Aerosol Effects on Simulated Storm Electrification and Precipitation in a Two-Moment Bulk Microphysics Model

The effects of cloud condensation nuclei (CCN) concentrations are found to strongly affect the microphysical and electrical evolution of a numerically simulated small multicell storm. The simulations reproduce the well-known effects of updraft invigoration and delay of precipitation formation as increasing CCN from low to intermediate concentrations causes droplet sizes to decrease. Peak updrafts increased from 16 m s(-1) at the lowest CCN to a maximum of 21-22 m s(-1) at moderate CCN, where condensation latent heating is maximized. The transition from low to high CCN first maximizes warm-rain production before switching over to the ice process as the dominant precipitation mechanism. Average graupel density stays fairly high and constant at lower CCN, but then drops monotonically at higher CCN concentration, although high CCN also foster the appearance of small regions of larger, high-density graupel with high simulated radar reflectivity.Graupel production increases monotonically as CCN concentration rises from 50 to about 2000 cm(-3). The lightning response is relatively weak until the Hallett-Mossop rime-splintering ice multiplication becomes more active at CCN > 700 cm(-3). At very high CCN concentrations (>2000 cm(-3)), graupel production decreases slowly, but lightning activity drops dramatically when the parameterization of Hallett-Mossop rime-splintering ice multiplication is based on the number of large cloud droplets collected by graupel. Conversely, lightning activity remains steady at extremely high CCN concentration when the Hallett-Mossop parameterization is based simply on the rate of rime mass accumulation. The results lend support to the aerosol hypothesis as applied to lightning production, whereby greater CCN concentration tends to lead to greater lightning activity, but with a large sensitivity to ice multiplication.