A sensitivity analysis of the MODIS Normalized Difference Snow Index

A study of the Normalized Difference Snow Index (NDSI) being used in mapping snow from the Moderate Resolution Imaging Spectroradiometer (MODIS) has been undertaken so as to understand via a modeling approach the sensitivity of the NDSI to variations within a MODIS pixel to snow grain size, atmospheric conditions, solar zenith and instrument viewing angle. This objective is being pursued using the MODTRAN-4 radiative transfer model along with the DISORT (Discrete Ordinance-method Radiative Transfer) approach for multiple scattering calculations. In this study the snow is added as a physically thin but optically thick aerosol layer at the bottom of the atmosphere. Results achieved to date with this approach, when compared to published results (e.g., Wiscombe and Warren, Journal of the Atmospheric Sciences, Volume 37, 1980), appear to adequately describe the variation of snow spectral albedo versus wavelength for different snow grain sizes, solar zenith angles and sensor view angles. Work is continuing to get more results over a wider range of atmospheric conditions. Further efforts have the objective extending this capability to understand more completely the errors and confidence limits due to varying viewing conditions that are encountered in the application of the MODIS NDSI fractional snow cover approach to local, regional and global observations of snow cover variability