Extended Abstract (288K)P5.1
Enhancement of the MODIS Daily Snow Albedo Product
Dorothy K. Hall, NASA, Greenbelt, MD; and C. B. Schaaf and G. A. Riggs
The MODIS daily snow albedo product is a data layer in the MOD10A1 snow-cover product that includes snow-covered area and fractional snow cover as well as quality information and metadata (Riggs et al., 2006; Hall and Riggs, 2007). The daily snow albedo algorithm was developed by Klein and Stroeve (2002), with heritage from the work of Stroeve et al. (1997), Liang (2000) and Nolin and Liang (2000). It was developed to augment the MODIS albedo product (MCD43) that provides 8-day maps of albedo globally at 500-m resolution (Schaaf et al., 2002). The MODIS daily snow albedo product provides more frequent, though less robust maps but only for pixels defined as “snow” by the MODIS snow-cover algorithm. Several validation efforts have shown that the MODIS daily snow albedo product is useful over large, relatively-flat areas (e.g., Sorman et al., 2006; Stroeve et al., 2006; Tekeli et al., 2006), but errors increase in more-complex terrain.
Daily snow albedo is calculated using various inputs such as the MODIS surface reflectance product (MOD09), Bidirectional Reflectance Distribution Function (BRDF), land cover product and a Digital Elevation Model (DEM) (Klein and Stroeve, 2002). Models of the BRDF of snow are created using the discrete-ordinate radiative transfer (DISORT) model of Stamnes et al. (1988) to correct for anisotropic scattering effects over non-forested surfaces. A narrowband or spectral albedo is calculated for each of the shortwave MODIS bands, then combined into a spectrally-integrated broadband albedo. Snow is treated as an anisotropic surface except in forests where it is treated as an isotropic surface.
There are important limitations to the MOD10A1 daily snow albedo product. For example, it is difficult to perform an atmospheric correction over a bright surface such as snow. In addition, utilizing the appropriate per-pixel BRDFs can be problematic, and correction for anisotropic scattering must be included. Also, narrow to broadband conversion specific for snow on different surfaces must be calculated and this can be difficult. In consideration of these limitations, we are exploring an enhancement to the daily snow albedo product algorithm by coupling the periodic per-pixel snow albedo (MCD43) with the daily surface reflectances. Use of the newly-reprocessed Version 005 data along with improved cloud discrimination will contribute to the improvement of the daily product. Some preliminary validation of the new daily snow albedo product is provided.
References
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Klein, A.G., Stroeve, J.C., 2002: Development and validation of a snow albedo algorithm for the MODIS instrument, Annals of Glaciology, 34:45-52.
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Nolin, A.W. and Liang, S., 2000: Progress in bi-directional reflectance modeling and applications for surface particulate media: snow and soils, Remote Sensing Reviews, 18: 307-342.
Riggs, G., Hall, D.K., Salomonson, V.V., 2006a: MODIS Snow Products Users Guide to Collection 5. http://modis-snow-ice.gsfc.nasa.gov (last accessed June 2008).
Schaaf C.B., Gao., F., Strahler, A.H., Lucht, W., Li, X., Tsang, T., et al., 2002 : First standard BRDF, albedo and nadir reflectance products from MODIS, Remote Sensing of Environment, 83 :135-148.
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Sorman, A.U., Akyurek, Z., Sensoy, A., Sorman, A.A., Tekeli, A.E., 2006: Commentary on comparison of MODIS snow cover and albedo products with ground observations over the mountainous terrain of Turkey, Hydrology and Earth System Sciences Discussion, 3:3655-3673 www.hydrol-earth-syst-sci-discuss.net/3/3655/2006/.
Stroeve, J.C., Box, J.E., Haran, T., 2006: Evaluation of the MODIS (MOD10A1) daily snow albedo product over the Greenland ice sheet, Remote Sensing of Environment, 105:155–171.
Tekeli, A.E., Sensoy, A., Sorman, A., Sula, A., Sorman, U., 2006: Accuracy assessment of MODIS daily snow albedo retrievals with in situ measurements in Karasu basin, Turkey, Hydrological Processes, 20:705-721 DOI: 10.1002/hyp.6114.
Poster Session 5, Validation of Remotely-Sensed Hydrometeorological Observations
Wednesday, 14 January 2009, 2:30 PM-4:00 PM, Hall 5
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