Noriyasu Honma

Spatial distribution of cold-season lightning frequency in the coastal areas of the Sea of Japan

The coastal areas of the Sea of Japan are a well-known hotspot of winter lightning activity. This study distinguishes between three common types of winter lightning in that region (types A–C), based on their frequency distributions and the meteorological conditions under which they occur. Type A lightning occurs with high frequency in the Tohoku district. It is mainly caused by cold fronts that accompany cyclones passing north of the Japanese islands. Type B, which occurs most frequently in the coastal areas of the Hokuriku district, is mainly caused by topographically induced wind convergence and convective instability, both of which are associated with cyclones having multiple centers. Type C’s lightning frequency distribution pattern is similar to that of type B, but its principal cause is a topographically induced wind convergence generated by cold air advection from the Siberian continent. Type A is most frequently observed from October to November, while types B and C tend to appear from November to January, consistent with seasonal changes in lightning frequency distribution in Japan’s Tohoku and Hokuriku districts.

Comparison of Tohoku LLS data and lightning current waveforms in winter

Performance of a lightning location system covering Tohoku region in Japan was validated by comparing the location data and lightning current waveforms observed around a coastal area of the Sea of Japan in 2012 winter. The current waveforms were composed of continuing currents, often superimposed with isolated narrow pulse currents, and pulses. The LLS detected electric field pulses associated with the isolated narrow pulse currents and accurately located their sources, but provided no information on the continuing currents. Peak amplitudes of the isolated narrow pulse currents were slightly underestimated. For current pulses associated with negative ground-to-cloud lightning events, the peak amplitudes were overestimated several times as much as the actual values.

Measurement of small variations of atmospheric electrical field signals in fair weather

The variations in atmospheric electrical field signals in fair weather were observed both at Aobayama in Sendai and at Tsukidate in the Miyagi Prefecture. The measurements were made using a slow antenna type electrical field meter composed of a metal plate electrode and a CR integrator. They were recorded using a portable pen-recorder. A field meter of this type is suitable for measuring relatively rapid variations. The observations were made in a period of a few minutes in fair breezy weather. The data collected using a bare metal electrode were compared with those using an insulated electrode in order to detect an air-earth current. Examination of the dependence of the amplitude of the variation to the electrode size and height showed that the atmospheric electrical field is affected by these parameters. Observation of signals at two different positions gave the time delay between positions. This indicated that the electrical field perturbation was about several hundred meters. The well known atmospheric circulation with a period of a few minutes suggests that the observed variations in the electrical field signals were caused by electrical charges moving together with the atmospheric circulation.