Simulation of transient eddies and flow separation over the Isle of Arran

A mesoscale meteorological model is used to simulate turbulent airflow over the Isle of Arran, south-west Scotland, UK. The topography of Arran is complex and in the north is dominated by sharp and steep mountains of up to 900 m height, while the southern part exhibits rolling hills with maximum heights of around 500m. Significant topography lies only a few tens of kilometres to the north and west of Arran and thus the airflow impinging on the island is affected by upstream mountains for northerly or north-westerly winds.

A mesoscale meteorological model is applied. The model is a finite-difference approximation to the anelastic, non-hydrostatic time-dependent Navier-Stokes equations. The high resolution model results are also compared with wave fields produced by a numerical model based on the linearised equations of motion and with surface-based field measurements of wind and pressure on the hill Tighvein (height 458 m, located on the southern part of the island).

The model is set-up with 2 nested domains. The outer domain has a grid resolution of 280 m in both the horizontal and the vertical. The second domain covers the island of Arran only, with horizontal and vertical resolutions of 70 m. The initialisation profile shows north-westerly winds and stable stratification in the lowest 2.5 km.

Figure 1 shows uy, the main wind component in the predominantly North-westerly flow, at the surface after 104 minutes, for the inner domain. Southerly flow is illustrated in red, northerly flow in blue. The maximum contours are +/- 15m/s. The turbulent surface airflow over Arran can be seen in fine detail. The figure shows further, that, over large parts of the island the flow is stagnant or even recirculating. The recirculation seems mostly associated with lee slopes of the extended valleys typical for the landscape of Arran. There is some indication, however, that the waves induced by the mainland and the terrain in the north of Arran also affect the flow separation. The relationship between the wave-induced surface pressure perturbations and the occurrence of flow separation is also being investigated in this study. Data from a new experimental campaign on Arran (which will finish by the beginning of March) will be compared with model results. Preliminary results and conclusions will be presented.