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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Gerrit Burgers and Cor van Oort Royal Netherlands Meteorological Institute
Experimental design and implementation:
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:
Two Gill R3A sonic anemometers
Infrared Fluctuation Meter
thermistor mounted on wire; Gill
Seawater temperature at 1.5m depth
Hs and wave age near platform
Short- and long wave radiationEddy correlation measurements
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?
Reject data if:
- wind direction outside 2100 and 3600
- alignment instruments into wind to much off
- heavy rain
- non-stationary runs
- conflicting sensor readings
This leaves about 5000 runs of 20 minutes
( 1 Dutch civil servant year)
- 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%
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)
z0 = 0.030u*2/g(u*/0.3)0.27+ visc.
z0 = 0.031u*2/g + visc.
No clear signal
Line: ASGAMAGE fit
contour plot of CDN
- little dependence on cp
- ``wrong’’ way
also little dependence on steepness and Hs
Lower in stable conditions
Indication for increase with wind speed