Preliminary results of the NORCOWE Direct Covariance Flux System for Ship based measurements

To cover the increased demand of renewable energy the development of offshore wind turbines that can be placed in deep water has started. Model results from Sullivan et al. [2] indicate that the ocean has the ability to interact with the lower part of the Marine Atmospheric Boundary Layer (MABL). This leads to turbulent horizontal and vertical motions of the air that potentially reduces turbine lifetime and energy reliability. This paper gives a description of the Norwegian Center for Offshore Wind Energy (NORCOWE) direct covariance flux system. Consisting of a sonic anemometer, a state-of-the-art inertial measurement unit (IMU) and a data logger the system can easily be mounted on all floating platforms, e.g. ships or buoys and operates autonomously. This will in the future enable researchers to investigate turbulent motion and turbulent momentum transport processes in the MABL in more detail.publishedVersio

Validation of boundary layer parameterization schemes in the Weather Research and Forecasting Model (WRF) under the aspect of offshore wind energy applications - Part II: Boundary layer height and atmospheric stability

Five different planetary boundary layer (PBL) schemes in the Weather Research and Forecasting (WRF) model have been tested with respect to their capability to model boundary layer parameters relevant for offshore wind deployments. For the year 2005, model simulations based on the YSU, ACM2, QNSE, MYJ and MYNN2 PBL schemes with WRF have been performed for the North Sea and validated against measurements from the FINO1 platform. In part I, the investigations had focused on the key parameters 100 m mean wind speed and wind shear in terms of the power-law exponent. In part II, the focus is now set on the capability of the model to represent height and stability of the marine atmospheric boundary layer.Considerable differences are found among the PBL schemes in predicting the PBL height. A substantial part of this variation is explained by the use of different PBL-height definitions in the schemes. The use of a standardized procedure in calculating the PBL height from common WRF output parameters, in particular the temperature gradient and the wind shear, leads to reduced differences between the different schemes and a closer correspondence with the FINO1 measurements. The study also reveals a very clear seasonal dependency of the atmospheric stability over Southern North Sea. During winter time, the marine atmospheric boundary layer is more or less neutral with several episodes of unstable periods. During spring and early summer, the occurrence of periods with very stable stratification becomes dominant with stable conditions up to 40–45% of the time when warm continental air is advected from the South. In general, the results of part II confirm again that the MYJ scheme performs slightly better than the others and can therefore be suggested as first choice for marine atmospheric boundary layer simulations without a priori information of atmospheric stability in the region of interest.acceptedVersio

First results of turbulence measurements in a wind park with the Small Unmanned Meteorological Observer SUMO

The Small Unmanned Meteorological Observer (SUMO), equipped with a miniaturized 5-hole probe for turbulent flow measurements with 100 Hz temporal resolution, has for the first time been operated in and around a wind farm. The 5 day campaign from May 9-13 took place at a small wind park of 21 1 MW-turbines close to Vindeby on Lolland, Denmark and was dedicated to the investigation of the effects of wind turbines on boundary layer turbulence. A total of 20 SUMO flight missions carrying the turbulence system have been performed during the campaign. In spite of a few pitfalls related to the fine-tuning of the autopilot system and in the configuration and synchronization of the corresponding data logging systems, this campaign provided promising results indicating the capability and future potential of small UAS for turbulence characterization in and around wind farms. Subsequent flights upwind and downwind of the park revealed qualitatively a distinct enhancement in the turbulence level behind the wind farm.publishedVersio

Sensor movement correction for direct turbulence measurements in the marine atmospheric boundary layer

Understanding of the turbulent exchange processes between the ocean and the overlying atmosphere is essential for wind shear and turbulence structure of the Marine Atmospheric Boundary Layer (MABL). This is highly relevant for the development of offshore wind projects with respect to the expected power output and acceptable structural loads and fatigue of turbines. This paper presents preliminary results from a recent field experiment off the coast of Martha's Vineyard, Massachusetts. Turbulent fluxes have been measured from both a discus buoy and the Air Sea Interaction Tower (ASIT). Using the correction algorithm of Edson et al. (1998) the buoy data are corrected and compared with the ASIT measurements. The comparison shows that the corrected fluxes measured from the mooring are in good agreement with measured fluxes from the tower and that the direct covariance flux method is applicable for offshore turbulent flux measurements.publishedVersio

