M. A. Uman

University of Florida, Gainesville, Florida, United States

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Publications (306)666.54 Total impact

  • F. L. Carvalho · M. A. Uman · D. M. Jordan · T. Ngin ·
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    ABSTRACT: We measured current and luminosity at the channel bottom of 12 triggered lightning discharges including 44 return strokes, 23 M-components, and 1 initial continuous current pulse. Combined current and luminosity data for impulse currents span a 10–90% risetime range from 0.15 to 192 µs. Current risetime and luminosity risetime at the channel bottom are roughly linearly correlated (τr,I = 0.71τr,L1.08). We observed a time delay between current and the resultant luminosity at the channel bottom, both measured at 20% of peak amplitude, that is approximately linearly related to both the luminosity 10–90% risetime (Δt20,b = 0.24τr,L1.12) and the current 10–90% risetime (Δt20,b = 0.35τr,I1.03). At the channel bottom, the peak current is roughly proportional to the square root of the peak luminosity (IP = 21.89LP0.57) over the full range of current and luminosity risetimes. For two return strokes we provide measurements of stroke luminosity vs. time for 11 increasing heights to 115 m altitude. We assume that measurements above the channel bottom behave similarly to those at the bottom and find that (1) one return stroke current peak decayed at 115 m to about 47% of its peak value at channel bottom, while the luminosity peak at 115 m decayed to about 20%, and for the second stroke 38% and 12%, respectively; and (2) measured upward return stroke luminosity speeds of the two strokes of 1.10 × 108 and 9.7 × 107 ms−1 correspond to current speeds about 30% faster. These results represent the first determination of return stroke current speed and current peak value above ground derived from measured return stroke luminosity data.
    Journal of Geophysical Research Atmospheres 09/2015; DOI:10.1002/2015JD023814 · 3.43 Impact Factor
  • D. Wang · N. Takagi · W. R. Gamerota · M.A. Uman · D.M. Jordan ·
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    ABSTRACT: 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 bi-directionally from that height, similar to the return strokes of artificially initiated (triggered) lightning previously reported by Wang at 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.
    Journal of Geophysical Research Atmospheres 09/2015; DOI:10.1002/2015JD023734 · 3.43 Impact Factor
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    ABSTRACT: This study examines coordinated storm and triggered lightning observations made 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 storm's 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.
    Geophysical Research Letters 06/2015; 42(13):n/a-n/a. DOI:10.1002/2015GL064203 · 4.20 Impact Factor
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    ABSTRACT: An acoustic camera comprising a linear microphone array is used to image the thunder signature of triggered lightning. Measurements were taken at the International Center for Lightning Research and Testing (ICLRT) in Camp Blanding, FL during the summer of 2014. The array was positioned in an end-fired orientation thus enabling the peak acoustic reception pattern to be steered vertically with a frequency-dependent spatial resolution. On 14 July 2014, a lightning event with 9 return strokes was succesfully triggered. We present the first acoustic images of individual return strokes at high frequencies (>1kHz) and compare the acoustically-inferred profile with optical images. We find: (i) a strong correlation between the return stroke peak current and the radiated acoustic pressure, (ii) an acoustic signature from an M-component with an unusual fast risetime. These results show that acoustic imaging enables clear identification and quantification of thunder sources as a function of lightning channel altitude.
    Geophysical Research Letters 06/2015; 42(14):n/a-n/a. DOI:10.1002/2015GL064451 · 4.20 Impact Factor
  • W. R. Gamerota · M. A. Uman · J. D. Hill · T. Ngin · J. Pilkey · D. M. Jordan ·
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    ABSTRACT: 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 1.5x108 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.
    02/2015; 120(4). DOI:10.1002/2014JD022815
  • W. R. Gamerota · M. A. Uman · J. D. Hill · D. M. Jordan ·
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    ABSTRACT: Corona streamers are a critical component of lightning leader step formation, and are postulated to produce the very high electric fields at their tips that produce runaway electrons resulting in the observed x-ray bursts associated with leader stepping. Corona emanating from the vicinity of the leader tip between leader steps was analyzed using three sequential high-speed video sequences of dart-stepped leaders in three different triggered lightning flashes during the summers of 2013 and 2014 in northeast Florida. Images were recorded at 648 kiloframes per second (1.16 µs exposure time, 380 ns dead time) at an altitude of 65 m or less. In each image sequence, the leader propagates downward in consecutive frames, with corona streamers observed to fan outward from the bright leader tip in less than the image frame time of about 1.5 µs. In 21 exposures, corona streamers propagate, on average, 9 m below the bright leader tip.
