A Study of Radar Echoes and their Relation to Lightning Discharges of Thunderclouds in the Hokuriku District

[摘要] In the present work, winter thunderclouds and active convective clouds were observed by means of radar with CAPPI (Constant Altitude Plan Position Indicator). The lightning activity is monitored by a local network of sferics direction finders around Komatsu Airport.In midwinter, from January to early March, active convective clouds, 30 dBZ echo tops of which develop up to the -20°C temperature level, exhibit very weak lightning activity called “Single-Flash Storm”, or sometimes fail to generate even a single lightning discharge. The altitude of the -10°C temperature level is between 1.8 to 1.4km in the former case. On the other hand, it is lower than 1.4km in the latter case. These significantly low altitudes of the -10°C temperature level are the main reason for keeping such clouds in a weak or non-lightning situation.Concerning the classification of the lightning activity, the author has proposed the following criteria:(a) When the altitude of the -10°C temperature level is higher than 1.8km, clouds exhibit strong lightning activity.(b) When the altitude of the -10°C temperature level is between 1.8 to 1.4km, clouds exhibit very weak or no lightning activity.(c) When the altitude of the -10°C temperature level is lower than 1.4km, clouds never generate a lightning discharge.Along with the results of thundercloud investigation stated in Part I of the present article (Michimoto, 1991), it is concluded that the necessary conditions for convective clouds to generate lightning discharge are as follows:(1) The 30 dBZ echo of the clouds has to develop at a level higher than the -20°C temperature level.(2) The altitude of the -10°C temperature level has to be higher than 1.4km.(3) The clouds have to involve rapid development of graupel particle precipitation; specifically, they have to involve formation and rapidly vertical movement of 40-to-50 dBZ echo cells.

[学科分类] 大气科学