MODIS albedo validation with higher spatial resolution estimates from FORMOSAT-2

Mira M., Courault D., Olioso A., Weiss M., Baret F., Hagolle O., Gallego-Elvira B., 2014. MODIS albedo validation with higher spatial resolution estimates from FORMOSAT-2. International Conference “Global Vegetation Monitoring and Modeling”, Avignon, France, 3rd - 7th February 2014. Abstract book pages 230-231. https://colloque.inra.fr/gv2m/content/download/1463/13907/version/1/file/GV2M_Abstract_Book.pdf https://colloque.inra.fr/gv2m/content/download/1404/13556/ https://colloque.inra.fr/gv2m/content/download/1506/14142/version/1/file/GV2M_Poster_S6.15_Mira.pdf [poster presentation]Mira M., Courault D., Olioso A., Weiss M., Baret F., Hagolle O., Gallego-Elvira B., 2014. MODIS albedo validation with higher spatial resolution estimates from FORMOSAT-2. International Conference “Global Vegetation Monitoring and Modeling”, Avignon, France, 3rd - 7th February 2014. Abstract book pages 230-231.https://colloque.inra.fr/gv2m/content/download/1463/13907/version/1/file/GV2M_Abstract_Book.pdf https://colloque.inra.fr/gv2m/content/download/1404/13556/https://colloque.inra.fr/gv2m/content/download/1506/14142/version/1/file/GV2M_Poster_S6.15_Mira.pdf[poster presentation]; Land surface albedo is among the main radiative uncertainties in current climate modeling. TheNASA Earth Observing System program provides series of high-level land surface productsincluding albedo at 1-km spatial resolution on a 16-day cycle derived from Moderate ResolutionImaging Spectroradiometer (MODIS) reflectances. Validation of such products still remainsproblematic because ground point-based measurements are not suitable for directcomparisons with coarse or middle spatial resolution satellite data over heterogeneouslandscapes. The objective of this study is to validate the MODIS albedo product with highspatial and temporal resolution data provided by FORMOSAT-2. To overcome the difficulty ofthe scaling problem, high-resolution images were aggregated by accounting for the PointSpread Function (PSF) of MODIS. The study area is a flat region called La Crau, covered by aspecific dry grass ecosystem located in the lower Rhône Valley, South-Eastern France. Weconsidered twenty-seven cloud free FORMOSAT-2 images (8-m spatial resolution) acquiredfrom March to October 2006 with a constant viewing angle. The albedo was estimated as alinear combination of waveband reflectances. It was previously validated against groundmeasurements over different surfaces from the same area presenting a RMSEA=0.015 and nobias (Bsaibes et al., 2009). About 12x24 equivalent MODIS pixels were finally considered foralbedo comparison after aggregation and geometric fittings. Actual albedo MODIS wasestimated from directional hemispherical albedo (black-sky albedo) and bi-hemisphericalalbedo (white-sky albedo), both included into the 1-km spatial resolution MCD43B3 product(Strahler and Muller, 1999). This standard product (which combines data from sensors onboardTerra and Aqua satellites) is a 16-days composite produced every 8 days. An estimate of thefraction of diffuse skylight (FDS) was also necessary to calculate the actual albedo. FDSchanges with solar zenith angle, optical depth, bands and aerosol model types. It wascalculated through a lookup table generated with the 6S code for MODIS bands. We obtainedvalues ranging from 0.048 to 0.313 for the optical depth at 550 nm (estimated using data fromthe Aerosol Robotic NETwork), and from 0.129 to 0.225 for the FDS. The PSF, modeled as aGaussian-like shape, was characterized by the value of its Full Width at Half Maximum(FWHM), both in the cross-track (x) and along-track (y) coordinates in the object space. Thebest performances were for PSFs with FWHMx varying from 1640 m to 2240 m, and withFWHMy from 920 to 1440 m. The cross-track size of the MODIS PSF is much higher than in thealong-track direction, as expected, because MODIS is a whiskbroom scanner. The largevariability of values obtained for FWHMx and FWHMy could be explained by different factors,such as the large homogeneity of the area or the complexity of the 16-days anisotropy modelused to create albedo MODIS by combining data from Terra and Aqua satellites (i.e., itGV2M: Global Vegetation Monitoring and Modelingused to create albedo MODIS by combining data from Terra and Aqua satellites (i.e., itconsiders data from different days, bands and viewing angles). The comparison between the 1-km FORMOSAT-2 aggregated map through the PSF and the albedo MODIS showed aRMSEA