Seminaire Jon Ander Arrillaga Mitxelena (Universidad Complutense de Madrid)

Thermally-driven slope winds develop in mountainous areas, when the large-scale flow is weak
and skies are clear, allowing greater incoming solar radiation during daytime and larger
outgoing longwave radiation during the night. Mountainous sites have struck the attention of
many studies for plenty of reasons. Among others, slope flows play a fundamental role in the
thermal and dynamical structure of the Atmospheric Boundary Layer (ABL) and its morning and
evening transitions. In our analysis, the interconnection of local downslope flows of different
intensities with the turbulent characteristics and thermal structure of the ABL is investigated
through observations.
Measurements are carried out in a relatively flat area 2-km away from the steep slopes of the
Guadarrama Mountain Range (Spain). A series of 40 thermally-driven downslope events is
selected from an observational database spanning the 2017-summer period, by using an
objective and systematic algorithm that accounts for a weak synoptic forcing and local
downslope wind direction. We subsequently classify the 40 downslope events into weak,
moderate and intense, according to their maximum intensity. 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 develop when soil moisture is low and the synoptic
wind is less weak 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 increases, preventing
the development of the surface-based thermal inversion, and giving rise to the so-called weakly
stable boundary layer. In the contrary, when the dynamical input is absent, buoyancy
acceleration drives the formation of a weak katabatic flow, and 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. In between, moderate downslope flows show
intermediate characteristics, depending on the strength of the dynamical input and the
occasional interaction with downbasin winds.

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