Dynamics of Local Circulations in Mountainous Terrain during the RHUBC-II Project

Abstract

he Radiative Heating in Underexplored Bands Campaign (RHUBC-II) project was held from August to October 2009 in the Atacama Desert in Chile at 5320-m altitude. Observations from this experiment and a high-resolution numerical simulation with the Weather Research and Forecasting Model (WRF) were used to understand the structure and evolution of the atmosphere over a region with complex terrain and extremely dry environmental conditions. The mechanisms driving the local circulations during synoptically unperturbed conditions at the field site were studied. The study suggests that the field site is mainly affected by a mountain-scale and a plateau-scale thermally driven circulation. The latter seems to dominate. The advection of warm air by downslope flows from higher heights during nighttime may be the mechanism that counteracts the longwave radiative cooling at the surface, causing a small decrease of near-surface temperature during the night. WRF represents the near-surface and upper atmosphere reasonably well above the RHUBC-II site. Important orographic features are misrepresented in the model terrain, which may cause the observed differences in near-surface winds. The zonal pressure gradient between both sides of the mountain and the static stability of the air mass on the windward side of the terrain control the local circulations over the field site. Consequently, a misrepresentation of these mechanisms in the model may cause differences between the simulated winds and observations.