ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Eddy-covariance flux measurements: Outline for the day Morning ChEAS sites and current research projects Methods(Berger et al, 2001; Yi et al, 2000) Results –Published or in press Yi et al 2000, Davis et al, Cook et al. –In preparation (with hypotheses) Various, PSU research group members Future plans/proposals

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Eddy-covariance flux measurements: Outline for the day Afternoon Eddy covariance flux calculation and Li-Cor demo Small group discussion. Suggested topics: –Causes of interannual variability at WLEF –Causes of differences among ChEAS tower flux measurements –Potential for instrument bias, errors, and improvements –Extension of interannual variability studies beyond ChEAS –Uses of sub-canopy flux and turbulence measurements –Two-dimensional flux experiments and analyses –Caterpillars – observed or imagined?

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University ChEAS eddy covariance flux measurements I: Sites and research projects

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Chequamegon Ecosystem-Atmosphere Study (ChEAS) flux towers WLEF tall tower (447m) CO 2 flux measurements at: 30, 122 and 396 m CO 2 mixing ratio measurements at: 11, 30, 76, 122, 244 and 396 m Forest stand flux towers: Mature deciduous upland (Willow Creek) Deciduous wetland (Lost Creek) Mixed old growth (Sylvania) All have both CO 2 flux and high precision mixing ratio measurements.

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University 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.

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University View from 396m above Wisconsin: WLEF TV tower

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University ChEAS web sites - Main page. ftp://ftp.essc.psu.edu/pub/workgroup/davis/ - Data access.ftp://ftp.essc.psu.edu/pub/workgroup/davis/ - Site descriptions. Also see: - Fluxnet’s home page, and, Map/index.cfm - AmeriFlux’s home page. Map/index.cfm

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Data availability SiteStart dateFull years of data WLEFSpring Willow CreekSummer Lost CreekFall SylvaniaSummer 2001 CeilometerSummer RadarSpring-Fall, 1998; Spring-Fall, 1999

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Research Funding Regional atmosphere/forest exchange and concentrations of carbon dioxide. –Study of net ecosystem exchange of carbon dioxide via eddy covariance measurements at the WLEF tower in northern Wisconsin, as well as the study of carbon dioxide transport and distribution within the boundary layer. –PI: P.S. Bakwin, U. Colorado/NOAA –Co-I: K.J. Davis Penn State –Department of Energy, National Institutes for Global Environmental Change –Duration: July, 1994–June, 1997; July, 1997–June, 2000; July, 2000–June Measuring and modeling component and whole-system CO 2 fluxes at local to regional scales. –Study of component processes which make up CO 2 fluxes in a forest ecosystem, and comparison to whole-ecosystem net flux measurements from small flux towers. Also a comparison between homogeneous ecosystem fluxes within the WLEF tower footprint and the WLEF net flux signal. –PI: P.V. Bolstad, U. Minnesota –Co-Is: K.J. Davis, Penn State, and P.B. Reich, U. Minnesota –Department of Energy, National Institutes for Global Environmental Change –Duration: July, June, July 2000 – June 2003.

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Research Funding Quantifying carbon sequestration potential of mid and late successional forests in the upper Midwest –Observations of CO 2 exchanges in an old-growth forest in the upper peninsula of Michigan and comparison to existing flux towers in younger forest stands northern Wisconsin. Davis's work would provide tower construction, instrumentation, and data analysis support for a 30m tower. –PI: Eileen Carey, U. Minnesota –Co-Is: P.V. Bolstad, U. Minnesota; K.J. Davis, Penn State –Department of Energy, Terrestrial Carbon Processes –Duration: January, December, 2003 Regional forest-ABL coupling: Influence on CO 2 and climate –Study of the coupling between the surface energy balance, boundary layer development, and net ecosystem exchanges of carbon dioxide, as well as the influence of the covariance between carbon dioxide fluxes and boundary layer development on boundary layer mixing ratios of carbon dioxide. Observations at the WLEF and the Walker Branch AmeriFlux sites using an NCAR radar. –PI: K.J. Davis, Penn State –Co-I: A.S. Denning, Colorado State University –Department of Energy, TECO/Terrestrial Carbon Program –Duration: September, August, 2002

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University ChEAS eddy covariance flux measurements II: Methods

