Fig. 1: Meteorological condition of tropical storm Haitang producing the outburst of negative NBEs.

a Cloud top temperature and lightning locations of the tropical storm at 18:00 UTC on 30 July 2017. The location is obtained from the combination of a local lightning array and the GLD360. The tropopause is inferred from the temperature profile obtained by radio-sounding observations at a site approximately 100 km from the storm at 00:00 UTC on 31 July 2017; the storm track and intensity are shown as cyan, and pink circles are tropical depression and storm. The green triangle presents the ground-based lightning VLF/LF stations. GZ denotes Guangzhou station (23.57° N, 113.61° E); TX represents Tunxi station (29.70° N, 118.28° E); WH denotes Wuhan station (30.51° N, 114.50° E); HF, HB, and JS stand for Hefei station (31.84° N,117.27° E), Huaibei station (33.98° N,116.91° E), and Jiashan station (32.81° N,118.27° E), respectively. b Vertical profile of radar reflectivity along 23.25° N latitude at 18:00 UTC. The S-band radar observation is obtained from the Shantou station (23.24° N, 116.89° E) approximately 100 km away from the storm (see “Methods”). c A negative NBE radio signal was recorded by four distant VLF/LF stations. The solid red line presents the arrival of the estimated ionospheric reflection waveform versus distance assuming that the reflection height of the ionosphere D layer is at 85 km altitude and that the source height of NBEs is 19.5 km.