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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

The Airborne Carbon in the Mountains Experiment (ACME): Initial Modeling Results

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 CO2 concentrations

• Measure carbon fluxes at the mountain-valley system scales using measurements of CO2 and other species from aircraft

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 CO2 in mountainous terrain using Lagrangian and Eulerian approach

• Estimate spatial and temporal pattern of surface CO2 fluxes in mountainous terrain using a variety of modeling approaches

Airborne Carbon in the Mountains Experiment (ACME)

Mountain forests represent a large portion of gross primary productivity within the United States and a significant potential net CO2 sink. We conducted the Airborne Carbon in the Mountains Experiment (ACME) in May and July of 2004 to explore methods for constraining regional-scale CO2 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 CO2, CO, O3, 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 CO2 in mountain valleys, and direct flux measurements.

Introduction

Lagrangian mass budget calculations

A Lagrangian mass budget calculation was performed to estimate the uptake of CO2 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. CO2 uptake 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. 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.

May flight July flights

ACME Carbon budget fluxes

0.25 g m-2 hour-1 0.45 – 0.60 g m-2 hour-1

• CO2 drawdown: 0.8 g C m-2

• Tower estimate: 1.4 g C m-2 (mean net flux, Niwot Ridge, May 18-22, 1999-2003)

• CO2 drawdown: 2.5 g C m-2

• Tower estimate: 1.5 g C m-2

(mean net flux, Niwot Ridge, July, 1999-2003)

• CO2 drawdown: 2.9 g C m-2

• Tower estimate: 1.0 g C m-2 (mean net flux, Niwot Ridge, July, 1999-2003)

ACME: Modeling Domain ACME: Flight Project Execution

(left Figure)

Valley CO2 and temperature profiles. Simulations show good agreement with observations

Forward Modeling

To investigate the processes underlying the observed horizontal and vertical distribution of CO2, 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 350x350 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 CO2. Model results are evaluated with aircraft data, surface measurements, and radiosonde data.

Surface flux fields of a passive tracer simulating CO2 fluxes are prescribed in space and time and the mesocale model RAMS is used to compute the spatio-temporal variations of the CO2 concentrations. Results are shown in the figures below. The nighttime accumulation of CO2 in the valleys is apparent. Vertical profiles of CO2 concentrations show a good agreement with vertical profiles of CO2 from the aircraft measurements.

Modeling approach and initial results

Composite of the NSF/NCAR C-130 research aircraft flying over the Rocky Mountains in Boulder. (Photo by Carlye Calvin.)

Inverse Modeling

Given the observations of CO2 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 CO2 concentration observed at the receptor to potential sources within and outside of the modeling domain.

(Figure above)

Modeled CO2 concentrations at the surface and vertical cross section of CO2 concentration in the ACME modeling terrain, showing accumulation of CO2 in the valleys at night

Summary

Acknowledgements

This work is supported by the National Science Foundation and NASA.

Nighttime accumulation of CO2 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.

The Airborne Carbon in the Mountains Experiment (ACME) was conducted successfully in May and July 2004, yielding a unique set of meteorological and CO2 observations over the Colorado Rockies. Several modeling approaches are used to understand and estimate CO2 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 CO2 in valleys occurs with concentrations exceeding 400 ppmv CO2, 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.

CO2 θ8 AM