ALBEDOMAP: MERIS Land Surface Albedo Retrieval Using Data Fusion with MODIS BRDF and Its Validation Using Contemporaneous EO and in Situ Data Products

A representative albedo is required by ESA for the improved retrieval of atmospheric products, such as water vapour, from the ENVISAT-MERIS instrument. This albedo is required at 13 of the 15 spectral channels of MERIS. An algorithm is described which has been applied to the generation of a global albedo for these 13 spectral bands of MERIS over 16-day time periods at a resolution of 0.05deg for the time period from June 2002 to December 2006. Inputs to this algorithm include improved cloud and snow detection, aerosol correction using the MERIS data itself and derived Spectral surface Directional Reflectances, SDRs (Schroder et ah, Preusker et al, see, http://envisat.esa. int/workshops/meris_aatsr2005/). Owing to the lack of sufficient directional samples for most of the Earth's land surface, even for a monthly compositing time period, direct inversion of BRDF parameters for MERIS are not possible worldwide. Instead the 16-day Collection 4 MODIS BRDFs at 0.05deg resolution (derived using either full inversions or magnitude inversions from the MOD43 values) were employed in a magnitude inversion scheme for the 4 common MERIS (490plusmn5 [b3], 560plusmn5 [b5], 665plusmn5 [b7], 865plusmn10 [bl3]) bands with MODIS (459-479 {b3}, 545-565 {b4}, 620-670 {bl}, 841-8766 {b2}). The impact of applying Terra-only compared to Terra+Aqua-combined are here compared and presented. In keeping with previously reported results, there are larger numbers of full MODIS inversions (and consequently better MERIS albedos) for the combined BRDF but the total number of MODIS BRDF retrievals remains stubbornly similar. Spectral interpolation to the remaining 9 MERIS bands and to 3 broadband regions (0.4-0.7 mum, 0.7-3 mum, 0.4-3 mum) is then performed using polynomial look-up tables derived by Dr Shunlin Liang (University of Maryland). As spectral albedo data is required on monthly time-steps for most applications, a simple- minded weighting function based on the fractional time-period of eac- h 16-day time period within a month was adopted for creating monthly products from 16-day products at 10km and 0.1deg. Global products on a 16-day time-step are being generated for the whole time period using both Terra-only and, where available, Terra+Aqua-combined Collection 4 MODlS-BRDFs. These EO products have been compared against contemporaneous MODIS spectral albedos (over 16-days) which have been gap-filled (Moody et al., 2005) as well as Collection 5 products, where available. Satellite intercomparisons have also been performed for monthly products of the monthly MERIS spectral albedo products for common bands against M1SR level-3 and POLDER level-3 products as well as for a subset of M1SR level-2 instantaneous products and the results will be shown here. Finally, inter-comparisons are shown of these EO products against ground-based albedometer measurements for a site in Finland will be presented as representative of one of several worldwide sites available through the CA VE taking into account, where necessary, issues associated with land cover heterogeneity.