Eddy Covariance Data Processing

Outline

➢ Overview of data processing

➢ Software installation and sample data preparation

➢ Hands-on EddyPro data processing with guidance

➢ EddyPro data processing exercise

Basic Data Processing ProceduresRaw data

Coordinate rotation

Uncorrected flux computation

WPL correction

Frequency correction

Corrected fluxes – with QA/AC flags

De-spike

Time delay removal

Sonic temperature correction

Footprint

Quality check – hard flags

Raw Data Formats

GHG file - .ghg

- High frequency data + metadata

- Biological and meteorological (Biomet) data + metadata

- Updated LI-7550 Analyzer Interface Unit

TOA5 - .txt

- Table Oriented ASCII Format 5

- PC logging

TOB1 - .dat

- Table Oriented Binary Format 1

- Campbell datalogger

Quality Check – Hard Flags

• Spike detection and removal test

• Amplitude resolution test

• Drop out test

• Absolute limit test

• Skewness and Kurtosis tests

• Discontinuities test

• Time lag tests

Quality Check – Hard Flags

statistical_flags

spikes amp_res drop_out

HFu/v/w/ts/co2/h2o/ch4/n2o HFu/v/w/ts/co2/h2o/ch4/n2o HFu/v/w/ts/co2/h2o/ch4/n2o

HF00000009 HF00000009 HF00000009

HF00000009 HF00000009 HF00000009

HF00000009 HF00000009 HF00000009

HF00000009 HF00000009 HF00000009

0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3 3.3 3.6

x 104

38.6

38.8

39

39.2

39.4

39.6

39.8

40

40.2

Spikes - Moving windows (1/6 of flux averaging

period with half window overlapped)

Not spikes - 4 or more consecutive points

Spikes are counted and removed.

Replaced through linear interpolation or not

Common σ range for scalars

Despiking

▪ (Vickers and Mahrt, 1997)

Time Delay and Removal

1.04 1.06 1.08 1.1 1.12 1.14 1.16 1.18

x 104

30.5

31

31.5

32

32.5

33

Signals

Tim

e

Signal 1

Signal 2

0.5 – average wind speed (m/s)

Coordinate Rotation

➢ Assumption:

- Mean vertical wind speed = zero

➢ Requirement:

- Level the sonic perfectly

➢ The reality:

- Cannot level the sonic perfectly

➢ The problem solver:

- Coordinate rotation

Coordinate Rotation - Methods

I. Natural wind coordinate rotation

- Done every averaging period

- Better for simple topography

- Crop land and grassland

II. Planar fit coordinate

- Long-term (monthly)

- Better for complex topography

- Forest site

0, rotating x axis, not recommended

Natural Wind Coordinate Rotation

z1 = z0

y0

y1α

x0x1

β

y2 = y1

z2z1

x1

x2

vw

00

1st step 2nd step

2D

3D

Planar Fit Coordinate Rotation

Paw et al. 2000, Bound.-Layer Meteorol. 97: 487-511

= b0 + b1 + b2

b0, b1 and b2

– regression coefficients

z axis is fixed

x and y axis are redefined

based on b1 and b2 in each

averaging period

H – sensible heat ρa – moisture air density

Cp – specific heat of air w - vertical wind speed

Ts – sonic temperature

F – uncorrected CO2 , H2O,or CH4 flux

c - CO2 , H2O,or CH4 molar density

λ - specific evaporation heat FH2O – water vapor flux

CO2 , H2O, or CH4 :

Latent heat :

Sensible heat :

F cw

LE F OH2

pc 'TswaH

Uncorrected Flux Computation

Ts - Sonic temperature (K)

Ta - Air temperature (K)

e - Water vapor partial pressure (kPa)

p - Air pressure (kPa)

Q – Specific humidity (mass ratio of water vapor to dry air)

Sonic Temperature Correction

Ts = Ta (1+0.32e/p)

Ts = Ta (1+0.51Q)

Frequency Response Correction

➢ Assumptions:

- Wind speed and mixing ratio measurements

Same time, same point

- All the eddies are measured

➢ The reality:

