A fine-mesh land approach for general circulation models and its impact onregional climate

Significant land processes occur on spatial scales too fine to be resolved by most current climate models. To address these subgrid-scale processes be tter, a model is introduced that represents them by a submesh imposed on ea ch atmospheric model grid. A water- and energy-conserving scheme disaggrega tes atmospheric conditions to the fine-mesh model grid and aggregates surfa ce fluxes back to the atmospheric grid. Results from a multiyear simulation using the fine-mesh model show that, in the Tropics, precipitation over la nd is sensitive to surface heterogeneities. Although surface temperatures o ver midlatitude continents are sensitive to subgrid variations in land cove r, precipitation there exhibits very little such sensitivity. The simulated climate over tropical Africa during the summer months is dras tically altered by the introduction of the fine-mesh model. Central Africa experiences a 22% reduction in precipitation through the period of July-Sep tember in the fine-mesh model simulations. West Africa, a region known for its sensitivity to land surface characteristics, experiences slight increas es in precipitation. Analysis of the model simulations shows that different physical mechanisms communicate land surface changes to the atmosphere ove r central and West Africa. Over central Africa, the decrease in precipitati on is driven mainly by an initial reduction in evaporation that results in decreased midtropospheric condensation and decreased low-level convergence. Over West Africa, on the other hand, the changes in the precipitation fiel d may be related to changes in the distribution of vegetation.