kdorheim
commented
10 months ago Implement aerosol forcing parameters to account for the new analysis by Zelinka et al. (2023)
Hector 3.0 is calibrated to forcing values in the AR6, which were informed by analysis of ESM model results as documented in Smith et al. (2021). We adjust aerosol forcing parameters in this version of the model from the values documented in Smith et al. (2021) to account for the new analysis by Zelinka et al. (2023). Zelinka et al. find, due to a coding error, a higher absorbing forcing, which is largely countered by a larger cloud indirect forcing. We therefore scale the magnitude of BC, other cooling aerosols, and F_ACI by the ratio of average CMIP6 model values from Zelinka et al. (2023, Table 2) and Smith et al. (2020) (Table 6). These changes largely offset each other, but result in a slightly higher net negative forcing for the current day and also result in slightly different total aerosol forcing pathways over time.
[Note two different references. Smith et al. 2021, and Smith et al. 2020. No relation :)
The resulting changes are to the following 5 aerosol constants:
SI Eq 12 -ρ𝐵𝐶 (new) = ρ𝐵𝐶(old) * ( (0.37+0.07) / (0.28 + 0.07) )**
SI Eq 13 through SI Eq 15 (for all the reflecting forcings) multiply by ( 0.65/0.63 )
SI Eq 16 -ρaci (new) = ρaci (old) * ( 0.88/ 0.81 )
The result will be a about a 5% larger net negative forcing, and a larger positive ARI from BC.
** Note that for BC we have assumed that the bulk of long-wave forcing is also due to BC absorption. Note that, while the long-wave forcing in Zelinka et al. (2023) and Smith et al. (2020) differ, this difference is not due to a coding change, but due to a different methodology in Zelinka et al. regarding how land surface warming is treated (Zelinka pers. communication). Because this is a definitional difference, we assume the LW forcing is the same between the two studies and allocate this to BC.
Smith et al. (2021) Smith, C. J., Kramer, R. J., Myhre, G., Alterskjær, K., Collins, W., Sima, A., Boucher, O., Dufresne, J.-L., Nabat, P., Michou, M., Yukimoto, S., Cole, J., Paynter, D., Shiogama, H., O'Connor, F. M., Robertson, E., Wiltshire, A., Andrews, T., Hannay, C., Miller, R., Nazarenko, L., Kirkevåg, A., Olivié, D., Fiedler, S., Lewinschal, A., Mackallah, C., Dix, M., Pincus, R., and Forster, P. M.: Effective radiative forcing and adjustments in CMIP6 models, Atmos. Chem. Phys., 20, 9591–9618, https://doi.org/10.5194/acp-20-9591-2020, 2020.
Zelinka et al. (2023) Zelinka, M. D., Smith, C. J., Qin, Y., and Taylor, K. E.: Comparison of methods to estimate aerosol effective radiative forcings in climate models, Atmos. Chem. Phys., 23, 8879–8898, https://doi.org/10.5194/acp-23-8879-2023, 2023.
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Related to #700, update the aerosol scalers