The Finnish Wind Atlas was prepared applying the mesoscale model AROME with 2.5 km horizontal resolution and the diagnostic downscaling method Wind Atlas Analysis and Application Programme (WAsP) with 250 m resolution. The latter was applied for areas most favourable for wind power production: a 30 km wide coastal/offshore zone, highlands, large lakes and large fields. The methodology included several novel aspects: (i) a climatologically representative period of real 48 months during 1989-2007 was simulated with the mesoscale model; (ii) in addition, the windiest and calmest months were simulated; (iii) the results were calculated separately for each month and for sectors 30°wide; (iv) the WAsP calculations were based on the mesoscale model outputs; (v) in addition to point measurements, also radar wind data were applied for the validation of the mesoscale model results; (vi) the parameterization method for gust factor was extended to be applicable at higher altitudes; and (vii) the dissemination of the Wind Atlas was based on new technical solutions. The AROME results were calculated for the heights of 50, 75, 100, 125, 150, 200, 300 and 400 m, and the WAsP results for the heights of 50, 75, 100, 125 and 150 m. In addition to the wind speed, the results included the values of the Weibull distribution parameters, the gust factor, wind power content and the potential power production, which was calculated for three turbine sizes. The Wind Atlas data are available for each grid point and can be downloaded free of charge from dynamic maps at www.windatlas.fi. Production of the Finnish Wind Atlas B. Tammelin et al.Accordingly, a strong need arose for a more accurate wind atlas. In Finland, the size of the country, its complex terrain and large seasonal differences generate strong demands for a wind atlas. The complexity of the terrain is not so much related to orography but to the complex shape of the almost flat coastline and archipelago, which generates a need for very high spatial resolution. Further, the differences in wind conditions between seasons are particularly large because in winter, the sea and lakes are frozen and the ground is covered by snow, which changes the surface roughness and stabilizes the atmospheric boundary layer (ABL). Stable stratification favours the generation of low-level jets. 4 In winter, wind power plants are also subject to ice accretion. The production of a new Wind Atlas for Finland has also been motivated by the need to evaluate the possible effects of climate change on wind conditions. In 2008, the Ministry of Labour and Economics released an international tender for production of the new Finnish Wind Atlas. The tender was won by the Finnish Meteorological Institute (FMI), with Risø DTU and Vaisala Ltd as subcontractors. The project started 1 June 2008, and the wind atlas was released 25 November 2009 (www.windatlas.fi).Many national wind atlases have recently been produced applying numerical weather prediction (NWP) models. In an ideal approach, all possible weather condition...

Wind gusts are traditionally observed and reported at the reference height of 10 m and most gust parametrization methods have been developed only for this height. In many practical applications, e.g. in wind energy, the relevant heights are, however, up to a few hundred metres. In this study, mean gustiness conditions were studied using observations from two coastal/archipelago weather masts in the Gulf of Finland (northern Europe) with observation heights between 30 and 143 m. Only moderate and strong wind cases were addressed. Both masts were located over relatively flat terrain but the local environment, and hence the surface roughness length, differed between the mast locations. The observations showed that above all the gust factor depended on the surface roughness. Stability had a more pronounced effect over the rough forested surface than over the smooth sea surface. At both locations the stability had a larger effect on gusts than the observation height. Two existing parametrization methods, developed for a 10 m reference height, were validated against the observations and a new parametrization was proposed. In the new method, the gust factor depends on the standard deviation of the wind speed, which is parametrized on the basis of the surface friction velocity, the Obukhov length and height and the boundary-layer height. The new gust parametrization method outperformed the two older methods: the effects of surface roughness, stability and the height above the surface were well represented by the new method.

