Transitions Between Closed and Open Marine Stratocumulus: Cloud Radiative Effect or Forcing?

Open and closed cells are the two main cloud regimes of marine stratocumulus clouds. The cloud cover is nearly 100% in the closed cells, whereas the open cells have cloud cover that is typically less than 65%. The closed cells are drizzling only lightly, and when they start to drizzle heavily they typically break into open cells. The open and closed cells regimes are related to high and low concentrations of CCN, respectively. We showed that the cloud radiatve effect (CRE) of closed cells is higher by nearly 100 Wm-2 than the CRE of adjacent open cells (calculated for the equinoctial day in 30° latitude). This is mainly caused by the cloud cover effect. When and where the cells are closed due to anthropogenic aerosols, this large CRE can be considered as radiative forcing.

Satellite snapshots frequently show patterns of open and closed cells with little indication for their origins, except for linear ship tracks. We tracked with the METEOSAT geostationary satellite the evolution of large areas of closed cells that could be clearly ascribed to anthropogenic aerosols. In the first case ship emissions caused ship tracks that expanded and merged to form vast area of closed cells in the size of about 600×600 km. In the second case closed cells formed within a European air mass that was advected into the Atlantic Ocean. The closed cells in this case covered the entire north east Atlantic Ocean, as if the clouds were a huge ship track off the British Islands and Western Europe. When possible, we used in situ measurements to identify the anthropogenic source of the aerosols.

Our observations suggest that continents can act as huge aerosol sources and form "continent tracks", similar to ship tracks, but at a much larger scale. These "continent tracks" are presently not associated with aerosol perturbations. The CRE of the clouds in such "continent tracks" is in fact radiative forcing, which might have a substantial impact on the global radiative forcing.