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High Meteorology: Wind throughout the boundary-layer

High Meteorology: Wind throughout the boundary-layer. Sven-Erik Gryning, Hans E Jørgensen, Poul Astrup, Lars Landberg Wind Energy Department Risø National Laboratory, Denmark Lars.Landberg@risoe.dk.

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High Meteorology: Wind throughout the boundary-layer

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  1. High Meteorology: Wind throughout the boundary-layer Sven-Erik Gryning, Hans E Jørgensen, Poul Astrup, Lars Landberg Wind Energy Department Risø National Laboratory, Denmark Lars.Landberg@risoe.dk

  2. Wind profiles over flat homogeneous terrainMap of the Høvsøre site at the west coast of Jutland with measuring sector shown EWEC 06, Athens

  3. 30 degrees 60 degrees 90 degrees views from the mast EWEC 06, Athens

  4. Measured wind profiles, sector 30 to 90 deg. EWEC 06, Athens

  5. Commonly used expression for the wind profile Neutral atmosphere Stable atmosphere (nighttime) Unstable atmosphere (daytime) . with the standard stability correction (Businger) based on measuremets at small masts (Kansas experiment): EWEC 06, Athens

  6. Monin-Obukhov wind profilesplanetary boundary layer only, constant flux and based on Businger (-1/4 power) EWEC 06, Athens

  7. but for unstable conditions actually, the theoretically correct correction for convective conditions reads: EWEC 06, Athens

  8. Monin-Obukhov wind profilesplanetary boundary layer only, constant flux and based on convective scaling (-1/3 power) EWEC 06, Athens

  9. Wind profile, common knowledge The wind profile for the boundary layer can be expressed as: where is the local friction velocity (proportional to the square root of the local Reynolds stress). The length scale is denotedit is a function of the state of the atmosphere and height EWEC 06, Athens

  10. Length scales The behaviour of the length scale is modelled by inverse summation of the three terms . which can be written . EWEC 06, Athens

  11. Length scales EWEC 06, Athens

  12. surface layer lower part of the boundary layer In the atmospheric surface layer (not influenced by and ) the above expression reduces to the logarithmic wind profile: and in the lower part of the boundary layer (not influenced by ) and for the entire boundary layer Neutral EWEC 06, Athens

  13. Stability correctionThe effect of atmospheric stability will be derived as a correction to the wind profile in neutral conditions. neglecting the (unknown) stability dependence on and For atmospheric stable conditions, Businger et al. (1971) EWEC 06, Athens

  14. Wind profile - unstable For atmospheric unstable conditions( negative ) where Businger et al. (1971) suggested and   and the theoretical correct value for convective conditions is p= -1/3 and a= -12. Then the length scale can be expressed as: EWEC 06, Athens

  15. Monin-Obukhov wind profilesplanetary boundary layer only, constant flux and based onconvective scaling (-1/3 power)and constant length scale in the middle layer EWEC 06, Athens

  16. for for Conclusion: the wind profile in the lower part of the boundary-layer over homogeneous terrain in near neutral conditions Both expressions reduces for neutral condtions, to EWEC 06, Athens

  17. Profiles of momentum (left) and kinematic heat flux (below), to determine the boundary layer height Stable conditions (nighttime, sometimes daytime winter) EWEC 06, Athens

  18. Unstable conditions (daytime) Profiles of momentum (far left) and kinematic heat flux (less left), to determine the boundary layer height EWEC 06, Athens

  19. Boundary layer height estimated from the measured profiles of momentum and kinematic heat fluxes. EWEC 06, Athens

  20. Which one is the better? It is clear that the height of the boundary layer is important for the stable cases where the height is about 200 metres. But it is not clear how to parameterize the length scale close to its top. EWEC 06, Athens

  21. Conclusions on wind profiles • Above the surface boundary layer the neutral wind profile deviates from logarithmic. It can be argued to be caused by the length scale not • being proportional to height (as in the surface layer) but approaching a • constant value. • Under very convective conditions use of a formulation for the stability correction that fulfills the theoretical requirements for the convective limit is seen to perform better than the commonly used Businger formulation. • Inclusion of the boundary layer height improves the wind profile, the effect was clearly seen during atmospheric the stable conditions where the boundary layer height was only slightly higher than the maximum measuring height. The effect is less well seen during unstable and neutral conditions where the boundary layer height is much higher than he measuring height. • The behaviour of the length scale near the top of the boundary layer is not clear. EWEC 06, Athens

  22. Conclusions on measurements • The measurements at 160 meters height were of decisive importance for the interpretation of the wind profiles. A 200 metre mast seems appropiate and wishful thinking for the national test station for large wind turbines • Measurements of the height of the boundary-layer are missing and should be added. Research on how to achieve this parameter should be initiated. EWEC 06, Athens

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