
with precipitation on a 0.5° by 0.5° grid over ocean. The algorithm applies a
Bayesian inversion method to the observed microwave brightness temperatures
using an extensive library of cloud-model-based relations between hydrometeor
profiles and microwave brightness temperatures. Each hydrometeor profile is
associated with a surface precipitation rate.
There is value in seeking to use the many alternative sources of remotely sensed
data relevant to estimating precipitation now available to provide a single best esti-
mate. The Global Precipitation Climatology Project (GPCP) seeks to derive such a
preferred satellite precipitation data set by selecting and merging data from many
sources.
Spaceborne radar
Good progress has been made toward remote sensing precipitation using
space-based radar broadly similar to that used in ground-based systems, and
there are plans to develop this approach further. The precipitation radar used
in the Tropical Rainfall Measuring mission (TRMM) was the first spaceborne
instrument designed to provide three-dimensional maps of storm structure.
These measurements yield information on the intensity and distribution of the
rain, on rain type and storm depth, and on the height at which the snow melts
into rain. TRMM had a horizontal resolution at the ground of about five kilom-
eters and a swath width of 247 kilometers. One important feature was its ability
to provide vertical profiles of the rain and snow from the surface to a height of
~20 kilometers. The TRMM radar was able to detect fairly light rain rates down
to about 0.7 mm hr
−1
, but it was less successful when detecting intense rain
rates.
Providing enough power to detect the weak return echo from the raindrops
when seen from orbital height is a fundamental challenge with spaceborne
radar. From the standpoint of provide routing information globally, the fact
TRMM had only intermittent coverage of the same location is also problematic.
However, multiple deployment of spaceborne radar systems is not a realistic
option for economic reasons. The proposed solution is the Global Precipitation
Measurement (GPM) mission. This will involve a core precipitation-measuring
observatory with both dual-frequency precipitation radar and a high-resolution,
multi-channel passive microwave rain radiometer. This observatory will serve
as the calibration reference system for a constellation of up to eight support
satellites conceived as being relatively small spacecraft that carry a single
high-resolution, multi-channel passive microwave rain radiometer which is
identical to that on the core satellite. In this way it is anticipated that the
GPM mission will frequently sample the diurnal variation in rainfall by
capitalizing on some satellite orbits that are synchronized with the sun and
others that are not.
