Yuji Takayanagi

Large bipolar lightning discharge events in winter thunderstorms in Japan

Using a low-frequency lightning location system comprising nine stations, we have observed and analyzed 374 large and bipolar electric field change waveforms that occurred during the winter of 2012–2013. Since the waveforms are different from those produced by any well-studied lightning discharge processes, we refer to these source discharge events using a new name: large bipolar events (LBEs). LBEs can be characterized by the following features: (1) All have the same polarity as negative return stroke. (2) All exhibit a single bipolar pulse with a pulse width around 15 µs and similar positive and negative cycles. (3) All are located on the land along the Japan Sea coast, indicating they are probably associated with high grounded objects. (4) Most LBEs produce very large electric field changes that are even larger than that of positive and negative return strokes. (5) Most LBEs are temporally isolated within several milliseconds but are frequently followed by intracloud discharges after tens of milliseconds. (6) Most LBEs produce a single well-distinguished ionospheric reflection pulse, and the time difference between LBE pulse and the corresponding reflection pulse can be used to calculate ionospheric reflection height. It is speculated that LBE is a type of powerful and transient lightning discharge event produced within a compact region of strong electric field formed when the negative charge layer in thunderclouds is very close to the top of a tall grounded object.

Leader process in 3D observed by VLF/LF broadband interferometer

We have been designing and developing a 3D lightning location system based on the broadband digital interferometry technique in VLF/LF band. The VLF/LF broadband digital interferometer (VLF/LF DITF) is able to locate lightning discharges such as return strokes, K events, and compact intracloud lightning discharges (CIDs), which are energetic breakdowns within thunderclouds several hundred kilometers away from the VLF/LF DITF. From July 2011 to now, a simultaneous observation of a VLF/LF band DITF and VHF broadband digital interferometers (VHF band DITF) were conducted in Kansai, Japan. We show the time series variation of an intracloud discharge initiated by a preliminary breakdown pulse train (PB), negative cloud to ground flash and intracloud discharge including K-event. VLF/LF DITF successfully located not only energetic breakdowns within thunderclouds but also leader processes.