Observations of Precursor Breakdown Prior to Intracloud Lightning Discharges

Using the New Mexico Tech VHF Lightning Mapping Array (LMA) we have detected weak precursor discharges occurring up to a few tens of seconds before the initiation of normal polarity intracloud (IC) lightning discharges. With the very sensitive LMA installed at White Sands Missile Range (WSMR) in south central New Mexico, we observe low-power VHF precursor events in about 10% of lightning discharges. Most of the precursors occur within horizontal distances of about 200 meters of the locations of the initial LMA sources observed in the subsequent discharges, with altitudes about the same as the altitudes of the initial LMA sources. Most precursors consist of a single LMA source, indicating that the duration of the RF emission is less than the 80 μs time window of the WSMR LMA. Occasionally, a given precursor event lasts for up to 400 μs. For most discharges with precursors, only one precursor is observed, occurring anywhere from a few hundred milliseconds to 30 seconds before the discharge. For some flashes, more precursors are observed, several seconds apart. In a few cases, some of the precursor sources are several kilometers from the initial LMA source. In these cases, the leader channels of the ensuing discharge propagate from the flash initiation point to the horizontal locations of the distant precursors. The precursors apparently occur in the regions of the highest pre-discharge fields in the cell, indicating high field regions before the lightning discharge. The precursors are observed only at altitudes of about 8 to 10 km, corresponding to the region between the mid-level negative and upper positive storm charges. We have not observed precursors at altitudes in the 5 to 7 km range, corresponding to the region below the mid-level negative charge where CG discharges are initiated. The altitude dependence of the precursors may result from the fact that breakdown is easier to initiate at lower pressures. Because of RF emission restrictions at WSMR, the background noise levels are about a factor of ten lower than other LMA arrays. We have observed precursors with less sensitive LMAs. In particular, we observed numerous IC precursors in an inverted polarity storm during the Severe Thunderstorm Electrification/Precipitation Study (STEPS 2000) project in western Kansas and eastern Colorado. Inverted polarity storms during STEPS and in Oklahoma were often observed to produce steady streams of temporally- and spatially-isolated single point discharges similar to precursors at the higher altitudes of the upper negative charge, and at even higher altitudes (up to 17-18 km) during convective surges in the storms. These observations also indicate that short duration precursor-type breakdown is easier to initiate at higher altitudes. If IC precursors are initiated by the same process that initiates the IC discharges, the abundance of precursors argue against cosmic ray induced air showers as the initiation mechanism which requires cosmic rays with energies of at least 1016 eV. The flux of such high energy cosmic rays is too low to initiate several precursors near the lightning initiation location within a period of a few tens of seconds. We thank the WSMR Meteorology Branch for data used for this work.