W. R. Gamerota

Current Waveforms for Lightning Simulation

From a survey of the literature on lightning characteristics, we present and discuss our recommendations for the median (50%) and severe (1%) values of the salient parameters of lightning current in both positive and negative cloud-to-ground discharges. We present general expressions, suitable for numerical simulation of lightning effects, for lightning current versus time, current derivative versus time, second current derivative versus time, charge transfer versus time, and action integral (specific energy) versus time. We give sets of constants for these expressions such that the resultant waveforms for positive and negative flashes and for their component strokes and continuing current exhibit approximately the median and severe lightning current parameters recommended, and otherwise resemble the measured waveforms found in the literature.

Evaluation of the GLD360 performance characteristics using rocket‐and‐wire triggered lightning data

We estimated the performance characteristics of the Global Lightning Dataset (GLD360) using rocket-and-wire triggered lightning data acquired at Camp Blanding, Florida, in 2011–2013. The data set consisted of 201 return strokes and 84 kiloampere-scale (≥1 kA) superimposed pulses (initial continuous current pulses and M components) in 43 flashes. All the events transported negative charge to ground. The GLD360 detected 75 strokes and 4 superimposed pulses in 29 flashes. The resultant detection efficiencies were 67% for flashes, 37% for strokes, and 4.8% for superimposed pulses. Out of 75 detected strokes, one (1.3%) was reported with incorrect polarity. The median location error was 2.0 km, and the median absolute current estimation error was 27%. This is the first comprehensive evaluation of GLD360 performance characteristics relative to absolute ground truth, with all previous evaluations being at least in part relative to other locating systems. The results presented in this work may be applicable to regions in and around Florida.

Lightning attachment processes of three natural lightning discharges

Using a high-speed optical imaging system specifically designed for observing the lightning attachment process, we have documented the attachment process for six strokes in three natural lightning flashes. All strokes initiate at a height above ground and propagate bidirectionally from that height, similar to the return strokes of artificially initiated (triggered) lightning previously reported by Wang et al. (2013, 2014). Though the data are quite limited, these natural return strokes suggest a correlation between larger peak current and greater initiation height. Initiation heights determined here span 12–60 m with a typical uncertainty of less than 10 m. The initial upward return stroke luminosity speeds range from (0.8 ± 0.2) to (2.0 ± 0.4) × 108 m/s. Two first return strokes downward luminosity speeds are assessed as (1.6 ± 0.3) × 107 m/s and (1.4 ± 0.3) × 108 m/s. One of the first return strokes appeared to be initiated with a stepping pulse discharge of its leader as an inseparable part of the return stroke.

Does the lightning current go to zero between ground strokes? Is there a current “cutoff”?

At the end of 120 prereturn stroke intervals in 27 lightning flashes triggered by rocket-and-wire in Florida, residual currents with an arithmetic mean of 5.3 mA (standard derivation 2.8 mA) were recorded. Average time constants of the current decay following return strokes were found to vary between 160 µs and 550 µs, increasing with decreasing current magnitude. These results represent the most sensitive measurements of interstroke lightning current to date, 2 to 3 orders of magnitude more sensitive than previously reported measurements, and contradict the common view found in the literature that there is a no current interval. Possible sources of the residual current are discussed.

Coordinated lightning, balloon-borne electric field, and radar observations of triggered lightning flashes in North Florida

This study examines coordinated storm and triggered lightning observations made in July–August 2013 at the International Center for Lightning Research and Testing to determine why triggered flashes in Florida typically transition from an upward vertical channel entering the cloud to horizontal structure near the storms melting level. Data from a balloon-borne electric field meter, a mobile 5 cm wavelength radar, and a small-baseline VHF Lightning Mapping Array acquired during a period in which three flashes were triggered on 1 August confirmed the hypothesis that the transition to horizontal lightning structure just above the melting level occurred in a layer of negative charge. This experiment was the first to provide vertical profiles of the electric field in Florida storms, from which their vertical charge distribution could be inferred. Three dissipating storms observed on different days all had negative charge near the melting level, but a growing mature storm had positive charge there.

Calibration of the ENTLN against rocket-triggered lightning data

We have estimated the ENTLN (formerly WTLN) performance characteristics using data for 245 negative return strokes in 55 flashes triggered from June of 2009 to August of 2012 at Camp Blanding, Florida. Performance characteristics are presented for both originally reported lightning data and reprocessed lightning data (the new processor was introduced in November of 2012). After the reprocessing, the ENTLN performance characteristics changed (relative to those corresponding to the originally reported data) as follows. Flash detection efficiency increased from 80% to 89%, stroke detection efficiency increased from 49% to 67% percentage of misclassified events decreased from 60% to 52% median location error increased from 621 m to 687 m, and median absolute current estimation error decreased from 51% to 17%.

