National Lightning Detection Network Research Articles

Rocket‐and‐wire triggered lightning in 2012 tropical storm Debby in the absence of natural lightning

AbstractLightning 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.

On the relationship between lightning peak current and Early VLF perturbations

Lightning strokes are known to cause direct heating and ionization of the D region, some of which are detected via scattering of VLF transmitter signals and are known as Early VLF events. The disturbed ionosphere typically recovers in many tens of seconds. New experimental evidence is presented demonstrating that the scattering pattern and onset amplitude of Early VLF events are strongly related to both the magnitude and polarity of causative lightning peak current. Observations of Early VLF events at nine Stanford VLF receiver sites across the continental United States are combined with lightning geolocation data from the National Lightning Detection Network (NLDN). During January and March 2011, NLDN recorded 7769 intense lightning discharges with high peak currents (>100 kA) generating 1250 detected Early VLF events. We show that the size of the scattered field due to the ionospheric disturbance increases with the peak current intensity of the causative lightning discharge. The most intense peak currents of >+200 and <−250 kA disturb VLF transmitter signals as far as ∼400 km away from the lightning stroke. Early VLF event detection probability also increases rapidly with peak current intensity. On the other hand, the observed VLF amplitude change is not significantly dependent on the peak current intensity. Stroke polarity is also important, with positive strokes being ∼5 times more likely to generate Early VLF disturbances than negative strokes of the same intensity. Intense positive cloud‐to‐ground lightning discharges, especially when occurring over the sea, are also more likely to produce Early VLF events with long recovery (many minutes).

Characteristics of lightning flashes with exceptional illuminated areas, durations, and optical powers and surrounding storm properties in the tropics and inner subtropics

Twelve years (1998–2009) of Lightning Imaging Sensor (LIS) observations are used to characterize lightning flashes by illuminated area, duration, and optical power, particularly for exceptional flashes defined as those above the 90th percentile of each parameter. Statistics of lightning are summarized over land, ocean, and coastal regions of the Tropical Rainfall Measuring Mission satellite's domain extending from 36°S to 36°N. The degree to which optical flash parameters are interrelated is discussed, as well as coincident environmental properties and the overall characteristics of parent thunderstorms. LIS flashes over the southern United States are also collocated to National Lightning Detection Network (NLDN) observations, and relationships between LIS optical flash properties and corresponding NLDN strengths are discussed. Daytime (nighttime) oceanic flashes are shown to be 31.7% (39.4%) larger and 55.2% (75.1%) brighter in terms of maximum event pixel radiance. At the same time, daytime (nighttime) coastal flashes have 22.1% (7.8%) longer durations than flashes over land and 15.6% (11.4%) longer durations than oceanic flashes. Particularly, large and bright flashes observed by LIS are typically centered in weak storm regions, but thunderstorms with exceptional flashes are, themselves, more intense overall than those with only small and dim flashes. Diurnally, the top 10% brightest lightning flashes peak about 2 h earlier than the top 10% largest and long‐lasting flashes over land, implying that lightning flash characteristics vary with the life cycle of thunderstorms. Larger and more radiant flashes are also shown to be associated with stronger NLDN flashes of positive and negative polarity.

On the peak amplitude of lightning return stroke currents striking the sea

Data gathered from the US National Lightning Detection Network (TM) (NLDN) show that the peak currents of lightning flashes striking the sea are significantly higher than those of lightning flashes striking the land. We suggest that the unfavorable conditions for the formation of positive charge pockets in maritime clouds lead to lightning initiation at higher cloud potentials compared to their land counterparts, resulting in larger peak currents in negative lightning flashes striking the sea. As the positive charge pocket does not promote positive ground strokes, no such discontinuity should be expected in positive first return strokes to ground between land and sea.

