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• Research based on electric field data for positive CG lightning. • Stepped leader and preliminary breakdown analysis done in spectral domain. • Spectrogram for preliminary breakdown is more complex than for return stroke. • Changing STFT parameters can reveal new features of lightning components. • PSD-based lightning identification can be implemented in lightning location systems. The main scope of this paper is spectral analysis of several uncommon preliminary events preceding positive cloud-to-ground lightning flashes. They were detected from two different dissipating thunderstorm cells passing over the Warsaw region on 30 July 2019. Three different electric field signatures of positive cloud-to-ground flashes preceded by the preliminary breakdown pulses were recorded and identified. The respective power spectrum density spectrograms were computed additionally to allow more detail analysis of such kind lightning discharge incidents in time and frequency domain. All considered single positive cloud-to-ground flashes were characterized by fast electric field pulse trains lasted in average 3.8 ± 3.7 ms and 4.7 ± 3.0 ms in the time and spectral domain respectively. [ABSTRACT FROM AUTHOR]