The Virtual Tall Towers approach: A link to the global CO 2 flux network Ken Davis, Natasha Miles, Scott Richardson, Weiguo Wang, Chuixiang Yi and colleagues The Pennsylvania State University Scott Denning, Joanne Skidmore and Marek Uliasz Colorado State University Peter Bakwin NOAA CMDL With support from: DoE Terrestrial Carbon Processes Program, DoE National Institutes for Global Environmental Change, NOAA

Outline Philosophy/hypotheses Method –Virtual tall towers Applications of existing VTT data New research activities –Regional flux project: ChEAS –Gashound-based mixing ratio sensor –AmeriFlux gets calibrated!

Philosophy/hypotheses Flux networks and mixing ratio networks should be complementary (and co-located?). Abundant, continuous terrestrial mixing ratio data will –enable regional, high-frequency inversions and –improve the accuracy of annual inversions. A moderate accuracy and precision, high density network complements a sparser, high precision and accuracy network.

Virtual tall towers method Calibrate flux tower LI-CORs! Bakwin et al, Zhao et al, Sub-sample for midday, well-mixed conditions in the atmospheric boundary layer. –Synoptic, seasonal and annual gradients resolved Micrometeorological correction applied to remove surface layer – boundary layer offset. –Improves data quality, helpful for annual and high resolution inversions

(One) motivation for virtual tall towers Estimated uncertainty of annual carbon budget from global inversion simulation using different North American observation network scenarios. Note that the assumed monthly random error for VTT sites was 2 ppm, about 5x greater than the expected level of random error. Systematic biases was assumed to be zero.

Photo credit: UND Citation crew, COBRA 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

Diurnal cycle of CO 2 in the ABL: midday vertical gradients are very small Bakwin et al, 1998

ABL CO 2 signals vs. the surface-layer - ABL bias at WLEF Time scaleSynoptic (days, within continent) Seasonal (amplitude of continental cycle, difference between the marine and continental boundary layers) Annual (marine- continental difference) Mixing ratio difference (ppm) Bias (midday surface layer to 24- hour mid-ABL, ppm) 1-4 (both peak in summer) 0.4/0.8 (half day-night, half vertical) Davis et al, in prep

Monthly average offsets between surface layer and mid-ABL CO 2

Monthly mean CO 2 mixing ratios in the convective boundary layer (CBL) at WLEF sub-sampled for convective afternoon hours at WLEF, free troposphere (FT) from aircraft flights at Carr, CO, and marine boundary layer (MBL) at 44.4N. Also shown is cumulative NEE at WLEF. Yi et al, to be submitted.

Synoptic variability in CO 2 Observed synoptic cycle of CO2 mixing ratios and temperature (for reference) at the WLEF tower. (a) Hourly, continuous data for 396m, data subsampled for daytime, well-mixed conditions at 30m. (b) Difference between daytime-averaged 30m and 396m CO2 mixing ratio data (+), and a virtual tall tower correction (diamond) using simple assumptions and gradient functions. Bias is just under 0.2 ppm (monthly mean). Standard error (monthly mean) is less than 0.2 ppm. Davis et al, in prep.

Vertical gradients are determined by fluxes, mixing depth, and vigor of mixing. Universal gradient functions relate these quantities to the vertical gradient (Wyngaard and Brost, 1984). Gradient functions have been computed via large eddy simulation (Wyngaard and Moeng, 1984; 1989; Patton et al, 2002; in prep) and observed (Davis et al, in prep). Micrometeorological correction: VTT methodology

gbgb gtgt Field data (Davis et al, in prep) LES results (Moeng and Wyngaard, 1984) z/h

LES results: Patton et al, in prep

Summary of VTT method VTT correction is modest (~2ppm max at WLEF). Maximum for large fluxes, and measurements close to the surface Significant uncertainty exists re: proper gradient functions over forest Gradient functions can be determined more precisely via further study of ABL turbulence

