Examination of aerosol effects on precipitation in deep convective clouds during the 1997 ARM summer experiment

It has been generally accepted that increasing aerosols suppress precipitation. The aerosol-induced precipitation suppression was suggested by the study of shallow stratiform clouds. Recent studies of convective clouds showed increasing aerosols could increase precipitation. Those studies showed that intense feedbacks between aerosols and cloud dynamics led to increased precipitation in some cases of convective clouds. This study expanded those studies by analyzing detailed microphysical and dynamical modifications by aerosols leading to increased precipitation. This study focused on three observed cases of mesoscale cloud ensemble (MCE) driven by deep convective clouds, since MCE accounts for a large proportion of the Earth's precipitation and the study of aerosol effects on MCE is at its incipient stage. Those MCEs were observed during the 1997 Atmospheric Radiation Measurement (ARM) summer experiment. Two numerical experiments were performed for each of the MCEs to simulate aerosol effects on deep convection. The first was with high aerosol number concentration, and the second was with low concentration. The results showed an increased precipitation at high aerosol, due to stronger, more numerous updraughts, initiated by stronger convergence lines at the surface in convective regions of the MCE. The stronger convergence lines were triggered by increased evaporation of cloud liquid in the high-aerosol case, made possible by higher values of cloud liquid necessary for autoconversion. The generality of these results requires further investigation. However, they demonstrate that the response of precipitation to increased aerosols in deep convection can be different from that in shallow cloud systems, at least for the cases studied here. Copyright (c) 2008 Royal Meteorological Society.