Entrainment, drizzle, and stratocumulus cloud albedo

Increased aerosol and hence droplet concentrations in polluted clouds are expected to inhibit precipitation and thereby increase cloud water, leading to more reflective clouds that partially offset global warming. Yet polluted clouds are not generally observed to hold more water. Much of the uncertainty regarding the indirect aerosol effect stems from inadequate understanding of such changes in cloud water. We use large-eddy simulations with bin microphysics and two-stream radiative transfer to show that the relative humidity of air overlying stratocumulus is a leading factor determining whether cloud water increases or decreases when precipitation is suppressed. We find that decreased precipitation leads to increased cloud water only at very low droplet concentrations or when the air overlying the planetary boundary layer is moist. Otherwise, decreased precipitation leads to a reduction in cloud water by increasing the entrainment of dry air into the boundary layer.