Influence of emissivity angular variation on land surface temperature retrieved using the generalized split-window algorithm

The angular variation of land surface emissivity (LSE) is rarely considered in the split-window algorithm for retrieving land surface temperature (LST), and this can cause large uncertainties in LST retrievals. To analyze the influence of angular LSE variation on LST retrievals, we built a look-up table (LUT) of directional emissivities from the MYD21A1 LST/LSE product in the Moderate Resolution Imaging Spectroradiometer (MODIS) split-window channels. The extracted directional emissivities were then input into the MODIS generalized split-window (GSW) algorithm to substitute for the classification-based emissivities. A simulation analysis was first conducted based on the LUT. Furthermore, the LST retrievals estimated from MODIS observations using the directional emissivities were compared with those estimated using the classification-based emissivities. In-situ measurements from the US SURFRAD and China's HiWATER networks were used to evaluate LST retrievals obtained using the two different emissivities. The results showed that angular LSE variations in the split-window channels for vegetated surfaces were generally minor during the daytime, but more pronounced during the night-time (approximately 0.005 between nadir and 60 degrees). For barren surfaces, the angular LSE variation in the (similar to)12 mu m channel was negligible but reached approximately 0.01 in the (similar to)11 mu m channel. In the simulation, the influence of angular LSE variation was minor for view-zenith angles (VZA) < 40 degrees, but pronounced for VZA > 40 degrees reaching approximately 1.0 and 0.7 K at VZA 65 degrees for barren and vegetated surfaces, respectively. In the evaluation, the LST estimated using the directional emissivities showed a higher accuracy than those estimated using the classification-based emissivities, especially over barren surfaces where the improvement reached > 1 K. We conclude that angular LSE variation cannot be ignored in LST estimation using the GSW algorithm when VZA is > 40 degrees, especially over barren surfaces. The accuracy of the GSW algorithm is improved pronouncedly by using the directional emissivities extracted from the MYD21 product.