Abstract:The aim of the research project "Innovative Strategies for Observations in the Arctic Atmospheric Boundary Layer (ISOBAR)" is to substantially increase the understanding of the stable atmospheric boundary layer (SBL) through a combination of well-established and innovative observation methods as well as by models of different complexity. During three weeks in February 2017, a first field campaign was carried out over the sea ice of the Bothnian Bay in the vicinity of the Finnish island of Hailuoto. Observations were based on ground-based eddy-covariance (EC), automatic weather stations (AWS) and remote-sensing instrumentation as well as more than 150 flight missions by several different Unmanned Aerial Vehicles (UAVs) during mostly stable and very stable boundary layer conditions. The structure of the atmospheric boundary layer (ABL) and above could be resolved at a very high vertical resolution, especially close to the ground, by combining surface-based measurements with UAV observations, i.e., multicopter and fixed-wing profiles up to 200 m agl and 1800 m agl, respectively. Repeated multicopter profiles provided detailed information on the evolution of the SBL, in addition to the continuous SODAR and LIDAR windAtmosphere 2018, 9, 268; doi:10.3390/atmos9070268www.mdpi.com/journal/atmosphereAtmosphere 2018, 9, 268 2 of 29 measurements. The paper describes the campaign and the potential of the collected data set for future SBL research and focuses on both the UAV operations and the benefits of complementing established measurement methods by UAV measurements to enable SBL observations at an unprecedented spatial and temporal resolution.

A new methodology is proposed for scaling Doppler lidar observations of wind gusts to make them comparable with those observed at a meteorological mast. Doppler lidars can then be used to measure wind gusts in regions and heights where traditional meteorological mast measurements are not available. This novel method also provides estimates for wind gusts at arbitrary gust durations, including those shorter than the temporal resolution of the Doppler lidar measurements. The input parameters for the scaling method are the measured wind‐gust speed as well as the mean and standard deviation of the horizontal wind speed. The method was tested using WindCube V2 Doppler lidar measurements taken next to a 100 m high meteorological mast. It is shown that the method can provide realistic Doppler lidar estimates of the gust factor, i.e. the ratio of the wind‐gust speed to the mean wind speed. The method reduced the bias in the Doppler lidar gust factors from 0.07 to 0.03 and can be improved further to reduce the bias by using a realistic estimate of turbulence. Wind gust measurements are often prone to outliers in the time series, because they represent the maximum of a (moving‐averaged) horizontal wind speed. To assure the data quality in this study, we applied a filtering technique based on spike detection to remove possible outliers in the Doppler lidar data. We found that the spike detection‐removal method clearly improved the wind‐gust measurements, both with and without the scaling method. Spike detection also outperformed the traditional Doppler lidar quality assurance method based on carrier‐to‐noise ratio, by removing additional unrealistic outliers present in the time series.

The increasing size of wind turbines, their height and the area swept by their blades have revised the need for understanding the vertical structure of wind gusts. Information is needed for the whole profile. In this study, we analyzed turbulence measurements from a 100 m high meteorological mast at the Danish National Test Station for Large Wind Turbines at Høvsøre in Denmark. The site represents flat, homogeneous grassland with an average gust factor of 1.4 at 10 m and 1.2 at 100 m level. In a typical surface-layer gust parametrization, the gust factor is composed of two components, the peak factor and the turbulence intensity, of which the turbulence intensity was found to dominate over the peak factor in determining the effects of stability and height above the surface on the gust factor. The peak factor only explained 15% or less of the vertical decrease of the gust factor, but determined the effect of gust duration on the gust factor. The statistical method to estimate the peak factor did not reproduce the observed vertical decrease in near-neutral and stable conditions and near-constant situation in unstable conditions. Despite this inconsistency, the theoretical method provides estimates for the peak factor when comparing gust durations of 1 and 3 s with averaging period lengths of 10 min and 1 h. A new technique to study the timing of maxima at different levels relative to the maximum gust at some level was developed. Results showed that a 10 m level maximum gust was typically preceded by maxima at higher levels and vice versa: a 100 m gust was usually followed by a maximum at lower levels.