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from IPython.display import Image

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."

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Image('Electrification-to-charge-structure.png', width="40%")


How do anomalous charge structures form?

Bruning et al. 2014 - Anomalous electrification (paywall)

  • 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.

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Image('Bruning2014-Fig3.png', width="60%")


What does a complete charge analysis look like?

Bruning et al. 2010 - Formation of charge structures in a supercell (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

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