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Methods: Eddy covariance flux measurements in ChEAS Basic theory of eddy covariance flux measurements. Tower flux instrumentation LI-COR calibration Sonic rotation Lag time correction Spectral corrections Random and systematic errors due to turbulence “Preferred” NEE algorithm (WLEF only) Filling missing data

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Publications describing methodology Yi, C., K.J. Davis, P.S. Bakwin, B.W. Berger, and L. C. Marr, The influence of advection on measurements of the net ecosystem-atmosphere exchange of CO 2 observed from a very tall tower, J. Geophys. Res. 105, Berger, B.W., K.J. Davis, P.S. Bakwin, C. Yi and C. Zhao, Long-term carbon dioxide fluxes from a very tall tower in a northern forest: Flux measurement methodology. J. Atmos. Oceanic Tech., 18, Davis, K.J., P.S. Bakwin, B.W. Berger, C. Yi, C. Zhao, R.M. Teclaw and J.G. Isebrands, The annual cycles of CO 2 and H 2 O exchange over a northern mixed forest as observed from a very tall tower. Global Change Biology, in press.

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Theory

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Theory Time rate of change (e.g. CO 2 ) Mean transport Turbulent transport (flux) Source in the atmosphere Integrate from the earth’s surface to the imaginary plane defined by the level of the flux sensor. “Reynold’s averaged” (= mean + turbulent components of all variables) scalar conservation equation.

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Theory Yi et al, 2000

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Theory: What is w’c’? Prime indicates departure from the mean. w’ > 0 is an updraft c’ > 0 is air rich in the scalar c w’c’ > 0 is upwards transport of the scalar Averaging this over time sums the transport observed due to all updrafts and downdrafts.

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Radar ABL depth WLEF fluxes CO 2 profile Davis et al, in press Daily cycle of ABL depth, and CO 2 fluxes and mixing ratios

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Diurnal cycle of CO 2 in the ABL Bakwin et al, 1998

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Instruments at WLEF Berger et al, 2001

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Instruments at WLEF Two “profiling” LI-CORs in the trailer, one sampling 396m, one cycling among all 6 levels. “Slow” time response. High-precision and accuracy calibration (Bakwin et al, 1998). C-bar. Vaisala humidity and temperature sensors at 3 levels (30, 122 and 396m). “Slow” Q-bar, T-bar. Three sonic anemometers (30, 122 and 396m). w’, T’ Three LI-CORs in the trailer, one for each sonic level. “Fast” time response. Long tubes, big pumps. Measure CO 2 and H 2 O. c’, q’ Two LI-CORs on the tower (122 and 396m). “Fast” time response. Short tubes, smaller pumps.

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Calibration of “fast” CO 2 and H 2 O sensors at ChEAS towers Calibration occurs using the fluctuations in the ambient atmospheric CO 2 and H 2 O mixing ratios. “Slow” sensors provide absolute values of these mixing ratios used to calibrate the “fast” LI-CORs. Ideal gas law corrections to LI-COR cell temperature, pressure and humidity are applied. Calibration slope and intercept are derived every 2 days. These values are smoothed (monthly running mean) to derive the long-term calibration factors used for the “fast” LI-CORs.

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Calibration of “fast” CO 2 and H 2 O sensors Berger et al, 2001

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University What’s up? (Sonic rotations) Sonic anemometers are oriented perfectly in the vertical, (and the wind’s “streamlines” aren’t always perpendicular to gravity). Data is collected over a long time (about a year) and we define “up” by forcing the mean vertical wind speed to be zero.

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Sonic rotations Berger et al, 2001

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Lag time calculation We must correct for the delay between the CO 2 and H 2 O measurements and the vertical velocity measurements. Lag time is determined by finding the maximum in the lagged covariance between vertical velocity and CO 2 /H 2 O for every hour. Level (m)IRGA position Tube length (m) Lag time (s) Tube inner diameter (m) Flow rate (L min -1 ) Reynolds number 396Trailer Trailer Trailer Tower Tower Berger et al, 2001

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Lag time calculation Berger et al, 2001

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Spectral corrections Flow through tubes smears out some of the atmospheric fluctuations, especially the small (high frequency) eddies. –Obvious for H 2 O. Much worse than theory predicts. –Not directly observed for CO 2. Small effect. The sonic anemometer (virtual) temperature measurement is not smeared out, so we use similarity between the virtual temperature spectrum and the water vapor spectrum to correct for the loss of high frequency eddies in H 2 O. We use past studies of flow in tubes to correct for the loss of high frequency eddies in CO 2.

