The Roles of Aerosols and Coupling State of Marine Stratocumulus in Determining Cloud Cover and Radiative Effects: Implications to Climate and Geoengineering

A new methodology to satellite retrieval of marine stratocumulus cloud drop concentrations (N_d) and cloud base updrafts (W_b) made it possible to calculate CCN(S) at cloud base. In addition the coupling state of the clouds with sea surface was retrieved. Ship observations validated the retrieved CCN(S) when the clouds were coupled. The ship measured surface CCN(S) was much higher than at cloud base when the clouds were decoupled. This reflects the fact that CCN originate from the sea surface and removed by the clouds mainly through precipitation. When decoupling occurs, the CCN accumulate below the inversion and do not reach the clouds that become very clean with low N_d. Coupled clouds tend to form solid decks when polluted to the extent of suppressing drizzle, and breakup when start drizzling heavily by a mechanism of precipitation driven downdrafts that spread at the sea surface into colliding gust fronts that trigger cumulus convection instead of the stratocumulus deck. This mechanism of breaking the cloud decks is inoperative for decoupled clouds. It has major implications for the conditions where pollution aerosols or cloud seeding with CCN can have strong negative radiative forcing of the order of 100 wm^-2, by enhancing cloud cover. Examples will be shown and the preferred locations with respect to synoptic conditions will be demonstrated.