Describe the dataset issue
For almost all CMIP6 models with a hybrid pressure coordinate, the bounds for the auxiliary coordinates a (or ap) and b are not set correctly, e.g.

        double ap(lev) ;
                ap:long_name = "vertical coordinate formula term: ap(k)" ;
                ap:units = "Pa" ;
                ap:missing_value = 1.e+20 ;
                ap:_FillValue = 1.e+20 ;
                ap:cell_methods = "time: point" ;

with missing

                ap:bounds = "ap_bounds" ;

As a consequence, for all these models the bounds for the derived coordinate air_pressure is missing.

• Data file has been changed by you in any way
• Project (CMIP5/CMIP6/OBS/obs4mips/etc): CMIP6
• Full dataset description (fill out as many as you know, please):
  • dataset: BCC-CSM2-MR, BCC-ESM1, CAMS-CSM1-0, GISS-E2-1-G, GISS-E2-1-H, MIROC-ES2L, MPI-ESM1-2-HR, MRI-ESM2-0, NESM3, SAM0-UNICON
  • experiment: historical
  • mip: Amon
  • variable used: cl
• Other notes and mentions: The listed datasets are the ones where this is the only issue for cl. I will open further issues for other datasets which have this problem plus additional issues.

Paths on DKRZ
/mnt/lustre02/work/bd0854/DATA/ESMValTool2/CMIP6_DKRZ/CMIP/BCC/BCC-CSM2-MR/historical/r1i1p1f1/Amon/cl/gn/v20181126/cl_Amon_BCC-CSM2-MR_historical_r1i1p1f1_gn_200001-201412.nc
/mnt/lustre02/work/bd0854/DATA/ESMValTool2/CMIP6_DKRZ/CMIP/BCC/BCC-ESM1/historical/r1i1p1f1/Amon/cl/gn/v20181217/cl_Amon_BCC-ESM1_historical_r1i1p1f1_gn_185001-201412.nc
/mnt/lustre02/work/bd0854/DATA/ESMValTool2/CMIP6_DKRZ/CMIP/CAMS/CAMS-CSM1-0/historical/r1i1p1f1/Amon/cl/gn/v20190708/cl_Amon_CAMS-CSM1-0_historical_r1i1p1f1_gn_200001-201412.nc
/mnt/lustre02/work/bd0854/DATA/ESMValTool2/CMIP6_DKRZ/CMIP/NASA-GISS/GISS-E2-1-G/historical/r1i1p1f1/Amon/cl/gn/v20180827/cl_Amon_GISS-E2-1-G_historical_r1i1p1f1_gn_200101-201412.nc
/mnt/lustre02/work/bd0854/DATA/ESMValTool2/CMIP6_DKRZ/CMIP/NASA-GISS/GISS-E2-1-H/historical/r1i1p1f1/Amon/cl/gn/v20190403/cl_Amon_GISS-E2-1-H_historical_r1i1p1f1_gn_200101-201412.nc
/mnt/lustre02/work/bd0854/DATA/ESMValTool2/CMIP6_DKRZ/CMIP/MIROC/MIROC-ES2L/historical/r1i1p1f2/Amon/cl/gn/v20190823/cl_Amon_MIROC-ES2L_historical_r1i1p1f2_gn_185001-201412.nc
/mnt/lustre02/work/bd0854/DATA/ESMValTool2/CMIP6_DKRZ/CMIP/MPI-M/MPI-ESM1-2-HR/historical/r1i1p1f1/Amon/cl/gn/v20190710/cl_Amon_MPI-ESM1-2-HR_historical_r1i1p1f1_gn_200001-200412.nc
/mnt/lustre02/work/bd0854/DATA/ESMValTool2/CMIP6_DKRZ/CMIP/MRI/MRI-ESM2-0/historical/r1i1p1f1/Amon/cl/gn/v20190308/cl_Amon_MRI-ESM2-0_historical_r1i1p1f1_gn_200001-200912.nc
/mnt/lustre02/work/bd0854/DATA/ESMValTool2/CMIP6_DKRZ/CMIP/NUIST/NESM3/historical/r1i1p1f1/Amon/cl/gn/v20190705/cl_Amon_NESM3_historical_r1i1p1f1_gn_185001-201412.nc
/mnt/lustre02/work/bd0854/DATA/ESMValTool2/CMIP6_DKRZ/CMIP/SNU/SAM0-UNICON/historical/r1i1p1f1/Amon/cl/gn/v20190323/cl_Amon_SAM0-UNICON_historical_r1i1p1f1_gn_200001-200912.nc

Fix provided in #529.