OWEMES 2006, Civitavecchia, Italy Jens Tambke, Lueder von Bremen, *Jörg-Olaf Wolff ForWind and *ICBM, Carl von Ossietzky University Oldenburg John A. T. Bye Physical Oceanography, The University of Melbourne, Australia Bernhard Lange ISET, Universität Kassel, Germany Lorenzo Claveri Finnish Meteorological Institute, Helsinki, Finland Francesco Durante, DEWI German Wind Energy Institute, Wilhelmshaven, Germany Modelling of Wind Fields above the North Sea Horns Reef FiNO1

Jens Tambke, University of Oldenburg / Slide 2 Overview  Offshore Wind Speed Profiles  Influence of Thermal Stratification at Horns Rev and FiNO1  Accuracy of Numerical Weather Analysis at FiNO1  New Model: Inertially Coupled Wind Profiles (ICWP)

Jens Tambke, University of Oldenburg / Slide 3 log(Height z) Speed u(z) neutral unstablestable Classical Profiles:  Logarithmic Profiles + Monin-Obukhov-Formula  Charnock Relation for variable Roughness © Elsam A/S

Jens Tambke, University of Oldenburg / Slide 4 Measurements at Horns Rev Cup anemometers at 4 heights Temperature at 3 heights (T) Investigated period: 10/2001– 4/2002 German Weather Service DWD at 10m, 34m, 110m - 15 m - 62 m - 45 m - 30 m (T) 55 m - (T) -4 m - (T) 13 m - Numerical Weather Analysis © Elsam A/S

Jens Tambke, University of Oldenburg / Slide 5 Wind Profiles and Predictions at Horns Rev  Different wind speed gradients! DWD-Model Observation RMSE = 15%

Jens Tambke, University of Oldenburg / Slide 6 Influence of Thermal Stratification at Horns Rev unstablestable  Binned wind speed ratios

Jens Tambke, University of Oldenburg / Slide 7 Influence of Thermal Stratification Measure for atmospheric stability: Bulk-Richardson-Number Ri b - Acceleration of Gravity - Temperature [K] - Measurement-Height - Difference of virtual potential temperature between sea-surface and height z - Wind speed at height z Calculation of Monin-Obukhov (MO) Length L: Grachev and Fairall (1997) allows calculation of Businger-Dyer stability functions at Horns Rev and FiNO1

Jens Tambke, University of Oldenburg / Slide 8 Influence of Thermal Stratification at Horns Rev unstable stable u(62m) u(15m) = 1.5 u(62m) u(15m) < 1.1

Jens Tambke, University of Oldenburg / Slide 9 Observations at FiNO1  Located in the German Bight 45km north of Borkum  Investigated period: 2004  Wind speed measurements at 33, 41, 51, 61, 71, 81, 91 and 103m height  Massive lattice mast causes strong flow distortion:  Corrections are very important

Jens Tambke, University of Oldenburg / Slide 10  Binned Wind-Speed Ratios Influence of Thermal Stratification at FiNO1 unstable stable

Jens Tambke, University of Oldenburg / Slide 11 Influence of Thermal Stratification at FiNO1 unstable stable u(51m) u(33m) = 1.15 u(51m) u(33m) < 1.05

Jens Tambke, University of Oldenburg / Slide 12 Influence of Thermal Stratification at FiNO1 unstable stable u(103m) u(33m) = 1.4 u(103m) u(33m) < 1.1

Jens Tambke, University of Oldenburg / Slide 13 Comparison of Modelled Profiles at FiNO1 RMSE(103m) = 1.4m/s MM5 (NCEP) Observation DWD analysis wind directions between 190° and 250°

Jens Tambke, University of Oldenburg / Slide 14 Accuracy of DWD Analysis at FiNO1 Observation Analysis Wind SpeedPotential Power Output  Mean Values vs. Hour of the Day, Average of 12 months, 2004 RMSE: 1.4 m/sRMSE: 13% of P(inst)

Jens Tambke, University of Oldenburg / Slide : Mean 103m Wind Speeds in the DWD Analysis Mean Potential Power Production in the German Bight 2004: 51% of the Installed Capacity

Jens Tambke, University of Oldenburg / Slide 16 New Air-Sea-Interaction Model: Inertially Coupled Wind Profiles (ICWP) 1.) Coupling of Ekman- and Log-Profile 2.) Coupling of wind and wave field Height Speed 0 z B < 20m : Matching height for speed, stress  and eddy viscosity Ekman Layer: (z) = A  u * 2 /f = const.  (z) = ρ ∂u/∂z Wave Boundary Layer: (z) =  u * z Φ (MO-Log.)  (z) =  (wave)

Jens Tambke, University of Oldenburg / Slide 17 Inertially Coupled Wind Profiles (ICWP) Similarity Assumptions: 2.) Ratio of drift velocities close to the air-sea interface 3.) Inertial Coupling Relation: a drag law with respect to the matching height z B in air and sea 1.) Ratio of eddy viscosities Ekman Spiral: Onset at z B ~ 1/29

Jens Tambke, University of Oldenburg / Slide 18 Comparison of theoretical and observed Profiles at Horns Rev ICWP Model Observation WAsP Model Input: time series of wind speed at 30m height WAsPbias = m/s ICWP bias = -0.1 m/s RMSE(62m) = 6% (3%) for wind directions between 135° and 360°

Jens Tambke, University of Oldenburg / Slide 19 Comparison of Mean Profiles at FiNO1 ICWP Observation WAsP for wind directions between 190° and 250° Model Input: time series of wind speed at 33m height WAsP bias = m/s ICWP bias = +0.1 m/s RMSE(103m) = 10% (5.5%)

Jens Tambke, University of Oldenburg / Slide 20 Comparison of Mean Profiles at FiNO1 Observation for wind directions between 190° and 250° Model Input: time series of wind speed at 33m height z0=0.2mm IEC-3

Jens Tambke, University of Oldenburg / Slide 21 Current Research: Analysis of Turbulence Intensities at FiNO1 Turbulence Intensity (σ u /u) vs. Wind Speed (u) at 103m, Jan-Dec 2004 for wind directions between 190° and 250°

Jens Tambke, University of Oldenburg / Slide 22 Conclusions 2.Observed Wind Profiles show higher wind shears above 45m height than expected 3.The ICWP-Model reproduces these higher wind shears with an Ekman-Approach 1.Thermal stratification has a crucial impact on offshore wind profiles Thank You for Your Attention! This work was funded by the EU within the Projects ANEMOS and POW’WOW.