Land-based remote sensing of snow for the validation of a snow transport model

As part of the effort of the Centre d'Etudes de la Neige (CEN), Meteo France, for the improvement in modelling snow cover evolution and avalanche risk forecasting, a numerical simulation of snow transport and related mechanical effects on snow particles morphology has been developed during the last years. The objective is to incorporate the wind effects into the Meteo France operational chain SAFRAN-CROCUS-MEPRA for avalanche risk forecasting. At present, an evaluation version of the model is applied to a well-instrumented site (Col du Lac Blanc 2700 m, French Alps), in order to assess its performance and validate its applicability. Here, in addition to nivo-meteorological stations, several specific snowdrift sensors and a range of snow depth poles, a novel remote sensing technique based on terrestrial photography is being applied. Oblique digital photographs of the area are taken periodically and after snowdrift events from an elevated viewpoint. These photographs are then georeferenced to a digital elevation model (DEM). Further radiometric corrections depending on topography and atmospheric conditions are applied to the georeferenced pixels to derive relative albedo. Spectral filtering applied to the digital camera permits detecting small variations in the associated snow grain characteristics. The effects of wind transport, erosion, accumulation and snow grain characteristics described by the model can then be monitored by measuring the associated changes in albedo, slope angles, texture and texture orientation and feature variations recorded by the photographs. This new tool seems to be an appropriate technique for spatial validation of snowdrift modelling, and it is useful in identifying both the strengths and weaknesses of the model. The results for the ongoing campaign on the 2002-2003 winter will be presented and discussed. (C) 2004 Elsevier B.V. All rights reserved.