Abstract

Raco is the local name given to a strong (gusts up to 17 m s(-1)), warm, and dry down-valley wind observed at the exit of the Maipo River Canyon in central Chile. Its climatology is documented based on eight years of surface measurements near the canyon exit together with a more complete characterization of its structure during an intensive observational period (IOP) carried out in July 2018. Raco winds occur in the cold season under well-defined synoptic conditions, beginning abruptly at any time during the night, reaching maximum hourly averages around 10 m s(-1), and terminating around noon with the onset of afternoon westerly up-valley winds. About 25% of the days in May-August have more than six raco hours between 0100 and 1200 LT, and raco episodes last typically 1-2 days. The sudden appearance of raco winds at the surface can be accompanied by conspicuous warming (up to 10 degrees C) and drying (up to 3 g kg(-1)). Raco winds are associated with a strong along-canyon pressure gradient, a regional pressure fall, and clear skies. During the IOP, radiosondes launched from both extremes of the canyon exit corridor showed a nocturnal easterly jet at 700 m AGL that occasionally descended rapidly to the surface, producing the raco. Transects along the canyon performed with a mobile ceilometer revealed a sharp frontlike feature between the cold pool over the Santiago Valley and the raco-affected conditions in the Maipo Canyon. Possible factors producing the easterly jet aloft and its occasional descent toward the surface are discussed, and a gap-wind mechanism is postulated to be at work.