Effects of Cumulus Parameterization Changes on the Behavior of Tropical Convection in the GISS CMIP6 GCMs

Description

For CMIP6, GISS will be contributing simulations from two different versions of our Model E GCM with updated convection physics. The first (Model E2) has a larger entrainment rate for the more strongly entraining part of the mass flux distribution than in our CMIP5 model (Model E2) and includes evaporation of convective rain into the environment above cloud base, and uses virtual temperature to assess downdraft buoyancy. It also includes revisions to the turbulence scheme that create generally shallower boundary layers in the tropics, and an adjustment to stratiform clouds to produce more supercooled liquid water. The more recent version of the GCM (Model E3) adds a cold pool parameterization to the moist convection scheme to regulate the occurrence of the less entraining part of the mass flux distribution and to isolate warm humid boundary layer air as the source of weakly entraining convection when cold pools are present. Evaporation of rain in the downdrafts that feed the cold pools is also modified, and the detrainment of convective condensate is updated to use more realistic ice particle size distributions and fall speeds. Model E3 also replaces the dry boundary layer with some features of the Bretherton-Park moist turbulence scheme, replaces the Sundqvist-based stratiform cloud microphysics with the second-generation Morrison-Gettelman two-moment scheme, and replaces the Sundqvist-based cloud macrophysics with a pdf-based diagnostic cloud fraction. We compare simulations of deep convection in Models E2, E2.1, and E3 for several ARM IOP periods including AMIE-Gan, TWP-ICE, and MC3E to determine the effects of the new physics on the shallow-deep transition, intraseasonal variability and clouds produced by deep convection.