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Eddy Correlation: Understanding Flux Measurement in Atmospheric Sciences

This quick course provides an in-depth exploration of the Eddy correlation method used to measure fluxes of sensible heat, latent heat (evaporation), carbon dioxide, and methane in the atmosphere. By utilizing turbulent properties of air, participants will learn to analyze raw signals and their correlations, determine covariance, and apply necessary corrections such as detrending and frequency response adjustments. Real-life data from the Horstermeer will illustrate concepts, enhancing understanding of how to accurately measure and interpret flux data across various atmospheric conditions.

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Eddy Correlation: Understanding Flux Measurement in Atmospheric Sciences

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Presentation Transcript

  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

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