Understanding the Differences Between AIRS, MODIS and ASTER Land Surface Emissivity Products

One of the key Earth Science Data Records identified by NASA is Land Surface Temperature and Emissivity (LST&E). LST&E data are key parameters in global climate change studies that involve climate modeling, ice dynamic analyses, surface-atmosphere interactions and land use, land cover change. The errors in retrievals of atmospheric temperature and moisture profiles from hyperspectral infrared radiances, such as those from the Atmospheric Infrared Sounder (AIRS) on NASA's Aqua satellite, are strongly dependent on using constant or inaccurate surface emissivities, particularly over arid and semi-arid regions where the variation in emissivity is large, both spatially and spectrally. LST&E standard products are available from spaceborne sensors such as AIRS, MODIS and ASTER at varying spatial, spectral, and temporal resolutions. Although these emissivity products represent the same measure, there are frequently discrepancies between the products associated with different scientific approaches used that need to be better understood. For example, ASTER provides LST&E data with the highest spatial resolution (90 m), compared with AIRS (50 km) and MODIS (1 and 5 km). AIRS has the highest spectral sampling and both AIRS and MODIS acquire data at much higher temporal frequencies (every 2-3 days) compared with ASTER (16 days). In this paper we present validation and intercomparisons of AIRS, MODIS and ASTER gridded emissivity products over North America. MODIS and ASTER data will be upsampled to the AIRS spatial resolution, and then compared to laboratory measured emissivities of in-situ rock/sand samples collected at ten validation sites in the Western USA during 2008. The directional hemispherical reflectance of the in-situ samples are measured in the laboratory using a Nicolet Fourier Transform Interferometer (FTIR), converted to emissivity using Kirchoff's law, and convolving to the appropriate sensor's spectral response functions. We present here some of the first quantitative results on the differences between emissivity products from different sensors as a result of differences in spatial, spectral and temporal resolutions, and furthermore, comparisons with laboratory results will give a measure of the accuracy of the emissivity products - a critical aspect for the broad scientific community in deriving accurate land surface temperatures.