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