Diagnostics Of Hydraulic Jump And Gap Flow In Stratified Flows Over Topography

Q. J. R. Meteorol. Soc.

Bastin

Guénard³

et al. 2005

SUMMARYThe paper examines the three-dimensional structure and dynamics of the mistral at the Rhône valley exit on 28 June 2001. The mistral refers to a severe wind that develops along the Rhône valley in southern France. This summer mistral event was documented in the framework of the ESCOMPTE field experiment. The dynamical processes driving the circulation of the mistral in the Rhône valley and particularly wake formation and planetary boundary layer (PBL) inhomogeneity at the scale of Rhône valley delta are investigated. Several important data sources are used (airborne Doppler lidar, radiosondes and surface stations) as well as non-hydrostatic mesoscale simulations.This paper analyses experimentally, numerically and theoretically the mechanism of wake formation. It shows that the flow impinging on the Alpine range and the Massif Central becomes supercritical all along the ridge line, including the Rhône valley and continues to accelerate in the lee regions until a hydraulic jump occurs. It leads to the formation of wakes behind and close to the mountain peaks. Compared to the Massif Central wake, the origin of the western Alps wake is rather complicated. In this study, the observations and simulations suggest a combined wall separation/gravity wave breaking mechanism to explain the western Alps wake. Indeed, it is shown that in addition to the flow descending the western Alps slopes and experiencing a strong hydraulic jump, the point where the mistral flow separates from the eastern flank of the Rhône valley located at about 44 • N is associated with a 'flank-shock' which is an oblique hydraulic jump (i.e. the downstream Froude number is supercritical).Wake formation in the lee of the Alps and the Massif Central causes large inhomogeneity of the PBL with differences between land and sea. In the Massif Central and western Alps wakes, the continental PBL is deeper (1.8 km) than in the mistral flow (1 km), which is consistent with a subcritical regime associated with enhanced turbulent mixing. The supercritical air flow, descending the Massif Central and Alps slopes and transitioning to subcritical flow, increases the near-surface air temperature due to the föhn effect. Over the Mediterranean, the surface heat fluxes are slightly negative (between −50 and 0 W m −2 ) and the main source of PBL turbulence is mechanical (wind shear). The PBL depth within the mistral flow does not vary over land (1 km), whereas the absence of convection but also of strong winds prevent PBL development over the sea in the wakes of the Massif Central and the Alps (PBL depth of about 0.5 km).

Section: Jet Structure and Wake Generation At The Rhône Valley Exit (mentioning

confidence: 91%

“…Figure 3 shows that the PBL is well mixed (nearly constant potential temperature in the PBL) probably due to enhanced dissipation in the expected jump (e.g. Drobinski et al 2001b). …”

Section: (B) Upstream Conditionsmentioning

confidence: 98%

See 1 more Smart Citation

Summer mistral at the exit of the Rhône valley

Q. J. R. Meteorol. Soc.

Bastin

Guénard³

et al. 2005

“…This can be caused by advection of cold air by the synoptic south-easterly flow, which stabilizes the lower troposphere near the shore and prevents the DSB development. In the northern district, the larger vertical extent of the DSB can be due to (1) the attenuated stabilizing effect that allows convection to contribute to the growth of the DSB between 1100 and 1400 UTC; (2) the sloping terrain that slows down the DSB as it starts ascending the hill (see Figure 6 between 3-4 km to the south of VAL) and thus deepens the DSB (characteristics of a supercritical flow, see Drobinski et al, 2001b). After the convergence front has passed beyond OBS, the wake downstream from the Saint Cyr massif is visible in Figure 6d and f with a core of very low radial velocities between 4 and 6 km to the south of VAL.…”

Section: Deep Sea Breezementioning

confidence: 99%

Vertical Structure of the Urban Boundary Layer over Marseille Under Sea-Breeze Conditions

Lemonsu

Masson

Boundary-Layer Meteorol

2006

During the UBL-ESCOMPTE program (June–July 2001), intensive observations were performed in Marseille (France). In particular, a Doppler lidar, located in the north of the city, provided radial velocity measurements on a 6-km radius area in the lowest 3 km of the troposphere. Thus, it is well adapted to document the vertical structure of the atmosphere above complex terrain, notably in Marseille, which is bordered by the Mediterranean sea and framed by numerous massifs. The present study focuses on the last day of the intensive observation period 2 (26 June 2001), which is characterized by a weak synoptic pressure gradient favouring the development of thermal circulations. Under such conditions, a complex stratification of the atmosphere is observed. Three-dimensional numerical simulations, with the Méso-NH atmospheric model including the town energy balance (TEB) urban parameterization, are conducted over south-eastern France. A complete evaluation of the model outputs was already performed at both regional and city scales. Here, the 250-m resolution outputs describing the vertical structure of the atmosphere above the Marseille area are compared to the Doppler lidar data, for which the spatial resolution is comparable. This joint analysis underscores the consistency between the atmospheric boundary layer (ABL) observed by the Doppler lidar and that modelled by Méso-NH. The observations and simulations reveal the presence of a shallow sea breeze (SSB) superimposed on a deep sea breeze (DSB) above Marseille during daytime. Because of the step-like shape of the Marseille coastline, the SSB is organized in two branches of different directions, which converge above the city centre. The analysis of the 250-m wind fields shows evidence of the role of the local topography on the local dynamics. Indeed, the topography tends to reinforce the SSB while it weakens the DSB. The ABL is directly affected by the different sea-breeze circulations, while the urban effects appear to be negligible

“…In downslope windstorms, Smith (1985Smith ( , 1989 found that wave breaking is associated with hydraulic jumps and in turn associated with the generation of potential vorticity (Smolarkievicz and Rotunno, 1993;Scha¨r and Smith, 1993). Drobinski et al (2001a) have shown that the location at which a hydraulic jump occurs depends on the upstream and downstream flow conditions and not only from upstream conditions. Recent high resolution simulations have shown that the western and eastern boundaries of the mistral are partly defined by gravity wave breaking over the Massif Central and the Alps, respectively (Jiang et al, 2003).…”

Section: Introductionmentioning

confidence: 99%

An Observational Study of the Mesoscale Mistral Dynamics

Guénard

Boundary-Layer Meteorol

Caccia

et al. 2005

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 [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