Mixed-phase cloud structure organizes winter return-stroke current and polarity

Winter thunderstorms are shallow yet can produce strong and frequently positive cloud-to-ground (CG) discharges. We matched 29,251 main or first return strokes from ten winters to terrain-relative temperature profiles and temperature-resolved cloud liquid water, cloud ice, and snow from the European Centre for Medium-Range Weather Forecasts Reanalysis version 5 (ERA5). In lower-profile environments, the 0, −8, and −25 ∘ C isotherms were 2.27–2.51 km closer to the ground, while the −8 to −25 ∘ C mixed-phase-region depth differed by only 56 m. Lower profiles preferentially enriched the strong-current tail: the fraction exceeding 200 kA increased from 14.6% to 21.3%, and the fraction exceeding 300 kA approximately doubled. This upper-tail response recurred in all ten winters and persisted after event-day weather adjustment. Positive CG probability responded most strongly to ERA5 cloud ice and snow at −8 to −15 ∘ C. A one-standard-deviation increase raised the probability by 6.45 percentage points across event days and regions and by 4.03 percentage points within event-day grid cells, with positive winter-specific responses throughout the record. The terrain-relative position of the mixed-phase region therefore organizes the high-current tail, while its warm-layer frozen-condensate structure organizes polarity composition.