Wildfire Pollution and its Effects on the Microphysical and Electrical Properties of Pyrocumulus

Pyrocumulus clouds form over wildfires when hot, smoke-filled air rises, cools and condenses. These smoke-polluted clouds have higher cloud condensation nuclei (CCN) concentrations, which affect their microphysical and electrical properties. Lightning generation processes in pyrocumulus are not well understood, but have implications for wildfire growth predictions and the radiative and chemical characteristics of the upper troposphere (Lang et al. 2014). Lang et al. (2014) documented an electrified pyrocumulus over the May 2012 Hewlett Gulch fire outside of Fort Collins, Colorado, which produced approximately twenty intracloud lightning flashes. Motivated by their work, we investigate the microphysical differences between low CCN clean clouds and high CCN pyrocumulus. Model simulations were made of a cloud forming under five different CCN concentrations, ranging from clean to extremely polluted. Moderately polluted pyrocumulus experience a complete shutdown of rain processes and an increase in graupel production. In extremely polluted pyrocumulus, however, graupel production is halted, which allows for large amounts of liquid water and small ice. Using these microphysical details, a possible charging mechanism will be inferred and compared to Lang et al. (2014). The main goal is to better understand the aerosol-induced cloud-scale microphysics that causes pyrocumulus electrification to occur.