Dust dry and wet deposition fluxes from CAM in bin 4 are zero in cam6_3_112, but are non-zero in later CAM versions

[oleson]

In a fairly recent CAM run intended to generate coupler history forcing to force CTSM:

/glade/campaign/cesm/cesmdata/cseg/runs/cesm2_0/f.cam6_3_112.FLT1850.ne30.landspinup.001

which has a compset of:

1850_CAM%DEV%LT%GHGMAM4_CLM51%BGC-CROP_CICE%PRES_DOCN%DOM_MOSART_SGLC_SWAV_SESP

the atmImp_Faxa_dstwet4 and atmImp_Faxa_dstdry4 fields in the *.cpl.hx.atm.24h.avrg.* have zero values.
I believe this is consistent with CESM2 results.

However, I see that in more recent versions of the model, these fields are non-zero.

E.g., I created a case with coupler history output using cesm3_0_alpha08c with this compset:

1850_CAM70%LT_CLM60%BGC-CROP_CICE%PRES_DOCN%DOM_MOSART_DGLC%NOEVOLVE_SWAV_SESP

and the coupler history fields generated for these dust bins, atmImp_Faxa_dstwet4 and atmImp_Faxa_dstdry4, are non-zero.

I'm curious to know if this is expected? Is this due to recent changes in CAM aerosols?
Thanks for any input!

 

[Danny Leung]

Hi Keith!
I think this is related to the recent aerosol code refactoring done by Francis Vitt and Simone Tilmes.
I just talked to Francis about this thread. Francis said he remembers fixing this issue with Simone a while ago to have all four bins carry dust depositions. Francis said it should be more correct now given the bug is fixed and you have 4 bins of dust dep.
I had a quick look at the CAM code myself. The main calculation happens under
src/chemistry/aerosol/aero_deposition_cam.F90, where dstwet1 through dstwet4 are filled with values before going to
src/cpl/nuopc/atm_import_export.F90 for exporting to CTSM.
You might know that CAM by default uses a modal aerosol model (MAM) with three modes of dust depositions. So, CAM needs to redistribute the dep fluxes from three modes to four bins, which happens in the aero_props%rebin_bulk_fluxes subroutine/procedure in aero_deposition_cam.F90.
I am guessing CTSM eventually needs the sum across these four bins for dust dep, so to me having dust dep in the 4th bin shouldn't be a big problem. But I suppose Francis and Simone did things the right way so things remain consistent or, if not, become better because the bug is now fixed.
Let me know otherwise, or if you have further inquiries!
Danny

 

[oleson]

Thanks for that information @Danny Leung !

 

[Danny Leung]

No problem! Just another quick note, Simone reminded me that dustwet and dustdry for different bins are for different dust aerosol sizes, so, that may change answers in CTSM if any module depends on dust sizes (e.g., I am not sure if the dust impact on snow albedo is dust size-dependent). Similarly, this dustdep change may change ocean BGC simulations too since bioavailable nutrients from dust are size-dependent.