Plume Dynamics, Turbulence and Volcanic Lightning

Abstract:

Volcanic lightning observations made with the Lightning Mapping Array (LMA) from the 2009 eruption of Redoubt Volcano, Alaska, USA are used to probe the kinematic structure of a volcanic eruption column. Bruning and MacGorman (2013) used lightning flash energy spectra to show that the electrical and kinematic components of a thunderstorm may be coupled. They found that the flash energy spectra showed a 5/3 slope over length scales consistent with the turbulent kinetic energy inertial subrange expected for thunderstorms. They proposed that turbulence may influence the charge distribution in a cloud by advecting charge-bearing precipitation, which would affect flash rate and size. This analysis has now been applied to the lightning storms that occurred during the series of explosive eruptive events in the 2009 Redoubt eruption. Results show that the spectral shape of the volcanic lightning changed over the course of the storms. While volcanic forcing was active the flash energy was concentrated at small flash sizes and the spectra did not have the 5/3 spectral shape at the scales observed by Bruning and MacGorman (2013). 5-10 minutes after the volcanic forcing ended, the spectra transitioned a shape similar to their observations. This delay was inferred to be a relaxation period where the volcanic flow began to equilibrate to and blend with the background atmospheric flow. The lack of a 5/3 spectrum during the period of volcanic forcing could be because the inertial range of the plumes was on scales smaller than the detection limit of the LMA. Alternatively, this may be due to the nature of the forcing. The turbulent volcanic forcing was highly impulsive and short duration compared to the supercell thunderstorms studied by Bruning and MacGorman, which would have been in a quasi-steady state. The 5/3 spectrum represents an equilibrium where energy is transferred from an energy-maximum integral length scale down to the inertial range. Therefore, we would not expect to see 5/3 spectra where the convective system was still coming to equilibrium. These results show how lightning observations may be indicative of the kinematic state of a volcanic eruption column.