Current surface vegetation atmospheric transfer (SVAT) models neglect the effects of surface slope and azimuth. There are significant subgrid differences in topographic features at the subgrid scales to which processes such as latent and sensible heating fluxes are sensitive. This study indicates the sensitivity of these processes due to sloping surfaces of varying azimuth (clockwise horizontal direction from north). A dynamically adaptive gridding system is implemented to determine subgrids based on surface slop and azimuth. A topographic index which accounts for four surface azimuth and five slope increments defines 21 characteristic types, including flat horizontal surfaces. A synthetic 10 km¿10 km watershed at latitude 38¿N and with a fixed diurnal cycle provides input for the surface temperature. Two find grid meshes, 1.0 km¿1.0 km and 2 km¿2 km, are embedded within the larger-scale cell. Digital evaluation data representing an area near Lake Tahoe, California, is also used in this sensitivity study of surface heat and moisture fluxes due to topographic forcing of incident radiation. A homogeneity test determines the minimum number of fine scale cells with matching characteristics and shared boundaries (but not strictly shared points) and forms groups of cells which represent regions of similar topographic characteristics. When the homogeneity test is set to 100% accuracy, there is a 72% reduction from individual fine grid cells to similar groups, while at 60% accuracy there is an 80% reduction, and at 10% accuracy there is a 99% reduction.

Each group computes a SVAT with the group defined characteristic topography. A set of test cases using the Biosphre-Atmosphere Transfer Scheme (Dickinson et al., 1986) is presented for north, south, east, and west facing 10¿ slopes with surfaces under dry and saturated conditions. The resulting latent heat fluxes are shown for grasslands and evergreen forests under dry and saturated conditions and for the synthetic watershed at varying degrees of accuracy. The Sierra topography is assumed to be all evergreen vegetation and dry. This study indicates that flat horizontal surfaces overpredict diurnally integrated, areally weighted sensible heat fluxes by 5 to 10% and latent heat fluxes by 1 to 2%. ¿ American Geophysical Union 1995