ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Eddy-covariance flux measurements: Outline for the day Morning ChEAS sites

Department of Meteorology, Penn State University ChEAS workshop. August, 2002

Eddy-covariance flux measurements: Outline for the day

Morning• ChEAS sites and current research projects• Methods (Berger et al, 2001; Yi et al, 2000)• Results

– Published or in press• Yi et al 2000, Davis et al, Cook et al.

– In preparation (with hypotheses)• Various, PSU research group members

• Future plans/proposals


Eddy-covariance flux measurements: Outline for the day

Afternoon• Eddy covariance flux calculation and Li-Cor demo• Small group discussion. Suggested topics:

– Causes of interannual variability at WLEF– Causes of differences among ChEAS tower flux

measurements– Potential for instrument bias, errors, and improvements– Extension of interannual variability studies beyond ChEAS– Uses of sub-canopy flux and turbulence measurements– Two-dimensional flux experiments and analyses– Caterpillars – observed or imagined?


ChEAS eddy covariance flux measurements

I: Sites and research projects


Chequamegon Ecosystem-Atmosphere Study (ChEAS) flux towers

WLEF tall tower (447m)CO2 flux measurements at: 30, 122 and 396 mCO2 mixing ratio measurements at: 11, 30, 76, 122, 244 and 396 m

Forest stand flux towers: Mature deciduous upland (Willow Creek) Deciduous wetland (Lost Creek) Mixed old growth (Sylvania)All have both CO2 flux and high precision mixing ratio measurements.


WISCONSIN

Coniferous

Mixed deciduous/coniferous

Wetland

Open water

Shrubland

General Agriculture

Willow Creek

WLEF

Lost Creek

Landcover key

North

Upland, wetland, andvery tall flux tower. Oldgrowth tower to the NE.

High-precision CO2

profile at each site.

Mini-mesonet, 15-20kmspacing between towers.


View from 396m above Wisconsin: WLEF TV tower


ChEAS web sites• http://cheas.psu.edu - Main page.• ftp://ftp.essc.psu.edu/pub/workgroup/davis/ - Data

access.• http://cheas.psu.edu/fieldsites.html - Site descriptions.Also see:• http://www.daac.ornl.gov/FLUXNET/fluxnet.html -

Fluxnet’s home page, and,• http://public.ornl.gov/ameriflux

/Participants/Sites/Map/index.cfm - AmeriFlux’s home page.




Data availabilitySite Start date Full years of data

WLEF Spring 1995 1997-2001

Willow Creek Summer 1999 2000-2001

Lost Creek Fall 2000 2001

Sylvania Summer 2001

Ceilometer Summer 1998 1998-2001

Radar Spring-Fall, 1998; Spring-Fall, 1999


Research Funding• Regional atmosphere/forest exchange and concentrations of carbon dioxide.

– Study of net ecosystem exchange of carbon dioxide via eddy covariance measurements at the WLEF tower in northern Wisconsin, as well as the study of carbon dioxide transport and distribution within the boundary layer.

– PI: P.S. Bakwin, U. Colorado/NOAA– Co-I: K.J. Davis Penn State– Department of Energy, National Institutes for Global Environmental Change– Duration: July, 1994–June, 1997; July, 1997–June, 2000; July, 2000–June 2003.

• Measuring and modeling component and whole-system CO2 fluxes at local to regional scales.

– Study of component processes which make up CO2 fluxes in a forest ecosystem, and comparison to whole-ecosystem net flux measurements from small flux towers. Also a comparison between homogeneous ecosystem fluxes within the WLEF tower footprint and the WLEF net flux signal.

– PI: P.V. Bolstad, U. Minnesota– Co-Is: K.J. Davis, Penn State, and P.B. Reich, U. Minnesota– Department of Energy, National Institutes for Global Environmental Change– Duration: July, 1997 - June, 2000. July 2000 – June 2003.


Research Funding• Quantifying carbon sequestration potential of mid and late successional forests in the upper Midwest

– Observations of CO2 exchanges in an old-growth forest in the upper peninsula of Michigan and comparison to existing flux towers in younger forest stands northern Wisconsin. Davis's work would provide tower construction, instrumentation, and data analysis support for a 30m tower.

– PI: Eileen Carey, U. Minnesota– Co-Is: P.V. Bolstad, U. Minnesota; K.J. Davis, Penn State– Department of Energy, Terrestrial Carbon Processes– Duration: January, 2001 - December, 2003

• Regional forest-ABL coupling: Influence on CO2 and climate– Study of the coupling between the surface energy balance, boundary layer development, and net

ecosystem exchanges of carbon dioxide, as well as the influence of the covariance between carbon dioxide fluxes and boundary layer development on boundary layer mixing ratios of carbon dioxide. Observations at the WLEF and the Walker Branch AmeriFlux sites using an NCAR radar.

