Vertical velocity and turbulence aspects during Mistral events as observed by UHF wind profilers Jean-Luc Caccia, V. Guénard, B. Benech, Bernard Campistron, P. Drobinski To cite this version: Jean-Luc Caccia, V. Guénard, B. Benech, Bernard Campistron, P. Drobinski. Vertical velocity and turbulence aspects during Mistral events as observed by UHF wind profilers. Annales Geophysicae, European Geosciences Union, 2004, 22 (11), pp.3927-3936. hal-00329404 HAL Id: hal-00329404 https://hal.archives-ouvertes.fr/hal-00329404 Submitted on 29 Nov 2004 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Annales Geophysicae (2004) 22: 3927–3936 SRef-ID: 1432-0576/ag/2004-22-3927 Annales © European Geosciences Union 2004 Geophysicae Vertical velocity and turbulence aspects during Mistral events as observed by UHF wind profilers J.-L. Caccia1, V. Guenard´ 1, B. Benech2, B. Campistron2, and P. Drobinski3 1LSEET, CNRS/Universite´ de Toulon, BP132, 83957 La Garde, France 2CRA/LA, CNRS/Obs. Midi-Pyren´ ees,´ 65300 Campistrous, France 3IPSL/SA, CNRS/Universite´ de Paris VI, Jussieu, 75252 Paris Cedex 05, France Received: 1 December 2003 – Revised: 26 April 2004 – Accepted: 12 May 2004 – Published: 29 November 2004 Part of Special Issue “10th International Workshop on Technical and Scientific Aspects of MST Radar (MST10)” Abstract. The general purpose of this paper is to exper- teristics. In addition, those vertical motions are found to be imentally study mesoscale dynamical aspects of the Mis- much less developed during the nighttimes because of the tral in the coastal area located at the exit of the Rhone-ˆ stabilization of the nocturnal planetary boundary layer due valley. The Mistral is a northerly low-level flow blowing to a ground cooling. The enhanced turbulent dissipation-rate in southern France along the Rhone-valleyˆ axis, located be- values found at lower levels during the afternoons of weak tween the French Alps and the Massif Central, towards the Mistral cases are consistent with the installation of the sum- Mediterranean Sea. The experimental data are obtained by mer convective boundary layer and show that, as expected UHF wind profilers deployed during two major field cam- in weaker Mistral events, the convection is the preponderant paigns, MAP (Mesoscale Alpine Program) in autumn 1999, factor for the turbulence generation. On the other hand, for and ESCOMPTE (Experience´ sur Site pour COntraindre stronger cases, such a convective boundary layer installation les Modeles` de Pollution atmospheriques´ et de Transports is perturbed by the Mistral. d’Emission) in summer 2001. Key words. Meteorology and atmospheric dynamics Thanks to the use of the time evolution of the vertical pro- (Mesoscale meteorology, turbulence) – Radio science file of the horizontal wind vector, recent works have shown (Instruments and techniques) that the dynamics of the Mistral is highly dependent on the season because of the occurrence of specific synoptic pat- terns. In addition, during summer, thermal forcing leads to a combination of sea breeze with Mistral and weaker Mistral 1 Introduction due to the enhanced friction while, during autumn, absence of convective turbulence leads to substantial acceleration as The Mistral is a northerly low-level, orography-induced, low-level jets are generated in the stably stratified planetary cold-air out-break over the Gulf of Lions, blowing off the boundary layer. At the exit of the Rhoneˆ valley, the gap flow shore of the south-eastern region of France at any season. dynamics dominates, whereas at the lee of the Alps, the dy- The climate of this area is under the influence of the Mistral namics is driven by the relative contribution of “flow around” which brings clear sky. It is frequently observed to extend and “flow over” mechanisms, upstream of the Alps. This pa- as far as a few hundreds of kilometers offshore and is one of per analyses vertical velocity and turbulence, i.e. turbulent the primary causes of storms over the northwestern Mediter- dissipation rate, with data obtained by the same UHF wind ranean. profilers during the same Mistral events. On meeting the Alpine range, a westerly to northerly syn- In autumn, the motions are found to be globally and signif- optic flow is deflected westward by the Coriolis force, as well icantly subsident, which is coherent for a dry, cold and stable as the pressure build up on the upstream edge of the range. flow approaching the sea, and the turbulence is found to be of As the flow experiences channelling in the Rhoneˆ valley sep- pure dynamical origin (wind shears and mountain/lee wave arating the French Alps, to the east, from the Massif Central, breaking), which is coherent with non-convective situations. to the west, by a gap of 200 km long and 40 km width (see In summer, due to the ground heating and to the interac- Figs. 1 and 2), it is substantially accelerated, giving birth to tions with thermal circulation, the vertical motions are less the Mistral (Pettre,´ 1982). pronounced and no longer have systematic subsident chara- Although some of the large-scale features of the Mistral are well understood, thus well forecasted, the mesoscale as- Correspondence to: J.