Wind, rain, and the closed to open cell transition in subtropical marine stratocumulus

Lagrangian transitions in mesoscale cellular convective (MCC) clouds beginning as closed cell MCC and transitioning to open cells or disorganized but cellular MCC are explored on timescales from 12 to 72 hr. Potential drivers of MCC transitions are shown to act on multiple timescales. Closed-to-open MCC cloud transitions are preceded by strong surface winds and large moisture fluxes at lead times of up to 72 hr; and by high cloud water content, reduced cloud drop concentrations, and intense rain rates at lead times of 12-36 hr. The relationship between intense rain and the formation of open cells is consistent with a cold pool convergence mechanism. A Lagrangian analysis shows that anomalously strong surface winds are associated with higher rain rates as well as subsequent increases in rain rates through modifications to moisture flux and content in the boundary layer. The closed-open MCC transition contrasts with the closed-to-disorganized transition which at long lead times is associated with warm sea surface temperature and at diurnal-scale lead times is associated with variables related to cloud top entrainment drying such as a deepening boundary layer, weakening subsidence and inversion strength, and a drier free troposphere. A conceptual model is proposed where excess boundary layer moisture associated with wind breaks up stratocumulus through the closed-to-open transition, while excess dry-air entrainment at cloud top breaks up stratocumulus through the closed-to-disorganized transition.