Modeling Mesoscale Cellular Structures and Drizzle in Marine Stratocumulus. Part I: Impact of Drizzle on the Formation and Evolution of Open Cells

A new modeling framework is used to investigate aerosol–cloud–precipitation interactions and dynamical feedbacks at the mesoscale. The focus is on simulation of the formation and evolution of cellular structures that are commonly seen in satellite images of marine stratocumulus clouds. Simulations are performed at moderate resolution in a 60 × 60 km2 domain for 16 h to adequately represent the mesoscale organization associated with open cells and precipitation. Results support the emerging understanding that precipitation plays a critical role in the formation and evolution of open cells. Evaporation of raindrops generates a dynamic response that manifests itself in cellular organization of updrafts and downdrafts and promotes and sustains the formation of an open cellular structure in cloud fields. Vertical motion in open-cell centers with thin clouds is minimal. It is shown that a mean surface rain rate as low as 0.02 mm day−1 is, for the case considered, sufficient to promote the formation of open cells. The maximum dimension of individual open cells ranges between 5 and 30 km. Individual cells grow at a mean rate of between 5 and 10 km h−1. Irregularity in the shape of open cells is caused by formation of new precipitating regions at the cell walls and interference with neighboring cells, which erode, and eventually eliminate, the old cells. The typical lifetime of large individual open cells is about 2 h, close to that observed by radar, although a collection of open cells as a whole may last for tens of hours.