Land surface VIS/NIR BRDF atlas for RTTOV-11: model and validation against SEVIRI land SAF albedo product
Vidot, Jérôme ; Borbás, Éva
This study describes the scientific approach and the validation of the visible and near-infrared snow-free land surface Bidirectional Reflectance Distribution Function (BRDF) atlas for Version 11 of the Radiative Transfer for the Television Infrared Observation Satellite (TIROS) Operational Vertical Sounder (TOVS) (RTTOV) Forward Model. The atlas provides a global (at a spatial resolution of 0.1°) and monthly mean land surface BRDF for any instrument containing channels with a central wavelength between 0.4 and 2.5 µm, as well as a quality index of the BRDF. It is based on the reconstructed hyperspectral BRDF from the seven channels of the operational and global Moderate Resolution Imaging Spectroradiometer (MODIS) 16 days BRDF kernel-driven product MCD43C1: a principal component analysis regression method was applied between the seven channels of the MODIS BRDF products and a set of the US Geological Survey hyperspectral reflectance measurements for soils, rocks, and mixtures of both and vegetation surfaces. The comparison of the RTTOV BRDF atlas against the Spinning Enhanced Visible and Infrared Imager (SEVIRI) surface black-sky albedo products of the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT) Land Satellite Application Facility on Land Surface Analysis (Land-SAF) shows good spatial and temporal consistency of the RTTOV BRDF atlas when applied on three SEVIRI visible and near-infrared channels. The RTTOV narrowband black-sky albedo is retrieved within ±0.01 in absolute accuracy at 0.6 and 1.6 µm and is overestimated by something between 0.01 and 0.03 at 0.8 µm. The temporal variation of the RTTOV broadband black-sky albedo is consistent with the EUMETSAT Land-SAF SEVIRI products but overestimated by somewhere between 0.01 and 0.02 when considering the best quality index of the RTTOV BRDF atlas. Less agreement is found in two cases: (i) for extreme geometrical conditions when the satellite zenith angle is greater than 65° and (ii) for lower quality indices of the RTTOV BRDF atlas.
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