Evaluation of the two Collection 6 MODIS LST products using ground measurements collected from a barren surface in Northwest China

In this study, the two latest Collection 6 (C6) MODIS Level-2 land surface temperature (LST) products (MxD11_L2 and MxD21_L2) from the Terra and Aqua platforms were validated against ground LST, which was acquired from a barren surface in northwest China during the Heihe Watershed Allied Telemetry Experimental Research (HiWATER) field campaign. Ground measurements obtained over a four-and-a-half-year period from June 2012 to December 2016 at four barren surface sites were used to perform the evaluation. The evaluation results indicate that the C6 MxD21 LST products have a better accuracy than the C6 MxD11 product during both daytime and nighttime for all four sites. For the daytime results, the LST is underestimated by the C6 MxD11 products at all four sites, with a mean bias of -1.77 K for Terra and -2.63 K for Aqua, whereas the mean biases of the MxD21 LST products are within 1 K, with a mean bias of 0.34 K for Terra and -0.67 K for Aqua, respectively. For the nighttime results, the LST is also underestimated by the C6 MxD11 products at all four sites, with a mean bias of -1.41 K for Terra and -1.01 K for Aqua, whereas the mean biases of the MxD21 LST products are 0.14 K for Terra and 0.54 K for Aqua, respectively. The emissivities in MODIS bands 31 and 32 for the two products were compared with each other and then compared with the ASTER emissivity. The results indicate that the underestimation of the C6 MxD11 LST product at all four sites is mainly results from the overestimation of the emissivities in MODIS bands 31 and 32. In addition, an experiment was carried out to test whether the physically retrieved dynamic emissivity of the MxD21 product can be used to improve the performance of the split-window algorithm for a barren surface. The results indicate that the biases were greatly improved compared with those of the C6 MxD11 LST product, with a mean bias decreased during the daytime from -1.80 K to 0.06 K for Terra and from -2.87 K to -0.87 K for Aqua, respectively. This study demonstrates that the physically retrieved emissivity of the MxD21 product is a useful input source for LST retrieval from different remote sensing data.