Diurnal circulation of the South American Altiplano: observations in a valley and at a pass

In 2003, two quasipermanent stations have been installed at the Bolivian Altiplano in order to study the diurnal circulation of this huge meridionally oriented plateau at an altitude of ∼4000 m above msl. The station Tambo Quemado is located at the eastern side of a pass leading from the Altiplano down to the Pacific in the west. The station Rio de La Paz is positioned in a valley leading from the eastern lowlands to the city of La Paz. The data from these stations are analyzed mainly on the basis of master equations. The coefficients of a master equation are based on the observations and give the transfer rates of flow states in the phase space of selected observed variables like the horizontal wind components at a station. At Tambo Quemado, there is easterly upslope wind after sunrise but westerly inflow to the Altiplano takes over in the afternoon. Perturbations of this diurnal system with westerlies throughout the day are not uncommon. The diurnal flow system in the valley of the Rio de La Paz is extremely regular in that upvalley flow invariably becomes intense around noon. The nocturnal flow is weak without a preferred direction. It is a surprising result that the downvalley flow is moister than the upvalley flow. This finding and additional measurements downstream from the station suggest that at least part of the upvalley flow does not originate in the lowlands but descends from higher levels.