Regional flow conditions associated with stratocumulus cloud-clearing events over the southeast Atlantic

Abstract

Large, abrupt clearing events have been documented in the marine stratocumulus cloud deck that resides over the subtropical Southeast Atlantic Ocean. In these events, clouds are rapidly eroded along a line hundreds–to–thousands of kilometers in length that generally moves westward away from the African coast. Because marine stratocumulus clouds exert a strong cooling effect on the planet, any phenomenon that acts to erode large areas of low clouds may be climatically important. Previous satellite-based research has suggested that the cloud-clearing events may be caused by westward-propagating atmospheric gravity waves rather than simple advection of the cloud boundary. The gravity waves are hypothesized to be excited by an interaction between offshore flow from the African continent and the stratocumulus-topped marine boundary layer. The Weather Research and Forecasting (WRF) model is used to explore the nature of the offshore flow, which is a fundamental physical mechanism behind the dramatic clearing events. Results are presented from two series of week-long simulations driven by ERA–Interim reanalysis in the month of May when cloud-clearing boundaries exhibit maximum frequency. One series covers a period containing multiple cloud-clearing episodes (active period), and the second series covers a period without any cloud-clearing episodes (null period). Synoptic analysis, Hovmöller diagrams, and passive tracers are used to assess the character of the diurnal west-African coastal circulation. Our results indicate that the active period regularly experiences offshore flow from the continent above the boundary layer overnight, whereas the null period is associated with predominantly onshore flow along the coast particularly in the afternoon. The offshore flow overrunning the boundary layer can extend hundreds of kilometers westward of the coast. We document 900-hPa disturbances in each period, which influence the coastal flow of the region. Additionally, we find that the boundary layer height is higher in the null period than in the active period, suggesting that the active periods are associated with areas of thinner clouds that may be more susceptible to cloud-clearing events.