Using Landsat-5 thematic mapper and digital elevation data to determine the net radiation field of a Mountain Glacier

An accurate evaluation of glacier energy balance requires a precise knowledge of surface-cover albedo and emittance. Usually these values are acquired from field measurements. However, the microclimatological effects in mountainous terrain greatly limit the potential for spatial extrapolation of such a set of values. This study overcomes the problem by using the upward radiance values registered on Landsat-5 Thematic Mapper (TM) images and information on the geometry of ter terrain extracted from a digital elevation model (DEM). Studies were performed in the Athabasca Glacier basin, one of the major glacier outlets of the Columbia Icefield in the Canadian Rocky Mountains. The methodology is composed, first, of an automated procedure for the physiographic description of the glacierized basin (cover type, elevation, slope, aspect, horizon profiles, sky-view factor, and the level of enclosing topography) in order to calculate the effects of topography on the radiation balance for each 30 m pixel. Because glacier surface covers usually have high reflective behaviors or distinct emission patterns, this study puts special emphasis on modelling the amount of terrain-reflected or terrain-emitted radiation received on a particular surface. Second, the topographic correction is applied to values of irradiance computed using the LOWTRAN-6 code, a spectrally based radiative transfer model, and atmospheric radiosonde measurements of the vertical temperature, air pressure, and relative humidity profiles. The measured upward radiance values from TM, corrected for path effects, are used to calculate surface-cover albedo and brightness temperature. Third, the daily net radiation field for snow and ice covers is computed to illustrate the contribution of the estimated surface radiative parameters to the glacier snow-and-ice melt analysis. The resultsshow that the irradiance values are accurately modelled within 10% of the radiattion values acquired from a field pyranometer. The calculated glacial surface-cover reflectance and albedo values compare favorably to published information, whereas the measured surface brightness temperature is well within the expected range of values.