– PI: K.J. Davis, Penn State– Co-I: A.S. Denning, Colorado State University– Department of Energy, TECO/Terrestrial Carbon Program– Duration: September, 1997 - August, 2002



II: Methods


Methods: Eddy covariance flux measurements in ChEAS

• Basic theory of eddy covariance flux measurements. • Tower flux instrumentation• LI-COR calibration • Sonic rotation• Lag time correction • Spectral corrections• Random and systematic errors due to turbulence• “Preferred” NEE algorithm (WLEF only)• Filling missing data


Publications describing methodology

• Yi, C., K.J. Davis, P.S. Bakwin, B.W. Berger, and L. C. Marr,

2000. The influence of advection on measurements of the net ecosystem-atmosphere exchange of CO2 observed from a very tall tower, J. Geophys. Res. 105, 9991-9999.

• Berger, B.W., K.J. Davis, P.S. Bakwin, C. Yi and C. Zhao, 2001. Long-term carbon dioxide fluxes from a very tall tower in a northern forest: Flux measurement methodology. J. Atmos. Oceanic Tech., 18, 529-542.

• Davis, K.J., P.S. Bakwin, B.W. Berger, C. Yi, C. Zhao, R.M. Teclaw and J.G. Isebrands, The annual cycles of CO2 and H2O exchange over a northern mixed forest as observed from a very tall tower. Global Change Biology, in press.


Theory

Flux of C across this plane

+ Rate of accumulation of C below the flux sensor

= Net Ecosystem-Atmosphere Exchange (NEE) of C

Net sideways transport = 0


Theory

Ci

i

i

i Sx

CU

x

CU

t

C

''

Time rate ofchange (e.g. CO2)

Mean transport

Turbulenttransport (flux)

Source in theatmosphere

Integrate from the earth’s surface to the imaginary planedefined by the level of the flux sensor.

“Reynold’s averaged” (= mean + turbulent components of all variables) scalar conservation equation.


Theory

turbulentstorage

advectionturbulentstorage

zs

zs

zs

FFNEE

FFFNEE

dzx

Cu

z

CW

x

CU

Cwdzt

CNEE

0

0

''

''

0

CCz NEEFcw 00''

Yi et al, 2000


Theory: What is w’c’?

• Prime indicates departure from the mean.

• w’ > 0 is an updraft

• c’ > 0 is air rich in the scalar c

• w’c’ > 0 is upwards transport of the scalar

• Averaging this over time sums the transport observed due to all updrafts and downdrafts.


Radar ABL depth

WLEF fluxes

CO2 profile

Davis et al, in press

Daily cycle of ABL depth, and CO2 fluxes and mixing ratios


Diurnal cycle of CO2 in the ABL

Bak

win

et a

l, 19

98


Instruments at WLEF

Ber

ger

et a

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01


Instruments at WLEF• Two “profiling” LI-CORs in the trailer, one sampling 396m,

one cycling among all 6 levels. “Slow” time response. High-precision and accuracy calibration (Bakwin et al, 1998). C-bar.

• Vaisala humidity and temperature sensors at 3 levels (30, 122 and 396m). “Slow” Q-bar, T-bar.

• Three sonic anemometers (30, 122 and 396m). w’, T’• Three LI-CORs in the trailer, one for each sonic level. “Fast”

time response. Long tubes, big pumps. Measure CO2 and H2O. c’, q’

• Two LI-CORs on the tower (122 and 396m). “Fast” time response. Short tubes, smaller pumps.


Calibration of “fast” CO2 and H2O sensors at ChEAS towers

• Calibration occurs using the fluctuations in the ambient atmospheric CO2 and H2O mixing ratios.

• “Slow” sensors provide absolute values of these mixing ratios used to calibrate the “fast” LI-CORs.

• Ideal gas law corrections to LI-COR cell temperature, pressure and humidity are applied.

• Calibration slope and intercept are derived every 2 days. These values are smoothed (monthly running mean) to derive the long-term calibration factors used for the “fast” LI-CORs.


Calibration of “fast” CO2 and H2O sensors

Ber

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What’s up? (Sonic rotations)

• Sonic anemometers are oriented perfectly in the vertical, (and the wind’s “streamlines” aren’t always perpendicular to gravity).

• Data is collected over a long time (about a year) and we define “up” by forcing the mean vertical wind speed to be zero.


Sonic rotations

Ber

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Lag time calculation• We must correct for the delay between the CO2 and H2O

measurements and the vertical velocity measurements.

• Lag time is determined by finding the maximum in the lagged covariance between vertical velocity and CO2/H2O for every hour.

Level (m) IRGA position

Tube length (m)

Lag time (s)

Tube inner diameter (m)

Flow rate (L min-1)

Reynolds number

396 Trailer 406 87 0.009 17.8 2640

122 Trailer 132 23 0.009 21.9 3250

30 Trailer 40 16 0.009 9.5 1420

396 Tower 5 1.7 0.0032 1.4 592

122 Tower 5 1.1 0.0032 2.2 915

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Lag time calculation

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Spectral corrections• Flow through tubes smears out some of the atmospheric

fluctuations, especially the small (high frequency) eddies.– Obvious for H2O. Much worse than theory predicts.– Not directly observed for CO2. Small effect.

• The sonic anemometer (virtual) temperature measurement is not smeared out, so we use similarity between the virtual temperature spectrum and the water vapor spectrum to correct for the loss of high frequency eddies in H2O.

