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

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.

The Effects of Offshore Wind Farms on Hydrodynamics and Implications for Fishes

“…This result disagrees with the findings by Grashorn and Stanev (2016) and Rivier et al (2016), who reported increased turbidity caused by OWF foundations. Lampert et al (2020) provide airborne observations of a large-scale regional reduction in wind stress downwind of the OWF in the German Bight, confirming results of previous studies (e.g., Djath et al, 2018;Djath and Schulz-Stellenfleth, 2019;Cañadillas et al, 2020;Siedersleben et al, 2020). Consistent with the observations by Lampert et al (2020), atmospheric model simulations by Siedersleben (2019) show a reduction of wind speed by 5%-25% inside the wind wake 5 km downwind of a wind farm.…”

In situ airborne measurements of atmospheric and sea surface parameters related to offshore wind parks in the German Bight

“…The unique data have been the basis for different studies, proving for the first time directly the horizontal extension of wakes downwind of offshore wind parks (Platis et al, 2018), quantifying the wind speed recovery in relation to stability Platis et al, 2020) and validating the WRF mesoscale model (Siedersleben et al, 2018a(Siedersleben et al, , b, 2020, which can then be used for larger scales and future wind energy scenario calculations.…”

“…Convective processes up to the formation of thunderstorms in the atmosphere were studied by direct measurements in the clouds and drop sondes released from the aircraft (Groenemeijer et al, 2009). The aircraft was deployed above the Mediterranean to investigate cyclones and mesoscale convective systems to understand which atmospheric conditions lead to devastating thunderstorms, and to improve the forecasting of such systems Ducrocq et al, 2014;Sodemann et al, 2017). A comparison of convective boundary layer conditions between wind lidar and airborne measurements was performed (Adler et al, 2019).…”

“…Weather Research and Forecasting (WRF) simula- tions were performed for some measurement flights and verified by the observations. First results of the measurements have been presented in Platis et al (2018), and WRF simulations and validation with observational data have been presented in Siedersleben et al (2018aSiedersleben et al ( , b, 2020. The importance of atmospheric stability for the development of wakes has been addressed by Platis et al (2019) and will be investigated in detail in another publication.…”

Observations and Simulations of a Wind Farm Modifying a Thunderstorm Outflow Boundary

“…Users can adjust the specifications of the parameterized turbine, including its rotor diameter, hub height, thrust coefficients, and power curve, as well as its latitude and longitude location. WFP simulations have been validated with power production data (Lee and Lundquist, 2017a) and airborne measurements of winds (Siedersleben et al, 2018b), temperature and moisture (Siedersleben et al, 2018a), and turbulence (Siedersleben et al, 2020) and have reproduced the observed localized, nighttime, near-surface warming produced by wind turbines mixing warmer air from the nocturnal inversion down to the surface (Fitch et al, 2013;Cervarich et al, 2013;Lee and Lundquist, 2017b;Xia et al, 2017Xia et al, , 2019. To our knowledge, the WRF WFP has not yet been applied to explore interactions between wind farms and transient phenomena like thunderstorm outflow boundaries.…”