Ronald W. Henderson

Global Distribution of Midnight Lightning: September 1977 to August 1978

Abstract A detailed study is reported of the distribution of global midnight lightning from 60°S to 60°N for 365 consecutive days. The period covered is from 1 September 1977 to 31 August 1978. More than 32 000 lightning locations obtained from DMSP photographic data are plotted with an accuracy of 100 km. The locations are in good agreement with features of the general circulation. Land-ocean lightning ratios vary from 2.2 (September) to 4.2 (July) with a mean of 3.2. If these values are corrected for the earths land-ocean area ratio of (2.4)−1, the land-ocean lightning ratios range from 5.3 (September) to 10 (July) with a mean of 7.7.

An East Coast Lightning Detection Network

A magnetic direction-finding network for the detection of lightning cloud-to-ground strikes has been installed along the East Coast of the United States. Most of the lightning occurring from Maine to Florida and as far west as Ohio is detected. Time, location, flash polarity, stroke count, and peak signal amplitude are recorded in real time. Flash locations, time, and polarity are displayed routinely for research and operational purposes. The data are being compiled in a data base to provide statistical information necessary for the prediction of the surge performance of electric power lines and the improvement of surge protection practices.

Performance evaluation of the U.S. National Lightning Detection Network in eastern New York: 1. Detection efficiency

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 1.2×l08 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 1.8×108 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.

Cloud‐to‐ground lightning in Hurricane Andrew

The spatial and temporal distribution of cloud-to-ground lightning was examined in Hurricane Andrew of 1992. Lightning locations available from the National Lightning Detection Network were superimposed on infrared satellite images to relate lightning activity to hurricane cloud structure. A distinct radial variation occurred in time-averaged flash density, with a weak maximum in the eye wall, a region of near-zero flash density 40 to 100 km from the center, and a steady increase to a large maximum in the outer rainbands 190 km from the center. This radial distribution is consistent with the convective structure of mature hurricanes. Eye wall lightning tended to be episodic, occurring almost exclusively prior to and during periods of intensification of the storm. During these periods, negative flashes occurred several kilometers inward from the highest eye wall cloud tops, in the region of the largest radar reflectivity. Positive eye wall flashes, while small in number, tended to occur directly under the highest cloud tops. The results are suggestive of a normal dipole in sign but outwardly tilted along the sloping eye wall. In general, hurricane flash characteristics resembled those for a background data set of nonhurricane flashes from the same area. The exception occurred for negative flashes in the eye wall, which had a much smaller mean peak current than the background (25.3 kA versus 44.9 kA).

Performance evaluation of the U.S. National Lightning Detection Network in eastern New York: 2. Location accuracy

We have evaluated the location accuracy of the U.S. National Lightning Detection Network (NLDN) via comparison of individual NLDN stroke locations with their corresponding stroke terminations as determined from multiple-site video recordings of local cloud-to-ground lightning flashes. The video records used in this analysis were acquired in the vicinity of Albany, New York, during the summers of 1994 and 1995, seasons corresponding to the initial and final phases of the upgrade of the NLDN to the improved accuracy from combined technology (IMPACT) configuration. For 1994 a total of 751 strokes were located in common between the video and NLDN data sets, yielding median and mean values of the NLDN-video location separation of 2.21 km and 3.74 km, respectively. Constraint of the 1994 data to the best video locations (those with a semimajor axis value of the 50% error ellipse less than 500 m) yielded a subset of 53 strokes with median and mean values of the NLDN-video location separation of 2.61 km and 4.74 km, respectively. For 1995 a total of 219 strokes were located in common, yielding median and mean values of the NLDN-video location separation of 442 m and 865 m, respectively. Constraint of the 1995 data to that years best video locations (those with a semimajor axis value of the 50% error ellipse less than 200 m) yielded a subset of 79 common strokes with median and mean values of the NLDN-video location separation of 435 m and 625 m, respectively. The IMPACT upgrade of the NLDN apparently has resulted in about a fivefold increase in location accuracy. A special subset of 11 strokes occurred in 1995; these were located by the NLDN and observed to terminate on local structures of accurately known location. For these 11 strokes, the median and mean values of the NLDN location error are 518 m and 484 m, respectively, a result quite consistent with the overall NLDN-video location comparison and the claimed 500-m median location accuracy for the upgraded network. Indeed, we argue that removal of an apparent residual bias in the NLDN locations ultimately could yield a median location accuracy in this region approaching 250 m. Finally, we find that the NLDN location error is a function of the estimated stroke peak current, with the weakest peak currents having the poorest locations for both years. For the 1995 data, strokes with peak currents of intermediate magnitude (20–30 kA) were the most accurately located.

