Aerosol-induced microphysical processes and electrification in pyrocumulus
To achieve the stated goal, a suite of idealized cloud-resolving model simulations has been performed using the Regional Atmospheric Modeling System (RAMS). To represent the environment at the time when the Hewlett Gulch pyrocumulus formed, the model was initialized with an average of the 12Z 16 May and 00Z 17 May observed Denver soundings, and the pyrocumulus was initiated with a thermal perturbation. An exponentially decreasing aerosol number concentration profile was used in which the surface concentration was varied from 100 to 10,000 mg-1 in order to span the wide range of possible aerosol concentrations associated with the smoke plume and the surrounding environment. The results demonstrate that with increasing background aerosol concentrations, the amount of large liquid and ice hydrometeor species (rain and hail) decreases, while the amount of small hydrometeors (cloud, aggregates, and pristine ice crystals) increases. However, graupel, which is often important for charge separation, is most predominant for moderately polluted conditions. Graupel production through riming rapidly becomes inefficient under more highly polluted conditions. The microphysical process rates responsible for these trends have been analyzed and will be presented. Additionally, electrification mechanisms have been inferred from the microphysical characteristics of the pyrocumulus, and simulated polarimetric model output has also been compared to the radar observations to deduce the most representative aerosol concentration for this case and hence the most probable charge separation mechanism in pyrocumulus.