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Mapping snow grain size on the ice sheets with a laser altimeter

Recent development of a spaceborne laser altimeter system for mapping changes in ice sheet elevation has also provided a surprising opportunity for mapping slow grain size. This new technique exploits the relationship between snow grain size and surface reflectance in the near-infrared wavelengths. A discrete-ordinates radiative transfer model is used to model the relationship between surface reflectance and grain size. Model inputs include illumination and viewing conditions thus requiring slope/aspect information. This method has been successfully tested using imaging spectrometer data at 1040 nm. An active sensor, as a Nd:YAG laser with a wavelength of 1064 nm could also be used. The Geoscience Laser Altimeter System (GLAS), due to launch in 2001, will provide all the required inputs for this grain size determination method including apparent surface reflectance and slope/aspect of the ground footprint. Though this method has not yet been experimentally validated, model simulations show that it should provide the most accurate estimates of snow surface layer grain size to date.