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The depth of the aerosol layer at the Villum Research Station at Station Nord in the high Arctic is analysed based on 8 years of observations from a ceilometer and one full year from a wind lidar. The layer is of particular interest for aerosol process modelling and atmospheric chemistry studies. The depth of the aerosol layer is assigned to the inflection point in the attenuated backscatter profile by two methods; one is based on polynomial approximation of the profile and the other is direct numerical differentiation. The analysis is based on two types of hourly profiles; one consists of averaging the attenuated backscatter observations and the other by computing the median. Due to sporadic occurrence of outliers in the ranges around 50 m in the ceilometer observations, this part of the profile is not used in this study. Restricting the observations to heights above 100 m, the depths of the aerosol layer are found to be typically ≈230 m. It varies little between winter and summer, but the spread in the depth is larger during the winter as compared to summer. To extend the study of the aerosol-layer depth below 100 m, a method is applied that combines the ceilometer measurements with the carrier-to-noise ratio from the wind lidar. The results are available for 2018 only, and they show aerosol-layer depths below ≈80 m as well as depths around 230 m and they show few observations between ≈80 and ≈230 m. Near the ground, the observed backscatter exhibits a pronounced seasonal variation, having low values during the summer and high values during the winter. The strength of the seasonal variability decreases with height, especially above the aerosol-layer depth, and is virtually absent at 1 km.

The depth of the aerosol layer at the Villum Research Station at Station Nord in the high Arctic is analyzed based on eight years of observations from a ceilometer. The depth of the aerosol layer is assigned to the inflection point in the attenuated backscatter profile by two methods; one is based on polynomial approximation of the profile and the other is direct numeric differentiation. The analysis is based on two types of hourly profiles; one consists of averaging the attenuated backscatter observations and the other by computing the median. It is noted that the depth of the aerosol layer in the Arctic might be different from the depth of the turbulent layer near the ground.Near the ground, the observed backscatter exhibits a pronounced seasonal variation, having low values during the summer and high values during the winter. The strength of the seasonal variability decreases with height and is virtually absent at 1 km.Due to sporadic occurrence of outliers in the range between 50 m and 80 m in the ceilometer observations, this part of the profile is not used in this study. Restricting the observations to heights above100 m, the depths of the aerosol layer are found to be typically 230-250 m. It varies little between winter and summer, but the spread in the depth was larger during the winter as compared to summer.To extend the profile of the attenuated backscatter below 100 m, the ceilometer measurements were combined with Carrier-To-Noise-Ratio observations from a Doppler wind Lidar. Thus, hourly profiles of attenuated backscatter starting at 40 m, are obtained. The results are available for 2018 only and they show aerosol-layer depths below 100 m as well as depths around 230-250 m and they show few observations of aerosol-layer depths between 100 m and 230 m.Acknowledgement: This article is based upon work from COST Action CA18235 PROBE, supported by COST (European Cooperation in Science and Technology). www.cost.eu.

Presentation given at a Webinar about lidar data filtering as part of the IEA Task 32 working group on lidars. The presentation discusses the effect of data filtering on the observed wind climate for an offshore site in the North Sea. It is found that filtering with a CNR threshold can significantly influence the measured wind speed distribution. The effect is found for both short and long-range wind lidars. Therefore, care should be taken when filtering the data of wind lidars.

Wind lidar observations are characterized by a Carrier-to-Noise-Ratio that is often used to filter the observations. The choice of the Carrier-to-Noise-Ratio threshold value for the wind lidar observations is found to have an effect on the climatological wind speed distribution in such a way that when the Carrier-to-Noise-Ratio (CNR) threshold value is increased the wind speed distribution is shifted to higher values. Based on one year of observations carried out with a wind lidar from 126&nbsp, m to 626 m height at the FINO3 (Forschungsplattform in Nord- und Ostsee Nr. 3) research platform in the North Sea, the effect that the choice of the Carrier-to-Noise threshold value has on the climatology of the wind speed and direction as well as the wind power density in relation to wind energy is illustrated and discussed. In the one-year data set considered here it is found that for thresholds larger than &minus, 29 dB, the mean wind speed and wind rose measured by the wind lidar become a function of the threshold value, and for values smaller than ~ &minus, 29 dB further decrease of the Carrier-to-Noise-Ratio threshold has a minor effect on the estimated mean wind speed and wind rose. The analysis of the data set from the North Sea shows that the limit for the Carrier-to-Noise-Ratio of the observations should be &minus, 29 dB or less to obtain a threshold independent estimate of the mean wind speed and wind rose. Alternatively, all valid observations should be used for the analysis. Although this study is specific for the conditions in the North Sea, we suggest that for a representative estimation of the wind resource with wind lidars, the effect of the CNR threshold filtering on the wind distribution should be studied when the recovery rate is less than 100%.

