Large-eddy simulations of trade wind cumuli: Investigation of aerosol indirect effects

The effects of aerosol on warm trade cumulus clouds are investigated using a large-eddy simulation with size-resolved cloud microphysics. It is shown that, as expected, increases in aerosols cause a reduction in precipitation and an increase in the cloud-averaged liquid water path (LWP). However, for the case under study, cloud fraction, cloud size, cloud-top height, and depth decrease in response to increasing aerosol concentration, contrary to accepted hypotheses associated with the second aerosol indirect effect. It is found that the complex responses of clouds to aerosols are determined by competing effects of precipitation and droplet evaporation associated with entrainment. As aerosol concentration increases, precipitation suppression tends to maintain the clouds and lead to higher cloud LWP, whereas cloud droplets become smaller and evaporate more readily, which tends to dissipate the clouds and leads to lower cloud fraction, cloud size, and depth. An additional set of experiments with higher surface latent heat flux, and hence higher LWP and drizzle rate, was also performed. Changes in cloud properties due to aerosols have the same trends as in the base runs, although the magnitudes of the changes are larger. Evidence for significant stabilization (or destabilization) of the subeloud layer due to drizzle is not found, mainly because drizzling clouds cover only a small fraction of the domain. It is suggested that cloud fraction may only increase with increasing aerosol loading for larger clouds that are less susceptible to entrainment and evaporation. Finally, it is noted that at any given aerosol concentration the dynamical variability in bulk cloud parameters such as LWP tends to be larger than the aerosol-induced changes in these parameters, indicating that the second aerosol indirect effect may be hard to measure in this cloud type. The variability in cloud optical depth is, however, dominated by changes in aerosol, rather than dynamics.