Looking for carbon in all the wrong places: Multi-scale carbon measurements in the Rocky Mountains-ACME 04
Dave Schimel, Bill Sacks, National Center for Atmospheric Research, Boulder CO, Britt Stephens, Steve Running, University of Montana, Jielun Sun NCAR, Dennis Ojima, Colorado State University and ACME Investigators*
Spatial scales
Introduction
Mountain ecosystems are major drivers of carbon cycling in the western United States. We are studying the importance of
montane ecosystems in regional carbon cycling. Our focus is on the Colorado Rocky Mountains and the Niwot Ridge LTER
site where the ground-based Carbon in the Mountains Experiment was carried out (July-Oct 2004) and where a
AMERIFLUX eddy covariance site provides long term flux data for context. We collected carbon concentration and flux
observations and related atmospheric information using aircraft and eddy flux towers. The airborne and ground-based
studies had a major emphasis on night-time and early morning observations to use advective flows to estimate nighttime
respiration. The length and timing of the seasonal cycle are key controls over carbon dynamics, and the ACME flights were
synchronized with the early and peak times in the growing season to determine whether seasonal changes in carbon fluxes
could be assessed regionally. The Airborne Carbon in the Mountains (May-July 2004) collected and utilized all these
observations together with remotely sensed information to constrain estimates by ecosystem models. Here we provide an
overview of integrated tower, airborne and remote sensing data that allow analysis of regional-scale carbon fluxes in a
mountainous landscape. Future research will focus on 1) estimating night-time respiratory fluxes from early morning
profiling of the nocturnal boundary layer, 2) integrating tower, airborne and satellite platform measurements into a
common assimilation model framework, and 3) assessing the requirements for a comprehensive regional cross-scale network
that could be implemented as part of a Rocky Mountain NEON system.
Several spatial scales are of interest:
1. Tower sites process studies and remote sensing products will be assessed relative to tower data
2. Carbonshed
•
analogous to watersheds, these areas are of the order of 10s to 100s of km2
3. Regional
•
A) “Regional-convective” sampled by mapping low altitude
concentrations and fluxes, 100s of km scale
•
B) “Regional-advective” sampled by semi-lagrangian
techniques, 1000s of km scale
Domain
Airborne Carbon in the Mountains Experiment (ACME)
Goal: to understand regional carbon fluxes and processes in montane forest regions by developing
new methods for measuring carbon fluxes at landscape to cordillera scales.
Activities:
• Make carbon flux measurements at the stand (tower) scale using eddy covariance and
process studies
• Make carbon flux measurements at the catchment or "carbonshed" scale using groundbased and airborne measurements of CO2 concentrations
• Make carbon flux measurements at the mountain-valley system scales using measurements of
CO2 and other species from aircraft
• Integrate spatially integrative measurements (fluxes) with spatially explicit measurements
(remote sensing) to link flux and process measurements within landscapes.
• Use multi-scaled measurements as input to a biogeochemical process assimilation model to
produce data-constrained estimates of both key carbon fluxes and controlling parameters.
ACME: Flight Project Design
Remote sensing for Carbon Science
Satellite based landcover for the ACME domain
showing, on the left, the full range of land cover
classes identified, and on the right, the spatial
distribution of the dominant Western Coniferous
Biome class.
The University of Montana developed a project-specific 1 km resolution, daily time-step MODIS GPP product for the
state of Colorado for 2004. This provides a data-constrained estimate of the time and space patterns of a key carbon
flux. The ACME MODIS GPP estimates are produced using the standard MODIS algorithm, using selected daily
level 2 satellite observations and gridded surface meteorology derived from observations. While none of the airborne
products correspond directly to GPP, we observe daytime Net Ecosystem Exchange (DNEE) which is equal to GPPdaytime respiration and whose seasonal cycle may be correlated with GPP.
MODIS GPP Seasonal Cycle
GPP is a major driver of NEE and ACME flights
sampled early and peak season fluxes
ACME: Flight Project Execution
Daytime concentration drawdown
Calculation of hourly rates from ACME flights and the Niwot Ridge tower show convergence of
estimates, especially in the early season. As the season progresses, the airborne estimates indicate that
the regional fluxes are getting to be higher and higher than the high-elevation fluxes observed at Niwot.
