Mapping land surface window (8-12 μm) emissivity from ASTER thermal data

Land surface window (8-12 μm) emissivity is an important parameter for estimating the longwave radiation budget in the study of earth-atmosphere system. This paper focuses on estimation and validation of the window emissivity using the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data. Using this sensor, it is possible to estimate surface spectral emissivity for five channels in thermal infrared region. An example is presented for a desert region in North Africa. In this paper, a multiple regression was used to relate the five ASTER emissivities to the window emissivity. This regression was developed using laboratory spectral measurement data. We validated this approach using a field radiometer that has a window channel and five spectral channels similar to ASTER´s wavelengths. The predicted window emissivities agreed within 0.01 RMSE of measured window emissivity. We applied this regression to emissivities extracted from ASTER data acquired in 2001 and 2002 over a 400 km by 1200 km area in the Sahara Desert. The derived emissivity map showed that the value widely ranges between 0.82 and 0.96 in desert region. These results show that ASTER data is useful for mapping the spatial variations of surface window emissivity over large area in the deserts of the world.