Determining the roles of microphysical and dynamical factors on precipitation efficiency

Previous studies have shown that precipitation efficiency tends to decrease when the aerosol concentration is increased, regardless of cloud type, albeit the decrease is more substantial for shallower clouds. From a purely microphysical perspective, an increase in the aerosol number concentration should lead to a decrease in droplet size (more numerous drops), suppressed precipitation formation, and decreased surface precipitation, effectively decreasing the precipitation efficiency (assuming that the total condensate remains unchanged). However, it is unclear how the effects on dynamics play into this problem. It seems plausible that the precipitation efficiency could actually increase when dynamical factors are considered. To address this problem, we are using the recently published piggybacking approach (Grabowski 2014) to separate the microphysical and dynamical effects. The initial results highlight the fact that the change in the precipitation efficiency is quite small in strong deep convection, with the largest differences being found for shallow tropical convection. Moreover, we have found clear increases in the precipitation efficiency for modest changes in the background aerosol number concentration, which has been related to the cloud deepening effect. For continental deep convection, increases in the aerosol number concentration monotonically decrease the precipitation efficiency.