Variability in the Kvísker Downslope Windstorms

Downslope windstorms at Kvísker in Southeast-Iceland are simulated with a mesoscale model and explored using observations from automatic weather stations as well as 40 years worth of analysis-data. Two different types of gravity-wave induced windstorms are detected. Type A is a westerly windstorm which is confined to the downslopes of Mount Öræfajökull while the northerly windstorm of type B continues some distance downstream of the mountain and ends in a hydraulic jump. The low-level flow in the type B windstorm is of arctic origin and close to neutral as is often the case with arctic airmasses. It has an inversion well above the mountain top level. The low-level flow in the type A windstorm is on the other hand of southerly origin and the airmass is stable from approximately mountain top level and upwards. Simulations of gravity waves in idealized flows show strong dependence of the flow and the resulting wave structure on different positions of the inversion height. The flow speed and the extent of the windstorm over the edges of the mountain is sensitive to the height of the interface between the lower neutral layer and the upper stable layer. The flow immediately downstrem of the top is however not as sensitive to the height of the interface.