Aerosol-induced microphysical processes and electrification in pyrocumulus

Pyrocumulus clouds that form over active fires often provide spectacular examples of aerosol indirect effects on deep convective clouds. In deep convection, changes in aerosol loading reduce the efficiency of warm rain processes, which impacts the supercooled liquid water content in the mixed phase region and the amount and size of ice hydrometeors that subsequently form. Such microphysical changes have important implications for latent heating rates, updraft strengths, precipitation production, anvil characteristics, and electrification mechanisms, particularly in pyrocumulus, with resulting impacts on the water vapor content, chemistry, and radiative cooling rates in the upper troposphere. A recent observational study that utilized polarimetric radar and Lightning Mapping Array observations documented an electrified pyrocumulus cloud that formed atop the Hewlett Gulch fire smoke plume near Fort Collins, CO in May 2012. The vertical charge structure and lightning activity in the pyrocumulus were anomalous relative to the surrounding convection that was not directly impacted by the smoke plume. This pyrocumulus case therefore provided a natural laboratory for investigating the effect of smoke aerosol on observed cloud characteristics and charge structure under similar meteorological conditions. However, the microphysical differences and mechanisms for charge separation were difficult to discern from the observations. The goal of this study is therefore to characterize the aerosol-induced microphysical processes and electrification mechanisms within pyrocumulus clouds using idealized simulations with a sophisticated microphysics scheme.

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.