Global Precipitation Monitoring - Satellite-based Applications on Climate Change - page 83

The SSM/I has been replaced with the Special Sensor Microwave Imager Sounder
(SSMIS) which is in operation at the time of this writing. Today, the Goddard
profiling algorithm (GPROF), a sophisticated inversion algorithm based on cloud
resolving models and radiative transfer calculations (Kummerow et al. 1996 , 2001 ),
is widely used on a variety of sensors such as the SSM/I, Tropical Rainfall Measuring
Mission (TRMM) Microwave Imager (TMI), and the Advanced Microwave Sound-
ing Radiometer-Earth Observing System (EOS) (AMSR-E) (Wilheit et al. 2003 ).
Due to the high and more varying emissivity of the land surface, the only reliable
means of detecting rainfall over land is by isolating depressed brightness
temperatures as a result of scattering by millimeter-sized ice particles that exist in
most rain clouds. Since the signal being captured is a result of ice particles instead
of raindrops, the scattering-based rainfall estimation is an indirect measure of
rainfall, as it relates the magnitude of the scattering near the freezing layer to
surface rainfall. The launch of the SSM/I in 1987 provided the first opportunity to
retrieve rain rate through scattering at higher frequency (85 GHz). Noteworthy
studies in this regard include Spencer et al. ( 1989 ), Grody ( 1991 ), and Ferraro and
Marks ( 1995 ). GPROF adopted this method over land as well (McCollum and
Ferraro 2003 ; Wang et al. 2009 ; Gopolan et al. 2010 ) and has been used for TMI
and AMSR-E.
Zhao and Weng ( 2002 ) took advantage of the highly scattering nature of 89 GHz
and in particular 150 GHz radiances from the Advanced Microwave Sounding Unit-B
(AMSU-B) and retrieved ice water path (IWP) using scattering parameters measured
at these two channels. The derived IWP is then converted into the surface rainfall rate
(RR) through an IWP and rainfall rate relationship developed from cloud model
results (Weng et al. 2003 ). This rain rate product is being operationally generated at
National Environmental Satellite, Data, and Information Service (NESDIS) of
National Oceanic and Atmospheric Administration (NOAA). This algorithm has
also been applied to Microwave Humidity Sounder (MHS) with some modification
since AMSU-B and MHS have very similar channels. Vila et al. ( 2007 )addedan
emission-based component to this rainfall algorithm to account for oceanic rain
systems that have little or no ice in them.
Figure 6.2 illustrates the use of passive MW measurements to retrieve rainfall.
Shown is the Naval Research Laboratory (NRL) passive MW composite, overlaid
on the global IR composite that was presented in Fig. 6.1 . The various color shades
indicate the intensity of rainfall derived from the array of MW sensors used in this
composite (e.g., SSM/I, AMSU, TMI, and AMSR-E).
6.2.3 Active Microwave Methods
In contrast to passive microwave radiometers, active microwave sensors provide
their own source of microwave radiation and are able to determine fine-scale and
vertical distribution of rainfall. In orbit since 1997, the precipitation radar (PR) on