Plasma discharge fluid simulations indicate a negative streamer can be initiated from the narrow tip of a cone‐shaped, realistically sized hydrometeor in a uniform electric field as low as 0.65Ek. A negative streamer, however, cannot be formed from a cone‐shaped hydrometeor with a slightly smaller base radius or a column hydrometeor under otherwise the same conditions. The simulation also shows the negative streamer is accompanied by a positive streamer. The results present implications for understanding lightning initiation and the physical mechanism of the recently discovered thunderstorm‐related phenomenon called fast negative breakdown. Finally, the initiated negative streamer develops with a relatively constant head radius and velocity in contrast with the expanding and accelerating positive streamer. It appears that the differences between the properties of each polarity of streamer are more pronounced in an electric field below Ek.
Lightning emits short wavelength radio waves from the very high frequency (VHF, 30–300 MHz) to ultrahigh‐frequency (UHF, 0.3–3 GHz) range. A lightning subprocess that has recently been shown to emit in this frequency range is streamer collisions. In this work, we report a modeling study of streamers colliding in ambient electric fields ranging from subbreakdown to overbreakdown values. The streamers are initiated from isolated hydrometeors similar in size to those typically seen in thunderstorms. In every case presented, the collision produces emissions extending into the UHF range. The emission spectrum from the subbreakdown ambient field cases falls off faster as frequency increases compared with the overbreakdown cases. It appears that the length of the streamers upon colliding, under the same ambient field, has a negligible effect on the shape of the spectrum. The results are important for interpreting observations of lightning processes that involve streamer collisions.
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