There is as yet no standard methodology for measuring wind gusts from a moving platform. To address this, we have developed a method to derive gusts from research aircraft data. First we evaluated four different approaches, including Taylor’s hypothesis of frozen turbulence, to derive the gust length scales that correspond to the gust time scales, namely, the gust duration (seconds) and the sample period (typically 10 min). The novelty of our method is in using peak factors (deviation of the gust from the mean wind speed normalized by the local turbulence) to convert between the scales. After devising a way to derive the gust length scales, we calculated the gust factors from aircraft observations and tested them against those from four parameterizations originally developed for weather stations. Three of them performed well (R2=0.66 or higher), while the fourth overestimated the gust factors in unstable conditions(R2=0.52). The mean errors for all methods were low, from -0.02 to 0.05, indicating that wind gust factors can indeed be measured from research aircraft. Moreover, we showed that aircraft can provide gust measurements within the whole boundary layer, if horizontal legs are flown at multiple levels over the same track. This is a significant advance, as gust measurements are usually limited to heights reached by weather masts. In unstable conditions over the open ocean the gust factor was nearly constant with height throughout the boundary layer, the near-surface values only slightly exceeding those at upper levels. Furthermore, we found gust factors to be strongly dependent on surface roughness conditions, which differed between the open ocean and sea ice in the Arctic marine environment. The roughness effect on the gust factor was stronger than the effect of boundary-layer stability.

Wind-speed observations from tall towers are used in combination with observations up to 600 m in altitude from a Doppler wind lidar to study the long-term conditions over suburban (Hamburg), rural coastal (Hovsore) and marine (FINO3) sites. The variability in the wind field among the sites is expressed in terms of mean wind speed and Weibull distribution shape-parameter profiles. The consequences of the carrier-to-noise-ratio (CNR) threshold-value choice on the wind-lidar observations are revealed as follows. When the wind-lidar CNR is lower than a prescribed threshold value, the observations are often filtered out as the uncertainty in the wind-speed measurements increases. For a pulsed heterodyne Doppler lidar, use of the traditional –22 dB CNR threshold value at all measuring levels up to 600 m results in a $$\approx $$ 7 % overestimation in the long-term mean wind speed over land, and a $$\approx $$ 12 % overestimation in coastal and marine environments. In addition, the height of the profile maximum of the shape parameter of the Weibull distribution (so-called reversal height) is found to depend on the applied CNR threshold; it is found to be lower at small CNR threshold values. The reversal height is greater in the suburban (high roughness) than in the rural (low roughness) area. In coastal areas the reversal height is lower than that over land and relates to the internal boundary layer that develops downwind from the coastline. Over the sea the shape parameter increases towards the sea surface. A parametrization of the vertical profile of the shape parameter fits well with observations over land, coastal regions and over the sea. An applied model for the dependence of the reversal height on the surface roughness is in good agreement with the observations over land.

Here we describe a nearly 1-yr meteorological campaign, which was carried out at the FINO3 marine research platform on the German North Sea, where a pulsed wind lidar and a ceilometer were installed besides the platform's 105-m tower and measured winds and the aerosol backscatter in the entire marine atmospheric boundary layer. The campaign was the last phase of a research project, in which the vertical wind profile in the atmospheric boundary layer was firstly investigated on a coastal and a semi-urban site. At FINO3 the wind lidar, which measures the wind speed up to 2000 m, shows the highest data availability (among the three sites) and a very good agreement with the observations of wind speed and direction from cup anemometers and vanes from the platform's tower. The wind lidar was also able to perform measurements under a winter storm where 10-s gusts were observed above 60 m s−1 within the range 400–600 m. The ceilometer and wind lidar have also the potential of detecting the marine boundary layer height based on, respectively, direct and indirect observations of the aerosol backscatter. About 10 % of the measured wind profiles are available within the first 1000 m, which allows the investigation of the behavior with height of the two horizontal wind speed components. From the preliminary analysis of these vertical profiles, a variety of atmospheric and forcing conditions is distinguished; from a number of 10-min mean profiles the wind is observed to turn both anti- and clockwise more than 50 °, likely indicating the influence of baroclinity.

