Numerical Simulations of MODIS Sensitivity Potential for Assessing Near Surface Mountain Snow Melt

Abstract

Variations in amount of near surface snow melt produce changes in snow grain size. We evaluated Moderate Resolution Imaging Spectroradiometer (MODIS) visible and near‐infrared bands for monitoring snow pack ripeness using a Mie scattering model, coupled with a simple approach for computing directional hemispherical reflectance (albedo or total flux leaving surface divided by total irradiance). Results suggest MODIS visible and near‐infrared bands may have the potential to detect changes in grain size as an indicator of mountain snow pack evolution during the melt season. Modeled at‐sensor radiance was produced for grain size changes ranging from 10μm to 1000μm. Differences in simulated at‐sensor radiance as a function of grain size, solar zenith angle, surface slope, and amount of diffuse irradiance were compared to band specific noise equivalent radiance (NEΔL) thresholds. MODIS wavelengths greater than 667nm may be sensitive to large changes in grain sizes, particularly those bands with coarse spatial resolution (1000m). Longer wavelengths showed greater sensitivity to small changes in smaller grains than to small changes in larger grains. Shorter wavelengths with small fine spatial resolution (500m) appear less effective overall for monitoring changes in grain size. Variation in solar zenith angle has a greater impact on changes in larger grains (100‐1000μm) than smaller (10‐100μm). The overall magnitude of difference in modeled at‐sensor radiance demonstrates that the affect of diffuse irradiance may not be substantial. Surface slope does not appreciably affect changes in model at‐sensor radiances for various grain sizes. This may not be true given the model does not account for snow BRDF.