M. A. Uman

University of Florida, Gainesville, Florida, United States

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Publications (301)647.09 Total impact

  • [Show abstract] [Hide abstract]
    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; DOI:10.1002/2014GL062911
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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; DOI:10.1002/2014JD022815
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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
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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
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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; DOI:10.1016/j.epsr.2014.06.007 · 1.60 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 · 4.46 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 · 3.44 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
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    ABSTRACT: We have estimated the NLDN performance characteristics using data for 388 negative return strokes and 185 kiloampere-scale superimposed pulses in 92 flashes triggered during 2004–2013 at Camp Blanding, Florida. The flash and stroke detection efficiencies were 94% and 75%, respectively. The superimposed pulse detection efficiency was 6% (for peak currents ≥1 kA) and 32% (for peak currents ≥5 kA). The NLDN misclassified 3% of return strokes and 18% of superimposed pulses as cloud discharges. The median location error for return strokes was 309 m. The median absolute current estimation error for return strokes was 14%. It appears that the systematic and random components of current estimation error were -6% and 22%, respectively. The median absolute event-time mismatch for return strokes was 2.7 µs. In addition, we examined the year-to-year variation in performance characteristics of the NLDN.
    23rd International Lightning Detection Conference (ILDC), Tucson, AZ; 03/2014
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    ABSTRACT: Using a high-speed optical imaging system specifically designed for observing the lightning attachment process, we have documented the process for stepped, dart, and dart-stepped leaders in an “anomalous” rocket-triggered lightning flash that terminated on a 10 m grounded utility pole. The initiation of the first return stroke was found to occur at a height of 23 ± 3 m above the top of the utility pole and was associated with three “slow-front” dE/dt pulses. A time of 1.5 µs later, a fast rise in luminosity at 18 ± 2 m was associated with a “fast-transition” dE/dt pulse. The first return stroke propagated bi-directionally from its initiation height, as did subsequent return strokes from their initiation heights of 8 ± 1 m to 16 ± 2 m above the top of the utility pole. The initial upward speed of the first return stroke was 1.4 × 108 m/s, while its initial downward speed was 2.2 × 107 m/s. The channel-bottom luminosity of the first return stroke rose more slowly to a two-or-more-times larger amplitude than that of the subsequent stroke luminosities. In contrast, the NLDN-derived first-return-stroke peak current is smaller than that of the second and the third strokes, and our electric field records at 45 km show similar behavior for the initial field peaks of the first and subsequent strokes.
    Journal of Geophysical Research Atmospheres 02/2014; 119(3):1524-1533. DOI:10.1002/2013JD020787 · 3.44 Impact Factor
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    ABSTRACT: [1] We investigate the structure of x-ray emissions from downward rocket-triggered lightning leaders using a pinhole-type x-ray camera (XCAM) located at the International Center for Lightning Research and Testing (ICLRT). This study builds on the work of Dwyer et al. [2011], which reported results from XCAM data from the 2010 summer lightning season. Additional details regarding the 2010 data are reported here. During the 2011 summer lightning season, the XCAM recorded 12 out of 17 leaders,five of which show downward leader propagation. Of those five leaders, one dart-stepped leader and two “chaotic” dart leaders are the focus of this paper. These three leaders displayed unique x-ray emission patterns: a “chaotic” dart leader displayed a diffuse structure (i.e., a wide lateral “spraying” distribution of x-rays spanning at least 12 m across) and a dart-stepped leader and a “chaotic” dart leader exhibited compact emission (i.e., a narrow lateral distribution of strong x-rayemission spanning about four m across). These two distinct x-ray emission patterns (compact and diffuse) illustrate the variability of lightning leaders. Using Monte Carlo simulations, we show that the diffuse x-ray source must originate from a diffuse source of energetic electrons or possibly emission from several sources. The compact x-ray sources originate from compact electron sources and the x-ray source region radius and electric charge contained within the x-ray source region were between 2-3 m and on the order of 10− 4 C, respectively. For the leaders under investigation, the x-ray source region average currents were determined to be on the order of 102 A.
