A distinct class of isolated intracloud lightning discharges and their associated radio emissions

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Abstract
Observations of radio emissions from thunderstorms were made during the summer of 1996 using two arrays of sensors located in northern New Mexico. The first array consisted of three fast electric field change meters separated by distances of 30 to 230 km. The second array consisted of three broadband (3 to 30 MHz) HF data acquisition systems separated by distances of 6 to 13 km. Differences in signal times of arrival at multiple stations were used to locate the sources of received signals. Relative times of arrival of signal reflections from the ionosphere and Earth were used to determine source heights. A distinct class of short‐duration electric field change emissions was identified and characterized. The emissions have previously been termed narrow positive bipolar pulses (NPBPs). NPBPs were emitted from singular intracloud discharges that occurred in the most active regions of three thunderstorms located in New Mexico and west Texas. The discharges occurred at altitudes between 8 and 11 km above mean sea level. NEXRAD radar images show that the NPBP sources were located in close proximity to high reflectivity storm cores where reflectivity values were in excess of 40 dBZ. NPBP electric field change waveforms were isolated, bipolar, initially positive pulses with peak amplitudes comparable to those of return stroke field change waveforms. The mean FWHM (full width at half maximum) of initial NPBP field change pulses was 4.7 μs. The HF emissions associated with NPBPs were broadband noise‐like radiation bursts with a mean duration of 2.8 μs and amplitudes 10 times larger than emissions from typical intracloud and cloud‐to‐ground lightning processes. Calculations indicate that the events represent a distinct class of singular, isolated lightning discharges that have limited spatial extents of 300 to 1000 m and occur in high electric field regions. The unique radio emissions produced by these discharges, in combination with their unprecedented physical characteristics, clearly distinguish the events from other types of previously observed thunderstorm electrical processes.

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