Expected / suggested improvements in CAM-Nor intended for NorESM3

[oyvindseland]

This discussion point is intended for sharing and engaging with the community on which model processes / parameterisations should be or should not be included in CAM-Nor for the NorESM3 release beyond what is already included in the current NorESM2.5 development version. The dead-line for the model freeze is not formally decided on, but expected to be June 2025: Each of the processes will have its own issue for implementation
I have started a list below in more or less random order. Please add missing processes, or suggested removals from the list

• Reverting the global maximum ice particle number maximum back to a local limiter in time and space(nimax) This parameter was used in an erroneous way in CESM2 and NorESM2. This was corrected in later versions of NorESM2.x The global limiter was found to give a better ECS in particular when working on LGM simulations, but has no particular physical argument to it. At the present time it is set to 1 cm-3. While re-introducing the old parameterisation is relatively straightforward from a technical point of view, but may have a large impact on the model climate sensitivity.

• New radiation package: Unclear amount of work for updating aerosol optical properties.

• Renaming the aerosol-free and cloud-free TOA radiation fluxes used for the Ghan aerosol diagnostic. Now these names are FSNT_DRF and FSNTCDRF.

• Making sure that the necessary TOA and TOM (top of the the model) radiation fluxes are diagnosed.

• Lower-resolution SE grid for the atmosphere (NE16 instead of NE30?)

• Introduce small correction to the DMS emission calculation in case of prescribed DMS ocean concentrations. Currently there is a small mismatch between the calculation in CAM (used when prescribing DMS ocean concentrations) and the calculation in iHAMOCC (used when running with fully-coupled ocean).

• Prognostic variables for all active GHG. CESM3 has / planned to calculate all GHG prognostically. This is can be done either by full ghg cycles including emissions, but also by nudged surface concentrations and prescribed loss rates. Increases the number of tracers by 5.

• Make the full-chemistry (from NorESM2.3) available in NorESM2.5. The aim is to update the NorESM full-chemistry with what is newly available in CESM (a CAM-CHEM type of chemical scheme).

• Change in chemical yield of secondary aerosol production from isoprene / monoterpene:

• Increased scavenging of sea-salt and sulphate in convective clouds:

• Emissions from forest fires: Should we use prognostic or prescribed fire emissions. CAM7 use prognostic fire emissions from CLM. CLM-Fates calculates fire emissions but with a different parameterisation (and yield factors)

• Update of CAM / CLUBB / PUMAS from ESCOMP: Should we set a dead-line for when we do the updates more or less automatically and then decide on a case by case basis.

• Dust parameterisation: Leung (new) or Zender (old)

• New droplet activation (Betancourt-Nenes) Not published yet and may have a strong impact on aerosol ERF.

• Parameterisation of secondary ice formation (RaFSIP)