Strong Wind Jets at the Exit of an Andean Valley in Central Chile: Observations and Preliminary Model Results

The interaction of atmospheric flow with topography produces a great variety of phenomena on a large range of temporal and spatial scales. In many cases, this interaction has effects that can be hazardous due to the intensity of the resulting winds or wind-shears, or on the contrary, topographically-induced stagnation and blocking can exacerbate air pollution problems in certain regions. To study these topographic-flow interactions in complex terrain, Chile offers a privileged venue, as the tall Andes Mountains run all along its eastern border, offering in the central part of the country mean altitudes above 4,000 m ASL at distances not larger than 200 km from the Pacific ocean. Thus, the relief map in the region provides a variety of topographic structures including coastal mountains, inland valleys, isolated peaks, transverse valleys intruding into the Andes Mountains, etc., all of them overlooked by the large Andes massif to the east. One example of atmospheric flows affected by this complex terrain is the existence of episodic strong easterly winds along the foothills of the Andes Cordillera, which depending on the region receive local names like Terral (~30 °S), Raco (~33° S), and Puelche (~35-40 °S). Only recently, scientific studies have begun addressing these local winds, although for now with emphasis on the large scale synoptic patterns affecting their occurrence (Rutllant and Garreaud 2004; Montes et al. 2016; Montecinos et al. 2016).

Raco is the name given by the locals to relatively strong, warm and dry easterly winds affecting the Maipo river Canyon in the southeastern region of Santiago’s valley in central Chile, at the foothills of the Andes Cordillera, which at these latitudes reaches heights above 5,000 m ASL. Raco winds occur mainly in the cold season, have a marked synoptic variability and are mostly felt at the surface during nighttime hours, when sudden increases in zonal winds are generally accompanied by concomitant positive jumps in temperatures and negative jumps in absolute humidity. In this work we summarize the climatology of Raco winds based upon about 6 years of surface meteorological observations at La Obra site. Here, Raco winds maximize their occurrence (50%) at the end of the night and early morning in June and July (austral winter), while being completely absent during the summer. The duration of Raco episodes is typically less than 24 hours, with initiation times distributed uniformly during the night and ending times most typically around noon. A direct relationship between duration and intensity is observed, although maximum hourly winds asymptote towards 10 m/s at this site. Temperature jumps of up to 10 C are perceived at the beginning of a Raco event, which is a conspicuous trait of these winds, as this sudden warming occurs in the middle of winter nights.

Results of a first intensive observational campaign of a Raco episode are also presented. The measurements took place 16-18 September 2017 at a site about 3 km west (downwind) of La Obra, where radiosondes and pilot balloons were launched every 6 and 2 hours, respectively. During the three nights sampled, an easterly jet was observed in the 00 and 06 LT radiosonde wind profiles, with maximum speeds above 10 m/s in the 500 - 1000 m height range. Occasionally, the easterly jet is brought down to the surface producing a Raco event. In the event of the night of 18 September, surface gusts up to 16 m/s were measured near the surface, with temperature suddenly jumping from 13 C to 25 C. Two future field campaigns are planned for winters of 2018 and 2019, in which vertical profiles at additional points along the valley are being considered.

Finally, we provide a brief characterization of the performance of the WRF mesoscale numerical model applied over the study region with maximum horizontal resolution of 1 km. The model run covers the full year of 2015. Although the original purpose of this model run was not the study of Raco events, it still provides a good starting point in order to assess the predictability of Raco winds by the WRF model. Indeed, the preliminary results show that while the wind events are reasonably reproduced by the model, the jumps in temperature and humidity are not. Future work includes the optimization of the model setup to simulate Raco winds and subsequent diagnosis in order to better understand their mesoscale dynamics.

Acknowledgments

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

References

Montecinos, A., R. Muñoz, S. Oviedo, A. Martínez, V. Villagrán, 2016: Climatological characterization of Puelche winds down the western slope of the extratropical Andes Mountains using the NCEP Climate Forecast System Reanalysis. Submitted to J. Applied Meteorology and Climatology.

Montes, C., J. Rutllant, A. Aguirre, L. Bascunan-Godoy, C. Juliá, 2016: Terral de Vicuña, a foehlike wind in semiarid northern Chile: meteorological aspects and implications for the fulfillment of chill requirements in deciduous fruit trees. J. Appl. Meteor. and Climatol., 55, 1183-1196.

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