Wind turbine parametrisation in HARMONIE-AROME

Offshore wind power production in the European Union (EU) and specifically the North-Sea region is steadily increasing: the Dutch offshore capacity is expected to grow from ±1 GW in 2019 to ±11.5 GW in 2030, as part of a total expected increase to ±70 GW in the entire EU (WindEurope, 2017). Wind turbines produce electric energy by extracting kinetic energy from the atmosphere, thereby decelerating (and agitating) the air. This typically results in a downstream decrease in wind speed and increase in turbulence (e.g. Baidya Roy & Traiteur, 2010; Fitch et al., 2012). As wind farms grow – both in size and number – the impact on weather and climate is expected to become more significant, requiring an adaptation of mesoscale models like HARMONIE-AROME (hereafter: HARMONIE) to account for the influence of wind farms on the local and regional meteorological conditions.

As part of the Dutch Offshore Wind Atlas (DOWA) project1 , we implemented the wind turbine parameterisation from Fitch et al. (2012) in HARMONIE. In the presence of wind turbines, this parameterisation adds an elevated drag term to the atmosphere, which locally decelerates the flow. The kinetic energy that is extracted from the atmosphere, but not converted into electric power, is used as a source term of turbulence kinetic energy (TKE).

As a first validation of the new wind turbine parameterisation in HARMONIE, four 48-hour experiments were compared to both experiments with the original code of Fitch et al. (2012) in WRF-ARW (hereafter: WRF), and available offshore measurements near the Dutch/Belgium coast. Next, we repeated 6 months of the DOWA reanalysis with the wind farm parameterisation and all current offshore wind farms in the North-Sea region included. The motivation for this experiment was twofold: first, to more thoroughly validate the wind farm parameterisation. During the chosen period from January to (including) June 2016, two floating lidars were available in the Borssele wind farm zone, one in the Westermost Rough wind farm, with additionally FINO1 tower measurements near the Alpha Ventus wind farm. Since all these measurements are in or near existing wind farms, they are ideal for validating the new wind farm parameterisation. Secondly, the six month experiment allowed us to quantify the impact of the offshore wind farms on the Dutch offshore and coastal meteorological conditions.

The content of this report is as follows: chapter 2 describes the wind farm parameterisation in HARMONIE, including a description of the code organisation and required input files. Chapter 3 contains the brief validation of HARMONIE with WRF, followed by the description and analysis of the 6 month HARMONIE reanalysis in chapter 4. Finally, this report is concluded in chapter 5.