Numerical simulations described in previous studies showed that adding cloud condensation nuclei to marine stratocumulus can prevent their breakup from closed into open cells. Additional analyses of the same simulations show that the suppression of rain is well described in terms of cloud drop effective radius (re). Rain is initiated when re near cloud top is around 12–14 μm. Cloud water starts to get depleted when column-maximum rain intensity (Rmax) exceeds 0.1 mm h−1. This happens when cloud-top re reaches 14 μm. Rmax is mostly less than 0.1 mm h−1 at re < 14 μm, regardless of the cloud water path, but increases rapidly when re exceeds 14 μm. This is in agreement with recent aircraft observations and theoretical studies in convective clouds so that the mechanism is not limited to describing marine stratocumulus. These results support the hypothesis that the onset of significant precipitation is determined by the number of nucleated cloud drops and the depth (D) above cloud base within the cloud that is required for cloud drops to reach re of 14 μm. In turn, this can explain the conditions for initiation of significant drizzle and opening of closed cells providing the basis for a simple parameterization for GCMs that unifies the representation of both precipitating and non-precipitating clouds as well as the transition between them. Furthermore, satellite global observations of cloud depth (from base to top), and cloud-top re can be used to derive and validate this parameterization.