Global Precipitation Monitoring - Satellite-based Applications on Climate Change - page 82

Fig. 6.1 Multi-satellite (i.e., numerous GEO and LEO satellites) composite of IR imagery from
1500 UTC, 2 June 2010 (Courtesy of the Naval Research Laboratory (NRL) in Monterey, CA
( http://www.nrlmry.navy.mil/sat_products.html ) )
that interacts with clouds and precipitation and is measured by a radiometer on
board a satellite.
Most passive microwave radiometers launched to date operate in frequencies
ranging from 6 to 190 GHz. At different frequencies, microwave radiometers observe
different parts of the rain profile. Below 20 GHz, emission by precipitation-size drops
dominates, and ice particles above the rain layer are nearly transparent. Above
60 GHz, ice scattering dominates, and the radiometers cannot sense the raindrops
below the freezing layer. Both emission and scattering effects are important for
frequencies between 20 and 60 GHz. In general, emission by liquid drops raises
brightness temperature, while scattering by ice particles has the opposite effect.
Window channels (i.e., wavelengths which have very little contamination due to
atmospheric constituents like water vapor and oxygen) can measure down to the
Earth's surface and are strongly influenced by surface properties (i.e., vegetation and
soil moisture). Other frequencies are sensitive to oxygen or water vapor/cloud droplet
absorption. These microwave properties set the foundation for the development of
rainfall estimation schemes.
There are two major categories in rainfall estimation using passive microwave
radiometry: emission-based method and scattering-based method. Emission-based
rainfall algorithms are mostly applicable over ocean because water surfaces are
relatively homogeneous and provide a cold background due to low emissivity. The
presence of raindrops allows for absorption and emission over the water surfaces and
results in a dramatic warming of the satellite measurements. Some of the earliest
emission techniques were developed with the Nimbus-5 and Nimbus-6 Electrically
Scanning Microwave Radiometer (ESMR) (Wilheit et al. 1977 ). Significant advances
were made through the Special Sensor Microwave/Imager (SSM/I) series, first
launched in 1987 and followed by five more instruments which operated effectively
through much of 2009 (e.g., Olson 1989 ; Ferraro and Marks 1995 ; Smith et al. 1994 ).