Evaluation of U.S. National Lightning Detection Network performance characteristics using rocket-triggered lightning data acquired in 2004–2009 (original) (raw)
Related papers
Journal of Geophysical Research, 1999
A model of return stroke detection by the U.S. National Lightning Detection Network (NLDN) magnetic direction finder (MDF) sensors is used to approximate the pulse width criterion modification made to the sensors during the 1994 upgrade. Decreasing the pulse width detection criterion used by the MDF sensors increases their effective detection range, which increases their sensitivity to weak flashes (because of NLDN network geometry, increasing sensitivity has little effect on detection of strong flashes). Consequently, we observe an increase in the weak flash counts. The increased detection of weak flashes accounts, in part, for the decrease in mean peak currents observed in subsequent years to 1994. In addition to decreasing the mean peak current of detected positive and negative flashes, the NLDN upgrade has apparently had the unwanted effect of increasing the contamination of the positive CG flash data with cloud flashes.
Journal of Geophysical Research: Atmospheres, 2014
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 the 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 mismatches (the difference in times at which the event is reported to occur 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 reilluminated 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, 1998
The detection efficiency (DE) of the U.S. National Lightning Detection Network (NLDN) has been evaluated using a large data set of video observations of cloud-to-ground lightning activity in the vicinity of Albany, New York. These data were acquired during the summers of 1993, 1994, and 1995, the latter being the year of completion of a major upgrade of the network to the improved accuracy from combined technology (IMPACT) configuration. For 1993, we find a flash DE value of 67% based upon 517 cloud-to-ground flashes documented on video. The latter two years yielded both flash and stroke DEs: in 1994, 86% of 893 flashes and 67% of 2162 strokes were detected; in 1995, 72% of 433 flashes and 47% of 1242 strokes were detected. The higher DEs of 1994 relative to 1995 are likely due to additional sensors deployed locally during the initial stage of the IMPACT upgrade. Detection efficiencies were found to vary significantly from storm to storm in each season, likely due to the inherent variability of return stroke characteristics between storms. For a special subset of 92 strokes of known location and measured electric-field change, peak current estimates were generated using the transmission-line model and a return stroke speed of 1.2x108 m/s. This speed was selected, as it is the effective speed used in present NLDN peak current estimates. For this 92-stroke data subset, the stroke DE depended upon peak current: strokes with peak currents greater than 14 kA were almost always detected (39 of 40); below 14 kA, the DE dropped until by 6-10 kA, the stroke DE was only 18% (three of 17). None of 14 strokes with estimated peak currents below 6 kA was detected. If the IMPACT design constraint of an effective 5-kA minimum peak current is applied to our 92-stroke subset, the respective flash and stroke DEs are 84% and 69%; this is consistent with NLDN model predicted performance in this area. As a faster return stroke speed, possibly 1.8x 108 m/s, would seem appropriate, the above cited current values would need to be scaled downward by a factor of 2/3, implying greater actual sensitivity of the NLDN to weaker strokes. However, a commensurate adjustment downward would be required of present NLDN-derived peak current estimates as well.
Journal of Geophysical Research, 2007
1] Four field campaigns were conducted in southern Arizona (AZ) and in northern Texas and southern Oklahoma (TX-OK) in 2003 and 2004 to evaluate the performance of the U.S. National Lightning Detection Network TM (NLDN) in detecting cloud-to-ground (CG) lightning after an upgrade in 2002 and 2003. The 2-year average flash detection efficiency (DE) in AZ was 93% (1024/1097), and the measured (first plus subsequent) stroke DE was 76% (2746/3620). The corresponding values in TX-OK were 92% (338/367) and 86% (755/882), respectively. After correcting for the time resolution of the video camera (16.7 ms), we estimate that the actual NLDN stroke DE and video multiplicities were about 68% and 3.71 in AZ and 77% and 2.80 in TX-OK. The average DE for negative first strokes (92%) was larger than the measured DE for subsequent strokes that produced a new ground contact (81%) and the DE for subsequent strokes that remained in a preexisting channel (67%). The primary cause of the NLDN missing strokes was that the peak of the radiated electromagnetic field was below the NLDN detection threshold. The average estimated peak current (I p ) of negative first strokes and the average multiplicity of negative flashes varied from storm to storm and between the two regions, but this variability did not affect the DE as long as the recording sessions had more than 60 flashes. By analyzing the NLDN locations of subsequent strokes that remained in the same channel as the first stroke we infer that the median random position error of the NLDN was 424 m in AZ and 282 m in TX-OK. An evaluation of the classification of lightning type by the NLDN (i.e., CG stroke versus cloud pulse) showed that 1.4-7% (6/420 to 6/86) of the positive NLDN reports with an I p 10 kA in TX-OK were produced by CG strokes; 4.7-26% (5/106 to 5/19) of the positive reports with 10 kA < I p 20 kA were CGs; and 67-95% (30/45 to 30/32) of the reports with I p ! +20 kA were CG strokes. Some 50-87% (52/104 to 52/60) of the negative, single-stroke NLDN reports in AZ and TX-OK with jI p j 10 kA were produced by CG flashes. Both the upper and lower bounds in these classification studies have observational biases.
