The ratio of column-integrated cloud content between ice and liquid phases (CR) is a good indicator of cloud dynamics (e.g., convective or stratiform) and radiative properties. Clouds are more stratiform when the CR is larger and more convective for smaller CR values. In this study, a tendency equation of the cloud ratio is derived. The tendency of the cloud ratio (∂CR/∂t ) is determined by six major cloud microphysical processes: vapor condensation and deposition as cloud sources, rainfall and evaporation of rain as the sinks of water clouds, and melting of graupel and accretion of cloud water by precipitation ice as the major conversion processes between water and ice clouds. Apparently, ∂CR/∂t is related to water cycling processes among vapor and different categories of clouds, and links the amount of water vapor and clouds with temperature through latent heating.

An analysis of the tendency of the cloud ratio is carried out using hourly zonal-mean data based on a 2D cloud-resolving simulation with the imposed forcing from the Tropical Ocean Global Atmosphere Coupled OceanAtmosphere Response Experiment. During the genesis and decay stages of clouds (a zonal-mean surface rain rate of smaller than 0.3 mm h^-1 ), the tendency of the cloud ratio is mainly controlled by the processes related to the vapor condensation and deposition. During the mature stage of clouds (a zonal-mean surface rain rate of larger than 0.3 mm h^-1 ), the tendency of the cloud ratio is determined by conversion between water and ice clouds through melting of graupel and accretion of cloud water by precipitation ice.