dc.description.abstract | Alpine valleys are rarely closed systems, implying that the atmospheric boundary layer
of a particular valley section is influenced by the surrounding terrain and large-scale
flows. A detailed characterisation and quantification of these effects is required in
order to design appropriate parameterisation schemes for complex terrains. The focus
of this work is to improve the understanding of the effects of surrounding terrain
(plains, valleys or tributaries) on the heat and mass budgets of the stable boundary
layer of a valley section, under dry and weak large-scale wind conditions. Numerical
simulations using idealised and real frameworks are performed to meet this goal.
Several idealised terrains (configurations) were considered: an infinitely long valley
(i.e. two-dimensional), and upstream valleys opening either on a plain (valley-plain),
on a wider valley (draining) or on a narrower valley (pooling). In three-dimensional
valleys, two main regimes can be identified for all configurations: a transient regime,
before the down-valley flow develops, followed by a quasi-steady regime, when the
down-valley flow is fully developed. The presence of a downstream valley reduces the
along-valley temperature difference, therefore leading to weaker down-valley flows. As
a result, the duration of the transient regime increases compared to the respective
valley-plain configuration. Its duration is longest for the pooling configuration. For
strong pooling the along-valley temperature difference can reverse, forcing up-valley
flows from the narrower towards the wider valley. In this regime, the average cooling
rate at the valley-scale is found to be a maximum and its magnitude is dependent
on the configuration considered. Therefore pooling and draining induce colder and
deeper boundary layers than the respective valley-plain configurations. In the quasisteady
regime the cooling rate is smaller than during the transient regime, and almost
independent of the configuration considered. Indeed, as the pooling character is more
pronounced, the warming contribution from advection to the heat budget decreases
because of weaker down-valley flows, and so does the cooling contribution from the
surface sensible heat flux. The mass budget of the valley boundary layer was found to
be controlled by a balance between the convergence of downslope flows at the top of
the boundary layer and the divergence of the down-valley flow along the valley axis,
with negligible contributions of subsidence far from the valley sidewalls. The mass
budget highlighted the importance of the return current above the down-valley flow,
which may contribute significantly to the inflow of air at the top of the boundary layer.
A case-study of a persistent cold-air pool event which occurred in February 2015 in
the Arve River Valley during the intensive observation period 1 (IOP1) of the PASSY-
2015 field campaign, allowed us to quantify the effects of neighbouring valleys on the
heat and mass budgets of a real valley atmosphere. The cold-air pool persisted as
a result of warm air advection at the valley top, associated with the passage of an
upper-level ridge over Europe. The contributions from each tributary valley to the
mass and heat budgets of the valley atmosphere were found to vary from day to day
within the persistent stage of the cold-air pool, depending on the large-scale flow.
Tributary flows had significant impact on the height of the inversion layer and the
strength of the cold-air pool, transporting a significant amount of mass within the
valley atmosphere throughout the night. The strong stratification of the near-surface
atmosphere prevented the tributary flows from penetrating down to the valley floor.
The evolution of the large-scale flow during the episode had a profound impact on
the near-surface circulation of the valley. The channelling of the large-scale flow at
night, can lead to the decrease of the horizontal temperature difference driving the
near-surface down-valley flow, favouring the stagnation of the air close to the ground. | en_US |