On remote measurements of lightning return stroke peak currents
Atmospheric Research, 2014
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 and the field-to-current conversion equation based on the transmission line model . 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 NLDNreported 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 × 10 8 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.
Characterization of the initial stage of negative rocket-triggered lightning
Journal of Geophysical Research, 1999
We performed a statistical study on the initial stage (IS) of negative rockettriggered lightning using 37 channel-base current recordings obtained during the summer of 1994 at Fort McClellan, Alabama, and during the summers of 1996 and 1997 at Camp Blanding, Florida. The IS can be viewed as composed of an upward positive leader (UPL) followed by an initial continuous current ( ICC ). The IS has a geometric mean (GM) duration of 279 ms and lowers a GM charge of 27 C to the ground. The average IS current in an individual lightning discharge varies from a minimum of 27 A to a maximum of 316 A with a GM value of 96 A for the entire sample of 37 discharges. We examined the current variation at the beginning of the IS in 24 flashes. In 22 out of 24 cases this initial current variation (ICV) includes a current drop, probably associated with the disintegration of the copper triggering wire and the subsequent current reestablishment. The GM time interval between the onset of the initial stage and the abrupt decrease in current is 8.6 ms, and the GM current level just prior to the current decrease is 312 A, a value about 3 times the GM value of average current for the whole IS, 96 A. Before this abrupt current decrease, a GM charge of 0.8 C has been lowered to ground with a corresponding GM action integral of 110 A 2 s. The abrupt current decrease takes typically several hundred microseconds and is followed, immediately or after a time interval up to several hundred microseconds, by a pulse with a typical peak of about 1 kA and a typical risetime of less than 100 •ts. The ICC usually includes impulsive processes that resemble the M processes observed during the continuing currents that follow return strokes in both natural and triggered lightning. We present statistics for the following parameters of current pulses superimposed on the ICC: magnitude, risetime, half-peak width, duration, charge transferred, preceding continuous current level, interpulse interval, and time interval between the onset of the IS and the first ICC pulse. The observed characteristics of ICC pulses varied significantly among the three data sets. For all data combined, the characteristics of the ICC pulses are similar to those of the M-component current pulses studied by . This latter finding suggests that ICC impulsive processes are of the same nature as M processes. of Florida, USA 2Consultants to the University of Florida 3Department of Electrical and Electronic Engineering, Gifu University, Japan from an examination of channel-base current records. If we assume that the average UPL speed is 2 x 105 m/s and that the primary negative charge involved in the ICC is located at a height of 6 to 8 km, then the first 30-40 ms (-•10%) of the iS is due to the UPL, and the rest of the IS is due to the ICC. The ICC is often followed bv dart leader/return stroke sequences, similar to those constituting subsequent strokes in natural downward initiated lightning. When there are no return strokes involved [e.g., Fieux et al., 1978; Laroche et al., 1985], the triggered lightning event consists of the IS only and is sometimes termed a "wire burn." Return strokes and M
Parameters of triggered-lightning flashes in Florida and Alabama
Journal of Geophysical Research, 1993
Channel base currents from triggered lightning were measured at the NASA Kennedy Space Center, Florida, during summer 1990 and at Fort McClellan, Alabama, during summer 1991. Additionally, 16-mm cinematic records with 3-or 5-ms resolution were obtained for all flashes, and streak camera records were obtained for three of the Florida flashes. The 17 flashes analyzed here contained 69 strokes, all lowering negative charge from cloud to ground. Statistics on interstroke interval, no-current interstroke interval, total stroke duration, total stroke charge, total stroke action integral ($ i 2dt), return stroke current wave front characteristics, time to half peak value, and return stroke peak current are presented. Return stroke current pulses, characterized by rise times of the order of a few microseconds or less and peak values in the range of 4 to 38 kA, were found not to occur until after any preceding current at the bottom of the lightning channel fell below the noise level of less than 2 A. Current pulses associated with M components, characterized by slower rise times (typically tens to hundreds of microseconds) and peak values generally smaller than those of the return stroke pulses, occurred during established channel current flow of some tens to some hundreds of amperes. A relatively strong positive correlation was found between return stroke current average rate of rise and current peak. There was essentially no correlation between return stroke current peak and 10-90% rise time or between return stroke peak and the width of the current waveform at half of its peak value.
