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Skyler Goldman, Meteorology, DMES

RELATIONSHIP BETWEEN ROUGHNESS LENGTH, STATIC STABILITY, AND DRAG COEFFICIENT IN A DUNE ENVIRONMENT. Skyler Goldman, Meteorology, DMES. Introduction to the subject. Drag (Wind) coefficient- quantifies the drag or resistance of an object in a fluid environment such as air or water.

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Skyler Goldman, Meteorology, DMES

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  1. RELATIONSHIP BETWEEN ROUGHNESS LENGTH, STATIC STABILITY, AND DRAG COEFFICIENT IN A DUNE ENVIRONMENT Skyler Goldman, Meteorology, DMES

  2. Introduction to the subject • Drag (Wind) coefficient- quantifies the drag or resistance of an object in a fluid environment such as air or water. • A lower drag coefficient indicates the object will have less aerodynamic or hydrodynamic drag. • Less Drag = Less Resistance!! • For small scale flow (dune environment), a better characterization of the surface drag can go a long way towards understanding and modeling the air flow. • Understanding what affects wind flow and magnitude is important to meteorology and engineering

  3. Data Collected • Temperature and wind collected using anemometer attached to stadia rod and elevated from 6.5 ft to 25 ft over seven different stations (vegetated and non-vegetated) on six different days Courtesy of Sarah Collins

  4. Neutral vs. Corrected Drag Coefficient • Overall goal is to draw comparisons between drag coefficient and wind profiles. • 1st Type of drag coefficient (Cd) • Neutral (dependent on roughness length (zo) The impact of surface friction on atmosphere extends upwards as zo increases. Increasing roughness length (zo)  Height (m) Wind Speed (m/s) (Figures from Beljaars, A. The Parameterization of the Boundary Layer, 5/1992. European Centre for Medium-Range European Weather Forecasts)

  5. Neutral Drag Coefficient K = von Karman constant (0.4) Zr = Reference height (6.5 ft) Zo = Roughness length Higher the roughness length, higher the drag coefficient

  6. 2nd Type of Drag Coefficient MOMENTUM • Corrected (dependent on stability factor and roughness) • Momentum moves downward • more easily in unstable • air than stable. • Near surface, unstable regime • produces steeper wind profiles Diagram from: Air-sea Interaction:  Laws and Mechanisms, Csanady Structure of the Atmospheric Boundary Layer, Sorbjan

  7. Corrected Drag Coefficient Neutral = Corrected = Where is a stability function But…how do we find this stability function???

  8. Correcting for Stability… • According to theory, the [drag] coefficients are a function of buoyancy and wind shear. • The exact relationship is not known but is approximated using observed wind, temperature and moisture profiles near the surface. - (Arya 2001)

  9. Profiles vary spatially and temporally WEST OF DUNE BEACH DUNE CREST Courtesy of Cory Hodes

  10. Finding Stability • To find stability function, a series of equations are used • 1st – Find static stability using temperature, pressure, and humidity data • 2nd – Determine Richardson Number • Ri Number = static stability / windshear • Richardson Number is then related to a buoyancy parameter • 3rd – Use buoyancy parameter to correct neutral drag coefficient

  11. What is Expected? • Neutral • Higher the roughness length, higher the drag coefficient • Roughness Lengths: • Vegetated (Ponce) • Station 1: 0.17 ft • Station 2: 0.21 ft • Station 3: 2.44 ft • Station 4: 0.59 ft • Non Vegetated (Denuded): • Station1: 0.45 ft • Station 2: 2.5 10^-3 ft • Station 3: 1.11 ft Station 2 Station 3 Station 1 WATER Roughness lengths from Sarah Collins Beach profile map from John Hearin

  12. What is Expected? • Corrected for Stability Our data • (Arya 2001)

  13. Putting it all together… • Based on formulas… • High stability = low drag • Low stability = high drag • High roughness length = high drag • Low roughness length = low drag • Yet…how does this affect the wind profile?

  14. Z Z GREATER CHANGE IN WIND SPEED OVER HEIGHT LOW DRAG LITTLE CHANGEIN WIND SPEED OVER HEIGHT HIGH DRAG Wind Vectors Wind Vectors decrease When subjected to more drag Wind Speed Wind Speed

  15. Final Question… • Evidence of direct relationship between roughness and stability • Evidence of indirect relationship between drag and stability • Wind profiles = little wind changes over height with low drag • Wind profiles = large wind changes over height with high drag • BUT…do these statements hold with actual data….

  16. Conclusions • Wind profiles on 26 May 2010 appear to be consistent with corrected drag estimates • This is not true of all sampled days as estimates of zo can vary due to: • wind speed • fetch • wave height for station 1 which is located near the high tide line.

  17. Questions… Break is next, followed by: Emily Teske!

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