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 Hovsore 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.

This chapter provides an overview of the present (2016) state of the art in ground-level remote sensing of the wind speed and boundary-layer height by use of lasers. The chapter covers a broad range of aspects of coherent wind-lidar measurements—from the basic measuring principles and calibration, to a discussion on the use of wind-lidar measurements for studies of the wind climate. A number of methods for the determination of atmospheric turbulence and wind gusts from wind-lidar measurements as well as ways to estimate the atmospheric boundary-layer height by use of lidar observations are presented. Finally the so-called wind-scanner system is described.

A new methodology is proposed for scaling Doppler lidar observations of wind gusts to make them comparable to 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 the 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 the method can be further improved 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.

We compare 4 planetary boundary layer (PBL) schemes of Weather Research and Forecasting model for high Artic conditions, documented during summer and winter campaigns at Station Nord, Greenland. During March 2012, 22 radiosonds were launched at 00 and 12 UTC. During July-August 2011, 25 radiosondes were launched at 00, 06, 12, and 18 UTC. The chosen PBL schemes are 3 TKE schemes: MYJ, MYNN and QNSE and non-local YSU. Comparison is performed between data from radiosoundings and corresponding in time model results up to different height from 100 m to 8000 m. Sensitivity of model to vertical and spatial resolution is examined with MYJ through 4 configurations combining 26 or 42 vertical levels and 4 km or 1.33 km horizontal grid step. Sensitivity to the resolution tests showed that increasing horizontal resolution from 4 km to 1.33 km did not improve model performance. Increasing the number of vertical layers lead to closer to observed profiles and slightly improved statistics by layers. Sensitivity to the lead time (24 h or 48 h) is examined with MYJ at 1.33 km grid step and 42 vertical layers. Quality of forecast for day 1 and day 2 is similar for the summer. Temperature and wind speed (WS) biases for the winter are with 1 K and 1 ms-1 larger for 48 h compared to 24 h lead time. The lack of diurnal variability during both campaigns is correctly simulated by all PBL schemes. The performed tests show that TKE schemes outperform YSU and as a whole MYNN gives the highest scores.

A range of large-eddy simulations, with differing free atmosphere stratification and zero or slightly positive surface heat flux, is investigated to improve understanding of the neutral and near-neutral, inversion-capped, horizontally homogeneous, barotropic atmospheric boundary layer with emphasis on the upper region. We find that an adjustment time of at least 16 h is needed for the simulated flow to reach a quasi-steady state. The boundary layer continues to grow, but at a slow rate that changes little after 8 h of simulation time. A common feature of the neutral simulations is the development of a super-geostrophic jet near the top of the boundary layer. The analytical wind-shear models included do not account for such a jet, and the best agreement with simulated wind shear is seen in cases with weak stratification above the boundary layer. Increasing the surface heat flux decreases the magnitude and vertical extent of the jet and leads to better agreement between analytical and simulated wind-speed profiles. Over a range of different inversion strengths and surface heat fluxes, we also find good agreement between the performed simulations and models of the equilibrium boundary-layer height, and of the budget of turbulent kinetic energy integrated across the boundary layer.

The influence of baroclinicity on wind within the planetary boundary layer was investigated using two years of wind lidar measurements collected at a suburban site in northern Germany (Hamburg) and a rural-coastal site in western Denmark (Hovsore). Measurements were made up to a height of 950 m. The surface geostrophic wind, the surface gradient wind and the gradient wind were estimated using the pressure and geopotential fields from a mesoscale model. At both sites the atmospheric flow was typically baroclinic. The distribution of the geostrophic wind shear was approximately Gaussian with a mean close to zero and a standard deviation of approximately 3 m s−1km−1. The geostrophic wind shear had a strong seasonal dependence because of temperature differences between land and sea. The mean wind profile in Hamburg, observed during an intensive campaign using radio sounding and during the whole year using the wind lidar, was influenced by baroclinicity. For easterly winds at Hovsore, the estimated gradient wind decreased rapidly with height, resulting in a mean low-level jet. The turning of the wind in the boundary layer, the boundary-layer height and the empirical constants in the geostrophic drag law were found to be dependent on baroclinicity for neutral conditions.

