Ogive

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  • Published : May 28, 2013
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Title

Some considerations about the

Application of ogive analysis for quality control and spectral correction of eddy covariance fluxes Christof Ammann, Albrecht Neftel

Content

Content
 Motivation for spectral/ogive analysis: methodological flux measurement problems at the Swiss CarboEurope grassland site  Some basics about ogives  Application examples:  High-frequency damping/correction  Analysis for intermittency/stationarity of turbulence  Conclusions

Site region

Measurement Site near Oensingen

0

N

annual distribution of wind directions

Site plots/fetch

Experimental Plots
200
0

N
150 100

EC systems

local SN scale [m]

Hi
50

g

y wa h

0

-50

- aerodyn. measurement height: z = 0.7 –1.2 m - sensor separation (lateral): s = 16 cm

-100

-150

-200 -300

-250

-200

-150

-100

-50

0

50

100

local WE scale [m]

Site wind

Nocturnal Wind Conditions
50.0 10

CO2 flux windspeed
8

CO2 flux [umol/m2/s]

windspeed ca. 1m above ground (Jun-Oct 2002)

6 0.0 4

frequency of occurrence [rel. units]

daytime nighttime

2

-50.0 20.07.2002 00:00

20.07.2002 12:00

21.07.2002 00:00

21.07.2002 12:00

22.07.2002 00:00

22.07.2002 12:00

0 23.07.2002 00:00

0

1

2

3

4

5

6

7

wind speed [m/s]

windspeed [m/s]

Site motivation

Motivation for spectral/ogive analysis
 Relatively small fields  low measurement height:  strong high-frequency damping  footprint problems (cf. talk by A. Neftel)  Frequent calm nights  weak/intermittent turbulence, non-stationary conditions  enhanced dew formation (on open-path sensor)

? Are parameterised spectral models for the correction of high-frequency damping accurate enough? ? Are integral turbulence characteristics (e.g.  /u*) specific enough for w optimum quality control?

Intro Ogive0

Wh tsa “ a i n ogive” ?
 statistics: cumulative frequency distribution curve (polygon)  architecture: diagonal rib of a gothic vault (or pointed arch)  in the context of atmos. turbulence: cumulative cospectrum (or power spectrum) 1.0

cumulative cospectral flux [rel. units]

ogive of cospectrum (rel. units) 0.8 Kaimal cospectrum (rel. units) 0.6

0.4

0.2

0.0 0.0001

0.001

0.01

0.1 normalised frequency n = f z/u

1

10

100

 ogive analysis = (integral) spectral analysis

Intro Ogive1

Characteristics of ogives
from: N. Beier and M. Weber (1992): Final Report of Research Project TUAREG (turbulent exchange processes in the boundary layer)

 ogives provide an effective visualisation of relevant spectral information - proportional to flux contribution - with integral smoothing effect  ogives allow a direct simple reduction of spectral information

Intro Ogive2

Application of ogive analysis
 Test of adequate flux averaging interval (land-, sea-, and aircraft based studies) proposed e.g. by Friehe et al. (1988), Oncley (1989), Brooks and Rogers (1997)

 Test/adjustment of delay time between vertical windspeed and scalar quantity Beier and Weber (1992)

 Test of stationarity of turbulence
Schulz and Sanderson (2004)

 Test and correction of high-frequency damping
Spirig et al. (2005)

Analysis criteria
 low-frequent ogive values/slope  turning points, flux contributions with opposite sign  overall ogive shape (comparison with Kaimal spectra or different scalar fluxes)

Ogive example

Daytime ogives (CO2 flux) for different wind speeds
1.2 green shades: 6 July 11:30-13:00 (u=3.4m/s) 1 red shades: 7 July 11:30-13:00 (u=0.85m/s) 0.8

normalised cumulative cospectrum of CO2 flux

0.6

0.4

0.2

0

-0.2 0.0001

0.001

0.01

0.1

1

10

frequency [Hz]

HF Moore theo

Damping of cospectra according to Moore (1986)
0.4 Kaimal cospectrum (z/L
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