Evaluation of snow extent and its variability in the Atmospheric Model Intercomparison Project

Simulations of monthly mean northern hemisphere snow extent from 27 atmospheric general circulation models (GCMs), run under the auspices of the Atmospheric Model Intercomparison Project (AMIP), are compared to observations. AMIP model runs have common values for sea surface temperatures specified from observations for the decade 1979 through 1988. Here AMIP GCMs are evaluated in terms of their simulations of (1) snow extent over northern hemisphere lands and (2) synoptic conditions associated with extremes in snow extent over particular regions. Observations of snow extent are taken from digitized charts of remotely sensed snow extent from visible imagery provided by the National Oceanic and Atmospheric Administration. In general, AMIP models reproduce a seasonal cycle of snow extent similar to the observed cycle. However, GCMs tend to underestimate fall and winter snow extent (especially over North America) and overestimate spring snow extent (especially over Eurasia). The majority of models display less than half of the observed interannual variability. No temporal correlation is found between simulated and observed snow extent, even when only months with extremely high or low values are considered. These poor correlations indicate that in the models, interannual fluctuations of snow extent are not driven by sea surface temperatures. GCMs are inconsistent in their abilities to simulate synoptic-scale tropospheric circulation patterns associated with extreme snow extent over North American regions, although some models are able to capture many of the observed teleconnection patterns.