Influence of the Realistic Description of Soil Water-Holding Capacity on the Global Water Cycle in a GCM

Abstract

The sensitivity of the hydrological cycle to soil water-holding capacity (WHC) is investigated using the Laboratoire de Meteorologie Dynamique General Circulation Model (LMD GCM) coupled to a land surface model (LSM). A reference simulation (REF), with WHCs equal to 150 mm globally (except in deserts where it is set to 30 mm), is compared to two perturbation simulations using datasets with realistic WHC distributions:the ?available WHC? (AWC) dataset is physically consistent with the definition of WHC in the LSM and has a global average close to 150 mm; the ?total WHC? (TWC) dataset is used as a secondary reference for a large WHC increase (more than a doubling from 150 mm). The average impact over land of the increase in WHC (from REF to both AWC and TWC) is an increase in annual mean evaporation, split between increased annual precipitation and decreased annual mean moisture convergence. The regional responses, however, are more complex: precipitation increases in summer over the midlatitude landmasses through the recycling of increased evaporation; in the Tropics, moisture convergence and precipitation decrease in the intertropical convergence zone and precipitation increases in the surrounding areas, both behaviors being related to the sensitivity of tropical convection to surface energy fluxes in the LMD GCM. Two important conclusions arise from these numerical results: first, the changes in the hydrological cycle are driven through evaporation by the WHC changes realized in the hydrologically active regions (continental midlatitude and tropical rainbelts); second, WHC increase of 10% to 20% in the rainbelts induces changes in the hydrologic cycle with similar patterns and almost the same amplitude as changes resulting from an increase greater than 100%. These results are strongly conditioned to the land?atmosphere feedbacks, which can only be allowed in a GCM environment.