Experimental characterization of the marine atmospheric boundary layer in the Havsul area, Norway

Offshore wind energy applications depend strongly on an improved knowledge of the physical processes taking place in marine atmospheric boundary layer (MABL). In particular the better understanding of the complex interactions between wind shear, atmospheric stability and turbulence and the effects of wind-wave interactions on offshore vertical wind profiles are essential for the development of offshore wind projects. This paper presents an analysis of the relation between turbulence parameters, such as horizontal and vertical turbulence intensity, and turbulence kinetic energy and average vertical wind profiles and wind shear. The investigations are based on 4 years of wind lidar measurements on the small island of Storholmen in the Havsul area about 8 km off the coast of the Norwegian mainland. The results show systematic dep endencies between the investigated turbulence parameters, both with respect to average wind speeds and average wind shear. The results indicate that in particular the horizontal turbulence intensity has the potential to act as a proxy for atmospheric stability in cases where corresponding temperature profiles are not available.publishedVersio

Processing of sonic anemometer measurements for offshore wind turbine applications

Quality assured measurements from offshore masts may provide valuable information of the characteristics of the offshore wind field, which is of high relevance for simulations of offshore wind turbines' dynamic response. In order to obtain these high quality data sets, a processing procedure tailored to offshore wind turbine applications must be followed. In this study, existing quality control routines applied in literature are evaluated, and a complete procedure is developed for sonic anemometer measurements. This processing procedure is applied to measurements at three heights from 16 months of measurements at FINO1. The processing procedure results in a data set of more than 6 000 30-minute periods of high quality time series showing a large variety in terms of wind speed and turbulence intensity. Together with an assessment of the stationarity, this processed data set is ready for use in offshore wind turbine research.publishedVersio

Interannual variability of air temperature inversions in ice-free area of northern James Ross Island, Antarctica

Air temperature inversions are common features in Antarctica, especially in the interior where they are observed nearly year-round. Large temporal variability of air temperature inversion incidence is typical for the coastal areas and little is known about its occurrence in the Antarctic deglaciated areas. Here we present a 12-year-long time series of near-surface air temperature inversion derived from two automatic weather stations situated at different altitudes (10 and 375 m a.s.l.) in ice-free part of northern James Ross Island (Antarctic Peninsula). The highest monthly relative frequency of temperature inversions during 2006–2017 was observed in July (38%) when the range between minimum and maximum monthly frequencies reached 34%. Both the lowest monthly relative frequency of temperature inversions and the range were found in December with values of 7% and 15%, respectively. The correlation between mean lapse rate and selected mesoscale flow characteristics were tested. The highest correlations were found between lapse rate and specific humidity for the yearly means (0.69 in the 925 hPa pressure level). Negative correlation coefficients were established between lapse rate and air temperature in summer (− 0.65 in the 500 hPa pressure level). Finally, we also used the Weather and Research Forecasting (WRF) model to ascertain its ability to simulate situations as complicated as near-surface air temperature inversion formation in complex terrain. For a strong winter air temperature inversion, simulated air temperature was compared with in situ observations to assess the model performance.publishedVersio

Velocity spectra and coherence estimates in the marine atmospheric boundary layer

