Bolts from the Blue
Creators: name: Edens, Harald
Publisher: name: Zenodo
Year: 2011
Subjects: Atmospheric sciences
Lightning
Imaging
Mapping
Physics
Electric field

description: A bolt from the blue (BFB) is a cloud-to-ground flash that initiates as a classic intracloud (IC) flash between midlevel negative and upper positive charge in a storm. It is of negative polarity, lowering negative charge to ground. BFB flashes can develop from IC flashes when the upper positive charge region of a storm is depleted relative to the midlevel negative charge region, due to the mixing with negative screening charge at the storm top. The leaders to ground of BFB flashes often exit the cloud at altitudes of 8 km or higher and can be observed visually. Leader characteristics are studied using Lightning Mapping Array (LMA) data, slow and fast electric field change (∆E) waveforms, broadband VHF recordings, and high-speed video recordings. Two types of BFB flashes are observed, which are termed Type-I and Type-II. In Type-I BFB flashes the leader to ground develops from the upward negative leader in a continuous manner. In Type-II BFB flashes the leader is initiated by fast negative breakdown along a preexisting channel and either branches off the channel or is an extension of it. Three stages are defined for BFB flashes: an intracloud stage, a cloud-to-ground stage, and a final stage. The intracloud stage encompasses all intracloud activity up to the initiation of the outward and downward leader, which propagates to ground during the cloud-to-ground stage. The final stage is characterized by continued positive breakdown, which often produces K-leaders into upper positive charge and dart leaders along the channel to ground. Of the 59 BFB flashes presented in this work, 49% produced more than one stroke to ground; 32% produced three or more strokes to ground. The mean initiation altitude of these flashes was 7.7 ± 0.7 km above MSL. The mean flash duration was 451 ± 173 ms while the time to the first return stroke was 221 ± 87 ms. The average number of strokes was 2.1 ± 1.7; one flash produced nine strokes to ground. The horizontal distance, or offset, of the cloud-to-ground leader from the flash initiation region ranged from 1 to 17 km with a mean distance of 6.5 ± 2.6 km. Also, a correlation was found between wind shear and the preferred cloud exit location of BFB flashes, with a preference for the upwind side of the storm. The wind shear is defined as the vector difference of the mean winds at 4–8 km altitude and at 8–12 km altitude. Some BFB flashes produce positive and negative breakdown in laterally displaced regions, indicating that the two major charge regions in a storm are displaced as a result of the wind shear. Several interesting observations are made of the cloud-to-ground leader of BFB flashes. Luminosity recoil waves are observed during the leader stepping process while the downward developing leader is between 9 and 6 km altitude. These recoil waves appear as bright luminosity events near the tips of the leaders, and appear to travel back up along the channel in a retrograde direction. The velocity of some of these recoil events at 6 km altitude is estimated at 2.5 × 10<sup>6</sup> m/s. Another observation is the occurrence of branch discharges, which are transient, bright discharges along inactive branches of the main leader channel at altitudes of 6–9 km after the main leader has propagated several kilometers farther downward and is approaching ground. These branch discharges are current surges that transport negative charge from the branch into and down along the main leader channel, heating it and making it more conductive. These cause brief intensifications in the luminosity of the main leader channel. It is found that fast K-leaders occur during the propagation of the negative leaders, both in the cloud and along the cloud-to-ground leader, which assist the negative breakdown by supplying negative charge to the leader tip(s). For one BFB flash in which these fast K-leaders occurred during the cloud-to-ground stage, a K-leader velocity of 1–2 × 10<sup>7</sup> m/s was found. Typical estimates of negative-leader velocities of BFB flashes are on the order of 0.8–2.0 × 10<sup>5</sup> m/s; dart leaders typically propagate at velocities of 5 × 10<sup>5</sup> to 1 × 10<sup>7</sup> m/s, consistent with leader velocities reported in the literature. BFB flashes typically produce one or more attempted dart leaders that do not succeed in reaching ground. Since BFB flashes have much longer leader channels than those in normal –CG flashes, they are expected to be less successful in producing multiple strokes to ground. BFB flashes show a characteristic transition in the VHF emissions by the negative cloud-to-ground leader as it propagates from high altitude (above 8 km) to low altitude (below 5 km). At high altitude the leader exhibits impulsive VHF emissions typical of regular IC flashes, while at low altitude the VHF emissions appear more continuous in nature, characteristic of those observed in normal –CG and low-altitude IC flashes. It is found that the rate of emissions increases with decreasing altitude, whereas the emitted source power does not appear to change significantly. The increase in the VHF emission rate by the negative leader appears to be caused by a change in the leader stepping process: The leader step length becomes shorter as the leader reaches lower altitude. From a photograph of a negative leader in clear air at 11 km altitude a step length on the order of 200 m is inferred, while the leader step length inferred from high-speed video at 6 km altitude is on the order of 15–30 m. From two photographs of negative cloud-to-ground leaders of BFB flashes a leader step length on the order of 16 m at 3 km altitude is inferred, which is an order of magnitude shorter than the step length at 11 km altitude. Since the velocity of a negative stepped leader does not appear to change between high and low altitudes, leader steps must occur at a higher rate at lower altitude, with a correspondingly higher rate of VHF emissions produced during the leader stepping process. This effect is enhanced by an increased number of active leader tips at low altitude from multiple branches; cloud-to-ground leaders of BFB flashes are generally branched profusely at low altitude but not at high altitude.