Frequency and evolution of Low Level Jet events over the Southern North Sea analysed from WRF simulations and LiDAR measurements

Transcription

1 Frequency and evolution of Low Level Jet events over the Southern North Sea analysed from WRF simulations and LiDAR measurements David Wagner1, Gerald Steinfeld1, Björn Witha1, Hauke Wurps1, Joachim Reuder2 1 Energy meteorology group, ForWind University of Oldenburg, Germany 2 Geophysical Institute University of Bergen, Norway EMS Annual Meeting DCU, Dublin, Ireland, 09 September 2017

2 Low Level Jets Wind maxima in the lower troposphere Typically lasting for hours Large (vertical) wind shears

3 Low Level Jets Low Level Jets (LLJs) generally frequently studied Only one WRF study in the North Sea at FINO1 Nunalee and Basu, Research Topics in Wind Energy, 2014, 2, LLJs may be crucial for offshore wind energy Research questions: For which wind directions do LLJs typically occur? What are typical heights of LLJ cores? How often do LLJs occur? How good are LLJs are represented in WRF? What are possible evolution mechanisms?

4 LLJ Statistics with WRF Results for year 2009 LLJs below 3000 m Frequency of LLJ events Frequency of LLJ events Northern Germany Height in m Height in m Grid point over Northern Germany: 20.8 % of the time. Grid point near FINO1: 10.5 % of the time. But over sea: more cases up to 100 m

5 OBLEX-F1: The Offshore Boundary Layer Experiment at FINO1 Source: a_ventus_lagekarte.png Source: FINO1, German Bight, May 2015 September 2016 (atmospheric part); May 2015 October 2015 (oceanic part) Trianel Borkum-Riffgrund 1 FINO1 Alpha Ventus

6 OBLEX-F1: The Offshore Boundary Layer Experiment at FINO1 Measurement setup (atmospheric part)

7 Method Available measurements: May 2015 April 2016 (incl. Weeks of LiDAR failure) Merged cup anemometer and LiDAR data (quality controlled and corrected), to 20 min averages (Cup up to 60 m, Lidar from 70 m) Detected LLJs with with Baas et al. (2009) method: Minimum above the maximum must be > 2 m/s and > 25 % less than maximum, height limit set to 990 m. Figure adapted from Baas et al. (2009): Boundary-Layer Meteorol., 97(3), )

8 Method 10 m temperature data from DWD weather stations near the coast

9 LLJ Directions In 15 % of all available data sets (634/4217) LLJs were detected But: LLJs occurred on 65 % of all days! Wind direction distribution for LLJ occurrences:

10 LLJ Heights Considerable amounts below upper blade tip height of modern offshore wind turbines

11 Case studies comparison with WRF simulations WRF V3.7.1 ERA-Interim 6h OSTIA-SST 6h MYNN (PBL and SL) 55 height levels D3 with 3 km resolution April 2016 and August 2015

12 Case study April 2016 Figure: 2016 Deutscher Wetterdienst

13 Case study April 2016 Measurements wind speed temperature

14 Case study April 2016 WRF vs LiDAR LiDAR WRF

15 Case study April 2016 WRF Cross Sec at 120 WD

16 Inertial oscillations Measured with LiDAR (Arrows are not vectors, but mark the chronology of oscillations)

17 Coastal zone baroclinicity Temperature differences of land and sea surface show: LLJ formation mainly at or slightly after point of maximum temperature difference

18 Conclusions Typical wind directions East to West (mostly SE and SW) Relative most jet core heights around 200 m height Jets occurred on 15 % of all measurements WRF represented two LLJ cases well Mechanisms: inertial oscillations, baroclinicity?

19 Outlook Deeper research on formation mechanisms with WRF (e.g. separation of inertial oscillations and baroclinicity, which role does the stability play) Finding solutions for better estimations of LLJs in WRF (e.g. PBL and surface layer scheme adaptions) Seasonal statistics and a lot more to investigate!

20 Acknowledgements Thanks to the Norwegian Centre for Offshore Research (NORCOWE) for data provision and support during this work. FINO 1 data has been obtained from the FINO database operated by BSH The simulations were performed on the HPC Cluster FLOW at the University of Oldenburg, funded by the Federal Ministry for Economic Affairs and Energy under grant number

21 Any Questions?

22 Low Level Jets Evolution Potential conditions leading to LLJs (Compare Stull (1988)): Synoptic-scale baroclinicity associated with weather patterns Inertial Oscillations Land and Sea breezes Fronts Advective Accelerations Mountain and valley winds Splitting, ducting and confluence around mountain barriers Baroclinicity associated with sloping terrain Additionally (e.g: Burk and Thompson (1996): Baroclinicity in the coastal zone

23 Low Level Jets Evolution by inertial oscillation Relatively warm air is advected over a relatively cold surface Stable/very stable conditions Presence of low to near-zero surface friction Theory of evolution by inertial oscillation: A sudden loss of frictional force leads to an imbalance between pressure gradient, Coriolis and frictional force. LLJs often located at the borders of high pressure systems Figure from Emeis (2014): Met. Z., 23(3),

24 Low Level Jets Evolution by coastal zone baroclinicity Low temperatures above the sea and high temperatures above the land even after sunset Embedded in a favoring synoptic situation Thermal wind, here: higher wind speed at lower pressure levels than above (supergeostrophic) Figure from Burk and Thompson (1996): Mon. Wea. Rev., 124,

25 LLJ Shear Wind Shear - Hellmann Exponent α IEC : α = 0.14 Design Requirements for Offshore Wind Turbines

26 Case study April 2016 WRF vs Radiometer Radiometer WRF

27 Case study April 2016 LiDAR & Radiometer Profiles

28 Case study April 2016 WRF Cross Sec along lon

29 Case study August 2015 Figure: 2015 Deutscher Wetterdienst

30 Case study August 2015

31 Case study August 2015 LLJ embedded within sloped marine capping inversion (e.g. Burk and Thompson (1996)) Jet formation likely due to baroclinicity