Southwesterly flows over southern Norway—mesoscale sensitivity to large-scale wind direction and speed

Abstract

Mesoscale structures have been identified and studied for simulations of ideal flows passing southern Norway. The large-scale wind direction was between south and west and the wind speed between 10 and 22.5 m s^—1. Flow data have been provided from simulations with a mesoscale numerical model with 10 km between the grid points horizontally. The results are found to be qualitatively in accordance with observational findings, including old forecasting rules for southern Norway. As expected, the influence of rotation is considerable. Accordingly, the flows are characterized by a jet on the left side of the mountains and a minimum on the right upstream side. In addition, a wind shadow extends far downstream of the main mountains, with signs of increased winds on the right side of the wind shadow. The wind shadow is connected to an inertio-gravity wave with downstream signatures caused by rotation. When the background wind direction was turned, the alignment of the structures was turned accordingly. For flows in the sector 200–270°, the action of the Coriolis force gave an efficiently narrower mountain (than without rotation). A similar action for southerly flows, on the other hand, resulted in an efficiently wider mountain. Different mountain widths resulted in different shape of the gravity waves and different acceleration of the jet on the left side. When the wind speed is increased, the amplitudes of the mesoscale structures are decreased with no abrupt change in the character of the flow.