IMPACT OF THE ONGOING AMAZONIAN DEFORESTATION ON LOCAL PRECIPITATION - A GCM SIMULATION STUDY

Numerical simulation experiments were conducted to delineate the influ ence of in situ deforestation data on episodic rainfall by comparing t wo ensembles of five 5-day integrations performed with a recent versio n of the Goddard Laboratory for Atmospheres GCM that has a simple bios phere model (SiB). The first set, called control cases, used the stand ard SiB vegetation cover (comprising 12 biomes) and assumed a fully fo rested Amazonia, while the second set, called deforestation cases, dis tinguished the partially deforested regions of Amazonia as savanna. Ex cept for this difference, all other initial and prescribed boundary co nditions were kept identical in both sets of integrations. The differe ntial analyses of these five cases show the following local effects of deforestation. 1) A discernible decrease in evapotranspiration of abo ut 0.80 mm d(-1) (roughly 18%) that is quite robust in the averages fo r 1-, 2-, and 5-day forecasts. 2) A decrease in precipitation of about 1.18 mm d(-1) (roughly 8%) that begins to emerge even in 1-2-day aver ages and exhibits complex evolution that extends downstream with the w inds. A larger decrease in precipitation as compared to evapotranspira tion produces some drying and warming. The precipitation differences a re consistent with the decrease in atmospheric moisture flux convergen ce and are consistent with earlier simulation studies of local climate change due to large-scale deforestation. 3) A significant decrease in the surface drag force (as a consequence of reduced surface roughness of deforested regions) that, in turn, affects the dynamical structure of moisture convergence and circulation. The surface winds increase s ignificantly during the first day, and thereafter the increase is well maintained even in the 2- and 5-day averages.