Examining the impacts of dynamic downscaling method and vegetation biophysical processes on the South American regional climate simulation

This study investigates three issues regarding South America's (SA) climate simulation; namely, the impact of the dynamic downscaling method (DDM) on seasonal mean and interannual variability predictions, and the effect of explicit vegetation biophysical processes (VBP) on the SA's monsoon system (SAMS) development. DDM was accomplished by nesting the NCEP 80-km regional Eta model in the NCEP T42 AGCM.; Results showed that the DDM can improve the simulation of seasonal mean lower- and upper-level circulations in both warm and cold seasons. The DDM produced better simulations of the SALLJ, both at regional and large scales, which in turn improved the precipitation predictions at subtropical latitudes. The DDM also produced better simulations of seasonal mean 2-m temperature in both seasons, as well as better summer precipitation intraseasonal variability. However, the simulation of the ITCZ by the nested model was weak due to weak moisture flux convergence. The possible causes for this deficiency are discussed.; The impact of the DDM on interannual variability simulations was also investigated. Results indicated that the DDM can improve the simulation of the circulation variability. The intensification of the SALLJ during El Nino was well simulated by the DDM, which consequently resulted in improved precipitation results over southeastern SA. In contrast, the nested model showed weakness in northern SA, especially during summer, where the DDM seasonal results were also weak. Spectral decomposition analysis suggested that the DDM's improvement resulted from improved small-scale moisture flux convergence interannual variability. The intramodel noise analysis indicated that the nested model (GCM) produced better precipitation predictability in the subtropics (tropics).; The effect of VBP in the SAMS was also tested. A stronger sensible heat flux gradient between continent and ocean in the VBP-included simulations helped generate enhanced ventilation effect over the northeastern SA during the pre-monsoon stages, resulting in more realistic precipitation results. It was further identified that the seasonality of savanna and shrub to the south and east of the rainforest contributed to the variability of heating gradients and influenced the SAMS onset.