A study on the characteristics of bolt-from-the-blue lightning flashes

A bolt from the blue (BFB) is an unusual type of cloud-to-ground (CG) lightning flash that develops from classical intra-cloud (IC) flashes but against the blue-sky background. As its name suggests, BFB always comes as a complete surprise to anyone close to the strike point and poses a great threat to human safety. Apart from the consideration of the safety aspect, the physical features of BFB are interesting enough to study because (1) BFB often indicates an imbalance of middle negative and upper positive charge regions in a storm and (2) the leaders in a BFB need to propagate outside the cloud edge and contact ground relatively far from a storm’s convection core. All of these make BFB behave much differently from classical CG or IC flashes. To examine these differences, we have used the lightning data observed by the Fast Antenna Lightning Mapping Array (FALMA) in Japan to study BFB flashes in a thunderstorm cell. The preliminary results show that BFB has an average initiation altitude of about 7.8 km which is much larger than typical negative CG flashes but comparable to IC-CG hybrid flashes. The horizontal extension of BFB is 7.6 km on average with the minimum and maximum values of 0.7 km and 14.0 km, respectively. A detailed charge structure analysis indicates that BFB appears to be initiated in a tilted charge structure, which is assumed to be related to the relatively strong lateral wind in the upper levels. More research on BFB and its comparison to hybrid flashes are underway and will be shown in future studies.