    02/2015; 42(6). DOI:10.1002/2014GL062911
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    ABSTRACT: Although the production of X-rays from natural and rocket-triggered lightning leaders have been studied in detail over the last 10 years, the energy spectrum of the X-rays has never been well measured because the X-rays are emitted in very short but intense bursts that result in pulse pileup in the detectors. The energy spectrum is important because it provides information about the source mechanism for producing the energetic runaway electrons and about the electric fields that they traverse. We have recently developed and operated the first spectrometer for the energetic radiation from lightning. The instrument is part of the Atmospheric Radiation Imagery and Spectroscopy (ARIS) project and will be referred to as ARIS-S (ARIS Spectrometer). It consists of seven 3′′ NaI(Tl)/photomultiplier tube scintillation detectors with different thicknesses of attenuators, ranging from no attenuator to more than 1′′ of lead placed over the detector (all the detectors are in a 1/8′′ thick aluminum box). Using X-ray pulses preceding 48 return strokes in 8 rocket-triggered lightnings, we found that the spectrum of X-rays from leaders is too soft to be consistent with Relativistic Runaway Electron Avalanche. It has a power law dependence on the energies of the photons, and the power index, λ, is between 2.5 and 3.5. We present the details of the design of the instrument and the results of the analysis of the lightning data acquired during the summer of 2012.
    Journal of Geophysical Research Atmospheres 01/2015; 120(20):10951-10963. DOI:10.1002/2015JD023217 · 3.43 Impact Factor
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    ABSTRACT: The correlations between channel-bottom light intensity and channel-base current of all discharge processes of a rocket-and-wire-triggered lightning flash, including initial continuous current (ICC) pulses, ICC pulse background continuing current (IBCC), return strokes, M components, and M component background continuing currents (MBCC), have been investigated. A rough linear correlation has been found between the current squared and the light intensity for ICC pulses (including peaks of different ICC pulses), IBCC, the initial rising stage (IRS) of return strokes (including current peaks of different strokes), and MBCC. The slopes of the correlation regression lines for the current squared versus light intensity of ICC pulses and IBCC are similar, but they are about 2-3 times smaller than the slopes of MBCC and 5-7 times smaller than the slopes of the IRS of return strokes. In contrast, a rough linear correlation has been found between the current and the light intensity for the later slow decay stage of return strokes and for the M components. The slopes of the correlation regression lines of the current versus the light intensity for these latter two processes are found to be similar. No simple correlation has been found between the current and the light intensity for the initial fast decay stage (IFDS) of return strokes. The duration of the IFDS of return strokes generally lasts from several microseconds to several tens of microseconds and is more or less directly proportional to the corresponding peak return stroke current squared. A time delay ranging from 12 mu s to 300 mu s has been found between the current and the light intensity of all ICC pulses and M components. The time delay decreases as the corresponding peak current increases.
    12/2014; 119(23):13,457-13,473. DOI:10.1002/2014JD022367
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    ABSTRACT: Lightning Mapping Array (LMA) data are used to compare the propagation paths of seven rocket-triggered lightning flashes to the inferred charge structure of the thunderstorms in which they were triggered. This is the first LMA study of Florida thunderstorm charge structure. Three sequentially (within 16 minutes) triggered lightning flashes, whose initial stages were the subject of Hill et al. [2013], are re-examined by comparing the complete flashes to the preceding natural lightning to demonstrate that the three rocket-triggered flashes propagated through an inferred negative charge region that decreased from about 6.8 to about 4.4 km altitude as the thunderstorm dissipated. Two other flashes were also sequentially triggered (within 9 minutes) in a thunderstorm that contained a convectively intense region ahead of a stratiform region, with similar observed results. Finally, two unique cases of triggered lightning flashes are presented. In the first case, the in-cloud portion of the triggered lightning flash, after ascending to and turning horizontal at 5.3 km altitude, just above the 0 °C level, was observed to very clearly resemble the geometry of the in-cloud portion of the preceding natural lightning discharges. In the second case, a flash was triggered relatively early in the storm's lifecycle that did not turn horizontal near the 0 °C level, as is usually the case for triggered lightning in dissipating storms, but ascended to nearly 7.5 km altitude before exhibiting extensive horizontal branching.
