View Angle Bias Corrections of Geostationary Satellite Land Surface Temperature Measurements Using an Empirical Mode
Friday, 19 December 2014
Knowledge of the surface emissivity is essential for retrieving surface skin temperature (Ts), a key parameter for understanding and modeling the surface energy budget, from satellite remote sensors. For a given region, land and ocean Ts is observed at a constant viewing zenith angle (VZA) by any geostationary satellite (GEOsat) imager. Emission from the surface and, hence, Ts is VZA-dependent, varying by 6 K or more with increasing VZA. Although commonly ignored, it has been established that methodologies for angular normalization of Ts are needed to better understand surface emissivity and reduce errors in Ts retrievals. In order to develop corrections for the VZA-dependence of Ts from GEOsats, inter-calibrated GEOsat and MODerate-resolution Imaging Spectroradiometer (MODIS) data are collocated, in time and space, for clear scenes observed at different viewing and illumination angles. The radiances in these temporally and spatially matched datasets are used to retrieve coincident Ts using the same one-channel retrieval algorithm, by which Ts is computed from the observed 11-μm brightness temperature (T11) through application of atmospheric absorption corrections appropriate for that spectral channel. Matches from the two instruments are used to build an empirical model that describes the dependence of Ts on VZA by calculating the radiance differences between the near-nadir views and off-nadir data. With matched T11 data from GOES-East, GOES-West, and Aqua-MODIS for North and South America, an adjustment can be computed using matched pairs, for which the Aqua-MODIS VZA is set less than 5º. Applying this correction to the same GOES data removes the angle dependence. Errors are assessed using independent matched land surface temperature datasets from Terra- and Aqua-MODIS and in situ measurements from SURFRAD. The approach can be used to develop corrections for each GEOsat, and should also be applicable to low-Earth orbit satellites. These corrections will be valuable for improving estimates of instantaneous surface emissivity, surface radiation, and surface heat exchange in observations and models.