Applications of existing tall tower, aircraft and vtt data Davis et al, Global Change Biology, –Seasonal offset exists between CO 2 fluxes and ABL mixing ratios measured at the WLEF tower. Bakwin et al, in review, Tellus –Similar offset exists at 3 of 4 flux towers, correlated with marine BL mixing ratios, can be used with reanalysis data to estimate regional surface fluxes of CO 2 Hurwitz et al, in press, J. Atmospheric Sciences. –Cause of seasonal offset appears to be vertical mixing caused by synoptic passages. Tropospheric mixing ratios very similar to marine BL mixing ratios. Helliker et al, to be submitted –Similarity between vertical mixing of water vapor and CO 2 can be used to derive a regional, synoptic, ABL to free troposphere flux- gradient relationship. CBL mixing ratios can estimate regional CO 2 fluxes with surprising long-term accuracy. Butler et al, in prep –Seasonal flux anomaly in the spring of 1998 shows up across much of northeast and northcentral North America. Flux anomaly is correlated with a mixing ratio anomaly measured both at the flux towers (VTT method) and in the marine flask network.

Net ecosystem-atmosphere exchange of CO 2 in northern Wisconsin Davis et al, Global Change Biology, 2003.

Common seasonal patterns across flux tower sites: link to marine flask network Results show that the sum of vertical and horizontal transport is related to the difference between the tower midday CO 2 mixing ratio and the marine boundary layer CO 2 mixing ratio. Bakwin et al, in review

Cold frontal passage and CO 2 advection (14 July, 1998) Hurwitz et al, in press, JAS

Helliker, Berry et al: Boundary layer cuvette,or Synoptic flux-gradient relationship Utilizes similarity in vertical mixing of scalars between the ABL and the free troposphere. Tropospheric mixing occurs with synoptic events. Synoptic events are analogous to ABL eddies, thus the method is analogous to surface layer similarity and flux-gradient relationships.

Synoptic flux-gradient vs. eddy-covariance fluxes of CO 2, WLEF tower. Helliker et al, to be submitted.

Early leaf-out, 1998, Wisconsin

Impact on atmospheric [CO 2 ]

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.

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. Butler et al, in prep.

Summary of VTT applications Seasonal and synoptic signals at terrestrial sites are large and robust. Vertical mixing dominates. VTT data capture large-scale mixing ratio data (consistent relationships vs. marine ABL mixing ratios, FT mixing ratios). Flux data capture large-area temporal patterns. Budget and flux-gradient approaches have promise for deriving regional fluxes. Analyses based on spatial gradients among VTT sites are lacking to date.

New research Regional tower network AmeriFlux VTT network

ChEAS regional flux experiment Derive daytime and daily seasonal fluxes using regional atmospheric inversions and relatively cheap, abundant, in situ CO 2 sensors. Overarching goal – evaluate/merge multiple approaches of studying terrestrial fluxes of CO 2. –Merge flux-tower based upscaling with downscaled inversion methodology. Regional integration and mechanistic interpretation. –Determine interannual variations in seasonal fluxes on a regional basis. Again, integrate with regional flux measurements/mechanistic interpretations. –If possible, derive net annual fluxes. Spatial resolution is limited by the magnitude of the annual signal.

Methods for determining NEE of CO 2 Methods for bridging the gap Upscale via ecosystem models and networks of towers. Move towards regional inverse modeling

100 km Chequamegon Ecosystem-Atmosphere Study (ChEAS) flux towers

NEE and Gross Fluxes: All Current Data NEE (gC m -2 )Rh (gC m -2 )Phot (gC m -2 ) WLEF WLEF WLEF WLEF WLEF Wcreek Wcreek Lcreek

Experimental design Deploy a regional network of in situ CO 2 sensors. Calibrate secondary standards for in situ sensors. Four standards/site, midday mixing ratio range. NOAA CMDL standards used to create secondary working standards. Sensors mounted on communications towers at a similar height, ~75m above ground. Differences in mixing ratio across the region reveal regional surface fluxes of CO 2. Compare results to upscaling via regional flux tower cluster.

