Isolating the role of evapotranspiration on regional moisture transport and precipitation on the eastern flanks of the Andes

The contribution of evapotranspiration (ET) to the regional moisture transport along the eastern flanks of the Andes was investigated using the Weather Research and Forecasting (WRF3.4.1) model, with nested simulations down to 1.2 km grid spacing to capture the ridge-valley structure of the Central Andes. Numerical simulations of strong and weak SALLJ (South American Low Level Jet) monsoon regimes as well as winter conditions were conducted, isolating separately the contribution of ET and associated boundary layer thermodynamic properties over the Andes and over the Amazon basin. To isolate the effects of ET from the surface sensible heat effects, a quasi-idealized approach was adopted, where at every time step the surface sensible heat effects are exactly the same as in the real-data case runs, but the surface moisture and latent heat fluxes do not enter the atmosphere. The results show that local ET along the eastern flanks of the Andes contributes considerably to cloud formation and precipitation (up to around 20%), thus supporting the hypothesis that Andean cloud forests play an active role in capturing the cloudiness they are immersed in, and consequently in rainfall harvesting at high elevations. However, even if the Amazon is upstream of the Andes, the shutting down of ET and the associated PBL variations over the low-land Amazon do not reduce, but on the contrary significantly increase heavier rainfall in the high Andes. That is, low level reduction in moisture transport from the foreland basin does not have an impact on central Andes orographic rainfall, but an increase in mid-level moisture transport due to rainfall reduction over the Amazon explains the increase in precipitation. Furthermore, a band of heavy rainfall develops along the outer rim of the Amazon foreland basin consistent with the displacement of strong low level convergence zones organized by the contrast between the thermodynamically “dry” and “natural” surfaces. To generalize these findings, highly idealized quasi-2D simulations were conducted, with topography matching the NE-SW cross section going through Cusco, Peru to manifest the geometry of the Andes.