Sumedhe Karunarathne

On the percentage of lightning flashes that begin with initial breakdown pulses

The initial breakdown (IB) stage of lightning flashes typically occurs in the first 20 ms of a flash and includes a series of IB pulses often detected with electric field change sensors. There is disagreement about the percentage of negative cloud-to-ground (CG) flashes that begin with IB pulses. This study includes new data on IB pulses in 198 CG flashes in Austria (latitude ~48˚N), Florida, USA (~29˚N) and South Dakota, USA (~44˚N) with, respectively, 100%, 100%, and 95% of the flashes having IB pulses. The data indicate that the amplitude of the largest IB pulse, range normalized to 100 km, is often weak, < 0.5 V m−1, with the lower latitude having a greater percentage (36%) of these weak maximum IB pulses than the higher latitude (11%). Since sensor noise levels are often larger than this value, detection of smaller amplitude IB pulses may be difficult. A similar result is seen in the amplitude ratio of the largest IB pulse to the first return stroke: at the lower latitude, 50% of flashes had a ratio < 0.1 versus 8% of flashes at the higher latitude. However, comparisons of the amplitude ratios from Austria (~48˚) and South Dakota (~44˚) do not support a simple latitude dependence. The data also show that 5–10% of IB pulses occur more than 100 ms before the first return stroke. These findings may explain why some previous studies found percentages <100%. Overall, the results indicate that all negative CG flashes probably begin with IB pulses.

Leader observations during the initial breakdown stage of a lightning flash

This paper describes luminosity and leader propagation during the initial breakdown (IB) stage of a cloud-to-ground flash, beginning at 6.06 km altitude and 31.86 ms before the return stroke (RS). High-speed video (50,000 frames per s) and time-correlated electric field change (E-change) data show multiple branch ends advance concurrently in the first 6 ms of the flash; each branch begins with IB pulses and propagates first via bursts as an initial leader. Burst luminosity (pixel intensity) is directly related to IB pulse amplitude. Some initial leader branches transition to advancing as stepped leaders after a few milliseconds. Each initial leader branch end makes the transition to a stepped leader branch end at a different time, resulting in a complex E-change waveform including relatively narrow step-type pulses during the IB stage and no apparent Intermediate stage prior to the Leader stage. There is no visible evidence of an upward propagating leader end prior to the RS nor of any light above the earliest visible IB luminosity prior to or during the RS. During the RS, the topmost visible portion of the channel that developed as an initial leader (above 5.1 km) behaves differently from the channel below, indicating it is less conductive. Radar and time-of-arrival lightning source data indicate that the IB luminosity visible to the camera comes from about 6 km inside the thundercloud echo. The 1276 m long initial leader transitions to a stepped leader at 4.9–5.0 km, near the altitude of the radar bright band.

Modeling initial breakdown pulses of CG lightning flashes

Electric field change waveforms of initial breakdown pulses (IBPs) in cloud-to-ground (CG) lightning flashes were recorded at ten sites at Kennedy Space center, Florida, in 2011. Six “classic” IBPs were modeled using three modified transmission line (MTL) models called MTLL, MTLE, and MTLK. The locations of the six IBPs were obtained using a time-of-arrival method and used as inputs for the models; the recorded IBP waveforms from six to eight sites were used as model constraints. All three models were able to reasonably fit the measured IBP waveforms; the best fit was most often given by the MTLE model. For each individual IBP, there was good agreement between the three models on several physical parameters of the IBPs: current risetime, current falltime, current shape factor, current propagation speed, and the total charge moment change. For the six IBPs modeled, the ranges, mean values, and standard deviations of these quantities are as follows: current risetime [4.8–25, (12 ±6)] μs, current falltime [15–37, (25 ±6)] μs, current speed [0.78–1.8, (1.3 ±0.3)]×108 m/s, and charge moment change [0.015–0.30, (0.12 ±0.10)] C km. Currents in the MTLL and MTLE models moved a negative charge −Q downward and deposited an equivalent positive charge +Q along their paths; the mean Q values were 0.35 C for MTLL and 0.71 C for MTLE. MTLK model deposited negative charge along its lower path and positive charge along its upper path with mean values of 0.27 C.

