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