The Raco Wind in Central Chile: A Recurring Gap Flow Interacting with a Cold Air Pool

Raco refers to a strong, warm and dry wind observed at the exit of the Maipo Canyon towards the Santiago valley in central Chile. A 7-year climatology of Raco winds has been recently published (Muñoz et al. 2020). These winds occur mainly during the night in the cold season, having a synoptic variability closely linked to the development of coastal lows (Rutllant and Garreaud 2004). A prominent characteristic of Raco winds is their abrupt appearance at the surface accompanied generally by a sudden increase in temperature and a drop in water vapor mixing ratio. Muñoz et al. (2020) show also the results of a 5-day field campaign carried out in July 2018 in which 6-hourly radiosondes were launched from the two extremes of the 12-km Maipo Canyon exit corridor. The existence of an upper level (700 m AGL) easterly wind jet is documented at the exit point, which occasionally rapidly descends producing the Raco wind at the surface. Muñoz et al. (2020) hypothesize that a gap wind mechanism is acting in the region between the two extremes of the exit corridor controlling the intensity and altitude of the wind flow exiting from the canyon. In the present contribution we show and analyze results from a field campaign carried out in 2019 in which an additional radiosonde launching site was operated close to the narrows of the exit corridor, just about 2 km downwind of a 3-1-3 constriction of the canyon topography. The results show that vertical profiles of the wind at the narrows have a jet shape similar to that at the downstream exit and differ from the nearly uniform wind profiles at the entrance of the corridor located just 6 km upwind. The topographic constriction is thus associated with a significant change in the wind profile. We also evaluate Froude numbers at the three sites showing the existence of a subcritical-supercritical transition occurring along the exit corridor. Additionally, we document the interaction of the exit flow with the Cold Air Pool (CAP) over the Santiago valley by means of along-canyon transects with a mobile ceilometer and concurrent surface and temperature measurements. A microfront with sharp changes of aerosols, temperature and humidity marks the boundary between the clean, warm and dry air affected by the Raco wind and the cold, humid and polluted air of the Santiago valley CAP. Available measurements during the field campaign allow following the movement of the front as it moves along the exit corridor.

Acknowledgments

This work is partially funded by FONDECYT Project 1170214 of the Chilean CONICYT agency.

References

Muñoz, R., L. Armi, J. Rutllant, M. Falvey, C. D. Whiteman, R. Garreaud, A. Arriagada, F. Flores, and N. Donoso, 2019: Raco wind at the exit of the Maipo Canyon in Central Chile: climatology, special observations, and possible mechanisms. Accepted in J. Applied Meteorology and Climatology. DOI: 10.1175/JAMC-D-19-0188.1

Rutllant, J., R. Garreaud, 2004: Episodes of strong flow down the western slope of the subtropical Andes. Monthly Weather Review, 132, 611-622.