An investigation of the long-term variability of wind profiles for wind energy applications is presented. The observations consists of wind measurements obtained from a ground-based wind lidar at heights between 100 and 600 m, in combination with measurements from tall meteorological towers at a flat rural coastal site in western Denmark and at an inland suburban area near Hamburg in Germany. Simulations with the weather research and forecasting numerical model were carried out in both forecast and analysis configurations. The scatter between measured and modelled wind speeds expressed by the root-mean-square error was about 10 % lower for the analysis compared to the forecast simulations. At the rural coastal site, the observed mean wind speeds above 60 m were underestimated by both the analysis and forecast model runs. For the inland suburban area, the mean wind speed is overestimated by both types of the simulations below 500 m. When studying the wind-speed variability with the Weibull distribution, the shape parameter was always underestimated by the forecast compared to both analysis simulations and measurements. At the rural coastal site although the measured and modelled Weibull distributions are different their variances are nearly the same. It is suggested to use the shape parameter for climatological mesoscale model evaluation. Based on the new measurements, a parametrization of the shape parameter for practical applications is suggested.

We present an analysis of data from a nearly 1-year measurement campaign performed at Hovsore, Denmark, a coastal farmland area where the terrain is flat. Within the easterly sector upstream of the site, the terrain is nearly homogenous. This topography and conditions provide a good basis for the analysis of vertical wind-speed profiles under a wide range of atmospheric stability, turbulence, and forcing conditions. One of the objectives of the campaign was to serve as a benchmark for flow over flat terrain models. The observations consist of combined wind lidar and sonic anemometer measurements at a meteorological mast. The sonic measurements cover the first 100 m and the wind lidar measures above 100 m every 50 m in the vertical. Results of the analysis of observations of the horizontal wind-speed components in the range 10–1200 m and surface turbulence fluxes are illustrated in detail, combined with forcing conditions derived from mesoscale model simulations. Ten different cases are presented. The observed wind profiles approach well the simulated gradient and geostrophic winds close to the simulated boundary-layer height during both barotropic and baroclinic conditions, respectively, except for a low-level jet case, as expected. The simulated winds are also presented for completeness and show good agreement with the measurements, generally underpredicting the turning of the wind in both barotropic and baroclinic cases.

By use of 1 yr of measurements performed with a wind lidar up to 600-m height, in combination with a tall meteorological tower, the impact of nudging on the simulated wind profile at a flat coastal site (Høvsøre) in western Denmark using the Advanced Research version of the Weather Research and Forecasting model (WRF) is studied. It was found that the mean wind speed, the wind direction change with height, and the wind power density profiles are underestimated with the configuration of WRF used and that the impact of nudging on the simulated mean values was minor. Nudging was found to reduce the scatter between the simulated and measured wind speeds, expressed by the root-mean-square error, by about 20% between altitudes of 100 and 500 m. The root-mean-square error was nearly constant with height for the nudged case (~2.2 m s−1) and slightly increased with height for the nonnudged one, reaching 2.8 m s−1 at 300 and 500 m. In studying the long-term wind speed variability with the Weibull distribution, it was found that nudging had a minor effect on the scale parameter profile, which is closely connected to the mean wind speed. Improvement by nudging was seen on the profile of the shape parameter. Without nudging, the shape parameter was underestimated at all heights; with nudging, the agreement was good up to about 100 m and above that height the shape parameter was underestimated.

T he growing body of knowledge and experience in weather and climate risk management in the energy industry has spurred a rapidly growing research interest at the nexus between weather, climate, and energy (Troccoli 2010; Troccoli et al. 2010; Ebinger and Vergara 2011; Marquis 2011). Although this increased attention has been stimulated by a renewed and fervent interest in renewable energy sources, weather and climate information is also critical to managing the energy supply from other energy sectors (e.g., offshore oil operations) as well as understanding and estimating energy demand. Until recently, such applied research was discussed in specialized sessions during conferences organized by industry-specific organizations related to wind energy (e.g., American Wind Energy Association), solar energy [e.g., Solar Power and Chemical Energy Systems (SolarPACES)], and meteorology (e.g., European Meteorological Society). To take advantage of the substantial overlap between these energy activities and their use of weather and climate information, the International Conference Energy and Meteorology (ICEM) 2011 (www.icem2011.org/) was convened in Queensland, Australia. The objective was to provide a dedicated forum where scientists, engineers, economists, policy makers, and other specialists and practitioners involved in research or implementation activities at the intersection between weather, climate, and energy could discuss recent research findings and emerging practices ranging from operational activities to longterm investment planning and to policy making. Such discussions were framed within the context of all energy sector uses, including climate change mitigation and weather/climate risk management practices. The four expected outcomes of this conference were as follows

