Numerical Simulation of Steep Terrain Influences on Flow Patterns at the Isle of Helgoland

Abstract

The isle of Helgoland is a small rocky island in the German Bight. The rock plateau is 1200 m long, 50 to 300 m wide and up to 62 m high. The cliffs in the north and west of the island are nearly vertical. The island influences the wind field around it and also meteorological routine measurements which should be representative for the surrounding sea. To study this influences the nonhydrostatic three-dimensional mesoscale model METRAS has been used (Schlünzen, 1990). It can be applied for terrain slopes up to 70 ° as is proved by systematic model runs over idealized hills for different terrain slopes.

Eight model simulations for different geostrophic wind directions with neutral atmospheric stratification have been performed to investigate the influences of the island. The results show that the wind velocity is strongly influenced by the island, much more than the wind direction. At several places the velocity can be increased or reduced by more than 50%. The vertical winds show strong upward motions on the windward and downward motions on the lee side of the island. They reach values of about ±2 ms ⁻¹ for a large-scale wind of 4.5ms ⁻¹ (Niemeier, 1992).

The German weather service measures wind velocity and direction at three sites on the island on a routine basis. The model results have been analysed especially at the grid points corresponding to the wind masts. The calculated speed-up values show the strong mesoscale influences of the island on the wind field. The differences between the calculated wind velocities for the measurement sites and over the undisturbed sea reach up to 30%. Only for one measurement point the calculated deviations are lower than 10% for all wind directions. The calculated speed-up values have been used for a correction of the measured data. The results show that mesoscale influences on the wind field are modelled correctly whereas some local effects are not included in the model calculations. For details on the results see Niemeier and Schlünzen (1993).