Eddy correlation quick-course

1. Eddy correlation quick-course Background Raw signals Time series covariantie Spectra Footprint Data processing angle of attack dependent calibration detrending rotation Frequency response corrections Schotanus Webb

2. Background of Eddy correlation • We want to measure the fluxes of sensible heat, latent heat (evaporation), carbon dioxide and methane • To measure them, we use the turbulent properties of the air • For example: during the day: • temperature humidity CO2 • high colder drier normal • 4 m 24 oC 17 g/kg 360 ppm • low warmer moister depleted • 0.1 m 25 oC 18 g/kg 355 ppm

3. Background of Eddy correlation 24 °C 17 g/kg H2O 360 ppm CO2 25 °C 18 g/kg H2O 355 ppm CO2 25 °C 18 g/kg H2O 355 ppm CO2 24 °C 17 g/kg 360 ppm

4. Measurements at the Horstermeer

5. The raw signals

6. The raw signals

7. correlation w - T r = 0.55 r2 = 0.30

8. covariance • covariance = (w – wmean) x (T – Tmean) • or: • when defining • w’ = (w – wmean) • T’ = (T – Tmean) • then • covariance = w’T’

9. 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

10. covariance H = ρ Cp w’T’ Similarly: LE = λ w’ρv’ = ρλ w’q’ fco2 = w’ρco2’

11. Angle of Attack Dependent CalibrationGash and Dolman, 2003van der Molen, Gash and Elbers, 2004

12. Detrending

13. Other corrections • rotation • Frequency response corrections • Schotanus

14. Webb corrections • rotation • Frequency response corrections • Schotanus • Webb