The role of topography in snow cover distribution inferred from satellite remote sensing and snowpack modelling

Snow cover distribution is ruled by the effects of topography and atmospheric forcings. Complex mountain topography makes snow distribution variable over short distances and the spatial resolution of observations plays a key role in understanding its dynamics. Among Earth Observation data, the MODIS (MODerate resolution Imaging Spectroradiometer on board Terra and Aqua satellites) Snow Covered Area product proved to be appropriate for hydrologic and climate modelling applications due to its high spatial (500 m) and temporal (1 day) resolution. Here, we consider ten years (2003-2012) of MODIS snow cover maps of the Po river basin, northern Italy. We investigate the issue of cloud obstruction, highlighting its dependence on altitude and season. In spring, the percentage area hidden by clouds is about 70% above 1000 m asl. We propose a cloud removal procedure and we apply it to the study area, which is characterized by high geomorphological heterogeneity. The regional snow line is estimated for detecting snow cover dependence on elevation and temporal filters aim to map snow cover in mid-altitude areas. The cloud-reduced and cloud-free MODIS maps are used as reference for assessing snow cover simulations by the Regional Climate Model (RCM) COSMO-CLM, run at 0.0715° (about 8 km) and 0.125° (about 14 km) resolution. The RCM is able to represent the seasonal and inter-annual variability of snow cover patterns. Advantages are provided by the higher spatial resolution, reflecting the improvements obtained in temperature and precipitation fields. Finally, we discuss the feasibility of using MODIS data for investigating the role of slope and aspect in snow dynamics in Central Apennines, Italy. The spatial analysis of snow cover patterns highlights the importance of orographic effects resulting from topographic barriers to moist air circulation. In spring, solar radiation distribution determines shorter snow cover duration on south-facing slopes than on north-facing slopes. The results are supported by validation of the snow cover maps with in-situ snow depth measurements (overall percentage agreement of 86% for Aqua and 88% for Terra MODIS).