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CAM5 ndrop bug

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steve.ghan@...
CAM5 ndrop bug

Affected releases - CESM1.0.z, CESM1.1.z, and CESM1.2.z
I found a bug in the heart of the droplet nucleation calculation in CAM5. There are two consecutive lines in ndrop.F90:
g1=g1sqrt*g1
g1=g1sqrt*g1
There should only be one.

This bug has been there for a long time.

The immediate impact depends on the aerosol concentration and updraft velocity. Although the impact is large for weak updraft velocity and polluted conditions, the minimum updraft velocity of 0.2 m/s prevents the bug from influencing the simulation much. 10-year simulations at 1 and 2 degrees (see the links below) produce changes in the aerosol radiative forcing of less than 0.05 W/m2. The present day global mean SWCF is weakened by a bit more than 0.3 W/m2.

I recommend that the second line be removed from ndrop.F90. Some tuning will be required to restore the energy balance, but it could be done in combination with tuning for other changes.

I will report this at Breckenridge.

Steve Ghan

1-deg FV, 10-year run: (Control: standard CAM5.3 (fc5_2000_f09_pd); Experiment: CAM5.3 with bug fix (fc5_2000_f09_pd1)):

AMWG diagnostics (PD): https://portal.nersc.gov/project/PNNL-PJR/bsingh/po-lun/1deg/fc5_2000_f09_pd1-fc5_2000_f09_pd/
Aerosol diagnostics (PD): https://portal.nersc.gov/project/PNNL-PJR/bsingh/po-lun/1deg/fc5_2000_f09_pd1-fc5_2000_f09_pd/aerosol.html
Aerosol Indirect Forcing for standard CAM5.3: https://portal.nersc.gov/project/PNNL-PJR/bsingh/po-lun/1deg/fc5_2000_f09_pd.eps.png
Aerosol Indirect Forcing for CAM5.3+bug fix: https://portal.nersc.gov/project/PNNL-PJR/bsingh/po-lun/1deg/fc5_2000_f09_pd1.eps.png

2-deg FV, 10-year run: (Control: standard CAM5.3 (fc5_2000_pd); Experiment: CAM5.3 with bug fix (fc5_2000_pd1)):

AMWG diagnostics (PD): https://portal.nersc.gov/project/PNNL-PJR/bsingh/po-lun/2deg/fc5_2000_pd1-fc5_2000_pd/
Aerosol diagnostics (PD): https://portal.nersc.gov/project/PNNL-PJR/bsingh/po-lun/2deg/fc5_2000_pd1-fc5_2000_pd/aerosol.html
Aerosol Indirect Forcing for standard CAM5.3: https://portal.nersc.gov/project/PNNL-PJR/bsingh/po-lun/2deg/fc5_2000_pd.eps.png
Aerosol Indirect Forcing for CAM5.3+bug fix: https://portal.nersc.gov/project/PNNL-PJR/bsingh/po-lun/2deg/fc5_2000_pd1.eps.png

Thanks to Po-Lun for running the simulations and Balwinder Singh for posting the results.

Steve Ghan

cacraig

This fix is in the CAM development trunk as of cam5_3_41.

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