A model of the development of droplet effective radius in convective cloud

Abstract A model is presented of a growing cumulus turret which is used to investigate the sensitivity of the droplet effective radius in a convective cloud to variations in the cloud‐base temperature, the liquid‐water content, the vertical wind speed at cloud base, the cloud condensation nucleus (CCN) spectrum entering the cloud base, and dry‐air entrainment. It is found that the existence of large quantities of dry‐air entrainment has a major impact on the evolution of the cloud microphysics. Near to the cloud base the growth of newly nucleated droplets causes a rapid increase in the droplet effective radius with height. Higher in the cloud, dry‐air entrainment results in new nucleation of small droplets at all heights in the cloud and the complete evaporation of some of the pre‐existing droplets. The effect of this is that is that the horizontally averaged effective radius becomes a very weak function of height. The value of the effective radius is most strongly influenced by the CCN spectrum entering through the cloud base. This mean that differences between oceanic and continental clouds are reflected in different values for the effective radius. The effects of entrainment do, however, tend to reduce the sensitivity of the effective radius to the initial CCN spectrum. In addition, the very broad droplet‐size distribution produced by the effects of entrainment may be very important in the development of the ice phase in these clouds. The droplent effective radius produced by the entraining model is less sensitive to temperature than the effective radius predicted by adiabatic growth. The result. The result is also insensitive to differences between the CCN distribution entering the cloud base and the CNC population entrained from the environment within reasonable limits.