Definition of AOA1 and AOA2 age tracers

[tmooring]

I am attempting to use the archived CESM2(WACCM6) output available at NSF NCAR GDEX Dataset d651045 for a study of stratospheric mixing and have a question about the tracers AOA1 and AOA2. Their names indicate that they are some sort of age of air, and the distributions of these tracers in the stratosphere suggests that they are both implemented as being conserved except at some boundary at which a time-dependent boundary condition is being imposed.

However, the rather odd tropospheric structures of these tracers as well as their actual numerical values mean that the exact nature of the boundary condition and the units of measure of the tracers are not at all obvious. I am a bit surprised that I have not managed to find anything in the literature or elsewhere online about the precise definitions of these tracers—can anyone here help?

Thanks in advance for any assistance you can provide, and please let me know if you have any questions.

Sincerely,

Todd Mooring
Research Associate
Linz Group
Department of Earth and Planetary Sciences
Harvard University

 

[yhanw]

Hello Todd, I run into this exact same confusion today with a different CESM2 dataset. Like you, I couldn't find it explicitly documented.

Just to add some info from my end if useful: In my case, AOA1 values are in a very narrow range (~2.3-2.4). I tried plotting AOA1 lat-lev field (using a 2.3~2.4 colorbar), and the expected physical structure does appear, so likely some sort of conversion is needed? The description says "Age-of_air tracer 1", but unit in netcdf metadata is "kg/kg".

Hope to echo this question, and I'd appreciate any insight from fellow forum users. Thanks!

Yuhan Wang
Postdoc
Earth System Science
Stanford University

 

[Jakub_Sypek]

Hello both,

Did you figure this out? I'm also trying to use the AOA to determine residence times in the mesosphere. If not, I'd also like to echo this question.

Jakub Sypek
Postgraduate Researcher
Department of Aeronautics and Astronautics
University of Southampton

 

[fairyeszhuo]

Hi All,

Did you figure this out? I am facing the same problem thus I'd also like to echo this question.

Best,
Zhihong

 

[mijeong]

In CESM2/WACCM6, AOA1 and AOA2 are two separate synthetic "age-of-air" tracers, which are designed for quantifying transport timescales. They're diagnostic tracers so they don't affect radiation or chemistry. AOA1 is a constant surface flux tracer that has a value of cflx= 1.e-6_r8 (kg/m2/s, flux density). Since there is no chemical sink, its global burden increases roughly linearly with time once the run pasts spin-up. Age or air is diagnosed by comparing a mixing ratio at a grid point to a reference time series from the model’s output and finding the time lag between them. AOA2 is a linearly increasing surface flux tracer that is driven by a flux that grows linearly with time and restricted to Northern Hemisphere land. Since the source itself has a known, prescribed time-dependence, AOA2 is close to an SF6-like tracer. The ages diagnosed from AOA1 and AOA2 should agree closely with each other and with theoretical expectations (e.g., increasing with altitude and latitude in the stratosphere, consistent with the Brewer-Dobson circulation).

To calculate age of air from AOA1, pick a reference location that best represents the well-mixed source region (e.g., tropical tropopause region). χ_ref(t) — time series of AOA1 mixing ratio at the reference location/level. For any grid point x and time t of interest, find the earlier time t' at which the reference timeseries had the same value: χ(x, t) = χ_ref(t'), AGE(x, t) = t - t'.

References
Plumb, R. A., and J. D. Mahlman, 1987: The Zonally Averaged Transport Characteristics of the GFDL General Circulation/Transport Model. J. Atmos. Sci., 44, 298–327, https://doi.org/10.1175/1520-0469(1987)044<0298:TZATCO>2.0.CO;2.

Yudin, Valery A., Sergey P. Smyshlyaev, Marvin A. Geller, Victor L. Dvortsov, 2000: Transport Diagnostics of GCMs and Implications for 2D Chemistry-Transport Model of Troposphere and Stratosphere. J. Atmos. Sci., 57, 673-699. doi: http://dx.doi.org/10.1175/1520-0469(2000)057<0673:TDOGAI>2.0.CO;2.