Proceedings with Extended Abstracts (single PDF file) - Radio ...

2003). They are related to the evolution of the upstream synoptic wind speed and directionconditions during the event, from west to northwest, in the first part of the event, and north, inthe second part. The progressive descent of the upstream inversion layer height (measured byradiosounding at Lyon), associated with the the upstream mountain elevation and the strongacceleration of the air masses along the Rhône-valley axis when the synoptic wind direction isnorth, are the causes of the passage from a “deep” to “shallow” Mistral situation. Then, thecessation of the Mistral at TLN, several hours before STC, is due to the blocking of this lowlevelflow by the Alps to the north of TLN (maximum upstream elevation around 3000 m),whereas STC still observed air masses directly exiting from the Rhône valley. This resultillustrates the important role played by the upstream orography to determine wether or not agiven site is under the influence of the Mistral.The vertical velocity field is presented here for the first time. Our results are consistentwith our basic knowledge of the Mistral, that is, the cold air masses arriving from the northtoward the Mediterranean sea are globally subsident. This is particularly true in Novemberwhere there is no significant ground heating. The right panels of Fig. 2 clearly show such abehaviour, i.e. quasi-systematic negative vertical velocities in the height-time regionconcerned by the Mistral episode with maximum values between –1 and –1.5 m s -1 aboveSTC and between –1.5 and –2 m s -1 , above TLN. The values have been temporally smoothedusing a 2h-width moving-averaging window in order to remove short period orographicallyinducedwavesFigure 2: Height-time diagrams of the horizontal wind speed (left panels), wind direction(middle panels) and 2h-averaged vertical velocity (right panels), obtained at STC(upper panels) and TLN (bottom panels), during the 6/8 November Mistral episode..Fig. 3 shows STC (top panels) and AIX (bottom panels) UHF-profiler observationsmade during the 21-to-23 June 2001 (ESCOMPTE). The maximum wind speed values arebetween 12 and 15 m s -1 below 1500 m.This event is a typical summer Mistral event, featured by WNW synoptic upstreamconditions (quasi-stationary during the whole event), weaker wind speeds and interactionswith thermal circulations. The left panels of Fig. 3 clearly show that the Mistral blows atlower levels during the nights and is “lifted up” during daytimes, especially the afternoonswhere the ground heating is maximum. Right panels of Fig. 3 show the turbulent dissipationrate ε which reveals the installation of the convective boundary layer at the same periodswhere the Mistral is “lifted up”. Moreover, still for the same periods, the near-surface wind is296