Summer mistral at the exit of the Rhône valley

Abstract

SUMMARYThe paper examines the three-dimensional structure and dynamics of the mistral at the Rhône valley exit on 28 June 2001. The mistral refers to a severe wind that develops along the Rhône valley in southern France. This summer mistral event was documented in the framework of the ESCOMPTE field experiment. The dynamical processes driving the circulation of the mistral in the Rhône valley and particularly wake formation and planetary boundary layer (PBL) inhomogeneity at the scale of Rhône valley delta are investigated. Several important data sources are used (airborne Doppler lidar, radiosondes and surface stations) as well as non-hydrostatic mesoscale simulations.This paper analyses experimentally, numerically and theoretically the mechanism of wake formation. It shows that the flow impinging on the Alpine range and the Massif Central becomes supercritical all along the ridge line, including the Rhône valley and continues to accelerate in the lee regions until a hydraulic jump occurs. It leads to the formation of wakes behind and close to the mountain peaks. Compared to the Massif Central wake, the origin of the western Alps wake is rather complicated. In this study, the observations and simulations suggest a combined wall separation/gravity wave breaking mechanism to explain the western Alps wake. Indeed, it is shown that in addition to the flow descending the western Alps slopes and experiencing a strong hydraulic jump, the point where the mistral flow separates from the eastern flank of the Rhône valley located at about 44 • N is associated with a 'flank-shock' which is an oblique hydraulic jump (i.e. the downstream Froude number is supercritical).Wake formation in the lee of the Alps and the Massif Central causes large inhomogeneity of the PBL with differences between land and sea. In the Massif Central and western Alps wakes, the continental PBL is deeper (1.8 km) than in the mistral flow (1 km), which is consistent with a subcritical regime associated with enhanced turbulent mixing. The supercritical air flow, descending the Massif Central and Alps slopes and transitioning to subcritical flow, increases the near-surface air temperature due to the föhn effect. Over the Mediterranean, the surface heat fluxes are slightly negative (between −50 and 0 W m −2 ) and the main source of PBL turbulence is mechanical (wind shear). The PBL depth within the mistral flow does not vary over land (1 km), whereas the absence of convection but also of strong winds prevent PBL development over the sea in the wakes of the Massif Central and the Alps (PBL depth of about 0.5 km).

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).…”

“…As for the mean surface wind field and wind variability, the PV field is very sensitive to the nudging time. The major source of PV, present in all simulations, is the mistral (generating positive PV)/tramontane (generating negative PV) winds, shooting through the Rhône and Aude valley gaps between the Alps and the Pyrenees, and bounded on both sides by mountain-induced PV banners shed from major peaks in the Pyrenees, the Massif Central and the Alps Drobinski et al, 2005). When τ increases, smaller-scale PV features appear in the western basin in the lee of Corsica-Sardinia (positive PV) and at the exit of the Ebro valley (Spain) where the cierzo blows.…”

North-western Mediterranean sea-breeze circulation in a regional climate system model

“…Alpert et al 1995;Trigo et al 1999), topographically-induced strong winds like Mistral (e.g. Drobinski et al 2005;Guénard et al 2005Guénard et al , 2006Corsmeier et al 2005) and Tramontane (e.g. Drobinski et al 2001), which are companion winds blowing south from the French Mediterranean coast to the African coasts (Salameh et al 2009).…”

Mistral and Tramontane wind systems in climate simulations from 1950 to 2100

“…The frequency of occurrence and intensity of Mistral and Tramontane winds are of great interest not only for risk assessments under changing climatic conditions, but also for scientific reasons. Many case studies have dealt with Mistral and Tramontane events (e.g., Drobinski et al 2005;Berthou et al 2016). Obermann et al (2016) found the sea-level pressure fields associated with Abstract The characteristics of the mesoscale Mistral and Tramontane winds under changing climate conditions are of great interest for risk assessments.…”