A VEGETATION-ATMOSPHERE INTERACTION STUDY FOR AMAZONIA DEFORESTATION USING FIELD DATA AND A SINGLE-COLUMN MODEL

The recent high deforestation rates in Amazonia have raised the questi on of how climate, mainly the precipitation patterns, might be affecte d as pasture land replaces the primary forest. This work initially sho ws how the dry season soil moisture and water stress in typical Amazon ian pasture modifies the energy partitioning at the surface. Low-level moisture convergence is a primary mechanism to trigger convective clo ud formation and precipitation. The relationship between moisture conv ergence and local surface evaporation in generating precipitation in A mazonia is studied utilizing a one-dimensional 'single column' model ( SiB-1D). SiB-1D couples a surface-vegetation model (SiB) to a physical parametrization of deep convection (Kuo scheme), radiation, turbulent diffusion and large-scale precipitation. Model simulations for short periods (2 days) show its ability to calculate the Amazonian surface e nergy-balance components and boundary-layer dynamics when compared wit h field observations from the Rondonia Boundary-Layer Experiment and t he Anglo-Brazilian Amazonian Climate Observation Study. The model was further used to test the sensitivity of the deep convection scheme to a range of typical low-level moisture convergence situations in a seco nd experiment, and a third test investigated the local convective prec ipitation generated over forest and grass vegetation as a function of available soil moisture. Results showed that the rainfall over forest vegetation appeared to be almost insensitive to soil water stress wher eas reduced precipitation was generated over pasture. When available s oil water fell below a threshold of 60% the calculated precipitation o ver the pasture sites rapidly declined. Although these results are con fined by the short integration period and the initial atmospheric prof iles, they help to strengthen the notion that deforestation reduces ev aporation and convective precipitation, especially during the dry seas on: a result already indicated by some previous general-circulation mo del experiments.