Three years of lightning impulse charge moment change measurements in the United States

We report and analyze 3 years of lightning impulse charge moment change (iCMC) measurements obtained from an automated, real time lightning charge moment change network (CMCN). The CMCN combines U.S. National Lightning Detection Network (NLDN) lightning event geolocations with extremely low frequency (≲1 kHz) data from two stations to provide iCMC measurements across the entire United States. Almost 14 million lightning events were measured in the 3 year period. We present the statistical distributions of iCMC versus polarity and NLDN‐measured peak current, including corrections for the detection efficiency of the CMCN versus peak current. We find a broad distribution of iCMC for a given peak current, implying that these parameters are at best only weakly correlated. Curiously, the mean iCMC does not monotonically increase with peak current, and in fact, drops for positive CG strokes above +150 kA. For all positive strokes, there is a boundary near 20 C km that separates seemingly distinct populations of high and low iCMC strokes. We also explore the geographic distribution of high iCMC lightning strokes. High iCMC positive strokes occur predominantly in the northern midwest portion of the U.S., with a secondary peak over the gulf stream region just off the U.S. east coast. High iCMC negative strokes are also clustered in the midwest, although somewhat south of most of the high iCMC positive strokes. This is a region far from the locations of maximum occurrence of high peak current negative strokes. Based on assumed iCMC thresholds for sprite production, we estimate that approximately 35,000 positive polarity and 350 negative polarity sprites occur per year over the U.S. land and near‐coastal areas. Among other applications, this network is useful for the nowcasting of sprite‐producing storms and storm regions.

Total Lightning Characteristics Relative to Radar and Satellite Observations of Oklahoma Mesoscale Convective Systems

Abstract The advent of regional very high frequency (VHF) Lightning Mapping Arrays (LMAs) makes it possible to begin analyzing trends in total lightning characteristics in ensembles of mesoscale convective systems (MCSs). Flash initiations observed by the Oklahoma LMA and ground strikes observed by the National Lightning Detection Network were surveyed relative to infrared satellite and base-scan radar reflectivity imagery for 30 mesoscale convective systems occurring over a 7-yr period. Total lightning data were available for only part of the life cycle of most MCSs, but well-defined peaks in flash rates were usually observed for MCSs having longer periods of data. The mean of the maximum 10-min flash rates for the ensemble of MCSs was 203 min−1 for total flashes and 41 min−1 for cloud-to-ground flashes (CGs). In total, 21% of flashes were CGs and 13% of CGs lowered positive charge to ground. MCSs with the largest maximum flash rates entered Oklahoma in the evening before midnight. All three MCSs entering Oklahoma in early morning after sunrise had among the smallest maximum flash rates. Flash initiations were concentrated in or near regions of larger reflectivity and colder cloud tops. The CG flash rates and total flash rates frequently evolved similarly, although the fraction of flashes striking ground usually increased as an MCS decayed. Total flash rates tended to peak approximately 90 min before the maximum area of the −52°C cloud shield, but closer in time to the maximum area of colder cloud shields. MCSs whose −52°C cloud shield grew faster tended to have larger flash rates.

Characterization and applications of VLF/LF source locations from lightning using the Huntsville Alabama Marx Meter Array

Arrays that detect and locate the four‐dimensional spacetime positions of radiation sources from lightning have largely utilized sensors sensitive to the very high frequency (VHF) regime with ∼ 15 km baselines or very low frequency/low frequency (VLF/LF) regime with ∼ 100 km baselines. This paper details initial results from the newly developed Huntsville Alabama Marx Meter Array (HAMMA), consisting of Marx meters (electric field change meters) sensitive to a frequency band ∼ 1 Hz to 400 kHz. The arrival time of HAMMA waveforms due to radiation sources from lightning are used to determine the spacetime position of these sources. The locations are compared with two well‐documented and operational arrays, the National Lightning Detection Network (NLDN) and the North Alabama Lightning Mapping Array (NALMA). The standard deviation of the difference between HAMMA and NLDN locations of return strokes is 305 and 266 m in x and y, respectively, while the standard deviation of the difference between HAMMA and NALMA sources is 237, 226, and 688 m in x, y and z, respectively. We further show that NLDN intracloud locations differ in horizontal distance from the corresponding HAMMA locations by a median value of 479 m. In addition, we use HAMMA source locations to map several lightning flashes in the VLF/LF and show HAMMA sources largely map out the same electrical extent as VHF sources and provide unique insights to the properties of the discharges occurring. Finally, we show that VLF/LF sources can determine the leader polarity in several example flashes but not necessarily whether a flash comes to ground. Copyright © 2013 American Geophysical Union. All rights reserved.

