8.3.4 Land Surface
Evaluation of the land surface component in coupled models is severely limited by the lack of suitable observations. The terrestrial surface plays key climatic roles in influencing the partitioning of available energy between sensible and latent heat fluxes, determining whether water drains or remains available for evaporation, determining the surface albedo and whether snow melts or remains frozen, and influencing surface fluxes of carbon and momentum. Few of these can be evaluated at large spatial or long temporal scales. This section therefore evaluates those quantities for which some observational data exist.
8.3.4.1 Snow Cover
Analysis and comparison of AMIP-2 results, available at the time of the TAR, and more recent AOGCM results in the present MMD at PCMDI, show that models are now more consistent in their simulation of snow cover. Problems remain, however, and Roesch (2006) showed that the recent models predict excessive snow water equivalent (SWE) in spring, likely because of excessive winter precipitation. Frei et al. (2005) found that AMIP-2 models simulate the seasonal timing and the relative spatial patterns of SWE over North America fairly well, but identified a tendency to overestimate ablation during spring. At the continental scale, the highest monthly SWE integrated over the North American continent in AMIP-2 models varies within ±50% of the observed value of about 1,500 km3. The magnitude of these model errors is large enough to affect continental water balances. Snow cover area (SCA) is well captured by the recent models, but interannual variability is too low during melt. Frei et al. (2003) showed where observations were within the inter-quartile range of AMIP-2 models for all months at the hemispheric and continental scale. Encouragingly, there was significant improvement over earlier AMIP-1 simulations for seasonal and interannual variability of SCA (Frei et al., 2005). Both the recent AOGCMs and AMIP models reproduced the observed decline in annual SCA over the period 1979 to 1995 and most models captured the observed decadal-scale variability over the 20th century. Despite these improvements, a minority of models still exaggerate SCA.
Large discrepancies remain in albedo for forested areas under snowy conditions, due to difficulties in determining the extent of masking of snow by vegetation (Roesch, 2006). The ability of terrestrial models to simulate snow under observed meteorological forcing has been evaluated via several intercomparisons. At the scale of individual grid cells, for mid-latitude (Slater et al., 2001) and alpine (Etchevers et al., 2004) locations, the spread of model simulations usually encompasses observations. However, grid-box scale simulations of snow over high-latitude river basins identified significant limitations (Nijssen et al., 2003), due to difficulties relating to calculating net radiation, fractional snow cover and interactions with vegetation.