[1] We investigate several lidar-type instruments and methodologies for boundary layer height (BLH) estimation during 2 days at a coastal site for winds that experience marine upstream flow conditions. Wavelet and profile fitting procedures on the aerosol backscatter signals from a ceilometer and an aerosol lidar reveal similar BLHs, but their agreement depends on the presence of clouds and the instrument signal, among others. BLHs derived by a threshold on the carrier-to-noise profiles of a wind lidar agree well with those derived by using a threshold on the backscatter profile of the ceilometer and are used as reference for a 10 day BLH intercomparison. Furthermore, the BLHs from the aerosol analysis are comparable to those derived from wind speed and direction profiles from combined mast/wind lidar measurements. The BLH derived from simulations performed with the Weather Research and Forecasting (WRF) model shows similar behavior compared to the lidar observations. The seasonal diurnal variation of the BLH for 2010, derived from the wind lidar and ceilometer thresholds, shows similar BLHs but generally higher values compared to that from WRF. No clear BLH diurnal variation is observed neither from the observations nor from the WRF model outputs, except in summer for the latter. Both observations and WRF model simulations reveal higher BLHs during autumn compared to spring time. These BLHs are used to evaluate the intra-annual variation and show high peaks in September, November, and February.

Traditionally it has been difficult to verify mesoscale model wind predictions against observations in the planetary boundary layer (PBL). Here we used measurements from a wind lidar to study the PBL up to 800 m above the surface at a flat coastal site in Denmark during a one month period in autumn. We ran the Weather Research and Forecasting numerical model with two different roughness descriptions over land, two different synoptic forcings and two different PBL schemes at two vertical resolutions and evaluated the wind profile against observations from the wind lidar. The simulated wind profile did not have enough vertical shear in the lower part of the PBL and also had a negative bias higher up in the boundary layer. Near the surface the internal boundary layer and the surface roughness influenced the wind speed, while higher up it was only influenced by the choice of PBL scheme and the synoptic forcing. By replacing the roughness value for the land-use category in the model with a more representative mesoscale roughness, the observed bias in friction velocity was reduced. A higher-order PBL scheme simulated the wind profile from the west with a lower wind-speed bias at the top of the PBL. For easterly winds low-level jets contributed to a negative wind-speed bias around 300 m and were better simulated by the first-order scheme. In all simulations, the wind-profile shape, wind speed and turbulent fluxes were not improved when a higher vertical resolution or different synoptic forcing were used.

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.

Operational since 2004, the National Centre for Wind Turbines at Høvsøre, Denmark has become a reference research site for wind-power meteorology. In this study, we review the site, its instrumentation, observations, and main research programs. The programs comprise activities on, inter alia, remote sensing, where measurements from lidars have been compared extensively with those from traditional instrumentation on masts. In addition, with regard to wind-power meteorology, wind-resource methodologies for wind climate extrapolation have been evaluated and improved. Further, special attention has been given to research on boundary-layer flow, where parametrizations of the length scale and wind profile have been developed and evaluated. Atmospheric turbulence studies are continuously conducted at Høvsøre, where spectral tensor models have been evaluated and extended to account for atmospheric stability, and experiments using microscale and mesoscale numerical modelling.

Vertical profiles of the horizontal wind speed and of the standard deviation of vertical wind speed from Large Eddy Simulations of a convective atmospheric boundary layer are compared to wind LIDAR measurements up to 1400 m. Fair agreement regarding both types of profiles is observed only when the simulated flow is driven by a both time- and height-dependent geostrophic wind and a time-dependent surface heat flux. This underlines the importance of mesoscale effects when the flow above the atmospheric surface layer is simulated with a computational fluid dynamics model.

Wind profiles up to 600 m height are investigated. Measurements of mean wind speed profiles were obtained from a novel wind lidar and compared to model simulations from a mesoscale model (WRF-ARW v3.1). It is found that WRF is able to predict the mean wind profile rather well and typically within 1–2 m s−1 to the individual measured values. WRF underpredicts the normalized wind profile, especially for stable conditions. The effect of baroclinicity on the upper part of the wind profile is discussed.

