Microphysics, water substance continuity, and horizontal variability of boundary-layer clouds

Strikingly different formations of stratocumulus clouds can occur within nearly the same environment, as evidenced by the common emergence of honeycomb-like “open” cells surrounded by overcast, or “closed”-cell areas. Open cells have been linked to the presence of precipitation and low concentration of droplets, but complete understanding of the many complex and interacting processes leading to their formation is lacking. In this presentation, we introduce a new conceptual model for the effect of the microphysics and dynamics on the transition between closed and open cloud cells. It postulates that the resulting cloud structure reflects the balance between two competing processes: mixing by turbulent eddies, which push the layer toward a vertically and horizontally uniform distribution of total (vapor + liquid) water, and precipitation, which tends to disturb that uniformity. When sufficient drizzle forms within updrafts, cloud water in the updrafts' outflow is depleted enough that overcast conditions cannot be sustained. The intensities of these processes are related to two timescales, an updraft timescale and a rain initiation timescale. Using a cloud parcel model, these can be linked to three observable parameters: droplet number concentration, cloud depth, and updraft speed. Despite its simplicity, the model demonstrates remarkable skill and correctly categorizes several observed cases of open and closed cell clouds in different environments. It also explains some ambiguities of previous studies that did not explicitly account for the effect of the dynamics. The relationship between microphysics, precipitation formation, and horizontal variability of boundary-layer clouds is further quantified using a two-dimensional kinematic model. Finally, implications of the results to the problem of parameterization of subgrid cloud variability in large-scale models are discussed.