Observations and Simulation of Downslope Windstorms and Gravity Waves over Northwest-Iceland

A windstorm over the complex terrain of Nortwest Iceland on 1-2 February is simulated with the numerical model MM5. The simulation has horizontal resolutions of 9, 3 and 1 km and is forced with boundaries from the ECMWF. Downwind of some of the mountains, observations and the simulation show local amplification of the surface winds. These local windstorms are associated with vertically propagating gravity waves. The waves are observed by satellite, and reproduced in the simulation where they are shown to break and generate strong turbulence between 700 and 500 hPa. The breaking of the waves is related to a reverse wind shear above 500 hPa. The numerical simulation correctly reproduces weak winds over some of the lee slopes. The absence of strong winds and wave breaking over these slopes appears to be related to the steepness of the slopes.