Understanding the surprising variation in storm charge structures on 4 June 2012 over West Texas

Date

2017-08

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Abstract

On the afternoon of 4 June 2012 several single and multicell storms developed within range of the West Texas Lightning Mapping Array (WTLMA). Some of these storms had anomalous charge structures (primarily positive charge in the mixed-phase region) and some normal (primarily negative at mid-levels), even through they existed concurrently. None of the expected controls on storm charge polarity explained the net charge differences; however, there had been previous convection in the regions that had normal polarity storms. A WRF ensemble suggested that this previous convection may have moistened the mid-levels of the atmosphere especially near cloud base. In this study we investigate whether entrainment can enhance positive graupel charging in the mid-levels and produce anomalous charge structures.

Idealized storm simulations in these environments provided two important insights. First, the updrafts' effective supercooled liquid water and rime accretion rates, important factors to electrification, reached their maximum values midway through the mixed-phase, meaning the inverted tripole charge structure can occur purely through non-inductive charging---an adjustment to the accepted paradigm. Second, through entrainment favoring smaller cloud droplets the rime accretion rates in the most active charging regions were more influenced by the magnitude of dry air at cloud base than the surface or instability characteristics. One tested electrification parameterization even captured different resulting charge structures but not at the heights or timing observed. The limiting factor of electrification within the simulations was cloud droplet freezing. Interpolations towards a colder, still reasonable freezing level produced profiles of updraft charging regions matching the different observed charge structures.

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Keywords

Lightning, Charge structure, LMA, Thunderstorm, Entrainment

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