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

Terrestrial snow cover is of significance to global geophysical systems because of its influence on both climatological and hydrological processes. Snow cover acts as a layer which modifies energy exchange between the surface and atmosphere, and as the frozen storage term in the water balance, affecting runoff and streamflow. This review addresses two challenges with regard 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 satellite-derived data products which complement in situ snow measurement procedures. Variability in data spatial resolution and domain, temporal repeatability, time series length and the level of snow-cover information derived (for example, snow extent vs. snow water equivalent) means that data application plays a large role in the utilization of an appropriate dataset. Given the variability in snow-cover data properties, the state of knowledge regarding interactions between snow cover and the atmosphere is similarly mixed. No 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 statistically moderate in strength, proving cause-and-effect relationships difficult 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 linkages between snow cover and the atmosphere.