Mesoscale Wind Regimes in Chile at 30°S

Abstract

In November of 1999, four permanent surface stations were installed in the vicinity of the surface ozone monitoring station on the summit of the Cerro Tololo (2200 m MSL) in Chile at 30°S. These stations were used to study the atmospheric flow conditions, which are important for the interpretation of the ozone measurements at Cerro Tololo. In addition, radiosonde ascents were performed in March of 2000 near the coast and about 60 km inland. Different wind regimes were distinguished. Above 4 km MSL, large-scale westerly winds prevailed, while northerly winds were observed in a band along the coastline between 2- and 4-km-MSL height. The upper boundary of the northerly wind regime corresponded to the mean height of the Andes mountain range. This wind regime resulted from the westerly winds being blocked and forced to flow in parallel to the Andes (when Froude number is less than 1). The phenomenon was also confirmed by model simulations. Seasonally varying, thermally induced valley winds and a sea breeze developed below the northerly wind regime. In summer, the valley winds reached the Cerro Tololo. Diurnal variation of the top boundary of the valley winds also influenced the lower boundary of the northerly wind regime, which was less than 2 km MSL during the night and greater than 2 km MSL during the day. Thus, this observational and modeling study has shown that in summer the baseline ozone monitoring site at Cerro Tololo can be contaminated by polluted air that is transported from the plains by the thermally induced wind systems.