A modelling study of aerosol impacts on cloud microphysics and radiative properties

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Abstract

A warm cloud microphysical parameterization was incorporated into a regional model to study the sensitivity to aerosols of cloud-radiative properties and precipitation. Assuming a trimodal lognormal aerosol size distribution, the aerosol numbers were explicitly calculated from prognostic aerosol masses, considering advection, diffusion, and cloud and raindrop activation/deactivation processes. Clean continental, average continental and urban aerosols, each with different modal parameters, were used to serve as condensation nuclei (CCN) of cloud- and raindrops, whose activations depended on supersaturation and aerosol composition.

Consistent with other studies, simulations conducted for a warm cloud system indicate that more aerosols result in more cloud water and more, but smaller, cloud drops, yielding increases in cloud albedo and decreases in surface precipitation. For example, the cloud drop effective radius decreased from ∼9 µm for clean continental aerosols to ∼5 and ∼2 µm, respectively, for average continental and urban aerosols, resulting in an increase in the respective cloud water path by ∼10% and ∼35% and cloud albedo by ∼6% and ∼12%. On the other hand, the accumulated precipitation decreased from 2.2 mm for clean continental aerosols to 1.9 and 1.2 mm, respectively, for average continental and urban aerosols. The presence of giant nuclei increased both the cloud drop effective radius and the precipitation, while the use of volumetric cloud drop radius tended to result in larger estimated cloud solar radiative forcing than the use of effective cloud drop radius. Copyright © 2007 Royal Meteorological Society

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