- Sensor separation

- Measurement path length

- Measurement frequency:

10 Hz – Loss of high frequency eddies;

30-minute averaging – Loss of low frequency eddies

➢ The problem solver:

- Frequency response correction

-4

-3

-2

-1

0

1

2

3

4

0 200 400 600 800 1000

Sin

ga

l

Time (s)

-1.5

-1

-0.5

0

0.5

1

1.5

0 200 400 600 800 1000

Sin

ga

l

Time (s)

Sin(x)

0.5sin(x)

0.1sin(x)

0.25sin(4x)

0.1sin(10x)

0.01sin(20x)

0.05sin(30x)

0.5sin(50x)

Fourier Data Transfer for Frequency Correction

X – time (second)

K – frequency (Hertz)

i – phasor

Time Frequency:

Frequency time :

Frequency Response Correction

Estimation of transfer functions:

(Input -> [tran. function] -> output)

Calculation of a true flux spectrum

- Moncrieff et al, 1997, Journal of Hydrology, 188-189: 589-611

Estimation of flux attenuation

Calculation of the spectral

correction factor

➢ Definition:- Mixing ratio is used in the eddy covariance formula

➢ The reality:

- Analyzers measure density instead of mixing ratio

- Density is affected by variations in temperature and humidity

➢ The problem solver:

- WPL correction

WPL (Webb-Pearman-Leuning) Correction

Stage

1

Stage 2Warming

Temperature

CO2 Density

CO2 Mixing Ratio

Open-path system CO2 and H2O fluxes:

Open-path system CH4 flux:

Closed-path system CO2 and H2O fluxes:

1) Using H = 0 and uncorrected E; 2) Using mixing ratio (LI-7200)

WPL Correction

𝑅𝑁𝑐𝑜𝑣 =

σ ƴ𝑤 ƴ𝑥𝑛𝑛 − ƴ𝑤 ƴ𝑥

ƴ𝑤 ƴ𝑥

II. Developed Turbulent Conditions tests Integral Turbulence Characteristics (ITC)

Time series is steady when RNcov<30%

𝐼𝑇𝐶𝜎 =𝐼𝑇𝐶𝑚𝑜𝑑𝑒𝑙 − 𝐼𝑇𝐶𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑

𝐼𝑇𝐶𝑚𝑜𝑑𝑒𝑙

Well developed turbulence when ITCσ < 30%

I. Steady State Tests Determine stationarity

Flux Quality Flags

➢ The normalized standard deviations of turbulent parameters over all frequencies of

the turbulent spectrum

➢ Measured: Modeled:

C1 and C2 – constants, Z – measurement height, L – Monin-Obukhov length, u* - Friction velocity

RNcov ITCσ QA/QC FLAG DATA QUALITY

≤30 ≤30 0 High

≤100 ≤100 1 Moderate

>100 >100 2 Low

Overall quality flags after Spoleto agreement, 2004 for CarboEurope-IP

Flux Quality Flags

Flux Footprint

I. EdiRe, University of Edinburgh, UK

2. ECpack, Wageningen University, the Netherlands

3. TK3, University of Bayreuth, Germany

4. Alteddy, Wageningen University, the Netherlands

5. EddySoft, Max Planck Institute for Biogechemistry, Germany

6. EddyUH, University of Helsinki , Finland

7. Self-written computer programs using Fortran or MATLAB

8. EddyPro , LI-COR Bioscience, released in April 2011

Data Processing Software

➢ Process CO2, H2O, CH4, N2O, CO… and energy flux data

➢ Support various raw flux data formats(GHG, ASCII, and TOB1)

, including biological and meteorological (Biomet) data

(radiation, soil heat flux ....)

➢ Express and Advanced modes

➢ Open source and free

➢ With LI-COR support

➢ Over 3800 downloads in 155 countries

➢ Flux networks are adapting EddyPro as a standard software

for flux processing

EddyPro Introduction

On-site Flux Data Processing - SMARTFlux

Thank You

Questions?