    12/2014; 119(23). DOI:10.1002/2014JD022139
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    ABSTRACT: The time delay between lightning return stroke current and the resultant luminosity was measured for 22 return strokes in 8 lightning flashes triggered by the rocket-and-wire technique during the summer of 2014 in Florida. The current-to-luminosity delay measured at the channel base at the 20 percent amplitude level ranged from 30 to 200 ns with an average of 90 ns and at the 50 percent amplitude level ranged from 30 to 180 ns with an average of 94 ns. The delays are significantly shorter than that predicted by Liang et al. (2014) from theory. The current-to-luminosity delays increase with increasing current rise-time, current rise-time varying from 190 ns to 570 ns, but the delay appears not to depend on the peak current value.
    11/2014; 41(22). DOI:10.1002/2014GL062190
  • W. R. Gamerota · M. A. Uman · J. D. Hill · T. Ngin · J. Pilkey · D. M. Jordan ·
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    ABSTRACT: 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% rise-time 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 ten 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.
    09/2014; 119(18). DOI:10.1002/2014JD021919
  • C. J. Biagi · M. A. Uman · J. D. Hill · D. M. Jordan ·
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    ABSTRACT: We present 63 high-speed video frames (108 kfps, 9.26 µs per frame) showing the development of the downward negative stepped leader in the initial stage of an altitude-triggered flash. The downward negative stepped leader initiated from the bottom of the triggering wire at a height of about 128 m above ground and, 553 µs later, it struck a lightning rod located at a distance of about 50 m from the launch tower. During the leader's development, electric-field-derivative pulses were detected associated with leader stepping. The interpulse intervals ranged from 3 to 27 µs with a mean value of 13 µs. Distinct segments of luminosity were observed ahead of the main leader channel that appear similar to space leaders were observed in the high speed video frames. A total of eight luminous segments were observed that were 1 m to 6 m in length, and were centered at distances from the main leader channel ranging from 3 m to 8 m. The new leader steps that appeared in the frames following the luminous segments were 5 m to 8 m in length. Two of the observed segments apparently never connected to the leader channel, and thus failed to produce a new leader step.
    07/2014; 119(13). DOI:10.1002/2013JD020281
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    ABSTRACT: We have evaluated the performance characteristics of the Earth Networks Total Lightning Network (ENTLN) using, as ground truth, data for 245 negative return strokes and 138 kiloampere-scale (≥1 kA) superimposed pulses in 57 flashes triggered from June 2009 to August 2012 at Camp Blanding, Florida. The performance characteristics were determined both for the ENTLN processor that had been in service at the time of acquiring triggered-lightning data (June 2009 to August 2012) and for the new ENTLN processor, introduced in November 2012. So, evaluation for the new processor simulates ENTLN output as if the new processor were in service from June 2009 to August 2012. For the same ground-truth data set and the same evaluation methodology, different performance characteristics for those two processors were obtained. For the old processor, flash detection efficiency was 77%, stroke detection efficiency was 49%, percentage of misclassified events was 61%, median location error was 631 m, and median absolute current estimation error was 51%. For the new processor, flash detection efficiency was 89%, stroke detection efficiency was 67%, percentage of misclassified events was 54%, median location error was 760 m, and median absolute current estimation error was 19%.
    Electric Power Systems Research 07/2014; 118. DOI:10.1016/j.epsr.2014.06.007 · 1.75 Impact Factor
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    ABSTRACT: We have estimated the ENTLN performance characteristics using data for 62 negative return strokes in 12 flashes triggered during 2013 at Camp Blanding, Florida. The flash and stroke detection efficiencies were 100% and 94%, respectively. The ENTLN misclassified 53% of detected return strokes as cloud discharges. The median location error was 603 m and the median absolute current estimation error was 20%. The 2013 results are compared to those for 2009–2012, both corresponding to the same processor, implemented in November of 2012.
    XV International Conference on Atmospheric Electricity (ICAE), Norman, OK; 06/2014
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    ABSTRACT: 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, 1 (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 evaluation of GLD360 performance characteristics relative to absolute ground-truth, with all previous evaluations being relative to other locating systems. The results presented in this work may be applicable to regions in and around Florida.
    Geophysical Research Letters 05/2014; 41(10):3636–3642. DOI:10.1002/2014GL059920 · 4.20 Impact Factor
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    ABSTRACT: At the end of 120 pre-return 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, two to three 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.
