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A Modeling Study of Narrow Electric Field Signatures Produced by Lightning Strikes to Tall Towers
The lumped voltage source model proposed by Baba and Rakov (2005b, https://doi.org/10.1029/2004JD005202) for studying the interaction of lightning with tall objects was used to examine the origin of earlier zero crossings observed in electric field signatures produced by lightning strikes to towers. Different return stroke models of transmission line type were used, and model parameters were varied in wide ranges. Lightning channel was assumed to be straight and vertical. It was found that the observed narrow field signatures cannot be reproduced by traditional models and require a narrower input current waveform or/and its faster decay with height. Contribution to the total electric field peak from a tower whose height exceeds 100 m or so is greater than that from the lightning channel. At distances of 2 to 50 km, the electric field signature due to the tower current was found to be bipolar, while that due to the lightning channel current was unipolar. The narrow bipolar electric field waveforms produced by lightning striking the 257-m tower in Florida were reproduced using two approaches. In the first one, we employed as input a typical channel base current waveform and the transmission line with exponential current decay with height model with a very small decay height constant. In the second approach, we used a narrow pulse followed by a steady-level tail as the input current waveform and the transmission line with linear current decay with height model. In both approaches, the computed electric field waveforms matched well the corresponding measured waveforms, at least for the initial half-cycle and opposite polarity overshoot.