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

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

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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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