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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

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

a year of ec flux measurements
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

eddy correlation measurements

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
what are we looking for
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?

cuts on the data
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)

corrections
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:

scatter in u physical or sampling
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)

drag coefficient
Drag coefficient

 z0 = 0.030u*2/g(u*/0.3)0.27+ visc.

Lower Gill

z0 = 0.031u*2/g + visc.

drag coefficient1

 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
sea state dependent fits
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.
charnock vs wave age
Charnock vs. wave age

No clear signal

Line: ASGAMAGE fit

sea state dependence
Sea-state dependence?

contour plot of CDN

orange lines:

observation density

- little dependence on cp

- ``wrong’’ way

Lower Gill

dependence on steepness hs
Dependence on steepness, Hs?

Upper Gill

Lower Gill

also little dependence on steepness and Hs

moisture flux
Moisture flux

Lower in stable conditions

Indication for increase with wind speed

heat flux 2

Heat fluxes from Gill

Heat flux (2)

No negative heat fluxes. Large values.

Indication for increase with wind stress

conclusions

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