Analysis of historicals 092 and 098b #561
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Does it seem like a lack of cloud water sensitivity through the period then? Maybe precipitation efficiency does not decrease in response to aerosols (smaller cloud drops) in CESM3 and it does in CESM2?? |
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Also maybe And |
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Following suggestions from @brianpm in the project meeting yesterday, here's a look at cloud liquid water path versus cloud droplet number concentration. But first, here's the cloud droplet number concentration and the anomalies which shows that our new runs start off with a higher cloud droplet number concentration and it increases less over time. Looking at high cloud liquid water varies with CDNUMC, we can also see that the cloud liquid water increases less with cloud number concentration than it did in CESM2 The plot below shows on the left the actual TGCLDLWP and then on the top right estimated TGCLDLWP assuming a linear relationship with CDNUMC where b is the slope of that linear relationship. That linear assumption works pretty well to explain the different behaviors. This final plot now focusses on 098b and shows the difference in TGCLDLWP between LENS2 and 098b (black). It then shows the difference that you'd get if you calculate the 098b value assuming it has the same CDNUMC as LENS2 but its own d TGCLDLWP/dCDNUMC slope (purple) i.e. the effect of the differing slopes alone and then it shows the opposite, where you assume that 098b has the same clope as LENS2 but it has it's differing CDNUMC (green). Both are contributing to the overall difference in TGCLDLWP with the difference in the rise in CDNUMC being a bit larger than the effect of the differing slopes. |
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As suggested by @adamrher, the slopes of TGCLDLWP versus CDNUMC are not that different if we restrict LENS2 to only the period where CDNUMC is higher. Indeed, that's the case. So our high CDNUMC in the new runs is probably the ultimate driver of the difference in the slopes as opposed to a difference in the physics. |
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The difference between 92 and 98 are:
For the BLT1850, we have: I examined these BLT1850 runs to distinguish the respective contribution of RRTMGP and clubb bugfix to the changes in AODVIS and in burden. (Note that I don't have the burdens for 92). It seems that the differences are coming from RRTMGP. I checked the namelists, and there are no major changes in tuning parameters or files (except those needed for RRTMGP and also the file used for Th difference in namelists are:
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The CESM2 historical simulations are warming too much in the 20th century. This seems like it might be related to a lack of influence of aerosols on shortwave cloud radiative effect:
Aerosols are increasing, so it's not that something has gone wrong with the rising aerosol forcing:
I noted in the CESM project meeting that the AODVIS in 098b was almost double that in LENS2 and we had some discussion about whether the different background aerosol state might be affecting the response to a further rise. But much of that difference in AODVIS seems to have come in between 092 and 098b, so I think we can rule out an importance of the background aerosols on their subsequent change since 092 and 098b are similar. 092 seems like it compares better with observed estimates of AODVIS e.g., this paper says global mean AOD from 2003-2012 is 0.126
It seems to be dust and sea salt that has changed the most between 092 and 098b.
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