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Sensitivity of a GCM Simulation of Global Climate to the Representation of Land-Surface Hydrology

John F. StammWater Resources Program, Princeton University, Princeton, New Jersey

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Eric F. WoodWater Resources Program, Princeton University, Princeton, New Jersey

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Dennis P. LettenmaierDepartment of Civil Engineering, University of Washington, Seattle. Washington

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Abstract

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 parsimonious parameterization of the subgrid-scale spatial variability of soil moisture capacity, as well as base flow, by means of soil moisture drainage during dry periods. Four experiments were performed using the VIC model. The first used a globally fixed soil moisture capacity of 15 cm to provide a comparison to the Budyko bucket. The second used a more realistic globally varying soil moisture capacity. The third and fourth were sensitivity experiments using globally fixed soil moisture 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 evaporation and precipitation were reduced, and surface air temperature was increased, especially in the Northern Hemisphere in summer. The greater sensitivity of the Northern Hemisphere land areas to the altered land hydrology is attributed primarily to recycling of summertime precipitation in the interior of these continents. The authors found, somewhat surprisingly, that the water-holding capacities of the VIC model had relatively little influence .on the simulated climates of northern Eurasia and North America. This is attributed to the fact that much of the soil moisture capacity is unutilized for evaporation, due to the dry period drainage to base flow. The results argue for representation of the surface hydrology in GCMs with two-layer sail models, which are capable of representing the cycling of moisture during dry periods by means of surface evaporation, which is generally underestimated by single-layer models.

Abstract

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 parsimonious parameterization of the subgrid-scale spatial variability of soil moisture capacity, as well as base flow, by means of soil moisture drainage during dry periods. Four experiments were performed using the VIC model. The first used a globally fixed soil moisture capacity of 15 cm to provide a comparison to the Budyko bucket. The second used a more realistic globally varying soil moisture capacity. The third and fourth were sensitivity experiments using globally fixed soil moisture 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 evaporation and precipitation were reduced, and surface air temperature was increased, especially in the Northern Hemisphere in summer. The greater sensitivity of the Northern Hemisphere land areas to the altered land hydrology is attributed primarily to recycling of summertime precipitation in the interior of these continents. The authors found, somewhat surprisingly, that the water-holding capacities of the VIC model had relatively little influence .on the simulated climates of northern Eurasia and North America. This is attributed to the fact that much of the soil moisture capacity is unutilized for evaporation, due to the dry period drainage to base flow. The results argue for representation of the surface hydrology in GCMs with two-layer sail models, which are capable of representing the cycling of moisture during dry periods by means of surface evaporation, which is generally underestimated by single-layer models.

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