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Summer Mistral at the Exit of the Rhone Valley

Drobinski, P. and Bastin, S. and Guénard, V. and Caccia, J.L. and Dabas, A. and Delville, P. and Protat, A. and Reitebuch, O. and Werner, C. (2005) Summer Mistral at the Exit of the Rhone Valley. Quarterly Journal of the Royal Meterological Society, 131, pp. 353-375. DOI: 10.1256/qj.04.63.

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Abstract

The 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).

Document Type:Article
Title:Summer Mistral at the Exit of the Rhone Valley
Authors:
AuthorsInstitution or Email of Authors
Drobinski, P.CNRS, Paris, F
Bastin, S.LSEET, La Garde, Toulon, F
Guénard, V.LSEET, La Garde, Toulon, F
Caccia, J.L.LSEET, La Garde, Toulon, F
Dabas, A.CNRM/Meteo-France, Toulouse, F
Delville, P.DT-INSU, Meudon, F
Protat, A.Inst. Pierre Simon Laplace, Vélizy, F
Reitebuch, O.UNSPECIFIED
Werner, C.UNSPECIFIED
Date:2005
Journal or Publication Title:Quarterly Journal of the Royal Meterological Society
Refereed publication:Yes
In ISI Web of Science:Yes
Volume:131
DOI:10.1256/qj.04.63
Page Range:pp. 353-375
Status:Published
Keywords:hydraulic jump, mountain wave, planetary boundary layer, valley flow
HGF - Research field:Aeronautics, Space and Transport (old)
HGF - Program:Space (old)
HGF - Program Themes:W EO - Erdbeobachtung
DLR - Research area:Space
DLR - Program:W EO - Erdbeobachtung
DLR - Research theme (Project):W - Vorhaben Atmosphären- und Klimaforschung (old)
Location: Oberpfaffenhofen
Institutes and Institutions:Institute of Atmospheric Physics > Lidar
Deposited By: elib DLR-Beauftragter
Deposited On:17 Mar 2008
Last Modified:14 Jan 2010 15:22

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