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Spectral corrections Berger et al, 2001 CO 2 H2OH2O TvTv

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Spectral corrections Level (m) IRGA position CO 2 (day) CO 2 (night) H2OH2O 396Trailer Trailer Trailer Tower< Tower< Table shows the typical % of flux lost due to smearing of small eddies. Berger et al, 2001

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Systematic errors – getting the large and small eddies Berger et al, 2001

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Random errors – a finite number of eddies are counted in one hour Random sampling errors for any one hour can be as large as the magnitude of the measured flux! Berger et al, 2001, following Lenschow and Stankov, 1986.

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University “Preferred” NEE (WLEF only) Data is taken from 30m at night and 122 or 396m during the day (the highest level where there is turbulent flow) when all data are available. If data are missing, any existing flux measurement is used. Data are screened out when the level of turbulence is very low. CO 2 is probably draining down hill. Early in the morning upper level data from WLEF is replaced with 30m data (Yi et al, 2000) because the flow appears to be systematically 2-D. Thus from 3 NEE measurements, one is derived as our “preferred” measurement for each hour.

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Nighttime drainage flows? Cook et al, submitted; Davis et al, in press Loss of flux at low turbulence levels at the Willow Creek tower.

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Morning advection at WLEF Yi et al, 2000

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Multiple level comparison at WLEF Davis et al, in press Comparison of all 3 levels, growing season 1997.

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University ChEAS eddy covariance flux measurements III: Results

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Missing data, gross fluxes, light and temperature response Nighttime NEE measurements (for CO 2 ) are fitted to soil or air temperature. This is assumed to describe the total respiration flux. Daytime NEE measurements are fitted to PAR after total respiration has been computed using the fits and “subtracted” from NEE. This fit describes the response of forest photosynthesis to sunlight. These fits are used to compute gross fluxes (respiration, photosynthesis) and to fill in missing NEE data needed to compute cumulative NEE.

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Gross fluxes and functional fits Davis et al, in press

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Example of gross fluxes fit to temperature and PAR at WLEF for one month. Davis et al, in Press.

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Hourly fluxes at WLEF for 1997, observed and filled. Davis et al, in press.

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Cumulative fluxes at WLEF, 1997 Davis et al, in press

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Gross fluxes at WLEF, 1997 Davis et al, in press RE -GEP NEE

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University (gC m -2 yr -1 = tC ha -1 yr -1 * 100) 1997 Cumulative NEE, GEP and RE vs. assumptions and methods /- 17Low U * screened, median fill /- 20Low U * retained /- 19Low U * screened, T-PAR fill REGEPNEEMethod Davis et al, in press

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Lack of energy balance: Are turbulent fluxes underestimated? Davis et al, in press Cook et al, submitted

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Monthly mean CO 2 fluxes at WLEF, Davis et al, in press

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Monthly mean latent heat fluxes at WLEF, Davis et al, in press

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Bowen ratio vs. time of year at WLEF, Davis et al, in press, following Cook et al, submitted.

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Drainage during stable conditions: What goes down must come up (somewhere).

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Very large positive turbulent fluxes from about 150 degrees. Blocking of flow. Occur during windy, weakly stable conditions when the canopy is decoupled from the ABL. Cook et al, sub.

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Is this venting of drainage? Can we capture these events across the landscape?

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Cook et al, submitted to Global Change Biology: Willow Creek

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Courtesy D. Hollinger

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Early leaf-out, 1998, Wisconsin

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Impact on atmospheric [CO 2 ]

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Spatial coherence of seasonal flux anomalies A similar pattern is seen at several flux towers in N. America and Europe. Three sites have high-quality [CO 2 ] measurements + data at Fluxnet (NOBS, HF, WLEF). The spring 98 warm period and a later cloudy period appear at all 3 sites. Temp CO 2 NEE Day of year 80200

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Detection of the spring 98 anomaly via oceanic flasks? 2 Alaskan flask sites have slightly higher [CO 2 ] in the spring of 98. Mace Head, Ireland shows a depression of [CO 2 ] in the spring of 98. Potential exists to link flux towers with seasonal inverse studies.

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University Synoptic variability in CO 2

ChEAS workshop. August, 2002 Department of Meteorology, Penn State University North American Carbon Plan (NACP)