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EGU 2005 Vienna, April 2005

EGU 2005 Vienna, April 2005. A Year of Eddy-Correlation Measurements of the Fluxes of Momentum, Heat and Moisture near the Dutch Coast.

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EGU 2005 Vienna, April 2005

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  1. EGU 2005 Vienna, April 2005 A Year of Eddy-Correlation Measurements of the Fluxes of Momentum, Heat and Moisture near the Dutch Coast Gerrit Burgers and Cor van Oort Royal Netherlands Meteorological Institute

  2. A year of EC flux measurements Experimental design and implementation: Wiebe Oost Cor Jacobs North Sea, 9 km off Dutch coast, depth 18m Boom from platform “Meetpost Noordwijk” December 2002 - December 2003 Westerly, on-shore winds

  3. 23m boom from platform Eddy correlation measurements: Two Gill R3A sonic anemometers Humidity sensor: Infrared Fluctuation Meter Temperature sensor: thermistor mounted on wire; Gill Other measurements: Seawater temperature at 1.5m depth Hs and wave age near platform Short- and long wave radiation Eddy correlation measurements

  4. What are we looking for? Bulk transfer coefficients - is CD consistent with a fixed Charnock coefficient? - do we observe a sea state dependence? - can we establish a wind dependence of the heat and moisture exchange coefficients? - how representative is our site? - what causes the scatter in the measurements?

  5. Cuts on the data Reject data if: - wind direction outside 2100 and 3600 - alignment instruments into wind to much off - heavy rain - non-stationary runs - conflicting sensor readings - birds This leaves about 5000 runs of 20 minutes ( 1 Dutch civil servant year)

  6. Corrections - apply results from wind tunnel calibration runs - “tilt” correction for distortion surrounding instruments - set mean w to zero Rough estimates for systematic measurements, from systematic differences between sensors: wind speed: 2% wind stress: 10% heat flux: 20% moisture flux:

  7. Scatter in u*: physical or sampling? Much scatter stems from sampling errors Sampling errors: # degrees of freedom N u*T/z Sreenivasan et. al (BLM 1978): sampling errors in u*measurements above sea, estimated from properties high-resolution time series: _u* /u* = sqrt[7.5 zobs/ (u(z) T)] We are estimating _u* from differences in subsequent runs, for runs of length T=5min and T=20min (in progress)

  8. Drag coefficient  z0 = 0.030u*2/g(u*/0.3)0.27+ visc. Lower Gill z0 = 0.031u*2/g + visc.

  9.  z0 = 0.013u*2/g (u*/0.3)1.3 + visc. Drag coefficient Upper Gill z0 = 0.014u*2/g + visc. • CDN North Sea higher • than open ocean • Similar to Geernaert • 11986, Oost et al. 2002

  10. Sea-state dependent fits • Sea-state dependent fits: • better than Charnock • not better thanz0 = a (u*)c/g + visc. • (a) z0 = a u*2 (u*/cp)c/g + visc. • (b) z0 = a u*2 (kp Hs)c/g + visc. • (c) z0 = a Hs (u*/cp)c + visc.

  11. Charnock vs. wave age No clear signal Line: ASGAMAGE fit

  12. Sea-state dependence? contour plot of CDN orange lines: observation density - little dependence on cp - ``wrong’’ way Lower Gill

  13. Dependence on steepness, Hs? Upper Gill Lower Gill also little dependence on steepness and Hs

  14. Moisture flux Lower in stable conditions Indication for increase with wind speed

  15. Heat fluxes from thermistor Heat flux (1) Negative heat fluxes?

  16. Heat fluxes from Gill Heat flux (2) No negative heat fluxes. Large values. Indication for increase with wind stress

  17. Many, many data - limitations from systematic errors • Measured drag over North Sea larger than over open ocean • Drag increases faster than for fixed Charnock parameter • Multivariate distribution CD(U10, cp): no support for wave age dependence • Indications for increase of CDH and CDE with windspeed Conclusions

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