[1] The fine structure of convective clouds leading to the generation of sprites and elves has been studied using radar images. Sprites and elves were observed on two days, 19 December 1998 and 27 January 1999, in lightning during winter thunderstorms on the Sea of Japan. Fractal analyses of the radar images (as an ensemble and as separate clusters) for those two days have been performed with respect to the occurrence of sprites and elves. We have found that a sprite and/or an elve is triggered when and just after the image cluster maximum size increases to a certain value ($150 km) and the ensemble fractal dimension approaches a threshold (i.e., percolation threshold).Citation: Hayakawa, M., T. Nakamura, D. Iudin, K. Michimoto, T. Suzuki, T. Hanada, and T. Shimura (2005), On the fine structure of thunderstorms leading to the generation of sprites and elves: Fractal analysis,

A case of aircraft triggered lightning, which occurred below the cloud base, was observed at Komatsu Airbase along the coast of the Sea of Japan during the intensive observation period using Doppler radar. The aircraft triggered lightning was accompanied by winter thunderclouds with graupel precipitation in the dissipating stage. The aircraft triggered lightning was initiated by a bi-directional leader. Lightning channels from the aircraft propagated simultaneously both upward and downward with return strokes. The surface electrical field was relatively weak (10 kV/m) compared with that of natural CG lightning.

Developing summer thunderclouds (mesoscale convective systems: MCSs) were observed using Doppler radar from the 23rd to the 30th of August, 1997. MCSs were generated around Mt. Akagi and Mt. Chichibu in the northern Kanto area. There were also a few MCSs around Mt. Haruna in the northern Kanto area. Most MCSs had a horizontal scale of 70-90km and a lifetime of four-five hours, and needed about one or two hours to reach MCSs. During the observation, the largest MCS occurred on August 29th, 1997. The wind shear line between the mountain wind from the northwest and the sea breeze from the southeast near the surface triggered MCS generation. Negative cloud to ground (CG) flashes occurred mainly in the strong echo area (above 32 dBZ) around the plains in this case. Many positive CG flashes in MCSs occurred around the weak echo area or the no echo area, and the outside area of the gust front. The first positive CG flash was associated with the generation of the divergence (downdraft). On the other hand, when the thunderclouds passed near a high mountain, both positive and negative CG flashes were concentrated on the high mountain side. After they left high mountain region; the negative CG flashes were concentrated exclusively in the strong echo area.

Two successive lightning strikes (over 200 kA) to the 200m-height stack were observed on December 21, 1995 during special observation period. The meteorological features of the thunderclouds, which caused the lightning strikes with large current, were revealed using weather radars, Doppler sodar and weather station data. The lightning strikes with large current were associated with the developing thunderclouds formed over the Japan Sea in cold air outbreak. A band-shaped radar echo with 100 km in length was developed near the coastline in meso-a.-scale low (cold vortex). The lightning strikes were observed twice at the top of the 200 m stack. The first strike occurred at the leading edge of the band-shaped echo (no echo region) at 0223 JST. After the landing of band echo cells, the second strike occurred in the core of the band system at 0234 JST. The mechanism of the large current lightning was involved that long term charge accumulation in the southwestern region of the band echo system, where is the formation area of new echo cells in its lifecycle, caused the lightning strikes with large current to the stack.

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