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Three-dimensional flows and NEE: Results from the Chequamegon Ecosystem-Atmosphere Study (ChEAS)

This study investigates the impact of three-dimensional flows on net ecosystem-atmosphere exchange (NEE) of carbon dioxide (CO2) in the Chequamegon ecosystem. It explores the contribution of humungous nighttime fluxes (HNFs) and advection terms on NEE, and highlights the uncertainties and systematic errors in estimating NEE.

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Three-dimensional flows and NEE: Results from the Chequamegon Ecosystem-Atmosphere Study (ChEAS)

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  1. Three-dimensional flows and NEE: Results from the Chequamegon Ecosystem-Atmosphere Study (ChEAS) Ken Davis, Weiguo Wang, Chuixiang Yi and others, The Pennsylvania State University Paul Bolstad, Bruce Cook and Jon Martin, University of Minnesota Peter Bakwin, NOAA/CMDL Jud Isebrands and Ron Teclaw, USDA-FS

  2. North Upland, wetland, and very tall flux tower. Old growth tower to the NE. 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  Willow Creek General Agriculture

  3. Progress • HNFs (humungous nighttime fluxes) identified and isolated. Very large contribution to NEE of CO2. Regional vent of nighttime drainage flows? • Advection terms computed from the ChEAS “mesonet” of CO2(x,z). Nighttime contributions of ~10% • Multiple flux levels at WLEF yield upper limit of ~20 gC m-2 yr-1 uncertainty in annual NEE of CO2.

  4. Humungous nighttime fluxes • Peak fluxes (turbulent flux, not rate of change of storage) of ~ 80 mmol m-2 s-1 detected under easterly winds and light stability. Strictly limited to easterly winds. Fluxes persist for several hours. • Impact on cumulative NEE of CO2 is large. If screened, annual sum ~ - 400 gC m-2 yr-1. No screening, sum ~ - 130 gC m-2 yr-1.

  5. Hypothesized flow

  6. Computing net ecosystem-atmosphere exchange (NEE)

  7. Mesonet advection calculations • Use only hours with winds along the line defined by WLEF and Willow Creek. • Assumes that CO2(x) is well-described by two points separated by ~15km. • Vertical advection computed both as a residual, and directly via sonic mean vertical velocity and WLEF CO2(z). • Hourly sonic resolution of 0.06 m s-1 determined from two dual-sonic deployments.

  8. Mesonet advection calculations Integrated effect on NEE is a 10% underestimate of nighttime flux. Order 50 gC m-2 yr-1 if extrapolated over a year

  9. Hours (LST) 30m 122m 396m Preferred gC m-2 d-1 All Day 0.89 1.18 1.08 -2.25 All Day w/ D6-9 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 June-August 1997 diurnal mean cumulative NEE at WLEF vs. level (Fraction of preferred NEE)

  10. 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 1997 Cumulative NEE, GEP and RE vs. assumptions and methods (gC m-2 yr-1 = tC ha-1 yr-1 * 100)

  11. WLEF 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

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