Estimation of triggered‐lightning dart‐stepped‐leader currents from close multiple‐station dE/dt pulse measurements

The modified transmission line model is used to derive the vertically propagating leader-step currents necessary to radiate measured dart-stepped-leader dE/dt pulses from triggered lightning at close range (<400 m) and low altitude (<70 m). The model-predicted dE/dt pulses were compared with measured dE/dt pulses at nine locations ranging from 27 to 391 m from the channel base for four dE/dt pulses radiated from two triggered dart-stepped leaders. The dE/dt pulses at the closest station, 27 m, were unipolar, dominated by electrostatic and induction components of the radiated dE/dt, and of opposite polarity to the more distant initial dE/dt peaks. The other, more distant, eight stations exhibited bipolar dE/dt pulses, being more or less dominated by the dE/dt radiation component. The derived leader-step current has a slow front that precedes a fast transition to peak amplitude followed by a slow decay to zero after several microseconds. For the four modeled dE/dt pulses, the estimated causative leader-step current peak amplitudes varied from 0.9 to 1.8 kA, the half-peak widths ranged from 370 to 560 ns, the charge transfers were about 1 mC, and the peak current derivatives were about 10 kA/µs. The upward propagation speeds of the leader-step current were from 1.1 to 1.5 × 108 m/s with exponential spatial current decay constants from 13 to 27 m. One dE/dt pulse is analyzed in more detail by studying changes in model-predicted waveforms versus current initiation altitude and by examining the effect of varying model input parameters.

Evaluation of ENTLN Performance Characteristics Based on the Ground Truth Natural and Rocket‐Triggered Lightning Data Acquired in Florida

The performance characteristics of the Earth Networks Total Lightning Network (ENTLN) were evaluated by using as ground-truth natural cloud-to-ground (CG) lightning data acquired at the Lightning Observatory in Gainesville (LOG) and rocket-triggered lightning data obtained at Camp Blanding (CB), Florida, in 2014 and 2015. Two ENTLN processors (data processing algorithms) were evaluated. The old processor (P2014) was put into use in June 2014 and the new one (P2015) has been operational since August 2015. Based on the natural-CG-lightning dataset (219 flashes containing 608 strokes), the flash detection efficiency (DE), flash classification accuracy (CA), stroke DE, and stroke CA for the new processor were found to be 99%, 97%, 96%, and 91%, respectively, and the corresponding values for the old processor were 99%, 91%, 97%, and 68%. The stroke DE and stroke CA for first strokes are higher than those for subsequent strokes. Based on the rocket-triggered lightning dataset (36 CG flashes containing 175 strokes), the flash DE, flash CA, stroke DE, and stroke CA for the new processor were found to be 100%, 97%, 97%, and 86%, respectively, while the corresponding values for the old processor were 100%, 92%, 97%, and 42%. The median values of location error and absolute peak current estimation error were 215 m and 15% for the new processor, and 205 m and 15% for the old processor. For both natural and triggered CG lightning, strokes with higher peak currents were more likely to be both detected and correctly classified by the ENTLN.

Do cosmic ray air showers initiate lightning? : A statistical analysis of cosmic ray air showers and lightning mapping array data

It has been argued in the technical literature, and widely reported in the popular press, that cosmic ray air showers (CRASs) can initiate lightning via a mechanism known as relativistic runaway electron avalanche (RREA), where large numbers of high energy and low energy electrons can, somehow, cause the local atmosphere in a thundercloud to transition to a conducting state. In response to this claim, other researchers have published simulations showing that the electron density produced by RREA is far too small to be able to affect the conductivity in the cloud sufficiently to initiate lightning. In this paper, we compare 74 days of cosmic ray air shower data collected in north central Florida during 2013, 2014, and 2015, the recorded CRASs having primary energies on the order of 1016 eV to 1018 eV and zenith angles less than 38 degrees, with Lightning Mapping Array (LMA) data, and we show that there is no evidence that the detected cosmic ray air showers initiated lightning. Furthermore, we show that the average probability of any of our detected cosmic ray air showers to initiate a lightning flash can be no more than 5 percent. If all lightning flashes were initiated by cosmic ray air showers, then about 1.6 percent of detected CRASs would initiate lightning, therefore we do not have enough data to exclude the possibility that lightning flashes could be initiated by cosmic ray air showers.

Electric field derivative waveforms from dart-stepped-leader steps in triggered lightning: DART-STEPPED-LEADER WAVEFORMS

Electric field derivative (dE/dt) pulse waveforms from dart-stepped-leaders in rocket-and-wire triggered lightning, recorded a distance of 226 m from the channel base, are characterized. A single dE/dt pulse associated with a leader step consists of a fast initial rise of the same polarity as the following return stroke followed by an opposite polarity overshoot of smaller amplitude and subsequent decay to background level, without superimposed secondary pulses. A “slow front” often precedes the fast initial rise. For 47 single dE/dt leader pulses occurring during the final 15 µs of 24 dart-stepped-leaders, the pulse mean half-peak width was 76 ns, mean 10-to-90% risetime 73 ns, and mean range-normalized peak amplitude 2.5 V/m/µs. For integrated dE/dt, the mean half-peak width was 214 ns and the mean range-normalized peak amplitude 0.21 V/m. Most dart-stepped-leader dE/dt pulses are more complex than a single pulse. Interpulse interval and average peak amplitude range normalized to 100 km were measured for both single and complex dE/dt pulses during the final 15 µs of 10 dart-stepped-leaders preceding triggered return strokes with peak currents ranging from 8.1 to 31.4 kA. The average range-normalized dE/dt and numerically integrated dE/dt (electric field) peak amplitude increased from 0.9 to 4.9 V/m/µs and 0.13 to 0.47 V/m, respectively, with increasing return stroke peak current while the interpulse interval remained relatively constant at about 2 µs. Strong positive linear correlations were found between both average range-normalized peak pulse amplitude and interstroke interval versus the following return stroke peak current.