Boundary layer impacts of wind farms under a temperature inversion

The ever increasing demand for wind power has led to a increased desire to understand the impact of wind turbines on local meteorology. With field data scarce to come by, most of our understanding of how wind farms impact their surroundings has come from numerical simulations or analytical models. The analytical work of Smith (2009) demonstrated how gravity waves can be generated by a large wind farm with the presence of a temperature inversion (Smith, 2009). The propagation of these gravity waves is dependent on the strength of the inversion, with subcritical flow generating upstream gravity waves on the inversion whereas super-critical can not. Also of interest is the phenomenon of 'choking'; with a Froude number close to unity, the gravity waves and pressure gradient can result in a significant reduction of winds within the wind farm. The analytical model used in this study is relatively simple though, and the results have not been confirmed with observations or with more detailed numerical simulations. The aim of this work is to address this.

Using the BLASIUS model from the UK Met Office, a wind farm parametrisation (based on Fitch et al 2012), has been implemented using a TKE source and momentum deficit. Numerical simulations have been undertaken using this scheme to reproduce the work of Smith (2009) and investigate the impact of a more detailed representation of the boundary layer and the wind turbines on the results. The work is then extended to regimes where the analytical solution of Smith (2009) is no longer valid. The results are important, both in terms of understanding the impact of wind turbines on local meteorology and gravity wave production, but also because the 'choking' phenomena could have significant impact on the power output of wind farms under the wrong conditions.