Mapping Surface Broadband Emissivity of the Sahara Desert Using ASTER and MODIS Data

Surface broadband emissivity in the thermal infrared region is an important parameter for the studies of the surface energy balance. This paper focuses on estimating a broadband window emissivity from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and Moderate Resolution Imaging Spectrometer (MODIS) data. Both sensors are on board the NASA Earth Observing System (EOS) Terra satellite, which was launched in 1999. First, several definitions of the broadband emissivity were investigated, and it was found that the emissivity integrated between 8 and 13.5 mu m is the best for estimating the net longwave radiation under clear-sky conditions. Then, a method to estimate broadband emissivity at the continental scale was developed. The method uses two regressions. The first regression is to relate the broadband emissivity to the emissivities for the five ASTER channels using measured emissivities in the laboratory from spectral libraries. The second regression relates the broadband emissivity map from ASTER data to the emissivity and reflectance derived from MODIS data. The first regression was used for mapping the broadband emissivity using ASTER data in a 500 km x 1400 km area of North Africa, which includes Tunisia, eastern Algeria, and western Libya. This emissivity map was used to calibrate the second regression, which was applied to MODIS data and generated a broadband emissivity map over North Africa and the Arabian Peninsula. The range of the broadband emissivity was found to be between 0.85 and 0.96 for the desert area. The resulting broadband emissivity maps and the methodology for generating them will contribute to future climate modeling studies.