Journal of Geophysical Research, 2000
This work compares simultaneous observations of lightning from two complementary systems. FORTE is a low-Earth-orbit satellite carrying radiowave and optical instruments for the study of lightning. The radio receivers aboard FORTE observe very high frequency (VHF) emissions from the air-breakdown process preceding (and sometimes accompanying) a lightning current. The National Lightning Detection Network (NLDN) is a ground-based array of sensors in the contiguous United States observing the low-frequency (LF) and very low frequency (VLF) radiation from vertical currents. Prior to the launch of FORTE in 1997, essentially no work had been done on the statistical correlations between (1) ground-based LF/VLF and (2) spaced-based VHF remote sensing of lightning. During a 6-month campaign in April-September 1998, FORTE took most of its triggered VHF data over and near the contiguous United States, and NLDN data were specially postprocessed in a loosened-criterion mode providing enhanced detection range beyond the coastline and borders of the array itself. The time history of reported events from the two systems was compared, and event pairs (each pair containing one event from FORTE, the other from NLDN) which were candidate correlations (closer than 200 ms from each other) were scrutinized to determine whether the members of a pair actually came from the same discharge process. We have found that there is a statistically significant correlation, for a subset of FORTE events. This correlation is most likely to occur for intracloud and less likely to occur for cloud-to-ground discharges. The correlated VHF and NLDN events tend to occur within +30 Us of each other, after correction for the propagation of the VHF signal to FORTE from the NLDN-geolocated stroke location. Most correlations outside of _+30 Us turn out to be merely a statistical accident. The NLDN-furnished geolocation allows the correlated FORTE-detected VHF pulses to be better interpreted. In particular, we can deduce, from the lag of the VHF groundreflection echo, the height of the VHF emission region in the storm. These workers also studied a very complex and varied interrelationship between the lowfrequency/very low frequency (LF/VLF) and VHF discharge signatures during the development and decay of the lightning flash. Furthermore, VHF pulses emitted by the storm have been found to be either "major," i.e., in a set of pulses grouped in time according to flashes and associated with LF/VLF signatures, or "minor," occurring higher in altitude and less ob-
A review of ten years of triggered-lightning experiments at Camp Blanding, Florida
Atmospheric Research, 2005
The principal results of triggered-lightning experiments conducted at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, Florida, from 1993 through 2002 are reviewed. These results include (a) characterization of the close lightning electromagnetic environment, (b) first lightning return-stroke speed profiles within 400 m of ground, (c) new insights into the mechanism of the dart-stepped (and by inference stepped) leader, (d) identification of the Mcomponent mode of charge transfer to ground, (e) first optical image of upward connecting leader in triggered-lightning strokes, (f) electric fields in the immediate vicinity of the lightning channel, (g) inferences on the interaction of lightning with ground and with grounding electrodes, (h) discovery of X-rays produced by triggered-lightning strokes, (i) new insights into the mechanism of cutoff and reestablishment of current in rocket-triggered lightning. Selected results are discussed.