The electrical structure of the hokuriku winter thunderstorms

Lightning charges, locations, and currents have been determined for 12 flashes from four winter storms observed on the Hokuriku coast of Japan during December 1977 through January 1978. Additional data is available from a total of eight winter storms. The heights and magnitudes of the charges in strokes‐to‐ground were calculated from simultaneous measurements of electric field changes made at seven stations covering an area of about 150 km 2 . Discharges lowering positive charge to earth often exhibit large continuing currents (>10 4 A) for periods up to 10 ms. One positive discharge involved a peak current value of ≃10 5 A and a charge that exceeded 300°C after 4 ms. Negative continuing current strokes are generally an order of magnitude smaller, ≃4×10 3 A, involving charges of 100°C or less. The positive lightning charges are located higher than the negative charges of the same storm, constituting a ‘normal’ bipolar system similar to the charge configuration found in summer thunderstorms. For the eight storms observed during this study, 26 out of a total of 63 strokes‐to‐ground were positive. A strong correlation exists between the fraction of positive ground strokes and the vertical wind shear in the cloud layer. On the basis of this study we suggest that the occurrence of positive strokes‐to‐ground is a consequence of the wind shear. The shear provides a horizontal displacement between the charges that helps to ensure that an initiating positive streamer will continue down to ground rather than into the negatively charged region that would normally be located directly below it. The data suggest that positive strokes‐to‐ground should appear at a threshold shear value in the cloud layer of about 1.5 m/s/km.