In this two-part paper, influences from environmental factors on lightning in a convective storm are assessed with a model. In Part I, an electrical component is described and applied in the Aerosol-Cloud model (AC). AC treats many types of secondary (e.g. breakup in ice-ice collisions, raindrop-freezing fragmentation, rime-splintering) and primary (heterogeneous, homogeneous freezing) ice initiation. AC represents lightning flashes with a statistical treatment of branching from a fractal law constrained by video imagery.

The storm simulated is from the Severe Thunderstorm Electrification and Precipitation Study (STEPS, 19/20 June 2000). The simulation was validated microphysically (e.g., ice/droplet concentrations and mean sizes, liquid water content [LWC], reflectivity, surface precipitation) and dynamically (e.g., ascent) in our 2017 paper. Predicted ice concentrations (~10 L-1) agreed—to within a factor of about two—with aircraft data at flight levels (−10 to −15 °C). Here, electrical statistics of the same simulation are compared with observations. Flash rates (to within a factor of two), triggering altitudes and polarity of flashes, and electric fields, agree with STEPS observations.

The ‘normal’ tripole of charge structure observed during an electrical balloon sounding is reproduced by AC. It is related to reversal of polarity of non-inductive charging in ice-ice collisions seen in lab experiments when temperature or LWC are varied. Positively charged graupel and negatively charged snow at most mid-levels, charged away from the fastest updrafts, is predicted to cause the normal tripole. Total charge separated in the simulated storm is dominated by collisions involving secondary ice from fragmentation in graupel-snow collisions.

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