Two years of continuous sonic anemometer measurements conducted in 2007 and 2008 at the FINO1 platform are used to investigate the characteristics of the single- and two-point velocity spectra in relation to the atmospheric stability in the marine atmospheric boundary layer. The goals are to reveal the limits of current turbulence models for the estimation of wind loads on offshore structures, and to propose a refined description of turbulence at altitudes where Monin–Obukhov similarity theory may be limited. Using local similarity theory, a composite spectrum model, combining a pointed and a blunt model, is proposed to describe the turbulence spectrum for unstable, neutral and stable conditions. Such a model captures the −1 power law followed by the velocity spectra at an intermediate frequency range in the marine atmospheric boundary layer. For the Monin–Obukhov similarity parameter ζ<0.3, the Davenport coherence model captures the vertical coherence of the horizontal velocity components well. A two-parameter exponential decay function is found more appropriate for modelling the coherence of the vertical velocity component. Under increasingly stable conditions, the size of the eddies in the vertical coordinate reduces, such that smaller separation distances than that covered in the present dataset may be required to study the coherence with sufficient accuracy.acceptedVersio

Characterization of Terrain-Induced Turbulence by Large-Eddy Simulation for Air Safety Considerations in Airport Siting

Topography-induced turbulence poses a potential hazard for aviation safety, in particular during the final approach and landing. In this context, it is essential to assure that the impact of topography-induced turbulence on the flight paths during take-off and landing is minimized already during the design and planning phase. As an example of the siting and planning of a potential new airport in complex terrain, this study investigates the distribution of terrain-induced boundary layer turbulence in the vicinity of the current Lofoten airport at Leknes (LKN). For that purpose, large-eddy simulations (LES) have been performed with the PAralellized Large-eddy Simulation Model (PALM) on a 40×45×4km3 computational domain around LKN. An initial parametric sensitivity study resulted in a grid spacing of 50 m and an overall simulation time of 12 h for our individual model runs. A suite of 32 model simulations for 16 different wind directions and two geostrophic wind speeds of 10 ms−1 and 20 ms−1 , was then performed and analysed. A turbulence risk analysis along idealized flight trajectories shows that the high-risk conditions are substantially determined by the wind conditions and their interaction with the topography. With respect to wind speed, the results indicate that for a geostrophic flow below 10 ms−1 , the risk of aviation critical, terrain-induced boundary layer turbulence (BLT), is rather low in the vicinity of LKN. At 20 ms−1 the situation has completely changed, as for 14 out of 16 investigated wind directions the 9 m2 s−2 aviation critical threshold of turbulent kinetic energy per unit air mass (TKE) is exceeded. In the northwesterly wind scenarios, the largest areas with critical turbulence in the vicinity of LKN are observed.publishedVersio

Downscaling an intense precipitation event in complex terrain: the importance of high grid resolution

Floods due to intense rainfall are a major hazard to both people and infrastructure in western Norway. Here steep orography enhances precipitation and the complex terrain channels the runoff into narrow valleys and small rivers. In this study we investigate a major rainfall and flooding event in October 2014. We compare high-resolution numerical simulations with measurements from rain gauges deployed in the impacted region. Our study has two objectives: (i) to understand the dynamical processes that drove the high rainfall and (ii) the importance of high grid resolution to resolve intense rainfall in complex terrain. This is of great interest for numerical weather prediction and hydrological modelling. Our approach is to dynamically downscale the ERA-Interim reanalysis with the Weather Research and Forecasting model (WRF). We find that WRF gives a substantially better representation of precipitation both in terms of absolute values as well as spatial and temporal distributions than a coarse resolution reanalysis. The largest improvement between the WRF simulations is found when we decrease the horizontal model grid spacing from 9 km to 3 km. Only minor additional improvements are obtained when downscaling further to 1 km. We believe that this is mainly related to the orography in the study area and its representation in the model. Realistic representations of gravity waves and the seeder–feeder effect seem to play crucial roles in reproducing the precipitation distribution correctly. An analysis of associated wavelengths shows the importance of the shortest resolvable length scales. On these scales our simulations also show differences in accumulated precipitation of up to 300 mm over four days, further emphasising the need for resolving short wavelengths. Therefore, our results clearly demonstrate the need for high-resolution dynamical downscaling for extreme weather impact studies in regions with complex terrain.publishedVersio