Improved cell_methods for land only variables?

[MartinDix]

Some variables like GPP are the average over only the land part of the grid box. The publihsed CMP6 data has

gpp:standard_name = "gross_primary_productivity_of_biomass_expressed_as_carbon" ;
gpp:cell_methods = "area: mean where land time: mean" ;

whereas um2nc output only shows

fld_s03i261:standard_name = "gross_primary_productivity_of_biomass_expressed_as_carbon" ;
fld_s03i261:long_name = "GROSS PRIMARY PRODUCTIVITY KG C/M2/S" ;
fld_s03i261:cell_methods = "time: mean" ;

Can it be made to show the extra information?

[MartinDix]

ESM1.6 (and AM3) GPP is weighted by land fraction so is the full grid box average, not the land only part.

However many other variables like runoff and snowdepth are the land average.