A comparison of the atmospheric stability and wind profiles using data from meteorological masts located near two wind farm sites in the North Sea, Egmond aan Zee (up to 116 m) in the Dutch North Sea and Horns Rev (HR; up to 45 m) in the Danish North Sea, is presented. Only the measurements that represent long marine fetch are considered. It was observed that within a long marine fetch, the conditions in the North Sea are dominated by unstable [41% at Egmond aan Zee Offshore Wind Farm (OWEZ) and 33% at HR] and near-neutral conditions (49% at OWEZ and 47% at HR), and stable conditions (10% at OWEZ and 20% at HR) occur for a limited period. The logarithmic wind profiles with the surface-layer stability correction terms and Charnock's roughness model agree with the measurements at both sites in all unstable and near-neutral conditions. An extended wind profile valid for the entire boundary layer is compared with the measurements. For the tall mast at Egmond aan Zee, it was found that for stable conditions, the scaling of the wind profiles with respect to boundary-layer height is necessary, and the addition of another length scale parameter is preferred. At the lower mast at HR, the effect was not noticeable. Copyright © 2011 John Wiley & Sons, Ltd.

We present the results of an analysis of simultaneous sonic anemometer observations of wind speed and velocity spectra over flat and homogeneous terrain from 10 up to 160 m height performed at the National Test Station for Wind Turbines at Hovsore, Denmark. The mixing length, l, derived from the wind speed profile, is found to be linearly proportional to the length-scale of turbulence, derived either from the peak of the vertical velocity spectrum, (λm)w, or from a three-dimensional turbulence spectral model, for a range of atmospheric stability conditions, friction Rossby numbers, and within the range of observational heights ((λm)w ∼ 7l). Under very unstable conditions and above 100 m, the local wind shear is low, and the relation between both length-scales is slightly nonlinear. Mixing-length and wind profile models, which depend on both atmospheric stability and friction Rossby number, generally show better agreement to the observations of the length-scale and wind speed profile than the models from surface-layer theory, which show good agreement with the observations for the first 80 m only. The results from this analysis demonstrate a close connection between these two types of length-scales. Copyright © 2010 Royal Meteorological Society

This work aims to study the seasonal difference in normalized wind speed above the surface layer as it is observed at the 160 m high mast at the coastal site Høvsøre at winds from the sea (westerly). Normalized and stability averaged wind speeds above the surface layer are observed to be 20 to 50% larger in the winter/spring seasons compared to the summer/autumn seasons at winds from west within the same atmospheric stability class. A method combining the mesoscale model, COAMPS, and observations of the surface stability of the marine boundary layer is presented. The objective of the method is to reconstruct the seasonal signal in normalized wind speed and identify the physical process behind. The method proved reasonably successful in capturing the relative difference in wind speed between seasons, indicating that the simulated physical processes are likely candidates to the observed seasonal signal in normalized wind speed.

The wind speed profile for the neutral boundary layer is derived for a number of mixing-length parameterizations, which account for the height of the boundary layer. The wind speed profiles show good agreement with the reanalysis of the Leipzig wind profile (950 m high) and with combined cup–sonic anemometer and lidar measurements (300 m high) performed over flat and homogeneous terrain at Høvsøre, Denmark. In the surface layer, the mixing-length parameterizations agree well with the traditional surface-layer theory, but the wind speed profile is underestimated when the surface-layer scaling is extended to the entire boundary layer, demonstrating the importance of the boundary layer height as a scaling parameter. The turbulence measurements, performed up to 160-m height only at the Høvsøre site, provide the opportunity to derive the spectral-length scales from two spectral models. Good agreement is found between the behaviors of the mixing- and spectral-length scales.

Models of the diabatic wind profile over homogeneous terrain for the entire atmospheric boundary layer are developed using mixing-length theory and are compared to wind speed observations up to 300 m at the National Test Station for Wind Turbines at Hovsore, Denmark. The measurements are performed within a wide range of atmospheric stability conditions, which allows a comparison of the models with the average wind profile computed in seven stability classes, showing a better agreement than compared to the traditional surface-layer wind profile. The wind profile is measured by combining cup anemometer and lidar observations, showing good agreement at the overlapping heights. The height of the boundary layer, a parameter required for the wind profile models, is estimated under neutral and stable conditions using surface-layer turbulence measurements, and under unstable conditions based on the aerosol backscatter profile from ceilometer observations.

