Analyzing the anisotropy of thermal infrared emissivity over arid regions using a new MODIS land surface temperature and emissivity product (MOD21)

The MOD21 Land Surface Temperature and Emissivity (LST&E) product will be included in forthcoming Moderate Resolution Imaging Spectroradiometer (MODIS) Collection 6. Surface temperature and emissivities for thermal bands 29 (8.55 μm), 31 (11 μm) and 32 (12 μm) will be retrieved using the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Temperature and Emissivity Separation (TES) method adapted to MODIS at-sensor spectral radiances, previously corrected with the Water Vapor Scaling method (MOD21 algorithm). We simulated MOD21 product estimates over two different sandy deserts (i.e. White Sands and Great Sands) using a series of MODIS scenes from 2010 to 2013. The objective of this study was to evaluate the anisotropy of the thermal infrared emissivity over semiarid regions, since angular variations of thermal infrared emissivity imply important uncertainties in satellite LST retrievals. The obtained LSEs and their dependence on zenith viewing angles were analyzed. Results from the MOD21 simulated algorithm showed that band 29 LSE decreased up to 0.038 from nadir to zenith angle of 60°, while LSEs for bands 31 and 32 did not show significant variation. MOD21 LSE for band 29 also showed mean differences between night and daytime retrievals of + 0.027 for WS and + 0.009 for GS. These differences can be attributed to the water vapor adsorption of the soil from the atmosphere. MOD21 nadir and off-nadir LSEs showed a good agreement with laboratory emissivity measurements, and were used to validate with satellite data a zenithal-dependent emissivity model proposed in a previous study.