The effect of soil moisture and vegetation on the 37 GHz microwave polarization difference index and on 91 and 150 GHz earth surface emissivities

The Microwave Polarization Difference Index (MPDI), defined as the difference between the 37 GHz vertically and horizontally polarized brightness temperatures (TBs) measured by the Defense Meteorological Satellite Program's (DMSP) Special Sensor Microwave/lmager (SSM/I), often is used for vegetation monitoring. However, variability in soil moisture can confound its interpretation. Better knowledge of surface emissivity is important over land for improving the retrieval accuracy of lower tropospheric water vapor profiles from DMSP Special Sensor Microwave/Water Vapor Sounder (SSM/T-2) 91 and 150 GHz TB observations. The focus of this research is evaluation of the effect of soil moisture and vegetation on the MPDI and on 91 and 150 GHz land surface emissivities. An Antecedent Precipitation Index (API) and the Normalized Difference Vegetation Index (NDVI) are used as surrogates for soil moisture and vegetation density, respectively.; For a U.S. data set, linear equations are derived that quantify the relationship between MPDI and API for three categories of NDVI. They demonstrate that soil moisture information can be extracted from the MPDI when the NDVI is used to account for the influence of vegetation and also that the effect of soil moisture on the MPDI should be considered if it is used as a vegetation index. The potential to normalize MPDI values for variations in soil moisture using lower frequency microwave TB measurements is high.; A 91 and 150 GHz emissivity retrieval scheme that uses radiosonde-measured atmospheric profiles, estimates of surface skin temperatures, and 91 and 150 GHz SSM/T-2 TB observations as inputs to a radiative transfer model is evaluated with a global set of measurements. Results show that it generally has limited sensitivity to uncertainty in the input data and that the retrieved values are reproducible for a given region, thus giving confidence in its use.; Inverse linear relationships are found between emissivity at 91 and 150 GHz and API when a U.S. data set is stratified by NDVI categories. It is demonstrated that TB measurements from the SSM/I, particularly from its 19 and 37 GHz horizontally polarized channels, are potentially useful for estimating 91 and 150 GHz emissivities.; Knowledge gained from this research will contribute to improved extraction of geophysical information from global passive microwave satellite observations, which will be valuable to future environmental studies.