On the relationship between snow grain morphology and in-situ near infrared calibrated reflectance photographs
Résumé
Seasonal and permanent snow cover a significant portion of our planet, and its impact on climate is significant. Through specific thermophysical properties, snow controls radiative and turbulent fluxes between the ground and the atmosphere, but many aspects of the energy balance are poorly understood due to lingering uncertainties regarding snow properties, such as grain size in particular. Rapid and accurate measurement method has yet to be developed given the reality of field and laboratory logistical constraints, and the sensitivity of snow to any sort of manipulation. In this paper, we investigate the relationship between snow grain morphology parameters measured from visible (traditional) snow grain photography and optical diameter estimated from Near-InfraRed (NIR) reflectance photographs of snow walls. A total of 51 snowpits were analyzed during our International Polar Year field campaign across a 1000 km South-to-North transect over Eastern Canada. We compared the NIR measurements with the theoretical snow albedo model of Kokhanovsky and Zege (2004). Results show the large difference between the snow specific surface area (SSA) of snow grains derived from snow albedo model and the geometrical (visual) diameter. From three different snow grain classes which can be distinguished from traditional photography, linkages can be made with shape factors required in the optical model in order to retrieve optical grain size from NIR photography.