Microphysics of Orographic Precipitation
and Vegetation Controls of Convection in the Andes

Observing and modeling cloud and precipitation systems in mountainous regions presents many challenges due to complex land-atmosphere interactions resulting in high spatial and temporal variability and heterogeneity. Here, we present first recent work focusing on microphysical processes in regions where fog habits and the vertical structure of clouds are strongly modulated by orography. In particular, we contrast the classical mechanism of orographic rainfall enhancement by forced ascent against orographic enhancement by Bergeron processes, and assess their relative impact on water resources and natural hazards (floods and landslides) as well as their response to climate variability and change. The role of the terrain in modulating moisture convergence patterns and constraining the thermodynamic environment in the lowest 2km will be illustrated in detail using observations and model results from the southern Appalachians. Second, findings from recent research on the role of evapotranspiration on the diurnal cycle of mountain precipitation are presented. Specifically, modeling studies were conducted using realistic and quasi-idealized ET withdrawal experiments using the WRF model at very high resolution (~1 km grid spacing). The results show that evapotranspiration fluxes modulated by landform govern convective activity in the lower troposphere, including cloud formation and precipitation processes that account for daily precipitation amounts as high as 50-70% depending on synoptic conditions and season. Using parsimonious model experiments and observations for the Amazon basin and the Central Andes, we demonstrate that vegetation acts as a “terrestrial hydrostat” regulating precipitation locally by controlling atmospheric moist instability, and remotely through changes in atmospheric transport patterns.

Ana P. Barros

 

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