Numerical simulations of local winds over steep orography in the storm over north Norway on October 12, 1996

Some power pylons (30 m high) broke down in a storm over north Norway in October 1996. The pylons were situated in a short, narrow valley (3 km long) surrounded by mountains reaching 600–850 m. The pylons fell in a direction opposite to the main southwesterly winds, and it is likely that the damage was caused by wind gusts of more than 50 m s −1 . High‐resolution simulations with the nonhydrostatic numerical model MEMO have been made to investigate the local winds. Initial conditions have been taken from successful limited‐area synoptic‐scale predictions of the damaging storm. For upstream southwesterly wind, leeside bluff separation was simulated in the valley. The recirculation filled the valley in a flow organized like a corkscrew down the valley. In the bottom of the valley the recirculation formed a return jet with a maximum wind speed between 17 and 20 m s −1 in the different simulations, and with a considerable component toward the separation line at the ground, downstream from the mountaintops on the southern side of the valley. The axis of the corkscrew was found at 0.4 of the valley height. A veering of the initial upstream wind direction of 20° changed the flow to be steered up the valley, with a shallow recirculation confined to the southern side of the valley only. The turbulence in the recirculation area was found to be large, and it is argued that the wind gusts in the southwesterly case might reach values 3 times the mean wind speed.