The Impact of Tree Line Shifts on Orographic Cloud Habits over the Andes

Idealized and real-data case simulations were conducted to investigate the impact of tree line shifts on cloud formation (e.g., cloud base height) over the Andes Mountains using the Weather Research and Forecasting model 3.1 (WRF3.1). To resolve fine scale structures under limited computational cost, high-resolution simulations (1km) were focused on Peru. To understand the influence of tree line shift on large scale circulations, especially on frictional convergence and moisture transport, simulations at coarser resolution (6km) covering the tropical and extratropical Andes (5S~35S) were carried out also. In the high resolution quasi-2D idealized simulations, topography matching the NE-SW cross section going through Cusco, Peru was implemented. 3D idealized simulations at coarser resolution with topography representing the shape and geometry of the Andes were conducted to characterize the impact of large-scale envelope orography. The real-data case simulations were carried out using one-way nesting for several events during the South American Low Level Jet Experiment (SALLJEX) period, for which most observations (e.g., aircraft, radiosonde, etc) are available. Three nests were implemented, with the first domain covering the whole South America, the second domain covering the tropical and extratropical Andes as well as the area where aircraft observations are available, and the third domain focusing on the Peru region. Simulations under strong and weak South American low level jet synoptic conditions as well as for relative calm conditions over Peru were conducted. Forcing was extracted from the ERA-Interim re-analysis from the European Centre for Medium-Range Weather Forecasts (ECMWF) data set. Besides investigating the impact of altitudinal shifts in tree line, additional scenarios of deforestation in the adjacent low lands, including the Amazon basin, were also conducted to understand their impact on cloud formation over the high Andes region.