The advantages and limitations of the ZephIR®, a continuous-wave, focused light detection and ranging (LiDAR) wind profiler, to observe offshore winds and turbulence characteristics were tested during a 6 month campaign at the transformer/platform of Horns Rev, the world's largest wind farm. The LiDAR system is a ground-based sensing technique which avoids the use of high and costly meteorological masts. Three different inflow conditions were selected to perform LiDAR wind profiling. Comparisons of LiDAR mean wind speeds against cup anemometers from different masts showed high correlations for the open sea sectors and good agreement with their longitudinal turbulence characteristics. Cup anemometer mean wind speed profiles were extended with LiDAR profiles up to 161 m on each inflow sector. The extension resulted in a good profile match for the three surrounding masts. These extended profiles, averaged over all observed stabilities and surface roughness lengths, were compared to the logarithmic profile. The observed deviations were relatively small. Offshore wind farm wakes were also observed from LiDAR measurements where the wind speed deficits were detected at all LiDAR heights. Profile-derived friction velocities and roughness lengths were compared to Charnock's sea roughness model. These average values were found to be close to the model, although the scatter of the individual estimations of sea roughness length was large. Copyright © 2008 John Wiley & Sons, Ltd.

We present measurements from 2006 of the marine wind speed profile at a site located 18 km from the west coast of Denmark in the North Sea. Measurements from mast-mounted cup anemometers up to a height of 45 m are extended to 161 m using LiDAR observations. Atmospheric turbulent flux measurements performed in 2004 with a sonic anemometer are compared to a bulk Richardson number formulation of the atmospheric stability. This is used to classify the LiDAR/cup wind speed profiles into atmospheric stability classes. The observations are compared to a simplified model for the wind speed profile that accounts for the effect of the boundary-layer height. For unstable and neutral atmospheric conditions the boundary-layer height could be neglected, whereas for stable conditions it is comparable to the measuring heights and therefore essential to include. It is interesting to note that, although it is derived from a different physical approach, the simplified wind speed profile conforms to the traditional expressions of the surface layer when the effect of the boundary-layer height is neglected.

An analysis tool for the study of wind speed profiles over the water has been developed. The profiles are analysed using a modified dimensionless wind speed and dimensionless height, assuming that the sea surface roughness can be predicted by Charnock’s roughness length model. In this form, the roughness dependency on wind speed is extracted and the variations on the wind profile are due solely to atmospheric stability. The use of the Charnock’s non-dimensional wind profile is illustrated using data collected from a meteorological mast installed in the Danish North Sea. The best fit with the observed mean non-dimensional wind profile under neutral atmospheric conditions is found using a value of 1.2 × 10−2 for Charnock’s parameter. The stability correction on the neutral wind profile suggested by the Businger-Dyer relations was found to perform well over the sea.

Analysis of profiles of meteorological measurements from a 160 m high mast at the National Test Site for wind turbines at Hovsore (Denmark) and at a 250 m high TV tower at Hamburg (Germany) shows that the wind profile based on surface-layer theory and Monin-Obukhov scaling is valid up to a height of 50–80 m. At higher levels deviations from the measurements progressively occur. For applied use an extension to the wind profile in the surface layer is formulated for the entire boundary layer, with emphasis on the lowest 200–300 m and considering only wind speeds above 3 m s−1 at 10 m height. The friction velocity is taken to decrease linearly through the boundary layer. The wind profile length scale is composed of three component length scales. In the surface layer the first length scale is taken to increase linearly with height with a stability correction following Monin-Obukhov similarity. Above the surface layer the second length scale (L MBL ) becomes independent of height but not of stability, and at the top of the boundary layer the third length scale is assumed to be negligible. A simple model for the combined length scale that controls the wind profile and its stability dependence is formulated by inverse summation. Based on these assumptions the wind profile for the entire boundary layer is derived. A parameterization of L MBL is formulated using the geostrophic drag law, which relates friction velocity and geostrophic wind. The empirical parameterization of the resistance law functions A and B in the geostrophic drag law is uncertain, making it impractical. Therefore an expression for the length scale, L MBL , for applied use is suggested, based on measurements from the two sites.

