CCN Pollution Impacts on Hailstorms

A triple moment(3MHAIL) hail scheme was recently implemented into RAMS by Loftus(2012). This scheme is used to investigate the impact of varying concentrations of aerosols that act as cloud condensation nuclei (CCN) on hail. The case simulated is the 29 June 2000 tornadic supercell hailstorm observed in northwestern Kansas during the Severe Thunderstorm Electrification and Precipitation Study (STEPS; Lang et al. 2004) field program. For an increase in CCN from 100 to 3000 cm-3 the simulated supercells exhibited an increase in the numbers and a decrease in the sizes of cloud droplets, as expected, yet the overall storm dynamics and evolution were largely unaffected. Increases in CCN lead to non-monotonic responses in the bulk characteristics of nearly all hydrometeor fields, surface precipitation, and cold-pool strength. However, higher concentrations of CCN also result in larger hail sizes and greater amounts of large diameter (≥ 2cm) hail both aloft as well as at the surface. Analyses of the hail formation and growth mechanisms for these simulations suggest that the combination of increased sizes of new hail particles and localized reductions in numbers of new hailstones forming near maximum growth regions with increasing CCN tends to promote conditions that lead to increased hail sizes and amounts of large hail. Other cases are now being explored.

It is interesting that these results are consistent with the results reported by Khain et al. (2011) using a two-dimensional cloud model with bin microphysics. They found an increase in hail size and amounts with increasing CCN whereas the study by Noppel et al. (2010) used a three-dimensional cloud model with two-moment bulk microphysics and found a general decrease in hail size and amount at the surface with increasing CCN.