ence (RFI) at levels too low to be otherwise detected could, in principle, introduce errors in such measurements.

PRECIPITATION RATE

Observations of global precipitation are important to both weather forecasts and climate studies. They are particularly useful in monitoring severe storms such as hurricanes and damaging fronts. Precipitation is important not only to safety, agriculture, and commerce, but also to hydrology and predictions of floods, soil moisture, and sea surface salinity (SSS). Since the locations of convective precipitating cells cannot be predicted well, and because they sometimes reside under higher cloud shields such as those obscuring hurricanes and other severe storms, only microwave sensors can reveal their intensities and locations. Precipitation is generally observed using the same sensors as those used for water vapor, which include (1) window-channel sensors at frequencies such as 18.7, 22, 23.8, 31.4, 37, and 89 GHz that observe raindrop emission against colder backgrounds such as ocean and low-emissivity soil (e.g., Special Sensor Microwave/Imager [SSM/I], Special Sensor Microwave Imager/Sounder [SSMI/S], Advanced Microwave Scanning Radiometer-Earth [AMSR-E]), and (2) the opaque water vapor resonance 176-191 GHz in combination with lower frequencies such as 89, 150, and 164-168 GHz; glaciated cell tops are particularly visible and sensitive to convective strength. In addition, the opaque oxygen bands 50-56 GHz are useful because they are sensitive to ice particle size distributions and therefore to the heavier precipitation rates (e.g., AMSU, SSMI/S).

SEA SURFACE SALINITY

Sea surface salinity is a critical missing parameter that scientists need in order to meet climate research goals. Measuring global SSS over time will contribute to scientists’ understanding of change in the global Earth system and how the system responds to natural and human-induced change. Global measurements of SSS can be achieved to ~0.2 practical salinity units using space-based passive microwave radiometry at 1.4 GHz and radar scatterometry at 1.26 GHz.1 These measurements can provide significant new information about how global precipitation, evaporation, and the water cycle are changing. Global SSS variability provides key insight regarding freshwater flow into and out of the ocean associated with precipitation, evaporation, ice melting, and river runoff. Global SSS measurements will also provide important background about how climate variation induces changes in global ocean circulation. The combination of global SSS and sea surface temperature

1

See http://aquarius.nasa.gov/science.php; accessed on January 15, 2010.



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