Understanding the Sensitivity of a GCM Simulation of Amazonian Deforestation to the Specification of Vegetation and Soil Characteristics

Abstract

The experiment reported on here presents a realistic portrayal of Amazonian deforestation that uses measurements of vegetation characteristics, taken as part of the Anglo?Brazilian Amazonian Climate Observation Study field campaigns, to define the forest and replacement pasture vegetation in the Hadley Centre GCM. The duration of the main experiment (10 yr) leads to greater confidence in assessing regional changes than in previous shorter experiments. Complete removal of the Amazonian forest produced area-mean changes that resemble earlier experiments with decreases in evaporation of 0.76 mm day?1 (18%) and rainfall of 0.27 mm day?1 (4%) and a rise in surface temperature of 2.3°C. However, the relative changes in magnitude indicate that increased moisture convergence partly compensates for the reduced evaporation, in contrast to many previous deforestation experiments. Results also showed large regional variations in the change in annual mean rainfall over South America, with widespread decreases over most of the deforested area and increases near the Andes. A better understanding of the mechanisms responsible for the final deforested climate has been gained by carrying out additional experiments that examine the response to separate changes in roughness and albedo. Increased albedo resulted in widespread significant decreases in rainfall due to less moisture convergence and ascent. The response to reduced roughness is more complex but of comparable importance; in this experiment it was dominated by an increase in low-level wind speeds resulting in decreased moisture convergence and rainfall near the upwind edge of the area and the opposite near the downwind boundary where the increased flow meets the Andes. In the standard deforestation scenario all vegetation parameters were modified together with one soil parameter?the maximum infiltration rate, which is reduced to represent the observed compaction of soil following deforestation. Results from a further experiment, in which the maximum infiltration rate was left unchanged, showed much smaller reductions in evaporation of 0.3 mm day?1 (7%) and indicated that the predicted regional changes in rainfall and evaporation were very sensitive to this parameter.