An idealized modeling study of the effect of continental air mass aerosol parameters on marine stratocumulus

Abstract A series of numerical simulations of a stratocumulus topped marine boundary layer (STBL) affected by advection of CCN from a continental origin were performed using a large eddy simulation (LES) model with explicit microphysics. We investigated the role of these continental air outbreaks as a function of continental aerosol size mode (giant, accumulation, and nucleation), location of the continental air mass in the vertical, and the presence of wind shear across the inversion. It was shown that giant CCN particles entrained into the cloud layer from an elevated continental air mass above the inversion significantly modulates drizzle efficiency, leading to profound effects on stratocumulus cloud layer dynamics and microphysics. These effects include breakup and decoupling of the cloud layer, attenuated turbulence, larger undulation of cloud top, and widening of the entrainment zone. In a marine air mass modified by continental aerosols of predominantly fine mode size, precipitation is suppressed. The suppression is less if the only source for the continental aerosol particles (in accumulation or nucleation mode) is via entrainment from a continental air mass above the inversion. The feedbacks between the continental aerosol forcing and drizzle efficiency, positive for the giant pollution particles and negative for accumulation and nucleation mode particles, are enhanced in the presence of wind shear. We conclude that the aerosol indirect effect cannot be characterized by the total aerosol particle load alone; the vertical variation in aerosol concentration, the size of the dominant particle mode, and the wind shear across the inversion each play significant roles in the physical processes underlying the indirect effect.