Impacts of Fractional Snow Cover on Surface Air Temperature in the NCAR Community Atmosphere Model (NCAR-CAM2)

The NCAR Community Atmosphere Model (CAM2) coupled with the Community Land Model (CLM2) shows a significant warm bias in surface air temperature over snow-covered areas in winter. An intercomparison of surface albedos from CAM2/CLM2 and MODIS reveals that in the northern mid-latitude grassland and cropland regions, the modeled values are much lower than observations mainly due to shallower-than observed snow depth. This underestimated snow depth is related to how snow cover fraction (SCF)is parameterized in the land module (CLM2). Most land surface models relate SCF to grid (or patch) averaged snow depth and surface roughness length. This study suggests that the SCF formulation of Yang et al. largely improves the simulations of snow depth, surface albedo and surface air temperature over non-forested regions in winter. However, snow ablation is delayed and a cold bias appears in spring, suggesting that the SCF-snow depth relationship may be different for accumulation and ablation, seasons. Guided by limited watershed-scale measurements, a different SCF-snow depth formulation for ablation season was tested in CLM2. Preliminary modeling results from CAM2/CLM2 indicated that using this SCF scheme for melting period and the SCF scheme of Yang et al. for accumulation period leads to the most encouraging results, such as a significant reduction of wintertime warm bias over snow-covered areas and few undesired effects on the springtime snow depth and air temperature. The caution of using these schemes will also be discussed.