Incorporation of a Two-Moment Warm Cloud Microphysics Scheme into Deep Cumulus Parameterization of NCAR Community Atmosphere Model

A two-moment warm cloud microphysical parameterization was incorporated into Zhang-McFarlane deep cumulus parameterization scheme in NCAR Community Atmosphere Model version 5.2 (CAM5.2). The unique aspect of this scheme is that rain moves downward at its terminal velocity and it can grow by collecting below cloud water before falling to the surface, while other schemes (e.g. Zhang and McFarlane, 1995; Tiedtke 1989) assume that rain falls to ground as soon as it forms.

Short-term CAM5.2 global simulation with this new scheme is conducted to investigate its effects. Sea surface temperature and cloud droplet number concentration in convective clouds are prescribed for simplicity. Simulated results are evaluated with available observational data and further processed through a satellite simulator for comparisons against CloudSat and CALIPSO data. Stratifrom precipitation rate, cloud ice and liquid water amount are much closer to observations compared to CAM5.2 default run, especially at lower latitudes because convective detrainment rate is enhanced in the modified scheme. Several sensitivity tests of cloud drop number concentration in convective clouds and particle size of convective detrained clouds are performed to examine possible aerosol effects. Higher droplet number concentrations or smaller detrained particle size tend to generate more cloud ice and liquid.