ESTIMATING SNOW GRAIN-SIZE USING AVIRIS DATA

Estimates Of snow grain size for the near-surface snow layer were calc ulated for the Tioga Pass region and Mammoth Mountain in the Sierra Ne vada, California, using an inversion technique and data collected by t he Airborne Visible/Infrared Imaging Spectrometer (AVIRIS). The invers ion method takes advantage of the sensitivity Of near-infrared snowpac k reflectance to snow grain size. The Tioga Pass and Mammoth Mountain single-band AVIRIS radiance images were atmospherically corrected to o btain surface reflectance. Given the solar and viewing geometry for th e time and location of each AVIRIS overflight, a discrete-ordinate mod el was used to calculate directional reflectance as a function Of snow pack grain size, for a wide range Of snow grain radii. The resulting r adius vs. reflectance curves were each fit using a nonlinear least-squ ares technique which provided a means of transforming surface reflecta nce in each AVIRIS image to optically equivalent grain size on a per-p ixel basis. This inversion technique has been validated using a combin ation of ground-based reflectance measurements and grain size measurem ents derived from stereologic analysis of snow samples for a wide rang e of snow grain sizes. The model results and grain size estimates deri ved from the AVIRIS data show that, for solar incidence angles between 0-degrees and 300, the technique provides good estimates of grain siz e. Otherwise, the local angle of solar incidence must be known more ex actly. This work provides the first quantitative estimates for grain s ize using data acquired from an airborne remote sensing instrument and is an important step in improving our ability to retrieve snow physic al properties independent of field measurements.