-L. Caccia pects, such as the temporal, vertical and horizontal variabil- ([email protected]) ities, the exact onset and cessation times have still to be in- 3928Figure J.-L. 2 Caccia et al.: Vertical velocity and turbulence aspects Figure 1 Fig. 2. Area in southern France targeted by our Mistral study. The UHF wind profiler sites, St-Chamas (STC), Aix-les-Milles (AIX) and Toulon (TLN), are indicated. The lower contour line, for the mountainous areas, is at 500 m of altitude. The dotted line schemat- ically indicates the Rhone-valleyˆ axis. The distances between STC and AIX, and between STC and TLN, are 25 km and 90 km, respec- tively. its interactions with the ABL (Atmospheric Boundary Layer) and thermal circulations mainly due to the proximity of the sea. In this paper, we have chosen to present and analyse wind and turbulence data obtained by two UHF-wind profilers dur- ing three Mistral cases (a strong autumn case and a moder- Fig. 1. Map of surface wind and15 geopotential at 1000 hPa above ate and a weak summer case) among the seven cases that western Europe in a typical Mistral situation from ECMWF-model occurred during MAP (three cases)16 and ESCOMPTE (four analyses. The autumn case (see the text) is presented : (a) 6 Novem- cases). In addition to earlier papers on mesoscale modelling ber 1999, 12:00 UTC and (b) 7 November 1999, 12:00 UTC. The aspects related to the Mistral (e.g. Blondin and Bret, 1986), wind speed is indicated in color code and the direction by arrows. The thick and narrow lines show the shore lines and the topography a recent one (Caccia et al., 2001) has shown, for the first levels every 500 m, respectively, used by the model. time, direct comparisons between time-height diagrams of wind fields directly obtained by UHF-profilers and simu- lated by the Meso-NH (mesoscale non-hydrostatic) model vestigated. During MAP (Mesoscale Alpine Program, au- (Lafore et al., 1998), run over a 10-km resolution domain tumn 1999, see Bougeault et al., 2001) and ESCOMPTE nested in a 50-km resolution domain. These comparisons (Experience´ sur Site pour COntraindre les Modeles` de Pol- have been made during the strong autumn case. The Mis- lution atmospheriques´ et de Transport d’Emissions, summer tral onset phase is correctly reproduced by meso-NH but the 2001, see Cros et al., 2004) field experiments, an UHF-wind temporal variation of the wind vertical structure is not and profiler network has been deployed in the coastal region of the event duration is strongly overestimated. These discrep- south-eastern France to document the spatial and temporal ancies illustrate the difficulty for mesoscale models to cor- structure of the tropospheric flow and in particular during rectly describe the ABL dynamics when the topography is some Mistral events. Guenard´ et al. (2002) reported prelim- very variable (here valleys, mountains, hills and land-to-sea inary results from UHF-profiler data during several Mistral transition) and plays a very important role. In summer, an events, and Drobinski et al. (2003) reported combined and encreased difficulty is expected due to the thermal forcings complementary observations of the wind field made by air- (convection and land/sea-breezes). borne Doppler Lidar (WIND) and UHF-wind profilers in a In addition to those numerical model aspects, very recent Mistral situation. The networking approach, combined with experimental work on some of those Mistral events, for dif- the high vertical (75 m) and time (5 min) resolutions of the ferent purposes have been made. Drobinski et al. (2003) UHF-wind profilers, enables the study of the mesoscale inho- investigated some 3-D aspects of the wind field obtained mogeneity and unsteadiness aspects of the Mistral, as well as during several legs of WIND. Druilhet et al. (2002) stud- J.-L. Caccia et al.: Vertical velocity and turbulence aspects 3929 Table 1. Characteristics of the three Mistral events reported in the present study. Marignane maximum surface wind Toulon maximum surface wind Field Campaign Period Duration (h) Speed (m/s) Direction (◦) Speed (m/s) Direction (◦) MAP 6–8 November 1999 43 20 350 16 310 ESCOMPTE 21–23 June 2001 72 8 340 12 270 ESCOMPTE 30 June–1 July 2001 30 14 340 10 280 ied Mistral-induced wind and temperature fluctuations us- to thermal circulations.