Absolute Spectral Irradiance Measurements of Lightning from 375 to 880 nm

Abstract A spectrometer-detector optical multichannel analyzer system capable of absolute spectral irradiance measurements has been used to record the time-integrated emissions (150 or 300 ms) from cloud-to-ground lightning. Two detectors, one operating in the 375–695 nm region and one in the 650–880 nm region, recorded emissions from the same lightning. Ten flashes containing 46 recorded strokes were selected for detailed analysis from 500 flashes recorded in 1981. Thew ten flashes occurred within three degrees of the optical axis of the spectrometer system. The average stroke spectral irradiance from 375 to 650 nm for flashes at approximately 15 km is 3.5 × 10−5 J m−2 per stroke with a standard deviation of 2.0 × 10−5 and a range from 0.7 × 10−5 to 6.8 × 10−5 J m−2 per stroke. The average stroke spectral irradiance from 650 to 880 nm for the same strokes is 1.2 × 10−5 j m−2 per stroke with a standard deviation of 0.7 × 10−5 and a range from 0.5 × 10−5 to 3.2 × 10−5 J m−2 per stroke. The ratio of the vis...

A reexamination of the peak current calibration of the National Lightning Detection Network

The peak current calibration of the National Lightning Detection Network (NLDN) recently reported by Orville (1991) has been reexamined with a roughly threefold larger data set of 57 directly measured stroke peak currents, Ipeak (kiloamperes), and their corresponding NLDN mean normalized magnetic signal strengths, M¯peak (LLP units). These 57 lightning strokes originated in 36 triggered lightning flashes initiated at the Kennedy Space Center (KSC), Florida, during the research campaigns of 1985–1991. Identification of corresponding Ipeak and M¯peak measurements was verified through accurate coincidence in absolute time of the two independent data sets. The Ipeak-M¯peak data (with one point excluded as an outlier) is apparently linearly related with a correlation coefficient of 0.881, consistent with that predicted by application of the transmission line model of the lightning return stroke. The regression equation for prediction of Ipeak from NLDN M¯peak measurements is Ipeak=4.20+0.171M¯peak where the slope is expressed in kiloampere/(LLP units). Examination of the overall Ipeak-M¯peak data set for the possible influence of two different models of signal strength attenuation with distance, D, (power law, Dβ, and exponential, [expαDD]−1) indicates negligible sensitivity to the proposed variations; other larger error sources likely mask the true attenuation effect. Twelve flashes were detected with four or more direction finders; a power law fit to the direction finder signal strength variation with distance of these individual flashes yields a mean β value of −1.09. Examination of the overall Ipeak-M¯peak data set for the possible effect of a nonlinear relation between the source stroke peak current and return stroke propagation speed indicates no obvious influence. The 95% confidence bounds for peak current prediction in the mean suggest a percent uncertainty of 10–15% for peak currents between 15 and 60 kA. Similar accuracy is expected for peak currents greater than 60 kA provided care is taken to minimize possible nonlinear amplification effects hi the NLDN data. Below 15 kA, the percent uncertainty rapidly increases suggesting that the inferred distribution of peak current values less than this limit may be quite unreliable.

The East Coast Lightning Detection Network

A magnetic direction-finding network for the detection of lightning cloud-to-ground strikes has been installed along the east coast of the United States. Most of the lightning occurring from Maine to Florida and as far west as Ohio is detected. Time, location, flash polarity, stroke count, and peak signal amplitude are recorded in real time. Flash locations, time, and polarity are displayed routinely for research and operational purposes. Flash density maps have been generated for the summers of 1983 and 1984, when the network only extended to North Carolina, and show density maxima in northern Virginia and Maryland. Computer-based display systems that access this network in real time are now in use to evaluate the usefulness of the data.

Ground Truth: A Positive Cloud-to-Ground Lightning Flash

Abstract We describe the circumstances and effects of a cloud-to-ground lightning flash that lowered positive charge and struck a residential home near Albany, New York. The characteristics of the flash were detected and recorded by a wide-band magnetic direction-finding system operating along the East Coast. The flash contained one stroke, had an estimated peak current of 70 kA and was apparently the only ground flash in the storm. A 70 mm2 hole in a section of iron roof flashing suggests that the charge transfer may have been between 25 and 150 coulomb, depending upon assumptions. Vertical shear in the horizontal wind at the time of the flash is estimated to have been 3 m s−1 km−1, a value consistent with previous observations of lightning lowering positive charge.