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Drizzle and turbulence in marine stratocumulus clouds

Marine boundary layer stratocumulus clouds persist almost year-round, blanket large portions of the oceans, and reflect much greater amount of sunlight back to space as compared to the ocean surface. Hence these clouds have a cooling effect on the Earth’s surface and need to be accurately represented in Earth System Model (ESM) simulations aimed at predicting the future climate. The lifetime of stratocumulus clouds is intimately coupled to the turbulence in the boundary layer that is modulated by radiation, surface fluxes, wind shear, and precipitation. Drizzle is ubiquitous in these clouds with much of it evaporating before reaching the surface, thereby cooling and moistening the sub-cloud layer. We used the data collected at the Atmospheric Radiation Measurement (ARM)’s Eastern North Atlantic (ENA) site to characterize the cloud, drizzle, turbulence properties during different stratocumulus mesoscale organizations. Data collected by vertically pointing radar, lidars and radiometers were combined to retrieve the cloud macro- and micro-physical properties together with the boundary layer turbulence and radiation fields. The results suggest on average 15% reduction in turbulence below the cloud base due to drizzle evaporation within the weakly drizzling closed cellular stratocumuli. The strongly drizzling open cellular stratocumuli exhibited much higher turbulence, precipitation induced downdrafts and surface density currents.