Abstract
This study examines the implications of Tropical Montane Forests (TMFs) loss on orographic precipitation in the Eastern slopes of the Andes (EADS). The focus is on moist processes for synoptic regimes associated with significant EADS precipitation: (1) monsoon rainfall for weak and strong South America Low-Level Jet (LLJ) conditions and (2) heavy rainfall associated with cold air intrusions (CAI) in the dry season. High-resolution simulations using the Weather Research and Forecasting (WRF) model were conducted for realistic and modified land-cover resulting from the conversion of TMFs to savanna. The deforestation scenarios result in 50–100% decrease (up to ∼400 J kg–1) in Convective Available Potential Energy (CAPE) spatially organized by land-cover change along the EADS. Analysis of the differences in simulated frequency distributions of rainfall intensity shows robust daytime increases in light rainfall (<2 mm h–1) and decreases in moderate rainfall rates (2–10 mm h–1) in 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 runoff decreases of 5–10% at the event-scale for the CAI case. Sensitivity simulations for lower initial soil moisture conditions indicate a 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.
Highlights
Tropical Montane Forests (TMFs)—prevalent in the altitudinal band between ∼500 and 3,500 m (e.g., Fadrique et al, 2018)—have a complex connection to weather and climate, both globally and regionally
The spatial co-organization of changes in the mean Maximum Convective Available Potential Energy (MCAPE) away from the eastern slopes of the tropical Andes (EADS) into the Amazon foreland basin along with larger standard deviations are attributed to the inherent chaotic properties of weather systems in those areas and do not represent a systematic cause-effect impact of land-cover change
The enhancement seen in the lower elevations and away from the foothills is not similar to the Weak Low-Level Jet (WLLJ) case, and the overall impact is much reduced
Summary
Tropical Montane Forests (TMFs)—prevalent in the altitudinal band between ∼500 and 3,500 m (e.g., Fadrique et al, 2018)—have a complex connection to weather and climate, both globally and regionally. Sun and Barros (2015a; 2015b) investigated the contribution of surface evapotranspiration to cloudiness, moist convection and precipitation along the eastern flanks of the tropical Andes (EADS) during the wet season through numerical experiments designed to elucidate the specific pathways of Evapotranspiration-Precipitation Interactions (EtPI) In their idealized experiments, the moisture input to the planetary boundary layer via local surface latent heat fluxes (i.e., evapotranspiration) was isolated and removed while land surface conditions were kept the same. The numerical experiments are designed to investigate the impact of replacing tropical montane forests by savanna on moist processes and atmospheric circulation in the eastern Andes at the storm-scale following previous studies (Sun and Barros, 2015a,b; Eghdami and Barros, 2019a,b). CAPE values during the second and third days of the simulations (October 4 18Z, 5 00Z and 5 18Z, 6 00Z) exceed 2,000 J Kg−1 (3,500 J Kg−1 in D03, not shown) consistent with highly unstable conditions and enhanced convective activity along and ahead of the CAI front
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