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Article Dans Une Revue Quarterly Journal of the Royal Meteorological Society Année : 2002

Gap flow in an Alpine valley during a shallow south föhn event: Observations, numerical simulations and hydraulic analogue

Résumé

This paper examines the three-dimensional structure and dynamics of southerly hybrid gap/mountain ow through the Wipp valley (Wipptal), Austria, observed on 30 October 1999 using high-resolution observations and model simulations. The observations were obtained during a shallow south föhn event documented in the framework of the Mesoscale Alpine Programme (MAP). Three important data sources were used: the airborne differential-absorption lidar LEANDRE 2, the ground-based Doppler lidar TEACO2 and in situ measurements from the National Oceanic and Atmospheric Administration P-3 aircraft. This event was simulated down to 2 km horizontal resolution using the non-hydrostatic mesoscale model Meso-NH. The structure and dynamics of the ow were realistically simulated. The combination of high-resolution observations and numerical simulations provided a comprehensive three-dimensional picture of the ow through the Wipptal: in the gap entrance region (Brenner Pass, Austria), the low-level jet was not solely due to the channelling of the southerly synoptic ow through the elevated gap. Part of the Wipptal ow originated as a mountain wave at the valley head wall of the Brenner Pass. Downstream of the pass, the shallow föhn ow had the characteristics of a downslope windstorm as it rushed down towards the Inn valley (Inntal) and the City of Innsbruck, Austria. Downhill of the Brenner Pass, the strongest ow was observed over a small obstacle along the western side wall (the Nösslachjoch), rather than channelled in the deeper part of the valley just to the east. Further north, the low-level jet was observed in the centre of the valley. Approximately halfway between Brenner Pass and Innsbruck, where the along-axis direction of the valley changes from north to north-northwest , the low-level jet was observed to be de ected to the eastern side wall of the Wipptal. Interaction between the Stubaier Alpen (the largest and highest topographic feature to the west of the Wipptal) and the south-westerly synoptic ow was found to be the primary mechanism responsible for the dee ection. The along-and cross-valley structure and dynamics of the ow were observed to be highly variable due to the in uence of surrounding mountains, localized steep slopes within the valley and outt ows from tributaries (the Gschnitztal and the Stubaital) to the west of the Wipptal. For that shallow föhn case, observations and simulations provided a large body of evidence that downslope ow created thinning/ thickening uid and accelerations/ decelerations reminiscent of mountain wave/hydraulic theory. Along the Wipptal, two hydraulic-jump-like transitions were observed and simulated, (i) on the lee slope of the Nösslachjoch and (ii) in the Gschnitztal exit region. A hydraulic solution of the ow was calculated in the framework of reduced-gravity shallow-water theory. The down-valley evolution of the Froude number computed using LEANDRE 2, P-3 ight level and TEACO2 measurements conn rmed that these transitions were associated with super-to subcritical transitions.
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Dates et versions

hal-01650713 , version 1 (28-11-2017)

Identifiants

Citer

Cyrille Flamant, Philippe Drobinski, L. Nance, R. Banta, L. Darby, et al.. Gap flow in an Alpine valley during a shallow south föhn event: Observations, numerical simulations and hydraulic analogue. Quarterly Journal of the Royal Meteorological Society, 2002, 128 (582), pp.1173-1210. ⟨10.1256/003590002320373256⟩. ⟨hal-01650713⟩
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