A Microphysics Parameterization for Convective Clouds in a Global Climate Model

An efficient two-moment microphysics parameterization scheme for convective clouds is developed to improve the representation of convective clouds, and its interactions with stratiform clouds and aerosol in global climate models (GCMs). The scheme explicitly treats mass mixing ratio and number concentration of four hydrometeor species (cloud water, cloud ice, rain and snow), and describes several microphysical processes, including autoconversion, self-collection, collection between hydrometeor species, freezing, cloud ice nucleation, droplet activation, and sedimentation. Thus this physically-based scheme is suitable for investigating the interaction between convection and aerosol, and the indirect aerosol effect on climate. An evaluation of the scheme in the single-column version of NCAR Community Atmospheric Model version 3.5 (CAM3.5) with the Tropical Warm Pool – International Cloud Experiment (TWP-ICE) data shows that the simulation of cloud microphysical properties in convective core is significantly improved, indicating that the new parameterization describes the microphysical processes in convection reasonably well. With more realistic convective cloud microphysical properties and thus their detrainment, the surface stratiform precipitation, which is seriously underestimated in the model, is increased by a factor of roughly 2, and therefore is much closer to the observations. In addition, the simulations of net surface shortwave radiation flux, OLR, high level cloud, specific humidity, and temperature are also improved to some extent. The sensitivity experiments show that the microphysics scheme is moderately sensitive to model vertical resolution and the lower boundary conditions of hydrometeor budget equations, but less so to numerics. The experiments with climatological aerosol distribution show that convective precipitation is suppressed with increasing aerosol amount, consistent with available observations.