Satellite observations of the microwave emissivity of a semi-arid land surface

Microwave emissivity is an important parameter for rainfall estimation over land. as well as for atmospheric temperature and humidity retrievals. However, over land surfaces, it varies over a considerable range depending principally on vegetation cover and soil moisture. This study examines the feasibility of estimating emissivity from satellite-based vegetation and moisture indicators for a semiarid region in the African Sahel. Microwave emissivity was calculated from SSM/I observations at 19, 37, and 85 GHz horizontal (H) and vertical (V) polarisation. The technique was validated by comparing the measured emissivity of a sea surface area with the theoretically predicted emissivity. For a dry atmosphere, there was good agreement between theory and measurement. However, the discrepancy was considerably higher in an area where the atmosphere was humid, particularly at 85 GHz. This is attributable to increased uncertainty in atmospheric correction. The land surface emissivity over a 5 degrees square area, which included the Hapex Sahel site, was studied from August to October 1992. The horizontally polarised emissivity CH and polarisation difference measured over dry land areas were found to be well-correlated with Normalised Difference Vegetation Index (NDVI) such that NDVI can be used to estimate Pixel e(H) to within +/-0.02. For a wet land surface, there is a general trend for the emissivity to increase with increasing NDVI and for the polarisation difference to decrease. However, the trend is much less well defined than in the dry case. A weak relationship was observed between areal averages of previous day's rainfall (PDR) and emissivity for various vegetation cover classes. A similar relationship was observed with ground-based soil moisture measurements. The results show that emissivity can be estimated with a S.E. < 0.015 at 19 GHz from a combination of NDVI and rainfall or soil moisture information. (C) 2001 Elsevier Science Inc. All rights reserved.