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. Howe ver, over land surfaces, it varies over a considerable range depending prin cipally on vegetation cover and soil moisture. This study examines the feas ibility of estimating emissivity from satellite-based vegetation and moistu re indicators for a semiarid region in the African Sahel. Microwave emissiv ity was calculated from SSM/I observations at 19, 37, and 85 GHz horizontal (H) and vertical (V) polarisation. The technique was validated by comparin g the measured emissivity of a sea surface area with the theoretically pred icted emissivity. For a dry atmosphere, there was good agreement between th eory and measurement. However, the discrepancy was considerably higher in a n area where the atmosphere was humid, particularly at 85 GHz. This is attr ibutable to increased uncertainty in atmospheric correction. The land surfa ce emissivity over a 5 degrees square area, which included the Hapex Sahel site, was studied from August to October 1992. The horizontally polarised e missivity CH and polarisation difference measured over dry land areas were found to be well-correlated with Normalised Difference Vegetation Index (ND VI) such that NDVI can be used to estimate Pixel e(H) to within +/-0.02. Fo r a wet land surface, there is a general trend for the emissivity to increa se with increasing NDVI and for the polarisation difference to decrease. Ho wever, the trend is much less well defined than in the dry case. A weak rel ationship was observed between areal averages of previous day's rainfall (P DR) and emissivity for various vegetation cover classes. A similar relation ship 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 El sevier Science Inc. All rights reserved.