Hi,
I was hoping to ask about how methane is dealt with in UKCA.
For the RCP scenario, where methane is treated as an LBC, there still appears to be an input emissions file (CH4_biomass) and output of methane emissions in stash-code 50157. I am wondering if the methane output is a combination of this biomass emissions file and the boundary condition, or if the boundary condition is over-writing this. Perhaps also this loop within ukca_add_emiss_mod.F90 is setting the field to zero:
IF (L_ukca_strat .OR. L_ukca_stratcfc .OR. L_ukca_strattrop) THEN
DO l = 1, n_boundary_vals
IF (advt(k) == lbc_spec(l)) THEN
em_field (:,:,:, k) = 0
em_field (:,:,surface_level,k) = &
(lbc_mmr(l) - tracers(:,:,surface_level,k)) * &
mass(:,:,surface_level) / surf_area(:,
/ timestep
But it looks like the diagnostics are created after this point and as the CH4 emissions output is non zero then I am confused as to whether this loop is setting the field to zero. I tried a run without this input file but received this error ‘Error message: CH4 missing in the emission files’.
I hope that this made some sense!
Many thanks for your help,
Hannah
The way lower boundary conditions (LBCs) are specified in UKCA is a bit complex - essentially, rather than taking the concentrations from the RCP and setting the lowest level of the corresponding tracer to be this value, instead UKCA calculates the flux required to adjust the lowest model level of the tracer to be the LBC value - this is what the code above is doing. It then uses the emissions code to apply this to the tracer using TR_MIX so this flux is actually distributed throughout the boundary layer. This is why the surface concentrations of fields like CH4 aren’t the same everywhere. This is also why the CH4 emissions flux has non-zero values - what is being output is the effective emissions required to make the lowest level match the LBC. It should have some negative as well as positive values.
Additionally, CH4 (for historical reasons) also has to have an emissions file supplied, even though this doesn’t want to be used in an LBC driven run. The data in this file shouldn’t be being picked up so long as CH4 is defined as an LBC - this is probably a hang-up from older UKCA versions which tried to consider CH4 emissions in addition to an LBC. I believe that the standard jobs should have the CH4 values in this file set to zeros to emphasise this fact. If you were worried you could put some large values in this file to double-check it’s being ignored.
Thanks,
Luke
Hi Luke,
Thanks for the explanation. I am using a copy of u-be303 from the Archer2 Standard Jobs and the file called is: $CMIP6_ANCILS/n96e/timeseries_1850-2014/AerosolChemistryEmissions/v3/CH4_biomass_low_1849_2015_time_series.nc
Which has non zero values. The output from my branch (right) (which has no methane edits currently) shows that the emissions output for methane matches the input from this file (left) (although is scaled):
I don’t think this is my branch interfering with the output as when I check the ainitial file: /work/y07/shared/umshared/ukesm/restarts/az513a.da19790101_00 the result is similar. As none of these have negative values, they look more like the input than what I expected an effective emission to look like.
The reason I am looking at this is that in another branch I am currently making edits to the LBC field for methane and in this case need to place a new file in the ukca_em_files and the biomass file is altering the surface concentrations I get slightly as is now being read in. I can take this out very easily, but just wanted to check about its purpose before doing so!
Thanks,
Hannah
Is this job a straight copy of the standard job, or are you intending to use methane emissions and you have purposefully removed the LBC aspects?
Thanks,
Luke
Yes a copy but I have edited the branch and some diagnostics. The output in the past answer was from a different job but for the one I am currently looking at methane in, I have a new latitudinal varying periodic input file for the methane LBC and have this being read in in the same way as I had previously done for hydrogen.
To get this to work, I have the emissions switched on but have code to get this to read in as a boundary condition later however I noticed that this additional biomass emissions file was also being read in!
(So not methane emissions, still boundary condition)
Thanks,
Hannah
Hi Hannah,
Sorry, I’m a bit confused here. What species is the biomass file going into?
If you want to prevent any CH4 emissions file from being used, could you set the contents to 0.0 to be safe, if the code raises an error when the file is removed?
Thanks,
Luke
Hi Luke,
Yes CH4- setting to zero works fine, I just wanted to check before doing so. I was also confused regarding the CH4 emissions file output appearing to not be effective emissions but as this is the same as in the dump file, I assume this is fine.
I also wanted to check that after copying over these input files to my work directory and creating new files, it is fine to delete the initial copied file (as this file is still write protected)?
Thanks,
Hannah
It should be fine to delete the file you initially copied if you aren’t using it anymore. You won’t be able to delete the file in the central location (or owned by somebody else), I assume here it’s just a copy you’ve taken to work with and the original is still elsewhere. The write protection was probably added to the original file to prevent it being accidentally deleted or overwritten.
Best wishes,
Luke
