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 University2NOAA CMDL, Boulder

3Colorado State University4USDA 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 inter-annual 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 Ecosystem-Atmosphere 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 ecosystem-atmosphere exchange (NEE)

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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)

30m 122m 396m Preferred gC m-2 d-1

All Day 0.89 1.18 1.08 -2.25

All Day w/ -9

0.98 0.96

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

(Fraction of preferred NEE)

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

Method NEE GEP RE

Low U* screened,

T-PAR fill

16 +/- 19 -1909 1924

Low U* retained

-48 +/- 20 -1681 1634

Low U* screened, median fill

-25 +/- 17 -1758 1733

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

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

• DoE – NIGEC – Midwest and Great Plains

• NOAA CMDL

• NASA – EOS Validation

• DoE – TCP/TECO

• NSF/NCAR

• NASA/NOAA GEWEX

• USDA-FS