Deforestation Impact on Orographic Precipitation in the Tropical Andes

Abstract:

This study examines the implications of Tropical Montane Forests (TMFs) loss on orographic precipitation in the Eastern slopes of the Andes (EADS). Numerical weather prediction models with convective permitting grid-spacing (~1 km) can produce high-intensity rain rates during extreme rainfall events. Based on medium-ranged simulations, the compound effects of deforestation and drought on moist processes are investigated for different key synoptic events in the EADS, including cold air intrusions (CAIs) from the mid-latitudes to the tropics, which are linked to extreme orographic rain. Model simulations with realistic and modified land-cover resulting from the conversion of TMFs to savannah are conducted using the Weather Research and Forecasting model (WRF). The deforestation scenarios result in 50-100% decreases (up to ~ 400 J kg^-1) in Convective Available Potential Energy (CAPE) spatially organized by land-cover change along the EADS foothills. Analysis of the differences in simulated frequency distributions of rainfall intensity shows a robust daytime reduction in intensity along the altitudinal band 500-2,000 m, where orographic enhancement is dominant. Whereas there are negligible changes in the spatial patterns of precipitation and hydrologic response for monsoon conditions, rainfall accumulations decrease for all cases, and the precipitation maxima shift downslope into the Amazon lowlands. Changes in rainfall amount and intensity result in a runoff decrease of 5-10% at the CAI event-scale, equivalent to the monthly discharge of headwater tributaries of the Amazon in the dry season. The results suggest strong positive feedback of forest loss to hydrologic drought along the EADS foothills in the austral spring when CAIs play a key role in the tropical EADS dry season hydrometeorology.