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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 17 Oct 2018

Research article | 17 Oct 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Weak and intense katabatic winds: impacts on turbulent characteristics in the stable boundary layer and CO2 transport

Jon A. Arrillaga1, Carlos Yagüe1, Carlos Román-Cascón1,2, Mariano Sastre1, Gregorio Maqueda1, and Jordi Vilà-Guerau de Arellano3 Jon A. Arrillaga et al.
  • 1Dept. Física de la Tierra y Astrofísica, Universidad Complutense de Madrid, Spain
  • 2Laboratoire d'Aérologie, University of Toulouse, CNRS, France
  • 3Meteorology and Air Quality Group, Wageningen University, Netherlands

Abstract. The role of thermally-driven local downslope or katabatic flows in the dynamics and turbulent features of the stable boundary layer (SBL) is investigated using observations. Measurements are carried out in a relatively flat area 2-km away from the steep slopes of the Guadarrama Mountain Range (Spain). Forty katabatic events are selected from an observational database spanning the 2017-summer period, by using an objective and systematic algorithm that is able to account for local and synoptic forcings. We subsequently classify the katabatic events into weak, moderate and intense according to the observed maximum wind speed. This classification enables us to contrast the main differences in dynamics and thermal structure. We find that the stronger katabatic events are associated with an earlier onset time of these flows. We relate it to very low soil-moisture values (<0.07m3m−3, i.e. smaller than the median during the analysed period) and a weak synoptic wind (V850<6ms−1) having the same direction as the katabatic. The relative flatness of the area favours the formation of very stable boundary layers characterized by a longwave radiative cooling of around 60–70Wm−2 and very weak turbulence (friction velocity (u*)<0.1ms−1). They occur when katabatics are weak, and are occasionally associated with the formation of skin flows, that are manifested as weak jets (U<1ms−1) at 3m. Intense katabatics, instead, are characterised by a strong and increasing bulk shear (the maximum u* is close to 1ms−1) that avoids the development of the surface-based thermal inversion, giving rise to the so-called weakly stable boundary layer. We identify the transition between the two regimes for a threshold katabatic wind speed of around 1.5ms−1, in agreement with the hockey-stick transition hypothesis. Our analysis is extended by calculating non-dimensional numbers to characterize the transition: the shear capacity, the bulk Richardson number and z/L. On the other hand, by inspecting individual weak and intense events, we further explore the interaction between katabatic flows and turbulence, and the impact on CO2 concentration. By relating the dynamics of the two regimes with 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 67ppm in 3h driven by the katabatic advection.

Jon A. Arrillaga et al.
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Jon A. Arrillaga et al.
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Short summary
Thermally-driven downslope winds develop in mountainous areas when the large-scale forcing is weak and skies are clear. In this work, we find that their onset time and intensity can be predicted from both the large-scale wind and soil moisture. We prove how their intensity has direct implications for the turbulent characteristics of the nocturnal atmosphere, which are fundamental for forecasting minimum temperatures and fog. We also quantify a strong CO2 transport due to intense downslope winds.
Thermally-driven downslope winds develop in mountainous areas when the large-scale forcing is...