CONICET | Buscador de Institutos y Recursos Humanos

Weather Research Forecasting (WRF) high resolution simulations were performed to evaluate the removal of water vapor from an atmospheric river impinging on the Subtropical Central Andes (30º-37ºS), and producing a heavy orographic precipitation event. The early winter precipitation event was associated with a strong landfalling atmospheric river on the west coast of southern South America raining out on the windward side of the Andes. On the leeward side of the Andes, light spillover precipitation, interrupted by intermittent downslope wind (i.e., zonda wind) was observed south of 35ºS, and a not intermittent downslope windstorm inhibiting completely the precipitation north of 34ºS. The WRF-ARW version 3.1 model with 3 km grid spacing reproduced a large fine ridge-valley variation in the spatial distribution of precipitation on the low coastal Chilean range and on the high Andes range, as well as reproduced well the evolution and strength of the atmospheric river during the storm. Then, the simulation was used to estimate the drying ratio, a parameter that evaluates the removal of water vapour as precipitation from the incoming moisture flow that crossed the Andes. As the height of the Andes decrease from 34º to south, we estimated the drying ratio in 33º-34ºS and 35º-36ºS latitude bands. The drying ratio was about 49% and 25% between 33º-34ºS and 35º-36ºS, respectively. This difference is consistent with surface and TRMM observations, and with the lower Andes in southernmost latitude band. In addition, the sensitive experiment with a flat terrain in the model (i.e., without the topography of the Andes) reproduced much less precipitation on the west of southern South America during the storm, as well as an almost cero drying ratio as the forced ascent of the moist air by the terrain disappeared. The results of this study suggest that Subtropical Central Andes plays a crucial role removing large amount of water vapor as orographic precipitation from atmospheric rivers coming from the Pacific Ocean, and hence, as water supplier in the region.