Vertical velocity and turbulence aspects during Mistral events as observed by UHF wind profilers

Abstract

Abstract. 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 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 chara-teristics. 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.

Investigation of the atmospheric boundary layer dynamics during the ESCOMPTE campaign

“…13, 9b). In addition, the numerical field at 14:00 UTC displays a deepening of the ABL in the advected continental layer, supporting the results from Caccia et al (2004), who showed that the Mistral is raised up by the sea breeze near the coast.…”

The effect of indiscriminate nudging time on large and small scales in regional climate modelling: Application to the Mediterranean basin

“…In the central Mediterranean, the north-north-west cold and dry mistral (e.g. Jansá 1987;Jiang et al, 2003;Caccia et al, 2004;Corsmeier et al, 2005;Drobinski et al, 2005;Guénard et al, 2005Guénard et al, , 2006Lebeaupin Brossier and Drobinski, 2009) and its companion wind the tramontane (e.g. Drobinski et al, 2001) blows in the Gulf of Lion, occasionally up to the African coasts (Salameh et al, 2007).…”

Aerosol distribution over the western Mediterranean basin during a Tramontane/Mistral event

“…The depth of the ABL increases westward from 500 to 1200 m with reflectivity values on the order of those observed on leg AF. Further to the west, a continental ABL is observed in the sheltered region in the lee of the Massif Central, with reflectivity values being significantly smaller than to the east of 43 • N. In this region, on leg CE, lidar measurements evidence the presence of gravity waves (at 2.2 km) inside the continental ABL and above, which are common features during Mistral events (Caccia et al, 2004;Drobinski et al, 2005). The vertical structure of the aerosol distribution in the atmosphere derived from LEANDRE-2 reflectivity is used to validate the CHIMERE simulations along the ARAT legs.…”