Produce Nitrogen (N2) forcing ancillaries for the ACCESS-ESM1.6 `piControl` experiments

[atteggiani]

From the CMIP7 inputs meeting notes of 08.10.2025:

• N2 forcing data is available. @penguian to check if NCI has downloaded this data.
• N2 forcing is a surface field, so we need to interpolate it and make sure it’s non-zero over land.

[MartinDix]

input4MIPS has 4 nitrogen deposition variables: drynhx, drynoy, wetnhx, wetnoy (same as CMIP6). From the Met Office CMIP6 ancillary creation https://code.metoffice.gov.uk/trac/ancil/wiki/CMIP6/ForcingData/NitrogenDeposition

The source data provide four different N deposition species, wet and dry, oxidesed and reduced forms (NOy / NHx). UKESM1 makes no distinction between these in terms of usefulness to plants, so they have been summed to provide the one N deposition fields required by the model. The units are unchanged (kgN/m2/s). At no point in the making of this ancillary has the field been scaled by the land fraction - the rate at which Nitrogen is deposited onto the land surface within a gridbox is not dependent on the extent to which the gridbox contains ocean.

https://input4mips-cvs.readthedocs.io/en/latest/dataset-overviews/nitrogen-deposition/ says

For pre-industrial control, please use the monthly climatology files (frequency value of monC). Simply apply this forcing on repeat.

Do they mean the climatology of the historical period? This would be different to CMIP6 where there were separate 1850 input4MIPS files that the Met Office used to create the PI control files

CMIP6 PI control
/g/data/vk83/configurations/inputs/access-esm1p5/modern/pre-industrial/atmosphere/land/biogeochemistry/global.N96/2020.05.19/Ndep_1850_ESM1.anc

Historical
/g/data/vk83/configurations/inputs/access-esm1p5/modern/historical/atmosphere/land/biogeochemistry/global.N96/2021.06.22/Ndep_1849_2015.anc

[MartinDix]

Using FZJ-CMIP-nitrogen-1-1, the sum of the species looks quite different between CMIP6 and CMIP7.

The ESM1.5 ancillary field matches if it's assumed to be in g m^-2 day^-1. From
https://github.com/ACCESS-NRI/UM7/blob/9d6c7869d6f54d3586a21dcf7946c732743d0fc2/umbase_hg3/src/atmosphere/CABLE/src/coupled/esm/cable_define_types.F90#L234

         Ndep,    & ! nitrogen deposition (gN m-2 d-1)

	Global total (kg/s)
CMIP6	1073
CMIP7	1161
ESM15	1074

Plotting the separate species shows CMIP7 has much more of the oxidised form and less reduced (CMIP6 solid lines, CMIP7 dashed).

[penguian]

The nitrogen deposition forcing datasets are now available as FZJ-CMIP-nitrogen-1-2. This fixes PCMDI/input4MIPs_CVs#385

[MartinDix]

The 1-2 dataset also corrects a double counting in the wetnoy term, #94 (comment)

Here comparing with the ESM1.6 field created by #104

	Global total (kg/s)
CMIP6	1073
ESM15	1074
CMIP7 1-1	1161
CMIP7 1-2	1004
ESM16	1004