Eddy correlation quick-course 1.Background 2.Raw signals Time series covariantie Spectra Footprint...
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Transcript of Eddy correlation quick-course 1.Background 2.Raw signals Time series covariantie Spectra Footprint...
Eddy correlation quick-course
1. Background
2. Raw signals • Time series• covariantie• Spectra• Footprint
3. Data processing• angle of attack dependent calibration• detrending• rotation• Frequency response corrections • Schotanus • Webb
Background of Eddy correlation
1. We want to measure the fluxes of sensible heat, latent heat (evaporation), carbon dioxide and methane
2. To measure them, we use the turbulent properties of the air
3. For example: during the day:
temperature humidity CO2
high colder drier normal4 m 24 oC 17 g/kg 360 ppm
low warmer moister depleted0.1 m 25 oC 18 g/kg 355 ppm
Background of Eddy correlation
CO2
360 ppm
U
CO2
355 ppm CO2
355 ppm CO2
360 ppm
U
I II
25 °C
18 g/kg H2O
355 ppm CO2
25 °C
18 g/kg H2O
355 ppm CO2
24 °C
17 g/kg H2O
360 ppm CO217 g/kg
360 ppm
24 °C
Measurements at the Horstermeer
The raw signals
The raw signals
correlation w - T
r = 0.55
r2 = 0.30
covariance
covariance = (w – wmean) x (T – Tmean)
or:
when defining
w’ = (w – wmean)
T’ = (T – Tmean)
then
covariance = w’T’
covariance
w’T’ = 0.33 m/s K
to calculate the energy content of this air stream we are actually interested in the covariance of
H = w’ (ρ Cp T)’ = (ρ– ρmean) Cp w’ T’
with ρ ~ 1.2 kg/m3 the air density and Cp ~ 1004.67 J/kg the heat capacity of air
But (fortunately) ρ does not correlate with w’T’, thus:
H = ρ Cp w’T’ = 1.2 * 1005 * 0.33 = 397 W/m2
covarianceH = ρ Cp w’T’
Similarly:
LE = λ w’ρv’ = ρ λ w’q’
fco2 = w’ρco2’
Angle of Attack Dependent Calibration
Gash and Dolman, 2003van der Molen, Gash and Elbers, 2004
Detrending
Other corrections
rotationFrequency response corrections Schotanus
Webb corrections
rotationFrequency response corrections Schotanus Webb