Simulations of Observed Lee Waves and Rotor Turbulence

Abstract

n 18 November 2008 a commercial aircraft encountered severe turbulence while flying in westerly flow along the southeastern coast of Iceland and descending from 2500 m down to the ground for a safe landing. Numerical simulations at horizontal resolutions of 9, 3, and 1 km are compared to the available observations. The simulations reproduce the situation, with an observed severe downslope windstorm at the ground as well as associated amplified lee waves and a rotor aloft, while climate data indicate that all observed westerly windstorms in the region are of the same type and occur in a similarly structured atmosphere. Strong shear turbulence is simulated at the interface of the lee wave and the rotor, as well as inside the rotor. The lee waves and the turbulence patterns are not stationary and as the upstream vertical wind shear increases, the lee wave becomes less steep, but the turbulence increases temporarily while the rotor circulation breaks down. From a forecasting perspective, this event could have been foreseen quite accurately, but not with the NWP tools that were in use for aviation forecasts, as their resolution was simply not adequate for resolving hazardous features of flow in and above complex terrain on the scale of this event. This event underlines the urgency of delivering products from finescale simulations over complex terrain to pilots and forecasters. Such products need to be developed taking into account the transient nature of the flows and the hazards.