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. Coniferous Mixed deciduous/coniferous Wetland Open water Shrubland General Agriculture Willow Creek WLEF Lost Creek Landcover key North Upland, wetland, and very tall flux tower. Old growth tower to the NE. High-precision CO 2 profile at each site. Mini-mesonet, 15-20km spacing between towers.

3. Progress HNFs (humungous nighttime fluxes) identified and isolated. Very large contribution to NEE of CO 2. Regional vent of nighttime drainage flows? Advection terms computed from the ChEAS “mesonet” of CO 2 (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 CO 2.

4. Humungous nighttime fluxes Peak fluxes (turbulent flux, not rate of change of storage) of ~ 80  mol 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 CO 2 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 CO 2 (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 CO 2 (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. June-August 1997 diurnal mean cumulative NEE at WLEF vs. level 2.211.151.011.0019-4 -0.581.161.011.00415-18 -2.011.060.940.9011-14 -1.871.161.290.985-10 0.960.98All Day w/  -9 -2.251.081.180.89All Day Preferred gC m -2 d -1 396m122m30mHours (LST) (Fraction of preferred NEE)

10. 1997 Cumulative NEE, GEP and RE vs. assumptions and methods 1733-1758-25 +/- 17Low U * screened, median fill 1634-1681-48 +/- 20Low U * retained 1924-190916 +/- 19Low U * screened, T-PAR fill REGEPNEEMethod (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