The ESA GlobAlbedo Prototype Sea-Ice Albedo Products from European & US Sensors
Muller, Jan-Peter1; Fisher, Daniel1; Watson, Gill1; Shane, Neville1; Kennedy, Tom1; North, Andreas1; Heckel, Andreas2; Danne, Olaf1; Brockmann, Carsten3; Stroeve, Julienne4; Pinnock, Simon5

A land surface broadband albedo map of the entire Earth's land surface, including snow and permanent ice, is required for use in Global Climate Model initialisation and verification as well as in hydrological modelling of Soil-Vegetation Transfer Schemes (SVATS). The land surface component of the GlobAlbedo project is described in an accompanying paper [1]. Sea-ice albedo is one of the largest sources of surface albedo changes on the planet and is intimately linked to climate processes involved in air-sea interactions including subsequent thinning of sea-ice [2]. Observations from satellites over the last decade indicate that sea-ice cover is reducing, especially in September months [2]. Albedo is difficult to measure using the N-day time compositing approach used for the land surface because sea-ice also moves more than 1-2km every day. Fortunately, the polar inclination of ENVISAT and NASA-Terra/Aqua means that there may be a large number of overpasses with different directional looks and more importantly a range of solar angles. There are two directional sensors in space measuring how sea-ice scatters incoming solar radiation into the hemisphere, the ESA AATSR, which has two directional looks, and the NASA MISR instrument which has 9 directional looks. We evaluate what is the impact on the BRDF/albedo values which arises from the diurnal compositing using the GlobAlbedo approach against the use of instantaneous multi-directional looks and how closely MISR and AATSR-derived albedos compare against in situ shortwave albedo measurements on sea-ice floes (collected by Donald K. Perovich, NSIDC) in 2004. A fundamental requirement for fusing several such datasets is that the co-registration/co-alignment of the pixels must be sub-pixel to ensure that there is no smearing of the fine-scale BRDF effects. Work will be reported on the sub-pixel co-registration of AATSR nadir pixels to MERIS based on the approach described in [3] as well as on the co-registration of MISR pixels based on Space Oblique Mercator and MODIS in polar stereographic projection. Another key issue is the detection of clouds over sea-ice and methods employing fusion of AATSR-MERIS will be compared against those based on stereo retrievals of cloud masks [4]. Finally, example products will be shown of prototype sea-ice albedo products and their potential for the future discussed. References cited [1] Muller, J.-P., López, G., Watson, G., Shane, N., Kennedy, T., Lewis, P., Fischer, J., Guanter, L., Domenech, C., Preusker, R., North, P.R., Heckel, A., Danne, O., Krämer, U., Zülhke, M., Brockmann, C., Cescatti, A., Pinnock, S. (2013) The ESA GlobAlbedo land surface albedo products from European sensors and their analysis and validation. This Symposium [2] Stroeve, J., Serreze, M., Holland, M., Kay, J.E., Barrett, A.P. (2012) The Arctic's rapidly shrinking sea ice cover: a research synthesis. Climatic change, 110, 1005-1027. DOI: 10.1007/s10584-011-0101-1 [3] Fisher and Muller. Global warping coefficients for improving ATSR co-registration. Remote Sensing Letters (2012) vol. 4 (2) pp. 151-160, DOI: 10.1080/2150704X.2012.713138 [4] Muller, J., M. Denis, R. D. Dundas, K. L. Mitchell, C. Naud, and H. Mannstein (2007), Stereo cloud-top heights and cloud fraction retrieval from ATSR-2, Int. J. Remote Sens., 28, 1921-1938. DOI: 10.1080/01431160601030975 * work supported under ESA/ESRIN contract 22390/09/I-OL