Multiscale Analysis of the Summertime Precipitation over the Central Andes

Precipitation over the central Andes in South America exhibits a marked annual march, with most of the rainfall concentrated during the austral summer season (December–February), when the atmospheric circulation favors the uplifting of moist air from the lowlands to the east of the mountain range. Within its rainy season, the central Andes experiences week-long rainy and dry episodes. The large-scale and local conditions during these episodes are investigated using satellite imagery, reanalyzed atmospheric fields, and in situ data. Despite the deep layer of conditional instability prevalent during most summertime afternoons, deep convection can occur only on those days in which the mixing ratio within the local boundary layer exceeds some threshold (∼7 g kg−1), yielding saturation of near-surface air parcels rising more than 600 m above ground. Convective cloudiness anomalies over the central Andes extend southeastward and tend to be concurrent with anomalies of opposite sign over the eastern part of the continent. Rainy (dry) episodes are also associated with anticyclonic (cyclonic) anomalies centered over subtropical South America that extend through the depth of the troposphere, accompanied by easterly (westerly) wind anomalies over the central Andes. These anomalies are presumably forced by planetary waves originating in the Southern Hemisphere extratropics. To gain insight into the regional processes linking the large-scale and local conditions, The Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model Version 5.2 was used to simulate contrasting rainy and dry episodes. The most marked and relevant differences are the strength and extent of diurnally varying flow over the eastern slope of the Andes. During the rainy simulation, strong easterly winds reach the upper part of the slope by midmorning, initiating an intrusion of warm and moist air (high θe air originating in the eastern lowlands) into the central Andes. In the dry case, the moisture transport from the east is restricted to the eastern slope of the Andes, and the central Andes is flooded by low θe air from the western foothills that cannot support deep convection even in the presence of localized updrafts. The momentum balance based on the model output indicates that turbulent momentum mixing from aloft (determined by the large-scale anomalies of the upper-level flow) into the convective boundary layer is the leading term causing the differences in the daytime upslope flow (and hence moisture transport) over the upper part of the eastern side of the Andes between rainy and dry simulations.