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Known issues in CESM2.0 CAM-chem compsets

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mmills
Known issues in CESM2.0 CAM-chem compsets

Affected Releases CESM2.0.0

Errors have been found in 3 surface emissions files used in WACCM and CAM-chem compsets with tropospheric chemistry (TS1 or TSMLT1). The affected compset are:

CAM-chem TS1 Compset Alias: Compset Long name

   FC2000climo     : 2000_CAM60%CCTS_CLM50%BGC-CROP_CICE%PRES_DOCN%DOM_MOSART_CISM2%NOEVOLVE_SWAV

   FC2010climo     : 2010_CAM60%CCTS_CLM50%BGC-CROP_CICE%PRES_DOCN%DOM_MOSART_CISM2%NOEVOLVE_SWAV

   FCHIST             : HIST_CAM60%CCTS_CLM50%BGC-CROP_CICE%PRES_DOCN%DOM_MOSART_CISM2%NOEVOLVE_SWAV

   FCSD                 : SDYN_CAM60%CCTS_CLM50%BGC-CROP_CICE%PRES_DOCN%DOM_MOSART_SGLC_SWAV

WACCM TSMLT1 Compset Alias: Compset Long name

   BW1850            : 1850_CAM60%WCTS_CLM50%BGC-CROP_CICE_POP2%ECO_MOSART_CISM2%NOEVOLVE_WW3

   FWHIST            : HIST_CAM60%WCTS_CLM50%BGC-CROP_CICE%PRES_DOCN%DOM_MOSART_CISM2%NOEVOLVE_SWAV

   FW1850             : 1850_CAM60%WCTS_CLM50%BGC-CROP_CICE%PRES_DOCN%DOM_MOSART_CISM2%NOEVOLVE_SWAV

   FW2000             : 2000_CAM60%WCTS_CLM50%BGC-CROP_CICE%PRES_DOCN%DOM_MOSART_CISM2%NOEVOLVE_SWAV

   FWSD                : SDYN_CAM60%WCTS_CLM50%BGC-CROP_CICE%PRES_DOCN%DOM_MOSART_SGLC_SWAV

Users should use corrected emissions in these compsets with the following steps:

1. Create a case, and run case.setup and preview_namelists in the case directory in order to create the atmosphere namelist file atm_in in the CaseDocs subdirectory.

2. Copy the entire "srf_emis_specifier" list from CaseDocs/atm_in into the user_nl_cam file

3. Edit the user_nl_cam specification to replace the 3 files:

a. Replace emissions-cmip6_BIGALK_bb_surface_1750-2015_0.9x1.25_c20170322.nc with emissions-cmip6_BIGALK_bb_surface_1750-2015_0.9x1.25_c20180611.nc

b. Replace emissions-cmip6_BIGENE_bb_surface_1750-2015_0.9x1.25_c20170322.nc with emissions-cmip6_BIGENE_bb_surface_1750-2015_0.9x1.25_c20180611.nc

c. Replace emissions-cmip6_NO_bb_surface_1750-2015_0.9x1.25_c20170322.nc with emissions-cmip6_NO_bb_surface_1750-2015_0.9x1.25_c20180611.nc

These corrected emissions files have been added to the inputdata repository, and CESM will automatically retrieve them.

Mike Mills
WACCM Liaison
Atmospheric Chemistry Division
NCAR Foothills Lab
Boulder, Colorado USA

mmills

Affected Releases CESM2.0.0

A bug has been found in the release code of CESM2.0.0 regarding the evaporation of rain number in Morrison-Gettelman V2 (MG2) cloud microphysics. This bug has been fixed in the cam6_0_005 tag, which was checked in to the CAM development trunk on July 16, 2018. 

Testing in a coupled piControl simulation reveals the effects of fixing this bug to be very small in the global average. A global average increase in RESTOM of the order 0.01 W m-2 is seen. However, there is more significant redistribution of shortwave cloud forcing over the Pacific Ocean. A 20th Century simulation is ongoing, and impacts will be reported here. 

The bug involves 3 lines (in bold below) in one subroutine: 

components/cam/src/physics/cam/micro_mg2_0.F90
 ______________________________
     do i=1,mgncol
        ! conservation of rain number
        !-------------------------------------------------------------------
        ! Add evaporation of rain number.
        if (pre(i,k) < 0._r8) then
           dum = pre(i,k)*deltat/qr(i,k)
           dum = max(-1._r8,dum)
           nsubr(i,k) = dum*nr(i,k)/deltat
        else
           nsubr(i,k) = 0._r8
        end if
     end do
______________________________
 

The intended behavior for these three lines is:

  1. Calculate the fraction of rain mass that is being evaporated in the current time step.
  2. Limit that fraction to no more than 1 (i.e. do not evaporate more rain than we actually have).
  3. Evaporate the same fraction of the rain number.

However, this code mixes quantities that are averaged over the grid cell, and those only covering the fraction of the cell with precipitation. The effect of the bug is to produce rain drops that are too small in some circumstances, and the correction will cause the particle size to increase.

The limiter is redundant with both the mass conservation code above it (which prevents pre from being too large in the first place), and the number conservation code below it (which would limit nsubr anyway). The bug fix removes this redundant limiter, replacing all three lines with one line (in bold below):

______________________________
     do i=1,mgncol
        ! conservation of rain number
        !------------------------------------------------------------------- 
        ! Add evaporation of rain number.
        if (pre(i,k) < 0._r8) then
           nsubr(i,k) = pre(i,k)*nr(i,k)/qr(i,k)
        else
           nsubr(i,k) = 0._r8
        end if
     end do
______________________________

Mike Mills
WACCM Liaison
Atmospheric Chemistry Division
NCAR Foothills Lab
Boulder, Colorado USA

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