Thick clouds and aerosol differences in a recent cesm tag (alpha08a + updated cam tag)

[adamrher]

Coupled simulations using a newer cesm tag (alpha08a) with an updated cam tag (cam6_4_142) has thicker clouds compared to the previous tag we were using, alpha07g. Run 280 is a B1850C_LTso run with this newer cesm tag, and run 271 uses the older alpha07g tag. The atmosphere parameter tunings are identical in these two runs, other than the increase of seasalt_emis_scale = 1.25->1.5 to compensate for lower sea salt burdens in the new cam tag, owing to a bug fix to the aerosol hygroscopicity (ESCOMP/CAM#1462).

As shown below, the cloud radiative forcings are larger in 280, w/ more negative SWCF and more positive LWCF:

The aerosol burdens are also different, with SOA being larger and sea salt being less:

While SO4 (sulfate) has the same burden, its vertical distribution in 280 is very different from 271, especially near the tropopause:

Back to the clouds, 280 has larger cloud numbers and mass, in both low altitudes (liquid clouds) and high altitudes (ice clouds), although ice cloud increases are primarily in the NH.

Map plots of LWP and IWP differences show the geographical distribution of the cloud changes:

I suspect that the aerosol changes are causing the cloud changes, since the cloud parameters are identical in 280 and 271. My first guess is that the hygroscopicity bug fix is leading to the aerosol changes, and so I currently have a clone of 280 running, but reverting the hygroscopicity bug fix, in order to see whether the aerosols and clouds looks more like 271. I'll update this thread as those results come in.

@cecilehannay @dlawrenncar @JulioTBacmeister @islasimpson @tilmes @jfasullo

[jfasullo]

Thanks Adam - I've now made output from my CMAT analyses available to the group at https://webext.cgd.ucar.edu/Multi-Case/CMAT/CMATv1_CESM3dev/

I'm happy to walk everyone through it to show how it can be used. The first thing I look at is the table - to gauge the overall score of a run and its breakdowns into components. Then I'll look at the flags (in the archive table below). Most of our recent runs have global and NH albedos/SWCF that are too high. The runs are also drifting cold. Evaporation over ocean versus land is also too high. You can click on the flag links to see a plot that's the basis for the flag. I think you and I are looking at the same issue, per your analysis above. The NH ice issue seems to be a reasonable cause. Please let me know when your clone of 280 is ready to be run through the package and I'll see if score improve. Thanks!

[tilmes]

can you point us to the atm/hist output of these cases?
Shawn can run the diagnostics to compare aerosol lifetimes.

[adamrher]

sure:

/glade/derecho/scratch/hannay/archive/b.e30_alpha07g.B1850C_LTso.ne30_t232_wgx3.271/
/glade/derecho/scratch/hannay/archive/b.e30_alpha08a.B1850C_LTso.ne30_t232_wgx3.280/

[dlawrenncar]

Are there results yet about whether or not this looks like related to the hygrosopicity bug fix?

…

On Thu, Jan 22, 2026 at 2:29 PM Adam Herrington ***@***.***> wrote: sure: /glade/derecho/scratch/hannay/archive/b.e30_alpha07g.B1850C_LTso.ne30_t232_wgx3.271/ /glade/derecho/scratch/hannay/archive/b.e30_alpha08a.B1850C_LTso.ne30_t232_wgx3.280/ — Reply to this email directly, view it on GitHub <#271 (reply in thread)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AFABYVDPDXSFKE3DBT2LEZL4IE6LVAVCNFSM6AAAAACSPLAEJGVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTKNJXGQ3TSOA> . You are receiving this because you were mentioned.Message ID: ***@***.***>

[adamrher]

I don't want to spike the football because the IWP & LWCF are still high, but it looks like SWCF is back to 271 levels after reverting the hygro bug fix (280.hygro). The run is only 8 years. I should have 12 years by the morning and will do a more thorough analysis for the CAM7 mtg.

[tilmes]

Are the SW cloud forcing changes mostly over low-cloud areas? In that case it may be cause just by the sea-salt change? Would it make sense for a test to rescale the seasalt that it would be more similar 176 in burden, so you could scale it up to see what happens. Another option, changes in sulfate are strong above 500hPa, since @shawnusaf has done the diagnostics for <500hPa. Maybe @shawnusaf could you rerun these again, and do it for the entire column?

[adamrher]

SW cloud changes are from low clouds and some contribution from high clouds. The sea salt doesn't work in the right direction -- 280 has more low clouds and less sea salt than 271. 280.hygro likely has more sea salt and less low clouds, relative to to 280.

[tilmes]

I agree, that is strange, but it could be because more mass went to the accumulation and Aitken mode and the main reduction is in the coarse mode, see below:

Here are changes to sea-salt: 280 - 271
Accumulation Mode: Increase in sea-salt up to 800hPa, slight decrease above, so increase in CCN for low clouds possible

Aitken Mode: Also in general, an increase in the Aitken mode below 800hPa

Coarse mode: stronger decrease in the coarse mode in the lower levels: the coarse mode does not affect the clouds that much, so, the burden change is mostly from the coarse mode, and therefore it is likely that even with less mass, we may get more SWCF due to the increase in accumulation and Aitken mode.

