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 [Advances in Geophysics, Vol. 31, 1989, Academic Press, 1-41] and the shallow water theory [Schar, 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 Rhone valley. The LLJ is strengthened by the channelling effect of the Rhone 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.