Effects of Number Concentration of Cloud Condensation Nuclei on Moist Convection Formation

We examined the sensitivity of the formation of moist convection to the number of aerosols that serve as cloud condensation nuclei (CCN) based on a set of numerical experiments using a nonhydrostatic model with a bin cloud microphysics model. Additionally, a linear stability analysis for an air parcel incorporating effects of the CCN number concentration (N_CCN) has been conducted to further demonstrate the findings in numerical experiments. The results of the numerical experiments show that moist convection does not form when N_CCN # 10 cm²³. The sensitivity to N_CCN can be divided into three regimes: when N_CCN # 10 cm²³, convection does not form or not fully develop; when 1 # N_CCN # 10² cm²³, maximum vertical velocity increases with N_CCN; and when N_CCN $ 10² cm²³, the intensity of convection does not largely depend on N_CCN. We demonstrate that the main reason convection does not form under environments with a small N_CCN is that the time scale for condensation is longer than that to change environmental conditions. Given a supersaturated environment, fewer droplets form when N_CCN is small and the size of droplets is potentially large. Consequently, the amount of latent heating is limited and the air parcel cannot obtain buoyancy within a reasonable time scale. Linear stability analysis using a parcel model considering the effect of N_CCN without ice-phase processes shows unstable and stable regimes as a function of the number of droplets. The analytically obtained critical droplet number for the convection formation well corresponds to the minimum N_CCN beyond which convection forms in the present numerical experiments.