    05/2014; 41(9). DOI:10.1002/2014GL059601
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    ABSTRACT: The performance characteristics of the WWLLN were evaluated using rocket-triggered lightning data acquired at Camp Blanding, Florida, in 2008–2013. This is the first absolute ground-truth evaluation for WWLLN. The data set consisted of 360 return strokes and 169 kiloampere-scale (≥1 kA) superimposed pulses in 80 flashes. The flash and stroke detection efficiencies were 8.8% and 2.5%, respectively. The stroke detection efficiency for strokes with peak current ≥25 kA is 29%. One superimposed pulse (an M-component with peak current of 12.3 kA) was detected by the WWLLN. The median location error is 2.1 km and the median absolute event-time mismatch is 15.3 µs. The median absolute error in WWLLN peak currents estimated from the empirical formula of Hutchins et al. [2012] is 30%.
    2014 Int. Conf. on Grounding and Earthing (GROUND) & 6th Int. Conf. on Light. Phys. and Eff. (LPE), Manaus, Brazil; 05/2014
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    ABSTRACT: We present a detailed evaluation of performance characteristics of the U.S. National Lightning Detection Network (NLDN) using, as ground-truth, Florida rocket-triggered lightning data acquired in 2004–2012. The overall data set includes 78 flashes containing both the initial stage and leader/return stroke sequences and 2 flashes composed of the initial stage only. In these 80 flashes, there are a total of 326 return strokes (directly-measured channel-base currents are available for 290 of them) and 173 kiloampere-scale (≥1 kA) superimposed pulses, including 58 initial continuous current pulses and 115 M-components. All these events transported negative charge to ground. The NLDN detected 245 return strokes and 9 superimposed pulses. The resultant NLDN flash detection efficiency is 94%, return stroke detection efficiency is 75%, and detection efficiency for superimposed pulses is 5% for peak currents ≥1 kA and 32% for peak currents ≥5 kA. For return strokes, the median location error is 334 m and the median value of absolute peak current estimation error is 14%. The percentage of misclassified events is 4%, all of them being return strokes. The median value of absolute event-time mismatch (the difference in times at which the event is reported by the NLDN and recorded at the lightning triggering facility) for return strokes is 2.8 μs. For two out of the nine superimposed pulses detected by the NLDN, we found optical evidence of a re-illuminated branch (recoil leader) coming in contact with the existing grounded channel at an altitude of a few hundred meters above ground.
    Journal of Geophysical Research Atmospheres 04/2014; 119(7):3825-3856. DOI:10.1002/2013JD021401 · 3.43 Impact Factor
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    ABSTRACT: [1] Dart-stepped-leader step formation in triggered lightning is documented with high-speed video recorded at 648 kilo-frames per second (1.16 µs exposure time, 380 ns dead time) and linear streak film with a temporal resolution of about 1 µs. Locally luminous points and segments of channel both separate and below the main descending leader tip were recorded on the high-speed video. Bidirectional leaders were imaged initiating at the locally luminous points below the main channel tip, points that remain stationary during the inter-step process. The average speed of five bidirectional leaders was 8.4x105 m/s upward and 4.8x105 m/s downward, assuming 1.5 µs between successive images. The main dart-stepped leader channel tip moved downward between steps. Leader steps extended below the bottom of the previous bidirectional leader. Processes that can be seen between steps on high-speed video are generally below the noise threshold of the streak film, which shows primarily the newly-formed steps.
    03/2014; 41(6). DOI:10.1002/2014GL059627
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    ABSTRACT: We have estimated the GLD360 performance characteristics using data for 201 negative return strokes in 42 flashes triggered during 2011–2013 at Camp Blanding, Florida. The flash and stroke detection efficiencies were 67% and 37%, respectively. Out of 75 detected-strokes, 1 (1.3%) was reported with incorrect polarity. The median location error was 2.1 km. The median absolute current estimation error was 27%.
    23rd International Lightning Detection Conference (ILDC), Tucson, AZ; 03/2014

Publication Stats

8k Citations
666.54 Total Impact Points


  • 1029-2015
    • University of Florida
      • Department of Electrical and Computer Engineering
      Gainesville, Florida, United States
  • 2005
    • University of Toronto
      Toronto, Ontario, Canada
  • 1995-1998
    • The University of Arizona
      Tucson, Arizona, United States
    • Texas A&M University
      College Station, Texas, United States