ChEAS regional flux experiment domain = LI-820 sampling from 75m above ground on a communications tower. = 40m Sylvania flux tower with high-quality standard gases. = 447m WLEF tower. LI-820, CMDL in situ and flask measurements.

Expected regional mixing ratio differences (winter to summer) Time scaleDaytimeDiurnalAnnual Flux magnitude 1 to 10  mol m -2 s -1 1 to 4 gC m -2 d -1 ~ 1 gC m -2 d -1 Mixing depth1 to 2 km ~10 km Advection time~10 hours~24 hours Advection distance ~180 km (half ring) ~400 km (full ring) 400 km (full ring) Change in ABL CO 2 1 to 5 ppm2 to 5 ppm~0.2 ppm

Influence functions for the WLEF tower (z=400m) for the June, July, August and September 2000 Simulation: RAMS v4.3 with two nested grids (Δx=100km and 20 km) + LPD (Lagrangian Particle Dispersion) model in a receptor- oriented mode. The 2 nd finer grid covers the domain around the WLEF tower used for dispersion calculations. Concentration sampling: The influence functions, travel time and influence frequency are presented for selected 2 hour sampling periods during the day during August The hour period represents the results for all sampling times during the month. All sampling times are local (GMT-6h). The influence frequency is derived in reference to the sampling period (i.e., how often the signal from a given source area is observed at the receptor during the sampling period). Travel distance is derived from the presented influence functions but averaged over 45 o sectors and shown in polar coordinates. Tracers: 1. Passive tracer with a constant flux – the spatial distributions are the same as for the respiration flux including dependence on the soil temperature. 2. A-tracer (assimilation tracer) with a daytime flux driven by shortwave radiation Reference: Uliasz, M. and A. S. Denning, 2002: Deriving mesoscale surface fluxes of trace gases from concentration data. submitted to: J. Appl. Meteor. Download:

Influence function climatology

CO 2 Measurement System Schematic

Deployment at Wittenberg

Why use the Licor 820? Fast time response is not required (feature of LI-6262, LI-7000) ~3 ppm peak-to-peak noise for 1-sec samples is large… –LI-6262 ~ 0.3 peak-to-peak noise But noise is random And noise reduces to < 0.2 ppm for a 5- minute average

LI-820 features Single cell IRGA Cost:$2,400 per sensor –LI-6262 ~ $10,000 –LI-7000 ~ $14,000 Size: 22 x 15 x 8 cm Weight: ~ 1 kg Temperature controlled emitter and detector Zero drift < 1 ppm in 24 hrs Power: VDC, 0.3 A 12 V (1.2 A warm- up) Output: Analog Voltage or mA or Digital TTL XML communication protocol

Planned intercalibration procedures Sample same air with all units for a limited period of time Circulate high-pressure target tanks Co-locate one sensor with the WLEF tower flask and in situ sensors

Preliminary results Systems deployed July, Site-to-site comparisons (sampling common air, travaling high pressure target tank) good. Offset with WLEF. Debugging in process. Expected fix later this month.

Kemp intercomparison test

First quiescent period

Second quiescent period

Preliminary results from deployment 10 days of concurrent data at 3 sites

AmeriFlux gets calibrated! A continental virtual tall tower network Employ same technology (essentially Bakwin et al, 1995) as the regional experiment at ~6 additional AmeriFlux towers. Utilize existing LI-CORs where possible. Conduct network-wide intercalibration activities. Apply VTT sub-sampling and micromet corrections to mixing ratio data from all sites. Apply data to continental inversion studies along with tall tower and aircraft data. Site instrumentation begins in the spring of 2004.

Potential VTT network: Selection of new sites to be based on optimization study, Skidmore et al, and plans for a Midwest regional intensive