Electromagnetic activity before initial breakdown pulses of lightning

Lightning flash initiation is studied using electric field change (E-change) measurements made in Florida. An initial E-change (IEC) was found immediately before the first initial breakdown (IB) pulse in both cloud-to-ground (CG) and intracloud (IC) flashes if the E-change sensor was within 80% of the reversal distance of the IEC. For 18 CG flashes, the IECs had an average point dipole moment of 23 C m and an average duration of 0.18 ms; these parameters for 18 IC flashes were −170 C m and 1.53 ms. The IECs of CG flashes began with a change in the slope of the E-change (with respect to time) from zero slope to a positive slope, consistent with downward motion of negative charge and/or upward motion of positive charge. For IECs of IC flashes, the beginning slope change was from zero to negative slope, consistent with upward motion of negative charge and/or downward motion of positive charge. During an IEC, the E-change monotonically increased for CG flashes and monotonically decreased for IC flashes. In 14 of 36 cases, the IEC beginning was coincident with a discrete, impulsive source of VHF radiation; another 13 cases had at least one VHF source during the IEC or the first IB pulse. Before the IECs, there were no preliminary variations detected in the 36 flashes. It is hypothesized that lightning initiation begins with an ionizing event that causes the IEC and that the IEC enhances the ambient electric field to produce the first IB pulse.

Observations of positive narrow bipolar pulses

Waveforms of 226 positive narrow bipolar pulses (NBPs) were obtained with five to eight stations of E-change meters covering an area of 70 × 100 km2. The NBPs had typical average parameters: 10–90% rise time of 2.6 μs, full width at half maximum time of 2.8 μs, zero cross time of 9.9 μs, and range-normalized amplitude at 100 km of 11.0 V/m. Four main types of positive NBP waveforms were identified: Type A had a simple bipolar waveform with a positive peak and a negative overshoot peak (1% of NBPs), Type B had extra peak(s) superimposed on the overshoot peak (67%), Type C had extra peak(s) on or just after the main positive peak (13%), and Type D had extra peak(s) before the main positive peak (19%). Regardless of type, each NBP waveform maintained its basic shape across a range of 10 to 130 km from its origin. NBP locations, obtained with a time of arrival technique, seemed unrestricted in their horizontal distribution (except for Type C), while NBP altitudes ranged from 7 to 19 km with an average of 13 km. Estimated peak currents were 2–126 kA with an average of 30 kA. Isolation of NBPs from other lightning events was determined for both temporal (660 ms) and spatial (>10 km) quantities; 37% of NBPs were isolated, 38% occurred within 660 ms before a flash, 19% occurred within flashes, and 11% occurred within 660 ms after a flash. The total RMS power radiated by NBPs within 1 kHz–2.5 MHz bandwidth had a range of 5.0 × 106–6.1 × 108 W with an average of 7.8 × 107 W.

Transient luminosity along negative stepped leaders in lightning

This study presents observations of abandoned stepped leader branches that briefly reconnect to the main stepped leader trunk or another active branch during the negative stepped leader advance in natural cloud-to-ground lightning strokes. The transient luminous features described are herein termed sparks. High-speed video data, with 20 µs image interval, show these sparks are common, bright, and fast. They typically reach maximum visible extent of a few hundred meters or less and peak intensity of one to three times that of their parent leader within 40 µs. Most sparks connect to a parent leader within their first 20 µs and are visible for less than 120 µs. Generally, there are several milliseconds (average 3.3 ms) before the spark during which its branch is visibly abandoned, i.e., apparently neither propagating nor connected to the active stepped leader system. There is a tendency for sparks to occur late in the stepped leader advance, averaging 900 µs before the return stroke for 90 sparks in 14 strokes. Sparks occur at altitudes at least as high as the visible stepped leader top (about 3000 m in these data), but they have not been observed below 500 m altitude. Parent leaders typically get brighter below the connection point after the spark, and in some cases, their speed of advance increases. Nearby time-correlated electric field change data show a distinct spark signature characterized by a relatively large bipolar pulse, followed by a slower decrease over 40–100 µs, ending with another relatively large pulse.

Branched dart leaders preceding lightning return strokes

This study describes the occurrence of branches in lightning dart leaders, based on data acquired in Florida using a high-speed video camera and electric field change sensors. More than half (57%) of 72 flashes with analyzable dart leaders show at least one successful branched dart leader (BDL), and 9 flashes have two successful BDLs. Overall, 18% of 282 visible successful dart leaders are branched. Most (42 of 50) cases of BDLs occur in the first dart leader after a stepped leader/return stroke sequence, and the data indicate 55% of first dart leaders are visibly branched. Compared to first dart leaders in the 31 flashes without any branched dart leaders, BDLs tend to follow stepped leader/return strokes with significantly larger average peak currents (-31.3 vs. -20.6 kA) and shorter average interstroke intervals (71.94 vs. 94.64 ms). Average peak current of BDL strokes is 62% larger (-17.8 vs -11.0 kA) than for unbranched first dart leader strokes. Branched dart leaders generally travel in the some of the most recently used lightning channels, but they are not always within the main channel of the prior return stroke. Successful BDLs may dart all the way to ground when in a prior stroke channel, or they may become stepped leaders when they reach the lower end of the prior stroke branch. Electric field change data for all the BDL cases exhibit an erratic pulse character for at least part of the leader duration; in some cases, the erratic character ends when the branches vanish. This paper is based on Stolzenburg et al. (2014).

