Aerosol effects on liquid-water path of thin stratocumulus clouds

Abstract

Thin clouds with mean liquid water path (LWP) of ∼50 g m-2 cover 27.5% of the globe and thus play an important role in Earth's-radiation budget. Radiative fluxes at Earth's surface and top of atmosphere are very sensitive to the LWP variation when the LWP becomes smaller than ∼50 g m-2. This indicates that aerosol effects on thin clouds can have a substantial impact on the variation of global radiative forcing if LWP changes. This study examines the aerosol indirect effect through changes in the LWP in three cases of thin warm stratocumulus clouds with LWP < 50 gm-2 . We use a cloudsystem resolving model coupled with a double-moment representation of cloud microphysics. Intensified interactions among the cloud droplet number concentration, condensation, and dynamics at high aerosol play a critical role in the LWP responses to aerosol increases. Increased aerosols lead to increased CDNC, providing the increased surface area of droplets where water vapor condenses. This increases condensation, and thus condensational heating, to produce stronger updrafts, leading to an increased LWP with increased aerosols in two of the cases where precipitation reaches the surface. In a case with no surface precipitation, LWP decreases with increases in aerosols. In this case, most of precipitation evaporates just below the cloud base. With decreases in aerosols, precipitation increases and leads to increasing evaporation of precipitation, thereby increasing instability around the cloud base. This leads to increased updrafts, and thus condensation, from which increased LWP results. Copyright 2009 by the American Geophysical Union.