Are there any linked Issues or Pull Requests?
No response
Brief description
With CASIM, the sub-grid fraction for rain and graupel is currently set to the max cloud fraction in the column above the current height. CoMorph currently works with a prognostic field for the sub-grid rain / graupel fraction, which it sets to a suitably small value where most of the precip is being produced by a fractionally-small convective cloud area.
Single Column Model tests using CASIM reveal that CASIM's existing assumption yields an unrealistically-large rain fraction in typical parameterised deep convection situations. Diffuse detrained cirrus readily spreads across the entire grid-area at cloud-top, which results in CASIM setting the rain fraction to 1 throughout the troposphere, even though the area occupied by the deep convection producing the rain is < 5%.
Unlike in the Wilson-Ballard microphysics scheme, the rain fraction does not have a strong influence on the assumed hydrometeor size distribution in CASIM. However it does have important effects on rain / cloud / ice overlap and on canopy throughfall versus re-evaporation where the rain falls on the land surface. Therefore we should try and couple CASIM consistently with CoMorph's precip fraction prognostic instead of using the max cloud-fraction in the column above.
This will build upon some of the relevant changes already coded in this branch:
cc86710
lfric_apps branch:
vn3.2_casim_precfrac
Equivalent changes in the UM:
vn14.2_casim_precfrac
Further details of the issue.
No response
Are there any linked Issues or Pull Requests?
No response
Brief description
With CASIM, the sub-grid fraction for rain and graupel is currently set to the max cloud fraction in the column above the current height. CoMorph currently works with a prognostic field for the sub-grid rain / graupel fraction, which it sets to a suitably small value where most of the precip is being produced by a fractionally-small convective cloud area.
Single Column Model tests using CASIM reveal that CASIM's existing assumption yields an unrealistically-large rain fraction in typical parameterised deep convection situations. Diffuse detrained cirrus readily spreads across the entire grid-area at cloud-top, which results in CASIM setting the rain fraction to 1 throughout the troposphere, even though the area occupied by the deep convection producing the rain is < 5%.
Unlike in the Wilson-Ballard microphysics scheme, the rain fraction does not have a strong influence on the assumed hydrometeor size distribution in CASIM. However it does have important effects on rain / cloud / ice overlap and on canopy throughfall versus re-evaporation where the rain falls on the land surface. Therefore we should try and couple CASIM consistently with CoMorph's precip fraction prognostic instead of using the max cloud-fraction in the column above.
This will build upon some of the relevant changes already coded in this branch:
cc86710
lfric_apps branch:
vn3.2_casim_precfrac
Equivalent changes in the UM:
vn14.2_casim_precfrac
Further details of the issue.
No response