Effect of aerosol on warm convective clouds: Aerosol-cloud surface flux feedbacks in a new coupled large eddy model

Abstract

We present a new large eddy simulation model that comprises coupled components representing size-resolved aerosol and cloud microphysics, radiative properties of aerosol and clouds, dynamics, and a surface soil and vegetation model. The model is used to investigate the effect of increases in aerosol on liquid water path LWP, cloud fraction, optical depth, and precipitation formation in warm, continental cumulus clouds. Sets of simulations that either neglect, or include the radiative properties of a partially absorbing aerosol are performed. In the absence of aerosol radiative effects, an increase in aerosol loading results in a reduction in precipitation. However, the clouds do not experience significant changes in LWP, cloud fraction and cloud depth; aerosol effects on LWP and cloud fraction are small compared to the dynamical variability of the clouds at any given aerosol concentration. Reasons for this response are discussed. When aerosol radiative effects are included, the modification in atmospheric heating profiles, and the reduction in surface latent and sensible heat fluxes resulting from the presence of these particles, have a significant effect on cloud parameters and boundary layer evolution. For the case considered, there is a significant reduction in the strength of convection, LWP, cloud fraction and cloud depth. Cloud optical depth responds non-monotonically to the increase in aerosol. These results indicate that in continental regions surface processes must be included in calculations of aerosol-cloud-precipitation interactions. Neglect of these surface processes may result in an overestimate of the second aerosol indirect effect. Copyright 2006 by the American Geophysical Union.