Transcript Multiple flux footprints, flux divergences and boundary

Multiple flux footprints, flux divergences
and boundary layer mixing ratios:
Studies of ecosystem-atmosphere CO2
exchange using the WLEF tall tower.
K.J. Davis1, P.S. Bakwin2, C. Yi1, B.D. Cook1, W.
Wang1, A.S. Denning3, R. Teclaw4 and J.
Isebrands4
1The
Pennsylvania State University
2NOAA CMDL, Boulder
3Colorado State University
4USDA Forest Service, Rhinelander
Background
• The global atmospheric CO2 cycle is not
closed. A missing sink exists.
• The magnitude of the sink is highly variable
from year to year.
• Northern terrestrial ecosystems are thought
to contribute both to the sink and its interannual variability.
• Tower-based eddy covariance flux
measurements provide direct observations
of ecosystem-atmosphere CO2 exchange.
Problems
• Flux tower NEE observations are troubled
with concerns about systematic errors.
• The footprints of eddy covariance flux
measurements are very small compared to
biomes and continents (the scale at which
we know there is a missing terrestrial sink).
Questions
• What can a tall tower (WLEF) tell us about
net ecosystem-atmosphere exchange (NEE)
of CO2 (that a small tower cannot?)?
• Can a tall tower bridge the gaps between
flux towers and atmospheric CO2
distributions used for inversion models?
Unique goals of WLEF
• Observe NEE of CO2 over a very large area
compared to a “small” tower. Include a
heterogeneous landscape for up-scaling
experiments.
• Directly observe boundary layer flux
divergence to study surface-ABL coupling.
• Merge CO2 flux and mixing ratio data at a
single continental site.
Chequamegon EcosystemAtmosphere Study (ChEAS) Region
• Vegetation
~ 70% upland forest: Aspen, maple, balsam fir,
red pine
~ 30% wetlands: alder, white cedar, tamarack,
black spruce, willow
• Terrain
– Sandy loam glacial soils
– Typical relief < 20m, few 100m across
– Vegetation cover follows the terrain + logging
ChEAS Studies
• WLEF tall tower (fluxes, flasks, O2, isotopes)
• Upland, wetland and old growth flux towers
• EOS validation site
• Sub-canopy microclimate network
• Soil and leaf CO2 flux measurements
• Sap flow measurements
• Boundary layer depth and cloud monitoring
• Airborne atmospheric CO2 profiling
• Land surface modeling of fluxes
• Mesoscale to global atmospheric modeling
(NOAA, PSU, UWisc, UMinn, USFS, CSU, UColo,
NCAR, Harvard, UUtah, NMSU)
Sources of error in cumulative NEE
measurements at WLEF
• Random
– Turbulent flux sampling (bigger eddies = bigger
errors)
– Weather (for seasonal to annual integrals of
NEE)
• Systematic
–
–
–
–
Dependence of fluxes on wind direction
Nighttime drainage flows
Persistent advection (tall tower helps!)
Methods of filling missing data
Computing net ecosystematmosphere exchange (NEE)
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NEE  Fstorage  Fturbulent  Fadvection
zs
NEE0  Fstorage  Fturbulent
Detecting advection
NEE0  NEE0 ( z2 )  NEE0 ( z1 )
' ' 
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Advection is revealed via the difference in NEE
between flux measurement at different levels.
June-August diurnal mean
cumulative NEE at WLEF vs. level
Hours
(LST)
All Day
30m
122m
396m
(Fraction of preferred NEE)
0.89
1.18
1.08
0.98
0.96
Preferred
gC m-2 d-1
-2.25
All Day
w/ 6-9
5-10
0.98
1.29
1.16
-1.87
11-14
0.90
0.94
1.06
-2.01
15-18
1.004
1.01
1.16
-0.58
19-4
1.00
1.01
1.15
2.21
1997 Cumulative NEE, GEP and RE
vs. assumptions and methods
Method
NEE
GEP
RE
(gC m-2 yr-1 = tC ha-1 yr-1 * 100)
Low U*
screened,
T-PAR fill
Low U*
retained
16 +/- 19
-1909
1924
-48 +/- 20
-1681
1634
Low U*
screened,
median fill
-25 +/- 17
-1758
1733
Summary
• WLEF region 1997 annual NEE is about 0!
• Identified systematic uncertainties
– Different levels: footprint/advection
– order 20 gC m-2 yr-1
– U* screen – order 50 gC m-2 yr-1
– Wind direction – didn’t appear to be large
– But surface energy balance isn’t obtained.
• Random errors (weather + sampling)
– Order 20 gC m-2 yr-1.
• GEP and RE values are very significant
Summary (continued)
• WLEF has much lower summer uptake
rates and cumulative NEE than most
AmeriFlux deciduous forest sites. Why?
– Wetlands? / Less productive landscape?
– Errors in our measurements?
– Tall tower yields different results than small
towers?
• Willow Creek (hardwood) data supports the
wetlands hypothesis. Lost Creek (wetland)
data is on the way! Helen Lake (old
growth) will follow.
Acknowledgements
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DoE – NIGEC – Midwest and Great Plains
NOAA CMDL
NASA – EOS Validation
DoE – TCP/TECO
NSF/NCAR
NASA/NOAA GEWEX
USDA-FS