An Observational Study of the Mesoscale Mistral Dynamics

We investigate the mesoscale dynamics of the mistral through the wind profiler observations of the MAP (autumn 1999) and ESCOMPTE (summer 2001) field campaigns. We show that the mistral wind field can dramatically change on a time scale less than 3 hours. Transitions from a deep to a shallow mistral are often observed at any season when the lower layers are stable. The variability, mainly attributed in summer to the mistral/land–sea breeze interactions on a 10-km scale, is highlighted by observations from the wind profiler network set up during ESCOMPTE. The interpretations of the dynamical mistral structure are performed through comparisons with existing basic theories. The linear theory of R. B. Smith [AdvancesinGeophysics, Vol. 31, 1989, Academic Press, 1–41] and the shallow water theory [Schär, C. and Smith, R. B.: 1993a, J. Atmos. Sci. 50, 1373–1400] give some complementary explanations for the deep-to-shallow transition especially for the MAP mistral event. The wave breaking process induces a low-level jet (LLJ) downstream of the Alps that degenerates into a mountain wake, which in turn provokes the cessation of the mistral downstream of the Alps. Both theories indicate that the flow splits around the Alps and results in a persistent LLJ at the exit of the Rhône valley. The LLJ is strengthened by the channelling effect of the Rhône valley that is more efficient for north-easterly than northerly upstream winds despite the north–south valley axis. Summer moderate and weak mistral episodes are influenced by land–sea breezes and convection over land that induce a very complex interaction that cannot be accurately described by the previous theories.

[1]  M. Rhein Deep water formation in the western Mediterranean , 1995 .

[2]  Ronald B. Smith On Severe Downslope Winds , 1985 .

[3]  Robert R. Long,et al.  Some Aspects of the Flow of Stratified Fluids: I. A Theoretical Investigation , 1953 .

[4]  R. Bleck,et al.  A preliminary analysis of the role of potential vorticity in alpine lee cyclogenesis , 1984 .

[5]  Ronald B. Smith Hydrostatic airflow over mountains , 1989 .

[6]  C. Kottmeier,et al.  The mistral and its effect on air pollution transport and vertical mixing , 2005 .

[7]  D. Heimann,et al.  On the influence of the Alps on a cold front , 1990 .

[8]  J. Egger Alpine Lee Cyclogenesis: Verification of Theories , 1988 .

[9]  R. Houze,et al.  The MAP special observing period , 2001 .

[10]  C. Millot Wind induced upwellings in the Gulf of Lions , 1979 .

[11]  S. Tibaldi,et al.  Cyclogenesis in the lee of the Alps: A case study , 1978 .

[12]  The nature of the mistral: Observations and modelling of two MAP events , 2003 .

[13]  Henri Wortham,et al.  The ESCOMPTE program: an overview , 2004 .

[14]  Piotr K. Smolarkiewicz,et al.  Low Froude Number Flow Past Three-Dimensional Obstacles. Part I: Baroclinically Generated Lee Vortices , 1989 .

[15]  Diagnostics Of Hydraulic Jump And Gap Flow In Stratified Flows Over Topography , 2001 .

[16]  Antonio Speranza,et al.  A Theory of Deep Cyclogenesis in the Lee of the Alps. Part I: Modifications of Baroclinic Instability by Localized Topography , 1985 .

[17]  C. Flamant Alpine lee cyclogenesis influence on air‐sea heat exchanges and marine atmospheric boundary layer thermodynamics over the western Mediterranean during a Tramontane/Mistral event , 2003 .

[18]  A. Tafferner,et al.  Test of Theories of Lee Cyclogenesis: ALPEX Cases. , 1990 .

[19]  E. Richard,et al.  Numerical Simulation of Flow Diversion around the Pyrenees: A Tramontana Case Study , 1996 .

[20]  Jacques Pelon,et al.  Observational Evidence And Modelling Of An Internal Hydraulic Jump At The Atmospheric Boundary-Layer Top During A Tramontane Event , 2001 .

[21]  H. Mannstein,et al.  Analysis of a potential‐vorticity streamer crossing the Alps during MAP IOP 15 on 6 November 1999 , 2002 .

[22]  C. Schär,et al.  Shallow-water flow past isolated topography. Part I: Vorticity production and wake formation , 1993 .

[23]  C. Schär,et al.  Low-Level Potential Vorticity and Cyclogenesis to the Lee of the Alps , 1998 .

[24]  P. Pettré On the Problem of Violent Valley Winds , 1982 .

[25]  C. Schär,et al.  Rotational aspects of stratified gap flows and shallow föhn , 2001 .

[26]  Dale R. Durran,et al.  Another Look at Downslope Windstorms. Part I: The Development of Analogs to Supercritical Flow in an Infinitely Deep, Continuously Stratified Fluid. , 1986 .