International audience

Precipitation and evaporation budgets over the Baltic Sea were studied in a concerted project called PEP in BALTEX (Pilot study of Evaporation and Precipitation in the Baltic Sea), combining extensive field measurements and modelling efforts. Eddy-correlation-measurements of turbulent heat flux were made on a semi-continuous basis for a 12 month period at four well-exposed coastal sites in the Baltic Proper (the main basin of the Baltic Sea). Precipitation was measured at land-based sites with standard gauges and on four merchant ships travelling between Germany and Finland with the aid of specially designed ship rain gauges (SRGs). The evaporation and precipitation regime of the Baltic Sea was modelled for a 12 month period by applying a wide range of numerical models: the operational atmospheric High Resolution Limited Area Model (HIRLAM, Swedish and Finnish versions), the German atmospheric REgional-scale MOdel, REMO, the operational German Europe Model (only precipitation), the oceanographic model PRO...

Two weeks of measurements of the boundary-layer height over a small island (Christianso) in the Baltic Sea are discussed. The meteorological conditions are characterised by positive heat flux over the sea. The boundary-layer height was simulated with two models, a simple applied high-resolution (2 km × 2 km) model, and the operational numerical weather prediction model HIRLAM (grid resolution of 22.5 km × 22.5 km). For southwesterly winds it was foundthat a relatively large island (Bornholm) lying 20-km upwind of the measuring site influences the boundary-layer height. In this situation the high-resolution simple applied model reproduces the characteristics of the boundary-layer height over the measuring site. Richardson-number based methods using data from simulations with the HIRLAM model fail, most likely because the island and the water fetch to the measuring site are about the size of the grid resolution of the HIRLAM model and therefore poorly resolved. For northerly winds, the water fetch to the measuring site is about 100 km. Both models reproduce the characteristics of the height of the marine boundary layer. This suggests that the HIRLAM model adequately resolves a water fetch of 100 km with respect to predictions of the height of the marine boundary layer.

Here we discuss the status and challenges in the development of atlases for the assessment of the regional and global wind resources. The text more specifically describes a methodology that is under development at DTU Wind Energy in Denmark. As the wind assessment is based on mesoscale modelling, some of the specific challenges in mesoscale modelling for wind energy purposes are discussed such as wind profiles and long-term statistics of the wind speed time series. Solutions to these challenges will help secure an economic and effective deployment of wind energy.

Knowledge on the forcing mechanisms (meteorological and chemical) that come into play in the Arctic environment is highly uncertain. We analyse data from measurements and mesoscale meteorological modelling for periods in summer 2011 and winter 2011/2012 to elucidate the boundary-layer features at Station Nord situated in Northern Greenland (81.6 N, 16.7 W). A major challenge for modelling is to connect local-scale observations with larger scales modelling of the atmosphere. In particular, in summer, bare soil in the vicinity of the station may not be present in the global boundary and surface conditions. Thus, in summer the deviations of modeled from measured values of temperature and humidity near the surface are larger compared to winter. We found that the underestimation of temperature near the ground is larger at clear sky compared to cloudy conditions; and the underestimation reached up to height 1-1, 5 km at clear sky and up to the first 100 m for the cloudy days. The measured wind speed profiles showed high variability, while the modeled were smoothed. During summer the modeled wind speed was close to or larger than the measured without clear indication for the role of clouds. In winter, the over-estimation of wind speed was more pronounced.

This paper summarises some of the key results from two European field programmes, WINTEX and LAPP, undertaken in the Boreal/Arctic regions in 1996–98. Both programmes have illustrated the very important role that snow plays within these areas, not only in the determination of energy, water and carbon fluxes in the winter, but also in controlling the length of the summer active season, and hence the overall carbon budget. These studies make a considerable advance in our knowledge of the fluxes from snow-covered landscape and the interactions between snow and vegetation. Also some of the first measurements of greenhouse gas fluxes (carbon dioxide and methane) are reported for the European arctic and sub-arctic. The measurements show a considerable variability across the arctic, with very high instantaneous values from sub-arctic birch and fen areas and extremely low fluxes reported from the polar desert in the high arctic. The overall annual budgets are everywhere limited by the very short active season in these regions. The heat flux over a high latitude boreal forest during late winter was found to be high. At low solar angles the forest shades most of the snow surface, therefore an important part of the radiation never reaches the snow surface but is absorbed by the forest. This indicates that in areas with sparse vegetation and low solar angles, absorption of direct solar radiation is due to an apparent vegetation cover, which is much greater than the actual one.