Electrification and Lightning within Pyrocumulus Clouds

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Sunday, 4 January 2015
Kendell LaRoche, University of North Dakota, Grand Forks, ND; and T. J. Lang

Pyrocumulus clouds are fire-initiated convective clouds that form from wildfires undergoing rapid growth. Pyrocumulus clouds can reach altitudes high enough to have an impact on aerosol concentrations and chemical composition within the upper atmosphere. Some pyrocumulus clouds become intense enough to produce lightning, and case studies of pyrocumulus clouds that both did and did not generate lightning were examined in this study. Dual-polarization weather radar, lightning detection network data, time series analysis, and geostationary satellite imagery were used to find and analyze pyrocumulus cases. Past research has shown that pyrocumulus cases are most likely to produce lightning when there is a decrease in differential reflectivity and an increase in the correlation coefficient, as measured by polarimetric radar, due to the transition from pure smoke/ash to frozen hydrometeors. Cases where lightning was produced by pyrocumulus were examined to see if they also had similar polarimetric characteristics. All pyrocumulus cases that contained lightning displayed the polarimetric characteristics of ice within the pyrocumulus cloud. Time series analysis of ice and smoke volume within the Hardluck Fire showed that lightning occurred when the pyrocumulus was ice-based and did not occur when the pyrocumulus was smoke-based. Enhanced understanding of lightning from pyrocumulus may help with identification of wildfires undergoing explosive growth, as well as help document wildfire impacts on the composition of the upper troposphere/lower stratosphere.