A SIMPLE-MODEL OF THE HYDROLOGIC-CYCLE AND CLIMATE .1. MODEL CONSTRUCT AND SENSITIVITY TO THE LAND-SURFACE BOUNDARY

In current General Circulation models (GCMs) and their proposed improvements, the fluxes of heat and moisture from soil storage and plant canopies are represented by a large number of parameters. Sensitivity testing of the candidate parametrizations is thus an important step in model development. Repeated simulations with the full GCM is burdened by both cost and the burial of sensitivity information in large volumes of generated data. Here we propose the use of a simple climate model with a full hydrologic cycle to perform some of the basic sensitivity experiments and thereby reduce the number of experiments for the three-dimensional GCM. The simple climate model is thus used to screen the multitude of possible parameter-sensitivity experiments. This model is derived from the column model of Koster and Eagleson (J. Climate, 3(6) (1990) 593-606) and it contains the important one-dimensional physical processes of GCMs. The large-scale convergence of heat and moisture for the atmosphere is parametrized; atmospheric dynamics are therefore neglected in that momentum equations are not solved in this simple climate model. In a series of sensitivity experiments, the influence of some land boundary parameters on the partitioning of the atmospheric precipitation and radiative forcing at the land surface are determined. The role of soil texture, soil column discretization and percolation boundary condition at the bottom of the soil column on surface water and energy balance components are investigated using the simple climate model.