Simulation of the effects of increasing cloud condensation nuclei on mixed-phase clouds and precipitation of a front system

Increasing cloud condensation nuclei (CCN) concentration usually leads to increasing cloud albedo and decreasing precipitation for warm clouds but its effect on the precipitation of mixed-phase clouds is less clear. Here, a bulk-formula semi-two-moment mixed-phase cloud scheme is incorporated with a regional model to study this aspect on a front system associated with convection. Although this cloud scheme is comparably simpler than most detailed bin-resolving cloud schemes used in cloud resolving models, certain features of CCN effects are reasonably simulated. Results show that both the first and the second indirect effects are simulated and that ice-phase precipitation effectively removes cloud water from the upper troposphere, leading to weaker first indirect effects. Sensitivity simulations show that although increasing CCN number results in more cloud drops and ice, its effect on surface precipitation depends on the interplay between the decreased warm-rain production and the decreased/increased ice-phase precipitation. The increase in cloud drops consequently enhances the diffusional growth of ice particles in water-subsaturated environments but inhibit it in water-supersaturated environments. Moreover, the change in riming depends on the balance between the enhancement due to more cloud drops and the suppression due to smaller cloud drops. For the case studied here, the precipitation responds nonlinearly to CCN number change, causing precipitation decrease in high CCN concentration environments but showing no clear tendency in low CCN concentration environments. (c) 2010 Elsevier B.V. All rights reserved.