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Turbulence intensity, wind shear and wind gusts assessed by meso-scale model AROME Context and objectives When prospecting, it is of major interest for wind industry to rely on correct estimations of turbulence and extreme wind gusts to help site selections at the first approach. However only a few pre-selected sites allow turbulence intensity estimation with in-situ observations at turbine height. To fill this gap, Meteo-France investigated the capabilities of a long term high resolution simulation with numerical weather prediction (NWP) model AROME to assess wind turbulence intensity, wind shear and extreme wind gusts. This poster aims to present the methods used to assess these parameters as well as the results of the comparison with available observations. Turbulence Intensity Wind shear component References Turbulence intensity, I, is used in the wind industry to estimate how the wind speed fluctuates in a given time period (usually 10 minutes). Turbulence intensity is the ratio of the standard deviation of wind speed (σ w ) divided by the mean wind speed (µ w ) over the same time period. Turbulence intensity can also be estimated from an atmospheric model (cf. Ben Wichers Schreur and Gertie Geertsema). In AROME, turbulent kinetic energy is proportional to standard deviation squared. Wind gust Input data set: 16 years simulation with high resolution NWP AROME The different parameters needed to achieve this work all come from a 16 years archive from AROME. This long term archive is built on a 10 years simulation (re-run) concatenated with the 6 years of operational archive of AROME (French operational NWP model). What are the limits? Authors: Cécile Gautier (1) , Béatrice Pouponneau (1) , Rachel Honnert (2) , Vivien Pourret (1) , Viviane Gouget (1) , Yann Seity (2) , Sophie Martinoni-Lapierre (1) (1) Météo-France, (2) CNRM Contact: [email protected] In short Few measurement sites are available (that include the record of the variance of wind speed over 10 min) to perform an in-depth validation of the estimation from NWP model AROME at heights between 40 and 200 m. Moreover, even if the results of the comparison show good consistency with observations, it is not enough to quantify the related uncertain ty. Another limit has to be pointed out: 50-year wind gust estimation are very sensitive to the method chosen. As it is performed at each grid point, the result is very dependant on the events recorded at this point. A single storm in a time-serie can have a significant impact on the estimated 50-year wind gust. Couvreux F. and al « Boundary layer turbulent processes and mesoscale variability represented by NWP models during the BLLAST caompaign » Wichers Schreur B. and Geertsema G. « Theory for a TKE based parameterization of wind gusts» Zhang M. « Wind Resource Assessment and Micro-siting » Wind shear component, α, allows the description of the wind profile and the estimation of a mean wind speed from one height to another. It depends on the roughness of the surface. For wind industry, best sites are sites with a low wind shear component. Wind shear component can be directly computed from AROME output parameters with the same formulae that is applied to observations. This formulae is based on a simple empirical power law that is not representative of all situations and terrains (ex: unstable air conditions + rough terrain). α = ln ( V Z 2 / V Z 1 ) ln ( Z 2 / Z 1 ) The dynamic downscaling process to achieve the re-run is the following: ERA-I (80 km) ALADIN (15 km) AROME (2.5 km) It includes a 6h surface analysis cycle in ALADIN to provide high consistency with observations of surface parameters, from 2500 measurement stations over Europe. Model parameters are available as 16 years time-series over a large French domain (offshore + onshore) at: ✔ 2.5 km horizontal resolution ✔ at 10, 20, 40, 50, 60, 80, 100, 120, 140, 160 m ✔ at an hourly time-step ✔ for 16 years, 2000 – 2015 This data set was used previously to build the latest French wind atlas. Validation was performed by comparison with in- situ measurements from 2 offshore sites and 2 onshore sites at different heights (see graph below). Duration of the comparison is about 1 year and a half. The daily cycle, very noticeable at the onshore location, is well represented in the model. For offshore sites, daily cycle is reduced or null due to smaller turbulence above sea than above earth. Wind gust are typically computed at 10 m high (V gust_10 ) in NWP model such as AROME. To estimate wind gust at 100 m high ( V gust_100 ) different diagnostics were tested and compared to observations to select the best one. The simpler the better! In AROME, wind gusts at 100 m high can be estimated from wind gusts at 10 m and average wind speed at 10 and 100 m respectively (V ave_10 , V ave_100 ): V gust_100 = V gust_10 – V ave_10 + V ave_100 The same approach is used at different heights. Validation was performed by comparison with in-situ measurements from the same offshore and onshore sites. Results of the comparison show very good consistency with observations. I = σ w µ w Hourly evolution of turbulence intensity from 02/2007 to 11/2008 on onshore site 2 50-year Extreme wind gust To provide estimation of 50-year extreme wind gust at 100 m height, a peaks over threshold method is used. A two days gap is set to ensure independence of maxima. Data from model simulation shows good consistency with observations. Turbulence intensity, 50-year wind gusts and wind shear component have been assessed by the 16 years simulation with AROME NWP model over a large French domain. Results of comparison show good consistency with observed indicators even if there was an insufficient number of observation sites to quantify uncertainty. These results can be provided by Meteo-France as well as long term wind speed statistics and 16-years hourly time-series on onshore and offshore sites. Average wind gust in m/s at 100 m (2000 - 2015) Wind shear component on the 40 to 160m layer (2000 - 2015) At this offshore site, in the Chanel, the 50-year wind gust at 100m is between 40 and 46 m/s with a confidence interval of 70%.
