Ground Measurements for the Validation of Land Surface Temperatures Derived from AATSR and MODIS Data

An experimental site was set up in a large, flat and homogeneous area of rice crops for the validation of satellite derived land surface temperature (LST). Experimental campaigns were held in the summers of 2002-04, when rice crops show full vegetation cover. LSTs were measured radiometrically along transects covering an area of 1 km. A total number of four thermal radiometers were used, which were calibrated and inter-compared through the campaigns. Radiometric temperatures were corrected for emissivity effects using field emissivity and downwelling sky radiance measurements. A database of ground-based LSTs corresponding to morning, cloudfree overpasses of Envisat/Advanced Along-Track Scanning Radiometer (AATSR) and Terra/Moderate Resolution Imaging Spectroradiometer (MODIS) is presented. Ground LSTs ranged from 25 to 32 oC, with uncertainties between ±0.5 and ±0.9 oC. The largest part of these uncertainties was due to the spatial variability of surface temperature. The database was used for the validation of LSTs derived from the operational AATSR and MODIS split-window algorithms, which are currently used to generate the LST product in the L2 level data. A quadratic, emissivity dependent split-window equation applicable to both AATSR and MODIS data was checked as well. Although the number of cases analyzed is limited (five concurrences for AATSR and eleven for MODIS), it can be concluded that the split-window algorithms work well, provided that the characteristics of the area are adequately prescribed, either through the classification of the land cover type and the vegetation cover, or with the surface emissivity. In this case, the AATSR LSTs yielded an average error or bias of -0.9 oC (ground minus algorithm), with a standard deviation of 0.9 oC. The MODIS LST product agreed well with the ground LSTs, with differences comparable or smaller than the uncertainties of the ground measurements for most of the days (bias of +0.1 oC and standard deviation of 0.6 oC, for cloud-free cases and viewing angles smaller than 60o). The quadratic split-window algorithm resulted in small average errors (+0.3 oC for AATSR and 0.0 oC for MODIS), with differences not exceeding ±1.0 oC for most of the days (standard deviation of 0.9 oC for AATSR and 0.5 oC for MODIS).