A prediction system for local wind variations in mountainous terrain

A three-level model system for the prediction of local flows in mountainous terrain is described. The system is based upon an operational weather prediction model with a horizontal grid spacing of about 10 km. The large-scale flow is transformed to a more detailed terrain, first by a mesoscale model with grid spacing of about 1 km, and then by a local-scale model with a grid spacing of about 0.2 km. The weather prediction model is hydrostatic, while the two other models are non-hydrostatic. As a case study the model system has been applied to estimate wind and turbulence over Vaernes airport, Norway, where data on turbulent flight conditions were provided near the runway. The actual case was chosen due to previous experiences, which indicate that south-easterly winds may cause severe turbulence in a region close to the airport. Local terrain induced turbulence seems to be the main reason for these effects. The predicted local flow in the actual region is characterized by narrow secondary vortices along the flow, and large turbulent intensity associated with these vortices. A similar pattern is indicated by the sparse observations, although there seems to be a difference in mean wind direction between data and predictions. Due to fairly coarse data for sea surface temperature, errors could be induced in the turbulence damping via the Richardson number. An adjustment for this data problem improved the predictions.