Role of Surface Evapotranspiration on Moist Convection along the Eastern Flanks of the Andes
The contribution of surface evapotranspiration (ET) to moist convection, cloudiness and precipitation along the eastern flanks of the Andes (EADS) was investigated using the Weather Research and Forecasting (ARW-WRF3.4.1) model with nested simulations of selected weather conditions down to 1.2 km grid spacing. To isolate the role of surface ET, numerical experiments were conducted using a quasi-idealized approach whereby at every time step the surface sensible heat effects are exactly the same as in the reference simulations, whereas the surface latent heat fluxes are prevented from entering the atmosphere.
Energy balance analysis indicates that local surface ET along the EADS influences moist convection primarily through its impact on conditional instability, because it acts as an important source of moist entropy in this region. The energy available for convection decreases by up to ~60% when the ET contribution is withdrawn. In contrast, when convective motion is not thermally driven, or under conditionally stable conditions, latent heating from the land surface becomes secondary. At the scale of the Andes proper, removal of surface ET weakens upslope flows by increasing static stability of the lower troposphere, as the vertical gradient of water vapor mixing ratio tends to be less negative. Consequently, moisture convergence is reduced over the EADS. In the absence of local surface ET, this process operates in concert with damped convective energy, suppressing cloudiness, and decreasing daily precipitation by up to ~50% in the simulations presented here.
When the surface ET is eliminated over the Amazon lowlands (AMZL), the results show that, without surface ET, daily precipitation within the AMZL drops by up to ~75%, but nearly doubles over the surrounded mountainous regions. This dramatic influence is attributed to a dipole structure of convergence-divergence anomalies over the AMZL, primarily due to the considerable cooling of the troposphere associated with suppressed convection. Further examination of moist static energy evolution indicates that the net decrease in CAPE (Convective Available Potential Energy) over the AMZL is due to the removal of surface ET that is only partially compensated by related regional circulation changes. Because of the concave shape of the Andean mountain range, the enhanced low-level divergence promotes air mass accumulation to the east of the central EADS. This perturbation becomes sufficiently strong around nightfall and produces significant eastward low-level pressure gradient force, rendering wind currents more away from the Andes. Moisture convergence and convection over the EADS vary accordingly, strengthened in the day but attenuated at night. Nocturnal convective motion, however, is more widespread. Analytical solutions of simplified diagnostic equations of convective fraction suggest that reduction of lower troposphere evaporation is the driving mechanism. Additional exploratory experiments mimicking various levels of thinning and densification of AMZL forests via changes in surface ET magnitude demonstrate that the connection between the AMZL ET and EADS precipitation is robust.
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