    01/2014; 119(2). DOI:10.1002/2013JD020266
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    ABSTRACT: Return-stroke peak current is one of the most important measures of lightning intensity needed in different areas of atmospheric electricity research. It can be estimated from the corresponding electric or magnetic radiation field peak. Electric fields of 89 strokes in lightning flashes triggered using the rocket-and-wire technique at Camp Blanding (CB), Florida, were recorded at the Lightning Observatory in Gainesville, about 45 km from the lightning channel. Lightning return-stroke peak currents were estimated from the measured electric field peaks using the empirical formula of Rakov et al. (1992) and the field-to-current conversion equation based on the transmission line model (Uman and McLain, 1969). These estimates, along with peak currents reported by the U.S. National Lightning Detection Network (NLDN), were compared with the ground-truth data, currents directly measured at the lightning channel base. The empirical formula, based on data for 28 triggered-lightning strokes acquired at the Kennedy Space Center (KSC), tends to overestimate peak currents, whereas the NLDN-reported peak currents are on average underestimates. The field-to-current conversion equation based on the transmission line model gives the best match with directly measured peak currents for return-stroke speeds between c/2 and 2c/3 (1.5 and 2 × 108 m/s, respectively). Possible reasons for the discrepancy in the peak current estimates from the empirical formula and the ground-truth data include an error in the field calibration factor, difference in the typical return-stroke speeds at CB and at the KSC (considered here to be the most likely reason), and limited sample sizes, particularly for the KSC data. A new empirical formula, I = − 0.66–0.028rE, based on data for 89 strokes in lightning flashes triggered at CB, is derived.
    Atmospheric Research 01/2014; 135-136:306-313. DOI:10.1016/j.atmosres.2012.08.008 · 2.42 Impact Factor
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    ABSTRACT: [1] Lightning Mapping Array source locations, channel base currents, and electric field waveforms are presented for a lightning flash triggered in the rainbands of 2012 tropical storm Debby. The National Lightning Detection Network reported no natural cloud-to-ground discharges within 60 km of the North Florida triggering site for at least 20 h before and 8 h after the triggered flash. Additionally, local electric field mill and wideband antenna networks show no close cloud or cloud-to-ground flashes. The triggering rocket was launched with negative charge overhead producing an electric field at the ground of 5 kV m−1 and in coordination with X-band, dual-polarimetric radar observations of streamers of enhanced precipitation descending from the melting level as they approached the site. The Debby flash consisted of an initial stage (IS) followed by eleven leader/return stroke sequences. The flash exhibited all the processes of normal triggered and natural cloud-to-ground lightning: leader/return stroke sequences, continuing currents, K events, and M components. Additionally, the flash exhibited several exceptional characteristics: three return stroke peak currents greater than 25 kA, one very long, 352 ms, continuing current that transferred about 35 C of charge to ground, and a relatively short, 202 ms, IS containing no initial continuous current pulses. Following a near-vertical upward positive leader attaining 2.8 km height, the IS branched and propagated horizontally at 3.5 km altitude. The flash, exhibiting strokes and continuing current, then ascended to and propagated horizontally at 5.5 km, extending about 25 km south and 15 km east. The 0°C level was near 4.5 km above sea level. It follows from the above that clouds that are not producing natural lightning can represent a triggered lightning hazard to launch vehicles and aircraft.
    12/2013; 118(23). DOI:10.1002/2013JD020501

Publication Stats

7k Citations
647.09 Total Impact Points

Institutions

  • 1029–2015
    • University of Florida
      • Department of Electrical and Computer Engineering
      Gainesville, Florida, United States
  • 2005
    • University of Toronto
      Toronto, Ontario, Canada
  • 1995
    • Texas A&M University
      College Station, Texas, United States
    • The University of Arizona
      Tucson, Arizona, United States
  • 1992
    • Louisiana State University
      • Department of Electrical Engineering
      Baton Rouge, LA, United States