Thunderstorm electrification basics

Collisions of ice crystals and graupel in the presence of supercooled water are the primary mechanism of thunderstorm electrification.

• Requires a mixed-phase cloud
• For any given updraft, there is a reversal temperature at which electrification switches polarity
• Does not require a background electric field. Due to ice surface microphysics alone.

After microscale electrification, sedimentation of charged, precipitation-sized hydrometeors results in net charge regions.

This is sufficient to explain the three main charge regions observed in the "normal tripole."

How do anomalous charge structures form?

Bruning et al. 2014 - Anomalous electrification (paywall) http://dx.doi.org/10.1016/j.atmosres.2012.10.009

• Figure 3 reproduced below.

Depletion rate of supercooled liquid water content controls length of time spent in positive charging region.

• New Mexico or Florida storms are lower trajectories
• High plains storms with deep or elevated positive charging are upper trajectories.

Suggests we should observe a continuous shift of position of lower positive charge through a range of altitudes depending on the depletion rate.

Depletion rate might be controlled by:

• Drier boundary layers with high cloud base heights
• Shallower warm cloud depth which reserves more liquid water for ice processes
• Entrainment of dry mid-troposhperic air into the cloud

Storms on the high plains of the US tend toward these low-depletion-rate conditions.

What does a complete charge analysis look like?

Bruning et al. 2010 - Formation of charge structures in a supercell http://journals.ametsoc.org/doi/pdf/10.1175/2010MWR3160.1 (PDF)

• Tracked new clusters of lightning that formed as modulations on quasi-steady updraft (Fig. 7)
• Clusters developed into three distinct charge regions (Fig. 9)
  • Initial -ICs
  • Ascending plume at 2247 UTC = 20 m/s upward
  • Descending bands 2250-2257

Illustrates storm-relative (Fig. 1, Fig. 8) organization of thunderstorm charge into different net structures.

• Elevated positive charge center in updraft core
• Additional "normal" charging on periphery of updraft
• +CGs out of forward flank
• "Normal" 6-layer structure to right of updraft (Stolzenburg et al. 1998 "outside updraft")

Explanation of these complicated charge structures

• Multiple electrificaiton regimes in different updrafts
• Transport of charge by storm-relative motion of precipitation and non-precipitating ice crystals