This paper describes how we identify various surface types, calibrate the emissivity adjustment for each unique SSM/I signature, verify of the accuracy of our approach to estimate shelter height temperatures from the passive microwave observations, and validate the utility of the wetness index with independent precipitation measurements. Hourly first order stations served as the reference for emissivity adjustments, and least absolute difference techniques were used to minimize the standard error for each surface type. A dense U.S. cooperative network of monthly mean in situ temperature anomalies provided validation of the temperature anomalies derived from the SSM/I channels.

The Global Precipitation Climate Program (GPCP) provides a reference or validation (Huffman et al. 1995) of the BWI. The GPCP precipitation values have been compiled over the same six year period, thereby allowing for a direct comparison of anomalies over the same base period. The GPCP values are derived from numerous sources: in situ point measurements, as well as infrared and microwave observations. The final product uses an interpolation scheme to produce global fields at 2.5° monthly resolution for each month. Where in situ data are available, they get considerably more weight than satellite observations. Since it would be inappropriate to compare the BWI in locations where the microwave observations have significant weight in the GPCP product, (i.e. the two products would not be independent), the comparisons were made over areas where in-situ observations were adequate to dominate the final product, thereby allowing us to assume that the two data sets are largely independent. The fact that these data are largely independent is substantiated by a map of weight for all data sources in the GPCP analysis and by the analysis performed is this study. If there was a lot covariance (cross talk) due to dependence, the correlations would not sharply increase when the precipitation is allowed to lag the wetness anomalies. A major reason the GPCP product is used in this study rests on the fact that it is probably the most comprehensive global analysis of precipitation, and that the same quality control and interpolation schemes were used across national borders and throughout all six study areas. A comparison of the two monthly anomalies provide a venue to validate the utility of the BWI, and its ability to serve as a proxy for precipitation over an area.

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