The Wake of St. Vincent

Smith, Ronald B.; Gleason, Arthur C. R.; Gluhosky, Paul A.; Grubı̆sı́c, Vanda

doi:10.1175/1520-0469(1997)054<0606:twosv>2.0.co;2

Cited by 98 publications

(89 citation statements)

References 34 publications

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“…The connection between mountain wakes and hydraulic jumps has been noticed by previous studies (e.g. Smith et al 1997;Rotunno et al 1999). The location of hydraulic jump occurrences are displayed by dashed lines in Fig.…”

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

confidence: 71%

Summer mistral at the exit of the Rhône valley

Drobinski

Bastin

Guénard³

et al. 2005

Q. J. R. Meteorol. Soc.

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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).

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

confidence: 71%

Summer mistral at the exit of the Rhône valley

Drobinski

Bastin

Guénard³

et al. 2005

Q. J. R. Meteorol. Soc.

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“…Overland (1984) showed that the momentum balances within gaps and their exit regions are highly dependent on the length and width scales of the gap. Smith et al (1997) used satellite images as well as boat and lowlevel aircraft surveys to investigate the wake of St. Vincent in the southeastern Caribbean. Pan and Smith (1999) examined the terrain-induced gap winds and wakes in the atmosphere using surface wind data from synthetic aperture radar (SAR) in the Unimak Island.…”

Section: Introductionmentioning

confidence: 99%

The interaction of northern wind flow with the complex topography of Crete Island – Part 1: Observational study

Koletsis

Lagouvardos

Kotroni

et al. 2009

Nat. Hazards Earth Syst. Sci.

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Abstract. The island of Crete with its mountain ranges is an excellent example of a major isolated topographic feature, which significantly modifies the regional airflow as well as the pressure and temperature fields. During summer, when northerly winds are blowing over the Aegean Sea (a large number of which are characterized as Etesians), the highly complex topography of Crete plays an important role in the modification of this northern wind flow. The main objective of this study is to determine the role of the topography of Crete Island during this wind flow on the strong downslope winds at the southern parts of the island as well as on the development of a gap flow between the two highest mountains of the island (Lefka Ori and Idi). For that purpose, observational data from four meteorological stations located along the aforementioned gap are used along with QuikSCAT satellite data. The observational analysis shows that the interaction of the northern wind flow with the mountains of Crete Island produces an upstream deceleration, a leftward deflection of the air as this approaches the mountains and an intensification of the winds at the southern coasts accompanied with a temperature increase. Furthermore, the maximum of the gap flow is observed at the exit region of the gap.

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“…Previous observational wake studies have been conducted for: Hawaii (Smith and Grubisic 1993), the Front Range in Colorado (e.g. Levinson and Banta 1995), the island of St. Vincent in the Caribbean (Smith et al 1997), the mountainous Lantau Island near Hong Kong (Lau and Shun 2000), small hills such as the Black Combe in Cumbria (Vosper and Mobbs 1997), islands of the Aleutian Chain (Pan and Smith 1999), and Greenland (Doyle and Shapiro 1999). Yet a complex wake such as the one of the Alps has only been explored during MAP.…”

Section: Introductionmentioning

confidence: 99%

Structure and dynamics of an Alpine potential‐vorticity banner

Schär¹,

Sprenger²,

Lüthi³

et al. 2003

Quart J Royal Meteoro Soc

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SUMMARYA detailed analysis is undertaken of the primary Alpine shear zone that occurred on 1 October 1999 during the Mesoscale Alpine Programme. This shear zone emanates from the south-western Alpine tip in north-westerly ow conditions, develops in response to the Alpine-scale ow splitting, and separates the northerly mistral wind to the south-west of the Alps from the quasi-stagnant air within the Alpine wake. The study is based on in situ ightlevel and dropsonde data from two research aircraft. The data are used for the construction of an objective analysis in a cross-section perpendicular to the shear zone, and for diagnostic computations of the potential-vorticity (PV) ux between vertically stacked ight legs. The observed ow structure is compared with simulation results of the Swiss Model and the non-hydrostatic Canadian Mesoscale Compressible Community Model (MC2), operating at horizontal resolutions of 14 and 3 km, respectively.Immediately downstream of the topography, the observed shear zone has a surprisingly quasi-steady structure that becomes evident provided a slow westward migration is accounted for. Further downstream, the shear zone shows increased signs of transience. Near the surface, it has an overall width of »150 km, but narrows to about 25 km near the top of the mistral inversion at a height of »2 km. Detailed analysis shows that the shear zone has a complex substructure consisting of at least three positive and three negative PV laments. These shear lines are characterized by a pronounced vertical coherence in the mixed layer and throughout most of the inversion layer. One of the banners is also evident in low-level tracer constituents. Diagnostic computations of the PV ux along the banners using the generalized Bernoulli theorem reveal that most of the associated PV ux is due to vertically oriented vorticity within the inversion layer. Within the shear zone, there is a spanwise circulation of appreciable strength. It consists of a deformation ow with the axes of deformation oriented perpendicular to the shear zone. This deformation ow in effect squeezes the PV banners and thereby makes them resilient with respect to dissipation and barotropic instabilities.Detailed intercomparison with numerical model results shows that models with a grid spacing below »15 km are able to capture the overall structure of the shear zone. The MC2 model in addition credibly replicates some of the ner-scale sub-structure of the shear zone, in particular at low levels. The high predictability and steadiness of the PV banners supports the idea that individual banners are generated by ow-separation and/or gravity-wave breaking events upstream. It is argued that the overall width of the shear zone is thus in effect determined by the geometry of the upstream topography and the dynamical processes that govern the downstream advection of the PV banners.

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The Wake of St. Vincent

Cited by 98 publications

References 34 publications

Summer mistral at the exit of the Rhône valley

Summer mistral at the exit of the Rhône valley

The interaction of northern wind flow with the complex topography of Crete Island – Part 1: Observational study

Structure and dynamics of an Alpine potential‐vorticity banner

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