北陸地方では冬季に雷雲が発生し，沿岸部に雷をもたらすことがある．夏季に発生 する雷雲と比較して，雷雲内の正電荷を中和する正極性落雷の割合が高いことが冬季 雷雲の特徴として挙げられる．しかし，雷雲内電荷を観測によって捉えることは難し く，落雷に直接関わる電荷の蓄積プロセスに関する理解が不足している．そこで本研 究では，正極性落雷をもたらす北陸冬季雷雲の電荷蓄積プロセスを明らかにすること を目的として，電荷生成過程を組み込んだ雲解像モデルを用いて，正極性落雷をもた らした冬季雷雲の再現実験を行った．また負極性落雷をもたらした冬季雷雲のシミュ レーションも行い，それらの再現実験結果の比較を行った． 負極性落雷をもたらした冬季雷雲は，発達期には-20 ℃高度付近に正，-10 ℃高度 付近に負，その下層に正という，電荷の弱い雷雲内三極電荷構造をとっていた．その 後上昇流は-30 ℃高度にまで発達し，霰粒子，雪粒子ともに上昇流によって持ち上げ られながら電荷分離が行われた．落雷発生時刻（成熟期）には，負電荷を帯びた霰粒 子，雪粒子による負電荷領域は，-10 ℃高度付近から-20 ℃高度付近へと，広範囲に 拡大していた．また，融解層付近に正電荷を帯びた霰粒子による小さな正電荷領域が 存在していた． 一方で正極性落雷を主にもたらした冬季雷雲は，発達期には負極性落雷をもたらす 冬季雷雲とよく似た電荷分布をとっていたが，上昇流は-10 ℃高度より下層で主に発 生しており，負電荷を帯びた霰粒子は上層へと持ち上げられず，-10 ℃層以下で霰粒 子が正に強く帯電していた．落雷発生時刻付近（成熟期）の雷雲内では，-10 ℃高度 付近に負電荷に帯びた霰粒子と雪粒子が分布しており，負極性落雷をもたらした冬季 雷雲と比較して負電荷領域が小さいことがわかった．また，融解層付近の正電荷領域 は水平に広く分布していた． 以上のように，正極性落雷をもたらす冬季雷雲の電荷蓄積プロセスは負極性落雷を もたらす場合のプロセスとは異なっていた．-10 ℃層以下で正に帯電した霰粒子によ って，大きな正電荷領域が生成されたことにより，下部の正電荷領域から正極性落雷 がもたらされたことが考えられる
In the winter season, thunderclouds develop in the Hokuriku Region and cause lightning flashes along the coast of the Sea of Japan. Occurrence ratio of positive cloud-to-ground (CG) lightning neutralizing positive charge in thunderclouds is higher in winter than in summer. Since it is difficult to observe time variation of charge distribution in clouds, the charge accumulation process in thunderclouds still remains to be revealed. In the present study, we performed simulation experiments of the winter thunderclouds in the Hokuriku Region using the Cloud Resolving Storm Simulator (CReSS) incorporating the riming electrification. The purpose of this study is to clarify the charge accumulation process in Hokuriku winter thunderclouds causing positive CG lightning. The results of the simulation experiments were compared with that of the thunderclouds which caused mainly negative CG flashes. The simulated thundercloud which caused mainly negative CG flashes formed a tripole structure of the charge distribution in the developing stage, and the updraft was predominant from -10 °C to -30 °C levels. Both charged graupel and snow particles were lifted upward. In the mature stage, negatively charged snow and graupel particles spread from -10 °C to -20 °C levels. The positively charged region around the melting layer was small. On the other hand, the simulated thundercloud which caused mainly positive CG flashes formed a similar charge distribution to that caused negative CG flashes in the developing stage. However, the updraft was predominant below -10 °C level. Consequently, negatively charged graupel particles were not lifted upward, and graupel particles in the low level were positively charged strongly. In the mature stage, the negatively charged snow and graupel particles formed negatively charged region around -10 °C level. The negatively charged region was smaller than that of the thundercloud of the negative CG flashes. Around the melting layer, strongly charged positive region spread widely. From the above results, the charge accumulation process of winter thunderclouds causing positive CG lightning is different from that of winter thunderclouds causing negative CG lightning. In the former thunderclouds, the positively charged graupel particles below -10 °C level formed the large positively charged region. Consequently, the positive charge region in the low level may cause the positive CG lightning.[BACK]