SENSITIVITY OF A GCM SIMULATION OF GLOBAL CLIMATE TO THE REPRESENTATION OF LAND-SURFACE HYDROLOGY

The sensitivity of global climate to the characterization of the land- surface hydrology is investigated using the Geophysical Fluid Dynamics Laboratory GCM at R15 resolution with the standard Budyko bucket and the Variable Infiltration Capacity (VIC) Model, which incorporates a p arsimonious parameterization of the subgrid-scale spatial variability of soil moisture capacity, as well as base flow, by means of soil mois ture drainage during dry periods. Four experiments were performed usin g the VIC model. The first used a globally fixed soil moisture capacit y of 15 cm to provide a comparison to the Budyko bucket. The second us ed a more realistic globally varying soil moisture capacity. The third and fourth were sensitivity experiments using globally fixed soil moi sture capacities of 5 and 25 cm. The results of the VIC fixed runs (15 cm) showed that global average soil moisture was considerably lower ( about 2.5 cm on average) as compared with the bucket runs, global evap oration and precipitation were reduced, and surface air temperature wa s increased, especially in the Northern Hemisphere in summer. The grea ter sensitivity of the Northern Hemisphere land areas to the altered l and hydrology is attributed primarily to recycling of summertime preci pitation in the interior of these continents. The authors found, somew hat surprisingly, that the water-holding capacities of the VIC model h ad relatively little influence on the simulated climates of northern E urasia and North America. This is attributed to the fact that much of the soil moisture capacity is unutilized for evaporation, due to the d ry period drainage to base flow. The results argue for representation of the surface hydrology in GCMs with two-layer soil models, which are capable of representing the cycling of moisture during dry periods by means of surface evaporation, which is generally underestimated by si ngle-layer models.