Observations and simulation of a windstorm below breaking gravity waves

An extreme downslope windstorm that lead to substantial damage in Southeast-Iceland is investigated. Observations in the outskirts of the windstorm show wind gusts of more than 50 m/s and a high-resolution numerical simulation reproduces to some extent the observations and shows a distinct maximum where the structural damage occurred. The simulation indicates that there were two local windstorms acting at the same time; a windstorm at the edge of the mountain and another windstorm at the foothills of the downslopes. The downslope windstorm appears to be created by strong gravity waves that are generated in a stably stratified layer and break below a reverse wind-shear well below 500 hPa. Applying Brasseur's gust method gives a predition of a gust factor of about 2, but the magnitude of the gusts is still somewhat underestimated.