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Volume 19, issue 7
Atmos. Chem. Phys., 19, 4615–4635, 2019
https://doi.org/10.5194/acp-19-4615-2019
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
Atmos. Chem. Phys., 19, 4615–4635, 2019
https://doi.org/10.5194/acp-19-4615-2019
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 08 Apr 2019

Research article | 08 Apr 2019

From weak to intense downslope winds: origin, interaction with boundary-layer turbulence and impact on CO2 variability

Jon Ander Arrillaga1,*, Carlos Yagüe1, Carlos Román-Cascón1,2, Mariano Sastre1, Maria Antonia Jiménez3, Gregorio Maqueda1, and Jordi Vilà-Guerau de Arellano4 Jon Ander Arrillaga et al.
  • 1Departamento de Física de la Tierra y Astrofísica, Universidad Complutense de Madrid, Madrid, Spain
  • 2Laboratoire d'Aérologie, CNRS, Université de Toulouse, CNRS, UPS, Toulouse, France
  • 3Departament de Física, Universitat de les Illes Balears, Palma, Spain
  • 4Meteorology and Air Quality Group, Wageningen University, Wageningen, the Netherlands
  • * Invited contribution by Jon Ander Arrillaga, recipient of the EGU Nonlinear Processes in Geosciences Outstanding Student Poster Award 2015.

Abstract. The interconnection of local downslope flows of different intensities with the turbulent characteristics and thermal structure of the atmospheric boundary layer (ABL) is investigated through observations. Measurements are carried out in a relatively flat area 2 km away from the steep slopes of the Sierra de Guadarrama (central Iberian Peninsula). A total of 40 thermally driven downslope events are selected from an observational database spanning the summer 2017 period by using an objective and systematic algorithm that accounts for a weak synoptic forcing and local downslope wind direction. We subsequently classify the downslope events into weak, moderate and intense categories, according to their maximum 6 m wind speed. This classification enables us to contrast their main differences regarding the driving mechanisms, associated ABL turbulence and thermal structure, and the major dynamical characteristics. We find that the strongest downslope flows (U > 3.5 m s−1) develop when soil moisture is low ( < 0.07 m3 m−3) and the synoptic wind not so weak (3.5 m s−1 < V850 < 6 m s−1) and roughly parallel to the direction of the downslope flow. The latter adds an important dynamical input, which induces an early flow advection from the nearby steep slope, when the local thermal profile is not stable yet. Consequently, turbulence driven by the bulk shear increases up to friction velocity (u*)  1 m s−1, preventing the development of the surface-based thermal inversion and giving rise to the so-called weakly stable boundary layer. On the contrary, when the dynamical input is absent, buoyancy acceleration drives the formation of a katabatic flow, which is weak (U < 1.5 m s−1) and generally manifested in the form of a shallow jet below 3 m. The relative flatness of the area favours the formation of very stable boundary layers marked by very weak turbulence (u* < 0.1 m s−1). In between, moderate downslope flows show intermediate characteristics, depending on the strength of the dynamical input and the occasional interaction with down-basin winds. On the other hand, by inspecting individual weak and intense events, we further explore the impact of downslope flows on CO2 variability. By relating the dynamics of the distinct turbulent regimes to the CO2 budget, we are able to estimate the contribution of the different terms. For the intense event, indeed, we infer a horizontal transport of 67 ppm in 3 h driven by the strong downslope advection.

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Thermally driven downslope winds develop in mountainous areas under a weak large-scale forcing and clear skies. In this work, we find that their onset time and intensity are closely connected with both the large-scale wind and soil moisture. We also show how the distinct downslope intensities shape the turbulent and thermal features of the nocturnal atmosphere. The analysis concludes that the downslope–turbulence interaction and the horizontal transport explain the important CO2 variability.
Thermally driven downslope winds develop in mountainous areas under a weak large-scale forcing...
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