Radar and lightning analyses of gigantic jet‐producing storms

An analysis of thunderstorm environment, structure, and evolution associated with six gigantic jets (five negative polarity, one positive) was conducted. Three of these gigantic jets were observed within detection range of very high frequency lightning mapping networks. All six were within range of operational radars and two‐dimensional lightning network coverage: five within the National Lightning Detection Network and one within the Global Lightning Detection (GLD360) network. Most of the storms producing the jets formed in moist tropical or tropical‐like environments (precipitable water ranged from 37 to 62 kg m−2, and 0–6 km shear from 3.5 to 24.8 m s−1), featuring high convective available potential energy (1200–3500 J kg−1) and low lifted indices (−2.8 to −6.4). The storms had maximum radar reflectivity factors of 54 to 62 dBZ, and 10 dBZ echo contours reached 14–17 km. Storms covered by three‐dimensional lightning mappers were near peak altitude of lightning activity (modes of the vertical distributions of radio sources were at altitudes colder than −50°C) and vertical reflectivity intensity, with overshooting echo tops around the times of their jets. Two of the other three jet‐producing storms produced their jet around the time of a convective surge as indicated by radar data and likely featured overshooting tops. The observations suggest a link between convective surges, overshooting tops, and the occurrence of gigantic jets, similar to prior modeling studies.

Optical power and energy radiated by natural lightning

AbstractCalibrated measurements of the visible and near‐infrared radiation produced by both negative and positive cloud‐to‐ground (CG) lightning strokes have been made at distances of 5 to 32 km in southern Arizona (AZ) and the central Great Plains using a photodiode sensor with a flat spectral response between 0.4 and 1.0 µm. Time‐correlated video images (60 fps) of the channel development provided information about the types of strokes that were detected and reports from the U.S. National Lightning Detection Network indicated their locations, polarities, and estimates of their peak current. In our sample of negative strokes that were suitable for analysis, there were 23 first (or only) strokes (FS), 19 subsequent strokes that created new ground contacts (NGC), and 101 subsequent strokes that re‐illuminated a preexisting channel (PEC). We also analyzed 10 positive strokes (in nine flashes), and 73 of the larger impulses that were radiated by intracloud discharges (CPs). Assuming that these events can be approximated as isotropic sources and that the effects of atmospheric extinction are negligible, the peak optical power (Po), total optical energy (Eo), and characteristic widths of the sources (tcw = Eo/Po) have been computed. Median values of Po for negative FS, NGC, and PEC strokes were 1.8 × 1010 W, 1.1 × 1010 W, and 4.4 × 109 W, respectively. Median values of Eo were 3.6 × 106 J, 3.5 × 106 J, and 1.2 × 106 J, respectively. The median characteristic widths of negative FS, NGC, and PEC strokes were 229 µs, 244 µs, and 283 µs, respectively. Positive CG strokes produced a median Po, Eo, and tcw of 1.9 × 1010 W, 9.3 × 106 J, and 497 µs, respectively. Estimates of the space‐and‐time‐average power per unit length () in the lower portion of negative FS, NGC, and PEC channels had medians of 2.8 × 106 W/m, 3.2 × 106 W/m, and 1.4 × 106 W/m, respectively, and the median for four positive strokes was 8.8 × 106 W/m. Median values for the estimated peak electromagnetic power (PEM) radiated at early times in the strokes are 2.0 × 109 W, 2.5 × 109 W, 1.0 × 109 W, and 9.1 × 109 W for FS, NGC, PEC and positive strokes, respectively. CP events produced a median Po, Eo, and tcw of 2.0 × 109 W, 0.7 × 106 J, and 311 µs, respectively, and are in good agreement with aircraft and satellite measurements. The values of Po, Eo, and for negative CG strokes in AZ are significantly larger than prior measurements in Florida, likely because there is less atmospheric extinction in our dataset, and due to extinction, all the above values of Po, Eo, and are lower limits at the source.

