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Article Dans Une Revue Quarterly Journal of the Royal Meteorological Society Année : 2003

Scale interaction processes during the MAP IOP 12 south föhn event in the Rhine Valley

Résumé

This paper examines the three-dimensional structure and dynamics of a south föhn flow in the Rhine Valley during its entire life cycle from 29 October 1999 until 31 October 1999. The south föhn event was documented in the framework of the Mesoscale Alpine Programme (MAP). This study investigates the synoptic-scale forcing sources, the dynamical processes driving the circulation of the föhn flow in the complex network of tributaries of the Rhine Valley, and the degree of inhomogeneity on the scale of the FORM (‘FOehn in the Rhine Valley during the MAP’ programme) target area at the bifurcation between the Rhine and Seez Valleys. Several important data sources were used (ground-based Doppler lidar, scintillometers, constant-volume balloons, radiosoundings and surface stations) as well as non-hydrostatic mesoscale simulations. The föhn penetrating the FORM target area on 30 October 1999 is preceded by a nocturnal shallow föhn phase which does not penetrate down to the ground due to katabatic drainage flow from the main transverse (east–west oriented) tributaries in which föhn cannot penetrate. This paper shows the contribution of the main tributaries of the Rhine Valley in directing the föhn flow towards the FORM target area during (i) the shallow föhn phase where the westerly upper-level flow is deflected in the main longitudinal (i.e. north–south oriented) tributaries; (ii) the penetrating föhn phase, where the south/south-westerly upper-level flow is nearly aligned with the longitudinal tributaries. The channelling efficiency of the main longitudinal tributaries of the Rhine Valley (particularly the Domleschg) is higher during the föhn phase when the south-westerly upper-level flow, experiencing mountain-wave-induced downward motion, penetrates these tributaries, than during the shallow föhn. In the FORM target area, the structure of the föhn flow varies on a 1-kilometre horizontal length-scale and the time evolution of the respective location of the cold air pool and the warm föhn air is investigated in detail. Also, flow splitting between the Rhine and Seez Valleys occurs during the entire föhn life cycle, but its vertical extension is maximum during shallow föhn, when lower- and upper-level flows are fully decoupled.
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hal-00137826 , version 1 (09-08-2021)

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Philippe Drobinski, Christian Haeberli, Evelyne Richard, Marie Lothon, Alain Dabas, et al.. Scale interaction processes during the MAP IOP 12 south föhn event in the Rhine Valley. Quarterly Journal of the Royal Meteorological Society, 2003, 129 (588), pp.729-754. ⟨10.1256/qj.02.35⟩. ⟨hal-00137826⟩
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