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The retrieval of rainfall intensity from radiances measured by spaceborne microwave radiometers can be understood in terms of well established physics. At frequencies below about 40 GHz over an ocean background the relationship between the rainfall and the observations is particularly well understood. In this part of the spectrum, the radiances are principally determined by the liquid hydrometeors with only a modest amount of ambiguity. In very intense convection, ice aloft may increase this ambiguity somewhat. At high frequencies, such as the 85.5 GHz channel of the SSM/I, scattering by the frozen hydrometeors becomes more significant and quantitative rainfall retrieval becomes more problematic. In spite of the ambiguities, the use of the higher frequencies is desirable on a number of counts including: applicability over land, spatial resolution and dynamic range.A total of 16 algorithms were submitted for the PIP-1. These include algorithms that are based on high frequency (scattering) measurements and low frequency (emission) measurements with a few combinations and variations on these themes. The calibration of the algorithms varies from mostly empirical to essentially first principles with most falling somewhere in-between. All of the algorithms retrieved rainfall and one also retrieved a profile of the liquid and frozen hydrometeors.

The global distribution of precipitation is an outstanding example of a pattern whose form cannot be deduced very satisfactorily from conventional observational data. Many investigations of the global hydrological cycle across real periods of time are based on broadly generalized assumptions concerning rainfall patterns, especially over the world's oceans. This paper explores the feasibility of employing observational data from meteorological satellites to yield more acceptable maps of rainfall across periods of 1 mo and upward than is possible using conventional surface measurements, whose distributions are less uniform, and whose derivations are more heterogeneous, than the satellite data coverage. The central problem is related to the fact that satellites cannot measure rainfall directly, and the solution of this problem necessitates the construction of a rainfall coefficient equation to be evaluated from nephanalysis indications of cloud cover. Evaluated coefficients for the months of March, April, May, and June 1966 were plotted against the corresponding rainfall recordings from a selected scatter of surface stations in the Australian region, and a best fit regression line was computed to relate the two sets of values. The regression equation was used subsequently b the compilation of a map of the estimated precipitation field for July 1966 covering Australia and adjacent areas. Finally, the implications, and some potential applications, of the method are discussed, and suggestions are made concerning its possible further development in association with satellite photographs and computer techniques of data processing and data analysis.

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