The Airborne Carbon in the Mountains Experiment ACME

The Airborne Carbon in the Mountains Experiment (ACME): Initial Modeling Results Stephan F. J. De Wekker (dewekker@ucar. edu), S. Aulenbach, B. Sacks, D. Schimel, B. Stephens, National Center for Atmospheric Research, Boulder CO; T. Vukicevic, and X. Warren-Laird, University of Colorado at Boulder, Boulder CO Introduction Modeling approach and initial results Inverse Modeling Mountain forests represent a large portion of gross primary productivity within the United States and a significant potential net CO 2 sink. We conducted the Airborne Carbon in the Mountains Experiment (ACME) in May and July of 2004 to explore methods for constraining regional-scale CO 2 fluxes over complex terrain and to collect measurements useful for devising and testing strategies for long-term monitoring of these fluxes. We flew a total of 54 hours on the NCAR C-130 aircraft over large regions of the Colorado Rocky Mountains, making continuous measurements of CO 2, CO, O 3, and water vapor concentrations among other measurements. The flights were conducted according to a combination of experimental designs, including morning to afternoon Lagrangian flux measurements, regional measurements for assimilation into a high-resolution atmospheric model, morning sampling of nocturnally respired CO 2 in mountain valleys, and direct flux measurements. Forward Modeling Given the observations of CO 2 concentrations from aircraft and at particular monitoring sites, we would like to investigate the range of possible surface source configurations that are consistent with the data. This problem requires the application of numerical inversion techniques. We plan to use the adjoint of the mesoscale model RAMS (RAMDAS) as well as a Lagrangian Particle Dispersion Model in the backward mode for this purpose. These techniques will allow us to calculate influence functions which relate the CO 2 concentration observed at the receptor to potential sources within and outside of the modeling domain. To investigate the processes underlying the observed horizontal and vertical distribution of CO 2, we use the Regional Atmospheric Modeling System (RAMS) mesoscale model. RAMS solves a set of dynamic equations in their nonhydrostatic, compressible form, a thermodynamic equation, and a set of cloud microphysical equations. It predicts the three velocity components, potential temperature, mixing ratio, and subgrid-scale turbulence kinetic energy (TKE) in a terrain-following coordinate system. The three-dimensional simulations are made centered over the Niwot Ridge Ameriflux site. The domain covers an area of about 350 x 350 km with a horizontal grid spacing of 1 km. Such a fine horizontal resolution is needed to resolve the topographic features in the investigation area, which are expected to play an important role in the accumulation, ventilation and transport of CO 2. Model results are evaluated with aircraft data, surface measurements, and radiosonde data. Surface flux fields of a passive tracer simulating CO 2 fluxes are prescribed in space and time and the mesocale model RAMS is used to compute the spatio-temporal variations of the CO 2 concentrations. Results are shown in the figures below. The nighttime accumulation of CO 2 in the valleys is apparent. Vertical profiles of CO 2 concentrations show a good agreement with vertical profiles of CO 2 from the aircraft measurements. Composite of the NSF/NCAR C-130 research aircraft flying over the Rocky Mountains in Boulder. (Photo by Carlye Calvin. ) Lagrangian mass budget calculations A Lagrangian mass budget calculation was performed to estimate the uptake of CO 2 by the mountain forests during the day. 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 mid-afternoon was sampled in the morning, and then resampled over Niwot Ridge in the afternoon. CO 2 uptake can be estimated from the change in the column integral of CO 2 between the morning and afternoon profiles, assuming that the forecast was accurate and the same airmass was sampled twice. Typical vertical profiles used in carbon budget calculations are shown below. In a subsequent study, we will use our mesoscale modeling tools to test the validity of the Lagrangian mass budget approach over orographically complex terrain. Airborne Carbon in the Mountains Experiment (ACME) Goal: To estimate and understand carbon fluxes in montane forest regions at valley to mountain range scales Field Study Activities: • Measure carbon fluxes at the stand (tower) scale using eddy correlation and process studies • Measure carbon fluxes at the catchment or "carbonshed" scale using ground-based and airborne measurements of CO 2 concentrations • Measure carbon fluxes at the mountain-valley system scales using measurements of CO 2 and other species from aircraft CO 2 8 AM (Figure above) θ Modeled CO 2 concentrations at the surface and vertical cross section of CO 2 concentration in the ACME modeling terrain, showing accumulation of CO 2 in the valleys at night (left Figure) Valley CO 2 and temperature profiles. Simulations show good agreement with observations Atmospheric Modeling Activities: • Simulate atmospheric flows in ACME modeling domain and capture complex terrain effects with appropriate parameterizations and model setup parameters • Simulate horizontal and vertical distribution of CO 2 in mountainous terrain using Lagrangian and Eulerian approach • Estimate spatial and temporal pattern of surface CO 2 fluxes in mountainous terrain using a variety of modeling approaches ACME: Modeling Domain Nighttime accumulation of CO 2 was observed on many of the morning research flights as shown in the figure below. These nighttime accumulations may be inverted using our modeling tools to provide a constraint on respiration. ACME: Flight Project Execution • CO 2 drawdown: 0. 8 g C m-2 • CO 2 drawdown: 2. 5 g C m-2 • CO 2 drawdown: 2. 9 g C m-2 • Tower estimate: 1. 4 g C m-2 (mean net flux, Niwot Ridge, May 18 -22, 1999 -2003) • Tower estimate: 1. 5 g C m-2 (mean net flux, Niwot Ridge, July, 1999 -2003) • Tower estimate: 1. 0 g C m-2 (mean net flux, Niwot Ridge, July, 1999 -2003) ACME Carbon budget fluxes May flight July flights 0. 25 g m-2 hour-1 0. 45 – 0. 60 g m-2 hour-1 Summary The Airborne Carbon in the Mountains Experiment (ACME) was conducted successfully in May and July 2004, yielding a unique set of meteorological and CO 2 observations over the Colorado Rockies. Several modeling approaches are used to understand estimate CO 2 fluxes at valley to mountain range scales using measurements obtained during ACME. Preliminary simulations using the mesoscale model RAMS indicate that significant nighttime accumulation of CO 2 in valleys occurs with concentrations exceeding 400 ppmv CO 2, in good agreement with the observations. RAMS performs well over the complex mountainous terrain in the Colorado Rockies and will be used as the central tool in our inverse modeling approaches. Calculations based on a Lagrangian mass budget approach show that as the season progresses, the airborne estimated regional fluxes become larger than the high-elevation fluxes observed at Niwot and approach the fluxes typical of low elevation forests. Acknowledgements This work is supported by the National Science Foundation and NASA. Airborne Carbon in the Mountains Experiment (ACME)