Running the Venus PCM with age of air
The age of air expresses the time since an air parcel has been in a user-specified source region, such as an altitude level. It is based on a passive tracer that circulates through the atmosphere. The calculation uses the ratio of the abundance of the tracer in each gridbox to the abundance in the source region to determine how long it has been since the air in the gridbox has been in the source region. It is a useful way to find transport timescales between regions and reveal teleconnections.
Basic settings in defs files
In gcm.def: iflag_trac = 1
In physiq.def: ok_aoa = y
In traceur.def: 10 10 aoa
These three settings are necessary to include age of air in the simulation. Currently, the age of air tracer cannot be included in a chemistry simulation, i.e. chemistry and age of air must be computed in separate simulations. This is because the increasing age of air tracer abundance appears to cause numerical instability when included in the tracer array sent to the photochemistry scheme.
Customisation
In physiq.def, the flag
reinit_aoa = y/n
controls whether the age of air is reinitialised (set to 0) at the start of the simulation. The default is y, so that each simulation starts fresh. If you change the setting to n, the simulation will read the existing value of age of air from restartphy and add it to the calculation in the new simulation as an offset. This makes it possible to chain together multiple restarts without losing track of the age.
In physiq.def, the flag
lev_aoa = 1
controls the source level where the age of air tracer is released. It can be set to any model level. Default is 1, the surface.
Output
Turning on ok_aoa adds two 3-D fields to the output: the tracer abundance ("aoa" or "aoaVL1") and the age of air in seconds ("age").
Bibliography
Maureen Cohen, James Holmes, Stephen Lewis, Manish Patel. Planetary Waves Drive Horizontal Variations in Trace Species in the Venus Deep Atmosphere, The Planetary Science Journal, 5 (219) (2024). DOI: https://doi.org/10.3847/PSJ/ad76a8
Maarten Krol, Marco de Bruine, Lars Killaars, Huug Ouwersloot, Andrea Pozzer, Yi Yin, Frederic Chevallier, Philippe Bousquet, Prabir Patra, Dmitry Belikov, Shamil Maksyutov, Sandip Dhomse, Wuhu Feng, and Martyn P. Chipperfield, Age of air as a diagnostic for transport timescales in global models, Geoscientific Model Development, 11 (8), 3109–3130 (2018). DOI: https://doi.org/10.5194/gmd-11-3109-2018