• We use past studies of flow in tubes to correct for the loss of high frequency eddies in CO2.


Spectral corrections

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CO2

H2O

Tv


Spectral corrections

Level (m)

IRGA position

CO2 (day)

CO2 (night)

H2O

396 Trailer 1 7 16

122 Trailer 1.5 9 19

30 Trailer 5 12 21

396 Tower <0.1 1 13

122 Tower <0.1 1 11

Table shows the typical % of flux lost due to smearing of small eddies.

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Systematic errors – getting the large and small eddies

Berger et al, 2001


Random errors – a finite number of eddies are counted in one hour

Random sampling errors for any one hour can be as large asthe magnitude of the measured flux!

Berger et al, 2001, following Lenschow and Stankov, 1986.


“Preferred” NEE (WLEF only)• Data is taken from 30m at night and 122 or 396m during

the day (the highest level where there is turbulent flow) when all data are available.

• If data are missing, any existing flux measurement is used.• Data are screened out when the level of turbulence is very

low. CO2 is probably draining down hill.• Early in the morning upper level data from WLEF is

replaced with 30m data (Yi et al, 2000) because the flow appears to be systematically 2-D.

• Thus from 3 NEE measurements, one is derived as our “preferred” measurement for each hour.


Nighttime drainage flows?

Coo

k et

al,

subm

itte

d; D

avis

et a

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pre

ss

Loss of flux at low turbulence levelsat the Willow Creek tower.


Morning advection at WLEF

Yi e

t al,

2000


Multiple level comparison at WLEF

Dav

is e

t al,

in p

ress

Comparison of all 3 levels, growing season 1997.



III: Results


Missing data, gross fluxes, light and temperature response

• Nighttime NEE measurements (for CO2) are fitted to soil or air temperature. This is assumed to describe the total respiration flux.

• Daytime NEE measurements are fitted to PAR after total respiration has been computed using the fits and “subtracted” from NEE. This fit describes the response of forest photosynthesis to sunlight.

• These fits are used to compute gross fluxes (respiration, photosynthesis) and to fill in missing NEE data needed to compute cumulative NEE.


Gross fluxes and functional fits

1

02

210 ))(exp(

bPAR

PARbbNEEREGEP

aTaaRE

GEPRENEE

s

Dav

is e

t al,

in p

ress


Example of grossfluxes fit to temperature andPAR at WLEFfor one month.

Davis et al, in Press.


Hourly fluxesat WLEF for1997, observedand filled.

Davis et al, in press.


Cumulative fluxes at WLEF, 1997



Gross fluxes at WLEF, 1997

Dav

is e

t al,

in p

ress

RE

-GEP

NEE


(gC m-2 yr-1 = tC ha-1 yr-1 * 100)

1997 Cumulative NEE, GEP and RE vs. assumptions and methods

867891-25 +/- 17Low U* screened, median fill

816864-48 +/- 20Low U* retained

96394716 +/- 19Low U* screened,

T-PAR fill

REGEPNEEMethod

Dav

is e

t al,

in p

ress


Lack of energy balance: Are turbulent fluxes underestimated?

Dav

is e

t al,

in p

ress

Coo

k et

al,

subm

itte

d


Monthly mean CO2

fluxes at WLEF, 1997.



Monthly mean latentheat fluxes at WLEF, 1997.



Bowen ratiovs. time of year at WLEF, 1997.

Davis et al,in press,following Cook et al,submitted.


Drainage during stable conditions: What goes down must come up

(somewhere).


Very largepositive turbulent fluxes from about 150 degrees.

Blocking offlow.

Occur duringwindy, weaklystable conditionswhen the canopyis decoupledfrom the ABL.

Cook et al, sub.


Is this venting of drainage?Can we capture these events

across the landscape?


-800

-700

-600

-500

-400

-300

-200

-100

0

100

200

0 50 100 150 200 250 300 350

Day of year

Cum

ula

tive N

EE

(g C

m-2)

no screening

screened for low u* andturbulent venting

screened for low u* only

screened for low u* and turbulent venting;corrected for energy-balance closure

Cook et al, submitted to Global Change Biology: Willow Creek


Courtesy D. Hollinger


Early leaf-out, 1998, Wisconsin


Impact on atmospheric [CO2]


Spatial coherence of seasonal flux anomalies

A similar pattern isseen at several fluxtowers in N. Americaand Europe.

Three sites have high-quality [CO2] measurements + dataat Fluxnet (NOBS,HF, WLEF).

The spring 98 warm period and a later cloudy period appear at all 3 sites.

Tem

pC

O2

NE

E

Day of year80 200


Detection of the spring 98 anomaly via oceanic flasks?

2 Alaskan flasksites have slightlyhigher [CO2] inthe spring of 98.

Mace Head, Irelandshows a depression of [CO2] in thespring of 98.

Potential exists to link flux towers with seasonal inverse studies.


Synoptic variability in CO2


North American Carbon Plan(NACP)

http://www.carboncyclescience.gov

Documents

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Eddy-covariance flux measurements: Outline for the day Morning ChEAS sites