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
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
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?
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
ChEAS eddy covariance flux
measurements
I: Sites and research projects
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Chequamegon Ecosystem-Atmosphere
Study (ChEAS) flux towers
WLEF tall tower (447m)
CO2 flux measurements at:
30, 122 and 396 m
CO2 mixing ratio measurements at:
11, 30, 76, 122, 244 and 396 m
Department of Meteorology, Penn State University
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.
ChEAS workshop. August, 2002
Upland, wetland, and
very tall flux tower. Old
growth tower to the NE.

North
WISCONSIN
High-precision CO2
profile at each site.

Mini-mesonet, 15-20km
spacing between towers.
Lost Creek
Landcover key
Open water
 WLEF
Wetland
Coniferous
Mixed deciduous/coniferous

Shrubland
General Agriculture
Willow Creek
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
View from 396m above Wisconsin: WLEF TV tower
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
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.
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Data availability
Site
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
Radar
Spring-Fall, 1998; Spring-Fall, 1999
Department of Meteorology, Penn State University
1998-2001
ChEAS workshop. August, 2002
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.
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
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
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
ChEAS eddy covariance flux
measurements
II: Methods
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
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
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
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.
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Theory
Flux of C across this
plane
+
Rate of
accumulation of C
below the flux sensor
Net sideways
transport = 0
=
Net EcosystemAtmosphere
Exchange (NEE) of C
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Theory
“Reynold’s averaged” (= mean + turbulent components of
all variables) scalar conservation equation.
U C
C
C
U i

 SC
t
xi
xi
'
i
Time rate of
change (e.g. CO2)
Mean
transport
'
Turbulent
transport (flux)
Source in the
atmosphere
Integrate from the earth’s surface to the imaginary plane
defined by the level of the flux sensor.
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Theory
w' c' z 0  F0  NEE
C

C
C
NEE  
dz  w'C '
t
0
zs

zs
'
' 
 C

C

u
C
dz
  U
W


x
z
x 
0 

NEE  Fstorage  Fturbulent  Fadvection
zs
NEE0  Fstorage  Fturbulent
Department of Meteorology, Penn State University
Yi et al, 2000
ChEAS workshop. August, 2002
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.
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Daily cycle of ABL depth, and CO2 fluxes and mixing ratios
Radar ABL depth
WLEF fluxes
CO2 profile
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Davis et al, in press
Bakwin et al, 1998
Diurnal cycle of CO2 in the ABL
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Berger et al, 2001
Instruments at WLEF
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
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.
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
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.
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Berger et al, 2001
Calibration of “fast” CO2 and H2O
sensors
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
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.
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Berger et al, 2001
Sonic rotations
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Lag time calculation
Level (m)
IRGA
position
Tube
length (m)
Lag time
(s)
Tube inner Flow rate
diameter
(L min-1)
(m)
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
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Berger et al, 2001
• 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.
Berger et al, 2001
Lag time calculation
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
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.
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
CO2
Tv
H2O
Department of Meteorology, Penn State University
Berger et al, 2001
Spectral corrections
ChEAS workshop. August, 2002
Spectral corrections
Level
(m)
396
IRGA
position
Trailer
CO2
(day)
1
CO2
(night)
7
H2O
122
Trailer
1.5
9
19
30
Trailer
5
12
21
396
Tower
<0.1
1
13
122
Tower
<0.1
1
11
Department of Meteorology, Penn State University
16
Berger et al, 2001
Table shows the typical % of flux lost due to smearing of small eddies.
ChEAS workshop. August, 2002
Systematic
errors –
getting the
large and
small eddies
Berger et al, 2001
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Random errors – a finite number
of eddies are counted in one hour
Random sampling errors for any one hour can be as large as
the magnitude of the measured flux!
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
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.
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Loss of flux at low turbulence levels
at the Willow Creek tower.
Department of Meteorology, Penn State University
Cook et al, submitted; Davis et al, in press
Nighttime drainage flows?
ChEAS workshop. August, 2002
Yi et al, 2000
Morning advection at WLEF
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Davis et al, in press
Multiple level comparison at
WLEF
Comparison of all 3 levels, growing season 1997.
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
ChEAS eddy covariance flux
measurements
III: Results
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
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.
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
NEE  RE  GEP
RE  a0 exp(  a1 (Ts  a2 ))
b0 PAR
GEP  RE  NEE  b2 
PAR  b1
Department of Meteorology, Penn State University
Davis et al, in press
Gross fluxes and functional fits
ChEAS workshop. August, 2002
Example of gross
fluxes fit to
temperature and
PAR at WLEF
for one month.
Davis et al, in
Press.
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Hourly fluxes
at WLEF for
1997, observed
and filled.
Davis et al,
in press.
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Cumulative fluxes at WLEF, 1997
Davis et al, in press
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
RE
NEE
-GEP
Department of Meteorology, Penn State University
Davis et al, in press
Gross fluxes at WLEF, 1997
ChEAS workshop. August, 2002
Method
Low U*
screened,
T-PAR fill
Low U*
retained
Low U*
screened,
median fill
NEE
GEP
RE
(gC m-2 yr-1 = tC ha-1 yr-1 * 100)
16 +/- 19
947
963
-48 +/- 20
864
816
-25 +/- 17
891
867
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Davis et al, in press
1997 Cumulative NEE, GEP and RE
vs. assumptions and methods
Davis et al, in press
Cook et al, submitted
Lack of energy balance: Are
turbulent fluxes underestimated?
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Monthly mean CO2
fluxes at WLEF,
1997.
Davis et al, in press
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Monthly mean latent
heat fluxes at WLEF,
1997.
Davis et al, in press
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Bowen ratio
vs. time of
year at
WLEF, 1997.
Davis et al,
in press,
following
Cook et al,
submitted.
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Drainage during stable conditions:
What goes down must come up
(somewhere).
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Very large
positive
turbulent fluxes
from about
150 degrees.
Blocking of
flow.
Occur during
windy, weakly
stable conditions
when the canopy
is decoupled
from the ABL.
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Cook et al, sub.
Is this venting of drainage?
Can we capture these events
across the landscape?
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Cook et al, submitted to Global Change Biology: Willow Creek
200
100
0
-2
Cumulative NEE (g C m )
screened for low u* only
-100
-200
no screening
-300
screened for low u* and
turbulent venting
-400
-500
-600
screened for low u* and turbulent venting;
corrected for energy-balance closure
-700
-800
0
50
100
150
200
250
300
350
Day of year
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Courtesy D. Hollinger
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Early leaf-out, 1998, Wisconsin
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Impact on atmospheric [CO2]
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Temp
Three sites have
high-quality [CO2]
measurements + data
at Fluxnet (NOBS,
HF, WLEF).
NEE
A similar pattern is
seen at several flux
towers in N. America
and Europe.
CO2
Spatial coherence of seasonal flux anomalies
Department of Meteorology, Penn State University
80
Day of year
200
The spring 98 warm
period and a later
cloudy period appear
atChEAS
all 3workshop.
sites. August, 2002
Detection of the spring 98 anomaly via oceanic flasks?
2 Alaskan flask
sites have slightly
higher [CO2] in
the spring of 98.
Mace Head, Ireland
shows a depression
of [CO2] in the
spring of 98.
Potential exists to link
flux towers with
seasonal inverse
studies.
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
Synoptic variability in CO2
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002
North American Carbon Plan
(NACP)
http://www.carboncyclescience.gov
Department of Meteorology, Penn State University
ChEAS workshop. August, 2002