The winter thunderclouds that frequently visit the southeastern coastal area of the Japan Sea were investigated by the field work, operating radars, the sferics direction‐finder system, and the field‐mill network. The clouds take the dipole electrical structure at their developing stage and then take the tripole structure at the mature stage. However, the period covering both dipole and tripole structures is very short (usually less than 10 min in early or late winter and less than several minutes in midwinter), because the graupel particles that carry the main negative charge and the lower positive charge do not stay stationarily in the clouds but fall off rapidly. For the remainder of the period of cloud duration, which lasts relatively long, the positive charge predominates in the clouds. The grade of charge separation and lightning activity is restricted by the altitude of −10°C temperature level. When the altitude is lower than 1.8 km, the clouds exhibit weak or no lightning activity. When it is lower than 1.4 km, the clouds exhibit neither natural lightning discharge nor tripole electrical structure.

No abstract

[1] Lightning and thundercloud systems leading to the generation of sprites were examined on the basis of analyses on the interrelation among radar reflectivity, lightning, and static electric field during the winter of 2004/2005 in the Hokuriku area of Japan. Eleven cases from the observed fifteen sprite events exhibit similar characteristics, so we present a typical case study of the thundercloud system. The analysis results are summarized as follows: (1)

The ascent rates of individual radar echoes of thunderclouds in the Hokuriku district were investigated in relation to lightning activity. Both CAPPI radar and the sferics direction-finding system were used for this investigation. Multicell-type thunderstorms containing several moderate to strong precipitating domains were observed by means of a conventional 5.7 cm weather radar and 100.5 MHz sferics direction-finding system installed at Komatsu Airport and its periphery. The results are briefly summarized as follows: 1) The first lightning discharge appears about five minutes after the 30 dBZ reflectivity echo exceeds the -20*C temperature level.2) The mean ascending velocity of echoes just before the initial reception of sferics from them is about the same both in summer and winter. By contrast, in cases without lightning activity, the 20-to-45 dBZ echoes have nearly the same ascending velocity as those with lightning activity in winter, but, in summer, the 20 and 25 dBZ echoes have a higher ascending velocity and the 30 and 35 dBZ echoes and equal or lower velocity than those with lightning activity. 3) In cases of very intense lightning activity, the 20-to-35 dBZ echoes ascend much faster than the 40 and 45 dBZ echoes in summer, while in winter, the 20-to-35 dBZ echoes ascend slowly or remain stationary and the 40 and 45 dBZ echoes ascend very fast. 4) In both summer and winter, the peak of lightning activity is observed when several strong echoes of 45 or 50 dBZ are formed at the -10°C temperature level and descend toward the 0°C temperature level. Takahashi (1984) established a rational thundercloud model through numerical calculation and clarified the characteristic convective and electrical activity of cloud cells in correlation with their life cycle. The present observational results can be interpreted as evidence that Takahashi's model corresponds well with actual thunderclouds observed in the Hokuriku district.

In the present work, winter thunderclouds and active convective clouds were observed by means of