Variability and change in terrestrial snow cover: data acquisition and links to the atmosphere

Terrestrial snow cover is of significance to global geophysical systems bec ause of its influence on both climatological and hydrological processes. Sn ow cover acts as a layer which modifies energy exchange between the surface and atmosphere, and as the frozen storage term in the water balance, affec ting runoff and streamflow. This review addresses two challenges with regar d to snow cover: how to monitor this variable adequately over time, and how to couple trends and variability in snow cover to atmospheric circulation. Developments in remote-sensing technology have provided a range of satelli te-derived data products which complement in situ snow measurement procedur es. Variability in data spatial resolution and domain, temporal repeatabili ty, time series length and the level of snow-cover information derived (for example, snow extent vs. snow water equivalent) means that data applicatio n plays a large role in the utilization of an appropriate dataset. Given th e variability in snow-cover data properties, the state of knowledge regardi ng interactions between snow cover and the atmosphere is similarly mixed. N o standardized trends in continental or hemispheric snow cover are evident and the direction of forcing between snow cover and the atmosphere is still ambiguous. Identified associations are typically regional in extent, and s tatistically moderate in strength, proving cause-and-effect relationships d ifficult to identify. Future research needs are outlined, with an emphasis on passive-microwave imagery. These data have the necessary characteristics (quantitative estimates of snow-water equivalent, all-weather imaging) to provide the input data to the process based studies necessary to isolate li nkages between snow cover and the atmosphere.