WRF Simulations of Episodes of Stratocumulus Clearing over the Southeast Atlantic

Large, abrupt clearing events have been recently documented in the marine stratocumulus cloud deck that resides over the subtropical Southeast Atlantic Ocean. The rapid cloud erosions that take place generally move westward away from the African coast and are on the order of hundreds to thousands of kilometers in length. Stratocumulus clouds exert a strong cooling effect on the planet and are not well represented in global climate models, so any phenomenon that acts to erode large areas of clouds is potentially important. Previous observational work has suggested that these events maybe caused by atmospheric gravity waves rather than simple advection of the cloud boundary. The gravity waves are hypothesized to be caused 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 physical mechanisms of the dramatic clearing events. Results are presented from a series of week-long simulations driven by ERA–Interim reanalysis covering a period containing multiple cloud-clearing episodes. The simulations focus on the character of offshore flow from the African continent, in particular its strength, depth, and diurnal cycle, in comparison to available satellite observations. Analyses of the interaction of the offshore flow with the marine boundary layer and model-produced cloud field are also presented.