The figure shows, for comparison, fluxes from the Harvard Forest eddy covariance site to indicate that
the regional Rocky Mountain fluxes increase over the growing season, approaching the fluxes typical of
low elevation forests. The convergence of estimates suggests that airborne measurements can be used
synergistically with localized tower fluxes for regional extrapolation and that the airborne data do
observe fluxes at larger scales and so provide additional information useful in coupling to spatially nonrepresentative but temporally continuous observations.
Vertical profiles were measured in upwind and downwind locations based on
a forecast of the winds. The airmass forecast to be over Niwot Ridge in midafternoon was sampled in the morning, and then resampled over Niwot
Ridge in the PM (Sacks et al this meeting). Daytime Net Ecosystem
Exchange can be estimated from the change in the column integral of CO2
between the morning and afternoon profiles, assuming that the forecast was
accurate and the same airmass was sampled twice. Retrospective analysis of
winds was used to establish the validity of this assumption. Typical vertical
profiles used in carbon budget calculations are shown below. Fluxes were
estimated from the difference in column integrals of carbon dioxide.
GPP estimated from
MODIS for May 20
GPP estimated from
MODIS for July 29
Spatial pattern of GPP for Colorado, early and peak growing season 2004
using satellite greenness and high resolution daily meteorology. Note the
emerging peak values in the montane forests.
Synthesis and conclusions
ACME Carbon
budget fluxes
May flight
July flights
0.25 g m-2 hour-1
0.45 – 0.60 g m-2 hour-1
Estimates of hourly carbon uptake from airborne and ground-based systems. The fluxes shown are for corresponding time
and days, using long-term averages for the towers as the actual fluxes for 2004 are not yet available. The blue bar indicates
the mean airborne flux estimate and the length of the bar indicates the time and duration of the flight. The vertical blue bars
indicate the estimated 40% uncertainty.
ACME Investigators: Steve Aulenbach NCAR/CGD, Rebecca McKeown Colorado State University, Teresa Campos NCAR/ACD, Dave Rogers NCAR/ATD/RAF, Dennis S. Ojima Colorado State University, David F. Baker NCAR/CGD, Alex B. Guenther NCAR/ACD, Heather Graven UCSD, Jeffrey Hicke Colorado State
University, Jia Hu University of Colorado, Julia Klein Colorado State University, Jielun Sun NCAR/MMM, Chun-Ta Lai University of Utah, Don Lenschow NCAR/MMM, Steve Oncley NCAR/ATD/RAF, Robert H Braswell University of New Hampshire,Robert Beyer University of Colorado, Russ Monson University of
Colorado, Ryan Jones U Colorado, William J. Sacks NCAR/CGD, Dave Schimel NCAR/CGD, Sean Burns NCAR/MMM, Stephan FJ de Wekker NCAR/CGD, Britt Stephens NCAR/ATD/RAF, Steve Knox Colorado State U,, Steve W Running U Montana, Tomislava Vukicevic Colorado State U, Thomas Karl NCAR/ASP
(ACD), Thomas Riley Colorado State U, Mark Tschudi NCAR/ATD/RAF, Andrew A. Turnipseed NCAR/ACD, Xylina Warren-Laird U Colorado
•Preliminary results suggest that airborne budget and ground-based eddy covariance approaches
produce comparable values in montane landscapes and so may be used synergistically.
• Airborne techniques can capture changes in the seasonal cycle of carbon fluxes, which, because
growing season length is such a critical control over carbon, is a key requirement for using regional flux
estimates to deduce regional processes.
• While towers and aircraft retrieve Net Ecosystem Exchange, and the MODIS algorithms retrieve
Gross Primary Productivity, the relative changes and timing of the different measures are correlated,
suggesting the possibility of their synergistic use.
• These preliminary results provide encouragement for the integration of space, air and ground-based
measurements into assimilation models for estimating carbon cycle dynamics in mountain landscapes.