Dynamics of the MAP IOP 15 severe Mistral event: Observations and high‐resolution numerical simulations

Abstract

SUMMARYThis paper investigates the fundamental processes involved in a severe Mistral event that occurred during the Mesoscale Alpine Program (from 6 to 9 November 1999). The Mistral refers to a violent north/north-westerly wind blowing in south-eastern France from the Rhône valley to the French Riviera. The study is based on measurements from radiosoundings launched from Lyon and Nîmes and from two UHF wind profilers located near Marseille and Toulon allowing a good description of the flow in the complex terrain formed by the southwestern Alps. Observational results are compared with RAMS non-hydrostatic numerical simulations performed with 27 km, 9 km and 3 km nested grids. The numerical simulations capture the flow complexity both upstream of the Alps and in the coastal area affected by the Mistral. They correctly reproduce horizontal wind speeds and directions, vertical velocities, virtual potential temperature and relative humidity documented by the observational network. The simulations are used to point out the main dynamical processes generating the Mistral. It is found that flow splitting around the Alps and around the isolated peaks bordering the south-eastern part of the Rhône valley (Mont Ventoux 1909 m, Massif du Lubéron 1425 m) induces the low-level jet observed near Marseille that lasts for 36 hours. The high-resolution simulation indicates that the transient low-level jet lasting for only 9 hours observed at Toulon is due to a gravity wave breaking over local topography (the Sainte Baume 1147 m) where hydraulic jumps are involved. A mountain wake with two opposite-sign potential-vorticity banners is generated. The mesoscale wake explains the westward progression of the large-scale Alpine wake.

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).…”

“…Smith, 1989;Smith et al, 2007). For instance, the mistral is qualified as a primary PV banner Guénard et al, 2006), contrary to smaller streams of vorticity which wrap up into pairs of eddies of opposite signs due to the irregular peak and pass structure of the mountain ridges forming eventually multiple PV banners (Aebischer and Schär, 1998;Schär et al, 2003;Flamant et al, 2004). Figure 5 displays the 850 hPa PV fields for the various values of τ for day 20 of the two-month simulation.…”

Dynamical and statistical downscaling of the French Mediterranean climate: uncertainty assessment

“…1) which is a key spot of occurrences of meteorological extremes, such as windstorms (e.g. Drobinski et al, 2005;Guénard et al, 2006;Lebeaupin Brossier and Drobinski, 2009), heavy precipitation (e.g. Ducrocq et al, 2008;Lebeaupin et al, 2006;Beaulant et al, 2011;or Vrac and Yiou, 2010) and heat waves and droughts (e.g.…”

“…In ALADIN-Climate, nudging is applied on temperature, wind vorticity, wind divergence and logarithm of the surface pressure. The maximum e-folding time depends on the variables (6 h for the vorticity, 24 h for the logarithm of the surface pressure, the specific humidity and the temperature, 48 h for the divergence) following the setting of Guldberg et al (2005). The maximum e-folding time is reached above 700 hPa and for scales larger than 1280 km.…”

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).…”