A Gray Zone GCM with Full Representation of Cloud Microphysics

This chapter describes the development procedure of general circulation model (GCM) with full representation of cloud microphysics at a medium-range horizontal resolution of 50 km and discusses the simulation results of their precipitation climatology and Madden and Julian Oscillation (MJO). One issue of developing such a GCM is to modify the cloud microphysics suitable to the horizontal resolution. In the present study, the modification is made based on sensitivity experiments for the parameters of the important processes sensitive to the model resolution, particularly the condensation process and the terminal velocity. It is demonstrated that shallow convection and scale-dependent deep convection are still needed in the present model of 50 km resolution with cloud microphysics. The present GCM is shown to simulate the precipitation statistic such as the light and heavy precipitation frequencies and the MJO reasonably well, although the MJO intensity is rather strong. Both cloud microphysics and scale-dependent deep convection play important roles in simulating a realistic MJO in the present GCM. Also noted is that the precipitation climatologies of the present atmospheric GCM (AGCM) and the coupled ocean–atmosphere GCM (CGCM) are quite different from each other, indicating that the air–sea interaction plays an important role in determining the climatology, and this result suggests us to tune the model physics and their parameters with CGCM rather than with AGCM.