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• An upward flash was triggered from a 257-m tower in florida by a +CG 45 km away. • Six return strokes of the upward flash were all initiated by bidirectional leaders. • All six return-stroke field waveforms at 8.8 km exhibited early zero-crossings. • Two scenarios giving rise to the narrow field waveforms are discussed. We examined in detail the morphology and evolution of an upward negative flash initiated from a 257-m tower in Florida. High-speed video camera images and wideband electric field records, as well as ENTLN data, were used. The upward negative flash was induced (triggered) by a single-stroke 50-kA +CG that occurred about 45 km from the tower. The 257-m tower flash contained 6 leader/return stroke sequences and 1 attempted leader that almost terminated on the tower. All the leaders exhibited bidirectional extension. Electric field waveforms produced by the return strokes (measured at a distance of 8.8 km) were bipolar and abnormally narrow (exhibited earlier zero crossings). In order to examine the origin of the observed earlier zero crossings, we used two return-stroke models of transmission line type (MTLL and MTLE) to see what model input parameters are responsible for this feature. Within the limits of those models, the observed earlier zero crossings could be explained only by a narrow input current waveform or/and its fast amplitude decay with height. [ABSTRACT FROM AUTHOR]