A Cloudless Land Atmosphere Radiosounding database for generating Land Surface Temperature retrieval algorithms

A database of global, cloud-free, atmospheric radiosounding profiles was compiled with the aim of simulating radiometric measurements from satellite-borne sensors in the thermal infrared. The objective of the simulation is to generate split-window (SW) and dual-angle (DA) algorithms for the retrieval of land surface temperature (LST) from Terra/Moderate Resolution Imaging Spectroradiometer (MODIS) and Envisat/Advanced Along Track Scanning Radiometer (AATSR) data. The database contains 382 radiosonde profiles acquired over land, with nearly-uniform distribution of precipitable water between 0 and 5.5 cm. Radiative transfer calculations were performed with the MODTRAN 4 code. Different viewing angles were considered in the simulation, taking into account the features of each sensor. The viewing capability of AATSR, with near simultaneous observations first at a forward angle (55 degrees from nadir) and then close to nadir, allows the implementation of DA algorithms. Using the simulation database, SW algorithms adapted for MODIS and AATSR data, and DA algorithms for AATSR data were developed. Both types of algorithms are quadratic in the brightness temperature difference, and depend explicitly on the land surface emissivity. A sensitivity analysis of all algorithms was made to obtain estimation of the algorithm errors. Furthermore the SW and DA algorithms developed from the simulation database were validated with actual ground measurements of LST collected, concurrently to MODIS and AATSR observations, in a site located close to the city of Valencia, Spain, in a large, flat and thermally homogeneous area of rice crops, where field campaigns were held during the summers of 2002-2006. Operational LST algorithms of each sensor were also validated in order to compare with the algorithms generated.