Accumulation model sulfate: more mixed picture

Here are percent changes, so there is no consistent trend but it could change the SWCF:

Coarse mode sulfate, also shows reductions below 800hPa, so likely there is a net increase of Aitken and Accumulation mode in sulfate too, since the modes are internally mixed, and that can be a result of the sea-salt changes

So, I would say, the hygro fix causes more removal of coarse mode seasalt but CCN could be still increased.
Simone

[adamrher]

This is interesting. Are you going to be at the CAM7 mtg this morning? It would be helpful to present this...

[shawnusaf]

Budget tables for these cases are located here: https://acomstaff.acom.ucar.edu/shawnh/ADF/b.e30_alpha07g.B1850C_LTso.ne30_t232_wgx3.271_60_70_vs_b.e30_alpha08a.B1850C_LTso.ne30_t232_wgx3.280_60_70/website/index.html.

[tilmes]

@shawnusaf Could you run the diagnostics table for the total column, not just below 500hPa, also could you run the ADF, including zonal means for the aerosols (all modes)?
Thanks Simone

[tilmes]

Thanks a lot. Here are some comments:
In 180: SOA burden is larger in 180 because of a little bit more production, probably from changes in biogenic emissions (land?).
Sea-salt is lower, wet removal has substantially increase, and lifetime removed, as expected from the change.
Dust has not changed much in general.
SO4 burden is the same, wet removal is very similar, and lifetime has not changed. I am not sure if the changes in sulfate you are seeing are significant. I will check the sulfate numbers later if I have a chance to see if something moved between modes, but it would show in the burden, lifetime etc.

[shawnusaf]

Hi Simone, See https://acomstaff.acom.ucar.edu/shawnh/ADF/b.e30_alpha07g.B1850C_LTso.ne30_t232_wgx3.271.column_60_70_vs_b.e30_alpha08a.B1850C_LTso.ne30_t232_wgx3.280.column_60_70/website/index.html .

…

-Shawn

On Fri, Jan 23, 2026 at 9:01 AM tilmes ***@***.***> wrote: @shawnusaf <https://github.com/shawnusaf> Could you run the diagnostics table for the total column, not just below 500hPa, also could you run the ADF, including zonal means for the aerosols (all modes)? Thanks Simone — Reply to this email directly, view it on GitHub <#271 (reply in thread)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AUIO2OW7MYOFKEXLQWSQE5D4IISUBAVCNFSM6AAAAACSPLAEJGVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTKNJYGE2DKNQ> . You are receiving this because you were mentioned.Message ID: ***@***.***>

[adamrher]

Providing a general update on the issue here. As Simone found that the hygroscopicity bug fix explains the redistribution of sea salt -- an increase in ncl_a1 & ncl_a2, but a reduction in ncl_a3 -- that appears to explain the SWCF changes, which occur over large swaths of the oceans. See 271-280 SWCF change plot below. We therefore started two runs to tune away the extra cloud brightening, runs 295 and 296:

296 is the preferred tuning, as it reduces microp_aero_wsub_scale = 1->0.8 and more directly corrects for over activation of cloud droplets. It also produces a cloud field that is more similar to 271 and we don't really want to thin the MarineSc as represented in 271. 296 is only 20 years, and we've stopped it because we are now focused on combining these new tunings with newly preferred ocean 287 to see where RESTOM ends up and if we need add'l tuning.

The other change in this new tag is the change in aerosols -- primarily sulfate and SOA -- that is not explained by the hygroscopicity bug fix. 271 used an older IVOC file and we moved to a newer file in the new tag. To test whether the new IVOC file explains the aerosol changes, I've reverted to the file in 271 in the new tag. This run is called 280.ivoc

The IVOC file change explains most of the increase in SOA. @wwieder also saw some changes in Isoprene emissions over land which may explain the residual change in SOA. The bottom line is the new IVOC file gives us more SOA and that's what we have to work with.

What's left to explain is the increase in Sulfate in the new tag, in particular in the UTLS region. I don't have a sense for whether these changes are large or small, only that the total burden seems mostly unchanged (see top of the thread). @tilmes if you have time to look into this, I would be interested in your take. If it helps at all, here's what the individual so4 modes look like:

[tilmes]

There are some dependencies on the modes, if num_a3 decreases from the sea-salt, that also increases in part the so4_a3, because the modes are internally mixed, and I think we see that below 800hPa. I am not exactly sure why SO4 increases in the UTLS, but that could be due to some changes in the modes. I do not see any significant changes in the burden or removal rates, or gas exchange, so production, else, I would check DMS emissions, since they could result in these changes, but I don't see any indications for that. I will think about it more...