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various shortcomings such as instrumental errors, beam blockage by orographic features,
and overshooting range effects (Krajewski and Smith
2002
). The only civilian active
spaceborne Tropical Rainfall Measuring Mission-Precipitation Radar (PR) sensor (TRMM-
PR) provides high-resolution reflectivity of rainfall fields (i.e., *4 9 4 km) over a narrow
band in the tropics with relatively low temporal revisiting frequency compared to the other
passive spaceborne sensors of lower resolution. The forthcoming Global Precipitation
Measuring (GPM) Mission, a constellation of nine satellites, promises to deliver obser-
vations of high precision precipitation and cloud dynamics at a global scale (3-h revisiting
time) and over varying resolutions and create opportunities for improving climate mod-
eling and hazard prediction at local scales (Flaming
2004
).
Precipitation observations from space are especially valuable in regions where no
ground observations are available either from rain gauges or from ground radars, such as
over the oceans or in underdeveloped regions of the world. It is over these regions,
however, that some extreme tropical storms develop for which high-resolution information
would provide important means for hazard prediction and warning as well as detailed
information on extremes, which could be used in nested models or in a data assimilation
setting. These tropical storms have distinct geometrical and statistical structures, as shown
below, posing extra demands on the methodologies of precipitation downscaling, data
fusion, and data assimilation.
As an illustrative example, Fig.
1
shows a snapshot of the two-dimensional rainfall
intensity patterns and the three-dimensional structure of precipitating clouds for typhoon
Neoguri, the first typhoon of the 2008 season in the western Pacific Ocean, on April 17,
2008, as observed by the TRMM-PR and the TRMM Microwave Imager (TMI). One
notices the geometrically structured precipitation bands embedded within the larger two-
dimensional storm system and the localized ''towers'' of high-intensity rainfall spatially
embedded within lower-intensity rainfall background. These localized high-intensity cells
and the steep sporadic gradients of precipitation intensity in such a storm are more clearly
demonstrated via a one-dimensional cross section as shown in Fig.
2
. Specifically, Fig.
2
b
Fig. 1 Left panel rainfall pattern of typhoon Neoguri in the western Pacific Ocean, on April 17, 2008. The
dark red bands indicate regions of the most intense rain. Rainfall rates in the inner swath are from TRMM's-
PR, while in the outer swath from the TRMM Microwave Imager (TMI); Right panel the three-dimensional
structure of precipitating clouds for typhoon Neoguri as observed by the TRMM-PR. This figure illustrates
the need for a downscaling scheme that has the ability to reproduce steep rainfall gradients embedded within
the storm. Source: NASA's Earth Observatory, available online through the TRMM extreme event image
archives (
http://trmm.gsfc.nasa.gov
)
