Cloud condensation nuclei (CCN) play a significant role in the evolution of marine stratocumulus cloud layers through their effect on cloud droplet concentration, and, thus, drizzle, cloud geometrical structure and lifetime. Over the oceans, the CCN concentration and size distribution often exhibit considerable variability, especially due to infrequent outbreaks of polluted continental air mass. A number of researchers have reported observations of elevated layers of higher CCN concentrations above stratus cloud decks even in clean marine regions off the California coast and in Pacific, Atlantic and the Southern Ocean during RITS, ASTEX and ACE field projects.

Most previous modeling studies of marine stratocumulus topped boundary layer (STBL) assume a constant CCN concentration and a fixed single spectrum at all vertical levels to represent either a clean or a polluted marine condition. In this paper, a set of numerical experiments are conducted using the CIMMS Large Eddy Simulation model to evaluate the effect of elevated highly polluted air on cloud evolution, drizzle formation, and cloud radiative properties. We consider the observed elevated CCN concentrations above the STBL inversion as resulting from:

a) heavy dust outbreaks with giant coarse mode particles,

b) continental air invasion with pollutant particles at the accumulation mode,

c) formation of new sulfate particles at the nucleation mode.

The continental CCN spectrum in the air above the inversion is superimposed on a clean background marine environment that has a vertically uniform CCN concentration. We examine the effect of the polluted air mixed into cloud layer through entrainment and large-scale subsidence and compare this effect with the effect of sea salt CCN originating from the ocean surface through winds and waves.

Preliminary results show that enhanced giant CCN descending gradually into the cloud layer significantly improved the drizzle efficiency, leading to accelerated stratocumulus breakup and decoupling of the subcloud layer resulting in attenuated turbulence, larger undulation of cloud top, and widening of the entrainment zone. In this case the increased CCN did not give rise to increased cloud droplet concentration and did not suppress drizzle as conventionally thought. On the other hand, enhancing the accumulation mode or nucleation mode CCN aloft produced more cloud droplets and less drizzle, preserving the solid stratocumulus cloud deck, causing intensified turbulent motions that lead to a rising cloud layer and inversion.