General Comments on SSM/I Data

Note: the NCAR Climate and Global Dynamics Climate Analysis Section may not have all data mentioned below.

Precipitation

The SSM/I rainfall algorithm developed at NOAA utilizes the 85V GHz channel to detect the scattering of upwelling radiation by precipitation sized ice particles within the rain layer. The scattering technique is applicable over land and ocean. Rain rate can be derived indirectly based on the relationship between the amount of ice in the rain layer to the actual rain fall on the surface. Care must be taken to remove anomalous surface scattering features (e.g., deserts and snow). Additionally, over ocean, an emission rain algorithm, based upon the absorption of the upwelling radiation by rain and cloud water (at 19 and 37 GHz) is blended with the scattering algorithm.

A scattering-based, global rainfall algorithm developed at the NOAA/SRL has been selected for use by the World Climate Research Programme/Global Precipitation Climatology Project (WCRP/GPCP) for land rainfall estimation. At present, monthly rainfall at 100 km and 250 km grids have been produced for the period of July 1987 to present. Additionally, the instantaneous rain rate produced from this algorithm has recently been implemented by Fleet Numerical Meteorology and Oceanography Center (FNMOC) as the operational SSM/I rainfall algorithm.

A new version of the 2.5 degree rainfall product has been developed for use by the GPCP, and denotes regions of indeterminate rainfall due to the presence of snow and ice cover (updated as of January 1997).

Due to the failure of the 85 GHz channels during the period of July 1990 to December 1991, no reliable retrievals can be made using the current algorithm. However, an alternate algorithm, which uses a 37 GHz scattering index over land and emission only over ocean can be used.

In addition to monthly rainfall estimates, the mean fractional coverage of rain (within a grid cell) can also be computed, which gives an indication of the relative frequency of rainfall. Finally, because the SSM/I measurements are made from a polar orbiting satellite, the estimates are subjected to sampling errors. As such, we provide maps of the SSM/I sampling frequency for the month, which can be used to assess this sampling error.

Total Precipitable Water

The SSM/I contains a vertical polarization channel at 22.235 GHz. This channel frequency is at the peak of a weak water vapor absorption line. Over ocean, this allows for the retrieval of integrated water vapor, commonly referred to as total precipitable water (TPW). Retrievals over land are impossible due to the high and varying emissivity of land surfaces. The TPW mainly corresponds to low level water vapor (i.e., 700 mb and below). Monthly TPW retrievals have been generated for July 1987 through July 1995.

It should be noted that the DMSP SSM/T2, with three channels near the 183 GHz absorption line allows for both ocean and land retrievals, as well as vertical profile information. These channels provide mainly information on upper tropospheric water vapor. NOAA/SRL possess SSM/T2 data sets beginning in March 1994 and is investigating their usefulness in several applications.

The monthly mean TPW is generated over ocean and have been screened for precipitation using the 85 GHz scattering algorithm.

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