Spatial and temporal variability of cloud-to-ground lightning over the continental U.S. during the period 1995–2010

We use 16years (1995–2010) of data from the National Lightning Detection Network (NLDN) to examine the spatial and temporal variability of major cloud-to-ground (CG) lightning days (defined as the 80days with the largest lightning activity) over the continental United States. Extreme lightning activity is concentrated over the central U.S. and west of the Appalachian Mountains. The largest frequency of major lightning days is concentrated during the summertime, with a tendency for these major days to have occurred in recent years. We also examine the presence of monotonic patterns over time in CG lightning flashes over the continental United States. Analyses are performed at the monthly scale (from April to September) and for total, negative-only, and positive-only flashes. The non-parametric Mann–Kendall test is used to examine the presence of monotonic patterns. The upgrades in NLDN during the study period complicate the separation between cloud-to-cloud flashes (CC) and the targeted CG lightning flashes. The results of the trend analyses are sensitive to the threshold used to discriminate between CC and CG flashes, in particular for positive-only flashes. The central U.S. is an area that exhibits statistically significant increasing trends independently of the selected threshold, while there is a general tendency towards decreasing trends over the Rocky Mountains. These results raise the question of whether the observed changes in lightning activity during the recent years are related to natural or human-induced changes in the climate system, and/or to inhomogeneities in the observational network.

Statistical patterns in the location of natural lightning

Lightning discharges are nature's way of neutralizing the electrical buildup in thunderclouds. Thus, if an individual discharge destroys a substantial fraction of the cloud charge, the probability of a subsequent flash is reduced until the cloud charge separation rebuilds. The temporal pattern of lightning activity in a localized region may thus inherently be a proxy measure of the corresponding timescales for charge separation and electric field buildup processes. We present a statistical technique to bring out this effect (as well as the subsequent recovery) using lightning geo‐location data, in this case with data from the National Lightning Detection Network (NLDN) and from the GLD360 Network. We use this statistical method to show that a lightning flash can remove an appreciable fraction of the built up charge, affecting the neighboring lightning activity for tens of seconds within a ∼ 10 km radius. We find that our results correlate with timescales of electric field buildup in storms and suggest that the proposed statistical tool could be used to study the electrification of storms on a global scale. We find that this flash suppression effect is a strong function of flash type, flash polarity, cloud‐to‐ground flash multiplicity, the geographic location of lightning, and is proportional to NLDN model‐derived peak stroke current. We characterize the spatial and temporal extent of the suppression effect as a function of these parameters and discuss various applications of our findings.

Toward Development of Improved QPE in Complex Terrain Using Cloud-to-Ground Lightning Data: A Case Study for the 2005 Monsoon in Southern Arizona

