A reexamination of the valley wind system in the A

Authors: G Zängl

Publish Date: 2004/01/28

Volume: 87, Issue: 4, Pages: 241-256

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Abstract

This paper presents idealized numerical simulations of the valley wind circulation in the Alpine Inn Valley which are compared with existing data and are used to improve our dynamical understanding of the valley wind The simulations have been performed with the Penn State/NCAR mesoscale model MM5 They use a highresolution realistic topography but idealized largescale conditions without any synoptic forcing to focus on the thermally induced valley wind system The comparison with the available observations shows that this simplified setup is sufficient to reproduce the essential features of the valley windThe results show that the tributaries of the Inn Valley have a considerable impact on the alongvalley mass fluxes associated with the valley wind circulation The upvalley mass flux is found to increase where tributaries enter the Inn Valley from the north that is from the direction where the Alpine foreland is located On the other hand the upvalley mass flux is reduced at the junctions with southern tributaries because part of the upvalley flow is deflected into these tributaries For the downvalley flow the situation is essentially reversed but the influence of the valley geometry on the flow structure is larger than for the upvalley flow The most important feature is a lateral valley contraction near the valley exit into the Alpine foreland It reduces the downvalley mass flux at low levels so that the wind maximum in the interior of the valley is shifted to a fairly large distance from the ground North of the valley contraction however the downvalley flow strongly accelerates and forms a pronounced lowlevel jet A dynamical analysis indicates that this acceleration can be interpreted as a transition from subcritical to supercritical hydraulic flow Another interesting feature is that the lowlevel jet maintains its structure for several tenths of kilometres into the Alpine foreland This appears to be related to the fact that the lateral wind shear on the flanks of the jet is associated with a strong dipole of potential vorticity PV Due to the conservation properties of the PV the downstream advection of the PV dipole leads to the formation of a bandlike feature that decays fairly slowly

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