Terrain Effects on Time-Frequency Characteristics of Negative Return Stroke Electric Fields: A Path-Incremental Method

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Abstract
Waveforms recorded by lightning location networks mix source variability with propagation effects, making terrain-associated distortion difficult to isolate. We developed a path-incremental method in which the additional segment between two stations observing the same stroke carries the propagation signal. From 4466 records of 554 negative cloud-to-ground return strokes, 469 near-collinear pairs were stratified by additional path length and split at the within-stratum median of a digital-elevation-model terrain complexity index. Higher-complexity (rugged) paths showed the clearest near–far increases in rise time and half-peak width, whereas fall time and zero-crossing time showed no stable ordering, indicating that terrain acts mainly on the pulse front and main peak. In the frequency domain, these paths retained lower normalized band-energy fractions at 50–150 and 100–300 kHz, most systematically at 100–300 kHz for subsequent strokes, whereas the 200–500 kHz response was unstable. Separate mixed-effects models linked path elongation, occlusion, and diffraction geometry individually to a 39.7–42.5% lower 100–300 kHz ratio for subsequent strokes. Twelve representative-path axisymmetric finite-difference time-domain simulations under matched source currents reproduced the same ordering in these two bands across three ground conductivities. Rise time, half-peak width, and the normalized 100–300 kHz ratio are the most consistent terrain-associated indicators, supporting terrain-aware waveform interpretation.
Funding Information
  • National Natural Science Foundation of China (U24B2099)

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