Abstract The relationship between convective precipitation and cloud-to-ground (CG) lightning is examined over a study area in southwest Arizona and northwest Mexico. Using seasonal-to-daily and hourly time resolution, the National Climatic Data Center (NCDC) stage IV precipitation product and the U.S. National Lightning Detection Network lightning data have been analyzed with the aim of developing an improved understanding of the relationship between these variables. A Gaussian method of spatially smoothing discrete lightning counts is used to estimate convective rainfall and improve the quality and spatial coverage of radar-derived precipitation in areas of complex terrain. For testing the dependence of the relationship between CG lightning and precipitation, a precipitation “sensor coverage” analysis has been performed. If locations that have poor sensor coverage are excluded, R2 between lightning and precipitation improves by up to 15%. A complementary way to estimate convective precipitation is proposed based on 1-h lightning occurrence intervals, which is the maximum time resolution in this study. We find that ~67% of the seasonal 2005 precipitation over the analysis domain is associated with CG lightning. Daily precipitation estimates are improved by specifying a “diurnal day” based on the diurnal maxima and minima in precipitation and CG lightning within the domain. Our method for improving quantitative precipitation estimation (QPE) using lightning is able to track and estimate convective precipitation over regions that have poor sensor coverage, particularly in both air mass storms and large multicellular events, with R2 up to 70%.

Charge rearrangement by sprites over a north Texas mesoscale convective system

Charge rearrangement by sprites is analyzed for a mesoscale convective system (MCS) situated in north Texas and east New Mexico on 15 July 2010. During the thunderstorm, electric field data were recorded by the Langmuir Electric Field Array (LEFA), while magnetic field data were recorded by the charge‐moment network near Duke University. A high speed (12500 fps) video system operated at Langmuir Laboratory recorded telescopic images of the sprites. Data from the National Lightning Detection Network (NLDN) show that each sprite was preceded by a series of cloud discharges and cloud‐to‐ground discharges. The triggering event preceding the sprite was typically a positive cloud‐to‐ground (+CG) stroke. For one out of the 10 sprites that were recorded, there was a positive hump in the electric field a few milliseconds after the +CG return stroke. The size and shape of the hump roughly matched the light intensity emitted from the sprite. The electric field hump is fit by a sprite current that originates in the ionosphere and propagates downward, producing the same effect as a downward moving positive current. The integral under the current hump was 23.9 C when the velocity of the current pulse was between 0.25 c and 0.55 c. The large sprite current was followed by impulsive electromagnetic radiation which has not been previously reported and could be a recoil effect similar to what is called a “K‐change” when it is observed in a lightning flash.

Upward lightning observations from towers in Rapid City, South Dakota and comparison with National Lightning Detection Network data, 2004–2010

We report on upward lightning observations from ten tall towers (91–191 m) in Rapid City, South Dakota, USA and compare with National Lightning Detection Network (NLDN) data. A total of 81 upward flashes were observed from 2004–2010 using GPS time‐stamped optical sensors, and in all but one case, visible flash activity preceded the development of the upward leaders. Time‐correlated analysis showed that the NLDN recorded an event within 50 km of towers and within 500 ms prior to upward leader development from the tower(s) for 83% (67/81) of the upward flashes. A preceding positive cloud‐to‐ground stroke (+CG) was detected in 57% (46/81) of the cases, and a preceding positive intracloud flash (+IC) in 23% (19/81) of the cases. However, 8 of the 19 NLDN‐indicated +IC events were actually +CG strokes based on optical observations. Preceding negative intracloud flashes (−IC) were recorded for 2% (2/81) of the cases. Analysis also showed that for 44% (36/81) of the upward flashes, the NLDN reported subsequent negative cloud‐to‐ground (−CG) strokes and/or −IC events at one or more tower locations. Of the 151 subsequent events, 70% (105/151) were −CG reports and 30% (46/151) were listed as −IC events. The geometric mean/median location accuracy and peak current for subsequent events were 194 m/206 m and −12.9 kA/−12.4 kA respectively. These correlated observations suggest that a majority of the upward lightning flashes were triggered by a preceding flash with the dominant triggering type being the +CG flash.

