The making of the New European Wind Atlas – Part 1: Model sensitivity

Abstract

Abstract. This is the first of two papers that document the creation of the New European Wind Atlas (NEWA). It describes the sensitivity analysis and evaluation procedures that formed the basis for choosing the final setup of the mesoscale model simulations of the wind atlas. The suitable combination of model setup and parameterizations, bound by practical constraints, was found for simulating the climatology of the wind field at turbine-relevant heights with the Weather Research and Forecasting (WRF) model. Initial WRF model sensitivity experiments compared the wind climate generated by using two commonly used planetary boundary layer schemes and were carried out over several regions in Europe. They confirmed that the most significant differences in annual mean wind speed at 100 m a.g.l. (above ground level) mostly coincide with areas of high surface roughness length and not with the location of the domains or maximum wind speed. Then an ensemble of more than 50 simulations with different setups for a single year was carried out for one domain covering northern Europe for which tall mast observations were available. We varied many different parameters across the simulations, e.g. model version, forcing data, various physical parameterizations, and the size of the model domain. These simulations showed that although virtually every parameter change affects the results in some way, significant changes in the wind climate in the boundary layer are mostly due to using different physical parameterizations, especially the planetary boundary layer scheme, the representation of the land surface, and the prescribed surface roughness length. Also, the setup of the simulations, such as the integration length and the domain size, can considerably influence the results. We assessed the degree of similarity between winds simulated by the WRF ensemble members and the observations using a suite of metrics, including the Earth Mover's Distance (EMD), a statistic that measures the distance between two probability distributions. The EMD was used to diagnose the performance of each ensemble member using the full wind speed and direction distribution, which is essential for wind resource assessment. We identified the most realistic ensemble members to determine the most suitable configuration to be used in the final production run, which is fully described and evaluated in the second part of this study (Dörenkämper et al., 2020).

The Making of the New European Wind Atlas – Part 2: Production and evaluation

“…the domain size) drove the selection of the setup. These aspects are discussed in more detail in Part 1 of this study (Hahmann et al, 2020b). It is important to mention here that the data from the 291 masts used for evaluation were not used for the selection of the WRF model setup used in the NEWA production simulations.…”

“…The EMD can generally be understood as the amount of physical work needed to move a pile of soil in the shape of one distribution to that of another distribution and is introduced in more detail in Part 1 of this study (see Sect. 4 in Hahmann et al, 2020b). Figure 12.…”

“…Storm et al, 2009;Jimenez et al, 2010;Liu et al, 2011;Horvath et al, 2012;Karagali et al, 2013;Hahmann et al, 2015;Draxl et al, 2015;Dörenkämper et al, 2015;Lundquist et al, 2019). The setup of the WRF model used for the NEWA wind atlas is based on the evaluation of a large number of sensitivity experiments using mast data, which is documented in the first part of this study (Hahmann et al, 2020b).…”

“…The mesoscale model simulations of the NEWA production run use a modified version of the WRF model version 3.8.1, with changes in the Mellor-Yamada-Nakanishi-Niino (MYNN) planetary boundary layer (PBL) scheme (for details see Sect. 5.2 in Hahmann et al, 2020b). Furthermore, additional code that estimates ice accumulation was added to the WRF model code.…”

“…This paper is the second of two parts describing the modelling involved in the making of the NEWA Wind Atlas: the first paper (Hahmann et al, 2020b) deals with the sensitivity experiments that were carried out to guide the selection of the WRF model configuration used for the production of the mesoscale model simulations of the wind atlas. This paper, the second part, focuses on the production of the wind atlas, including a selection of the results and the evaluation of the wind atlas model chain using measurements from tall meteorological towers covering most of Europe.…”