An Improved Convective Ice Parameterization for the NASA GISS General Circulation Model

Upper tropospheric cloud ice, a substantial fraction of which is the result of detrainment from convection, impacts the radiation budget and can affect General Circulation Model (GCM) climate sensitivity.  Recent studies showed that the CMIP5 configurations of the Goddard Institute for Space Studies (GISS) GCM simulated an upper tropospheric ice water content (IWC) that exceeded an estimated upper-bound by a factor of ~2.  Partly in response to this bias, a new GCM parameterization of convective cloud ice has been developed that incorporates recently developed ice particle fall speed information and particle size distributions (PSDs) from aircraft flights sampling deep convective outflow during NAMMA, TC4, MC3E, and SPartICus field campaigns.  A habit-independent mass-area relationship is used to derive ice particle mass distributions, and so to facilitate analytical integration of mass distributions, particle maximum diameters are converted to equivalent melted diameters.  Particle volume- and projected area- weighted equivalent melted diameters (D_m and D_a, respectively) exhibit substantial variation as a function of in situ IWC and temperature (r ~ 0.8). 

Regression fits allow D_m and D_a to be diagnosed as a function of temperature and IWC in the GCM convective plume.  Together, D_m and D_a provide sufficient information to solve for the fit parameters of the assumed normalized gamma PSD.  A formulation for translating ice particle fall speeds as a function of maximum diameter into fall speeds as a function of equivalent melted diameter is also developed and used in the parameterization. The diagnosed PSDs and fall speeds are combined with the GCM’s parameterized convective updraft vertical velocity to physically partition convective updraft condensate into precipitating and detrained components.  The new parameterization is incorporated into a candidate CMIP6 GISS GCM, and a 5-year prescribed sea surface temperature simulation shows a 30 – 50% decrease in upper-tropospheric deep convective IWC, bringing the tropical and global mean ice water path climatologies into closer agreement with CloudSat best estimates.