Dynamical structures for southwesterly airflow over southern Norway: the role of dissipation

Authors

  • IDAR BARSTAD

    Corresponding author
    1. Department of Geology and Geophysics, Yale University, CT, USA; SIGBJØRN GRØNÅS, Geophysical Department, University of Bergen, 5007 Bergen, Norway
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*Corresponding author.
e-mail: idar@gfi.uib.no

Present address: Geophysical Department, University of Bergen, 5007 Bergen, Norway.

ABSTRACT

Earlier studies have revealed mesoscale structures in southwesterly flows over the mountains of southern Norway (Rossby number ∼1): a left-side jet and an upstream wind minimum as signs of the influence of rotation; a downstream wind shadow connected to inertio-gravity waves; a weak jet on the right side of the wind shadow and a shallow coastal wind shadow between the left-side jet and the main wind shadow. In the present study, the dynamics of the structures have been further examined from the results of experiments performed by a mesoscale numerical model and computations using a linear model with rotation included. Ideal atmospheric conditions for a large-scale wind direction from southwest have been used to initialize the numerical model. The sensitivity of mesoscale structures was studied with respect to dissipation from wave breaking and surface friction. Nonlinearity and dissipation in breaking waves are needed to explain the location, depth and strength of the downstream wind shadow. Increased wind to the right of the shadow was found to be generated by the effect of the Coriolis force as air was pulled towards the low-pressure perturbation behind the mountains. The coastal wind shadow was found to be a direct result of differential friction between land and ocean. Nonlinear experiments and an attempt to include boundary layer effects in linear theory showed that friction is reducing the effective height of the mountain and the signal of the mesoscale structures.

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