Turbulent kinetic energy over large offshore wind farms observed and simulated by the mesoscale model WRF (3.8.1)

Simon K Siedersleben,Stefan Emeis,Astrid Lampert,Jens Bange,Julie K Lundquist,Tom Neumann,Andreas Platis,Bughsin Djath,Beatriz Cañadillas,Konrad Bärfuss,Johannes Schulz-Stellenfleth

doi:10.5194/gmd-13-249-2020

Abstract

Abstract. Wind farms affect local weather and microclimates; hence, parameterizations of their effects have been developed for numerical weather prediction models. While most wind farm parameterizations (WFPs) include drag effects of wind farms, models differ on whether or not an additional turbulent kinetic energy (TKE) source should be included in these parameterizations to simulate the impact of wind farms on the boundary layer. Therefore, we use aircraft measurements above large offshore wind farms in stable conditions to evaluate WFP choices. Of the three case studies we examine, we find the simulated ambient background flow to agree with observations of temperature stratification and winds. This agreement allows us to explore the sensitivity of simulated wind farm effects with respect to modeling choices such as whether or not to include a TKE source, horizontal resolution, vertical resolution and advection of TKE. For a stably stratified marine atmospheric boundary layer (MABL), a TKE source and a horizontal resolution on the order of 5 km or finer are necessary to represent the impact of offshore wind farms on the MABL. Additionally, TKE advection results in excessively reduced TKE over the wind farms, which in turn causes an underestimation of the wind speed deficit above the wind farm. Furthermore, using fine vertical resolution increases the agreement of the simulated wind speed with satellite observations of surface wind speed.

Highlights

Offshore wind energy in Europe has gained importance every year in the last decade
Given the success with the simulation CNTRb – case study II, we explore the sensitivity of the wind farm parameterizations (WFPs) of Fitch et al (2012) with respect to horizontal grid size, the turbulent kinetic energy (TKE) source, vertical resolution, TKE advection and thrust coefficient in Sect. 5.1, 5.2, 5.3 and 5.4
Using airborne measurements of wind speed and turbulent kinetic energy near offshore wind farms, we evaluate the wind speed and turbulent nature of the wind farm wakes as well as the parameterization of those wind farm wake effects enabled by the Weather Research and Forecasting Model (WRF) wind farm parameterization (WFP) of Fitch et al (2012)

Summary

Introduction

In 2017, the wind energy market experienced a new record in investments, with 3148 MW additional net installed offshore energy equal to 560 new offshore wind turbines at 17 wind farms. Two-thirds of these turbines were installed at the North Sea, equal to an increase of 2105 MW in net installed capacity (WindEurope, 2017). Compared to onshore wind farms, offshore wind farms are larger in size; the efficiency of large offshore wind farms is mainly driven by the turbulent vertical momentum flux (e.g., Emeis, 2010, 2018). Wind turbines extract kinetic energy from the mean flow and convert it partly into electrical energy. The resulting wind deficit downwind is balanced by the advection of momentum of the mean flow and the turbulent momentum fluxes.

Results

Discussion

Conclusion