| The general purpose of this paper is to experimentally study mesoscale
dynamical aspects of the Mistral in the coastal area located at the exit of
the Rhône-valley. The Mistral is a northerly low-level flow blowing in
southern France along the Rhône-valley axis, located between the French
Alps and the Massif Central, towards the Mediterranean Sea. The experimental
data are obtained by UHF wind profilers deployed during two major field
campaigns, MAP (Mesoscale Alpine Program) in autumn 1999, and ESCOMPTE
(Expérience sur Site pour COntraindre les Modèles de Pollution
atmosphériques et de Transports d'Emission) in summer 2001.
Thanks to the use of the time evolution of the vertical profile of the
horizontal wind vector, recent works have shown that the dynamics of the
Mistral is highly dependent on the season because of the occurrence of
specific synoptic patterns. In addition, during summer, thermal forcing
leads to a combination of sea breeze with Mistral and weaker Mistral due to
the enhanced friction while, during autumn, absence of convective turbulence
leads to substantial acceleration as low-level jets are generated in the stably
stratified planetary boundary layer. At the exit of the Rhône valley,
the gap flow dynamics dominates, whereas at the lee of the Alps, the
dynamics is driven by the relative contribution of "flow around" and
"flow over" mechanisms, upstream of the Alps. This paper analyses vertical
velocity and turbulence, i.e. turbulent dissipation rate, with data obtained by
the same UHF wind profilers during the same Mistral events.
In autumn, the motions are found to be globally and significantly subsident,
which is coherent for a dry, cold and stable flow approaching the sea, and
the turbulence is found to be of pure dynamical origin (wind shears and
mountain/lee wave breaking), which is coherent with non-convective
situations.
In summer, due to the ground heating and to the interactions with thermal
circulation, the vertical motions are less pronounced and no longer have
systematic subsident charateristics. In addition, those vertical motions are
found to be much less developed during the nighttimes because of the stabilization of
the nocturnal planetary boundary layer due to a ground cooling. The enhanced
turbulent dissipation-rate values found at lower levels during the
afternoons of weak Mistral cases are consistent with the installation of the
summer convective boundary layer and show that, as expected in weaker
Mistral events, the convection is the preponderant factor for the turbulence
generation. On the other hand, for stronger cases, such a convective
boundary layer installation is perturbed by the Mistral. |