Why does the methane concentration keep increasing in the GEOS-Chem CH4 forward run?

[XunZhangNJU]

Your name

XunZhang

Your affiliation

Nanjing University

GEOSChem v14.3.1 CH4 forward run .

When I use two global transport models to simulate CH4 with the same inventory data, initial CH4, and CH4 sinks, why does the CH4 concentration from GEOS-Chem keep increasing? What is causing this? Is there something wrong with my GEOS-Chem inventory settings? Are there other factors involved?
For example, I compared TM5 and GEOS Chem CH4 forward run.
Figure 1 shows time series of the same layer daily CH4 conc. in july, 2015.

TM5 shows stable monthly fluctuations, whereas GEOS-Chem's CH4 fluctuations differ significantly from TM5.
What is causing this situation? Could you provide me with some advice or literature references to address the issue of continuously increasing GEOS-Chem CH4 concentration? Thanks a lot.

[msulprizio]

It was recently discovered that the CH4 simulation does not conserve mass (see #2409). We recently merged a fix (#2428) into the 14.5.0 development branch. This version is undergoing final benchmarking and will be released in the coming weeks.

[XunZhangNJU]

Thanks @msulprizio. I have another question, which is about the order in which GEOS-Chem simulates CH4. Let me give a simple example: the model performs operations advection/convection/diffusion/emission/chemistry as a sequential series, thus for example: a c d e s. So, what operations does the GEOS-Chem model perform?

[msulprizio]

@XunZhangNJU I would refer you to main.F90 which is the driver routine for GEOS-Chem (in GCClassic). There you can see the order of operations which is currently: transport, dry deposition, emissions, mixing, convection, chemistry, wet deposition.