Electrostatic field changes and durations of narrow bipolar events

Using an array of ten sensors, electric field change measurements of 35 positive narrow bipolar events (NBEs) were obtained at close range (≤10 km). At the closest sensor all 35 NBEs had a net electrostatic change (ΔEfast) associated with the main bipolar pulse with amplitudes of 0.4 to 16.3 V/m (not range normalized). At the closest sensor the bipolar pulse of each of the 35 NBEs was followed by a relatively long, slow electrostatic change (ΔEslow) with amplitudes of 0.1 to 43.4 V/m and durations of 0.7 to 33.7 ms. For ΔEfast, estimated 3-D charge moments for 10 NBEs ranged from 0.46 C kmto 1.81 C km with an average and standard deviation of (1.09 ± 0.36)C km. Seven 3-D charge moments were essentially vertically oriented, and the other three moments were tilted at 10°-20° from vertical. The ten 3-D charge moments were overlaid on vertical radar cross sections; six NBEs occurred in weak reflectivity near the upper reflectivity boundary; the other four NBEs occurred near the top of the high-reflectivity core of the thunderclouds. For ΔEslow, we estimated 3-D charge moments for only 3 NBEs; they ranged from 1.11 C km to 2.69 C km with an average of (1.83 ± 0.80)C km. A two-current transmission line model matched the bipolar pulse and the following slow change (ΔEslow) of one NBE reasonably well. The slow change mechanism may be different from the NBE mechanism and similar to the initial E-change before typical lightning flashes.

Narrow Bipolar Pulse locations compared to thunderstorm radar echo structure

The locations of 172 positive narrow bipolar pulses (NBPs) found on one day in Florida are superimposed on radar reflectivity data from that day. All 172 NBPs were found within the reflectivity of a thundercloud or at the edge of the reflectivity. The NBPs were classified into three groups: (I) in or above the high-reflectivity core of the storm, (II) in the convective region but not Group I, or (III) in the anvil region. Groups I, II, and III had, respectively, 79%, 17%, and 4% of the NBPs. Of the 136 NBPs in Group I, 43% occurred within the reflectivity core and 57% occurred above the core. A sequence of 34 positive NBPs during 1 h of one thunderstorm suggests that the majority of NBPs occurred during the rapid growth of two thunderstorm cells. Positive NBPs seem to recur in some storm locations; 67 (39%) of the NBPs were part of a recurrent set. We found 28 cases of NBPs recurring in approximately the same location, including 22 doublets, 3 triplets, 2 quadruplets, and 1 sextuplet. Analyses of one quadruplet and one sextuplet showed that these 10 positive NBPs occurred just above and/or right beside the high-reflectivity core on the downshear side of the core. Our data lead us to a hypothesis that NBPs occurring between the thunderstorms upper positive charge and upper negative screening charge are initiated by small-scale charge regions with positive charge above negative charge, or opposite the orientation of the large-scale storm charges.

Modeling stepped leaders using a time‐dependent multidipole model and high‐speed video data

A full negative stepped leader and portions of four negative stepped leaders preceding negative cloud-to-ground lightning return strokes were modeled; each model was constrained to match electric field change measurements recorded at three or four sites located within 30 km of the leader. The time evolution and 2-D locations of stepped leaders were obtained from data collected with a high-speed video camera operated at 50,000 frames/s. The Lu et al. (Charge transfer during intracloud lightning from a time-dependent multidipole model, Journal of Geophysical Research, 2011) time-dependent multidipole model was used with some modifications. The model used a time step equal to one video frame, 20μs. At each time step, negative charges were deposited at stepped leader tips based on measured light intensity, and an equivalent positive charge was deposited at one of the locations of the initial breakdown pulses that preceded the stepped leaders. The method has the unique advantage of obtaining locations of CG stepped leaders including its branches all the way to the ground. Three main quantities were obtained from the model: total charge transfer of −1.50 to −7.51 C, average line charge density of −0.113 to −0.413 mC/m (mean =− 0.196 mC/m), and average current of −0.084 to −0.456 kA (mean =− 0.31 kA). From the video data, the estimated 2-D speeds were 2.43–4.95×105 m/s (mean 3.34 × 105 m/s), and the cumulative lengths of the all branches were 3.5–9.2 times the vertical distance traveled by the visible stepped leader.