A catchment-based approach to modeling land surface processes in a generalcirculation model 1. Model structure

A new strategy for modeling the land surface component of the climate syste m is described. The strategy is motivated by an arguable deficiency in most state-of-the-art land surface models, namely, the disproportionately highe r emphasis given to the formulation of one-dimensional, vertical physics re lative to the treatment of horizontal heterogeneity in surface properties, particularly subgrid soil moisture variability and its effects on runoff ge neration. The new strategy calls for the partitioning of the continental su rface into a mosaic of hydrologic catchments, delineated through analysis o f high-resolution surface elevation data. The effective "grid" used for the land surface is therefore not specified by the overlying atmospheric grid. Within each catchment, the variability of soil moisture is related to char acteristics of the topography and to three bulk soil moisture variables thr ough a well-established model of catchment processes. This modeled variabil ity allows the partitioning of the catchment into several areas representin g distinct hydrological regimes, wherein distinct (regime specific) evapora tion and runoff parameterizations are applied. Care is taken to ensure that the deficiencies of the catchment model in regions of little to moderate t opography are minimized.