Charge moment change and lightning‐driven electric fields associated with negative sprites and halos

Sprites are structured high altitude optical emissions produced by lightning‐driven electric fields. Both strong positive and negative cloud to ground flashes (CGs) are capable of initiating sprites. However, reported sprites are almost exclusively produced by +CGs. The very limited number of negative polarity sprites makes it difficult to reveal their morphologies and mechanisms. Since 2008, we have operated low light cameras at 5 locations in the United States to detect lightning‐driven transient luminous events (TLEs). At Duke University, two pairs of magnetic sensors simultaneously record lightning‐radiated magnetic fields. During 4 years of observations, the low light cameras collectively captured 1651 sprite events. Among them, 6 were produced by −CG lightning, which was confirmed by both the National Lightning Detection Network (NLDN) and magnetic field measurements. All of these negative sprites show similar features in their morphology, lightning source current, and lightning‐driven ambient electric fields. They all initiate within a few ms from their parent lightning discharges and always are accompanied by sprite halos. Compared to positive sprites, the downward streamers in negative sprites terminate at higher altitudes, about 55–60 km. The extracted source current of their parent lightning discharges is very impulsive and produces at least 450 C km charge moment change in 0.5 ms or less. Unlike most +CG strokes, essentially no continuing current follows these −CGs. Thus the uniformity of negative sprite morphology appears to reflect the uniformity of the characteristics of high charge transfer negative strokes. Numerical simulation shows these impulsive source currents produce very high (>2 Ek, where Ek is the local air breakdown field) but short‐lived electric fields at halo altitudes between 70 km and 90 km. At streamer termination altitudes, the inferred background electric field is 0.2–0.3 Ek, which is close to but below the critical field (0.4 Ek) for negative streamer propagation. The simulations also show that the timescale of the lightning source current has a significant impact on the high altitude electric fields and thus the sprite initiation and morphology. With the same amount of charge transfer, a more impulsive current source produces a higher electric field of shorter duration at halo and sprite altitudes. The short timescales may explain why a larger threshold of total charge moment change is required to initiate negative sprites than short‐delayed positive sprites, whose parent lightning source current is usually less impulsive.

On remote measurements of lightning return stroke peak currents

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 45km 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 2×108m/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.

Lightning in the Anvils of Supercell Thunderstorms

AbstractThis study uses data from the Oklahoma Lightning Mapping Array (OK-LMA), the National Lightning Detection Network, and the Norman, Oklahoma (KOUN), prototype Weather Surveillance Radar-1988 Doppler (WSR-88D) radar to examine the evolution and structure of lightning in the anvils of supercell storms as they relate to storm dynamics and microphysics. Several supercell storms within the domain of the OK-LMA were examined to determine whether they had lightning in the anvil region, and if so, the time and location of the initiation of the anvil flashes were determined. Every warm-season supercell storm had some flashes that were initiated in or near the stronger reflectivities of the parent storm and propagated 40–70 km downstream to penetrate well into the anvil. Some supercell storms also had flashes that were initiated within the anvil itself, 40–100 km beyond the closest 30-dBZ contour of the storm. These flashes were typically initiated in one of three locations: 1) coincident with a local reflectivity maximum, 2) between the uppermost storm charge and a screening-layer charge of opposite polarity near the cloud boundary, or 3) in a region in which the anvils from two adjoining storms intersected. In some storms, anvil flashes struck ground beneath a reflectivity maximum in which reflectivity ≥20 dBZ had extended below the 0°C isotherm, possibly leading to the formation of embedded convection. This relationship may be useful for identifying regions in which there is a heightened risk for cloud-to-ground strikes beneath anvil clouds. In one storm, however, anvil lightning struck ground even though this reflectivity signature was absent.

Forecasting the onset of cloud‐to‐ground lightning using radar and upper‐air data in Romania

AbstractA method for forecasting the onset of cloud‐to‐ground (CG) lightning flashes is examined. Reflectivity data and upper‐air measurements were analysed for 49 convective storms observed in 36 different days over southern Romania during the convective season (May–September) from 2003 to 2005. The radar reflectivity data were associated with the CG lightning flash locations obtained from the Romanian National Lightning Detection Network. The CG lightning initiation signature is based on reflectivity thresholds (35 and 40 dBZ) at a given environmental temperature aloft (−10 and − 15 °C). We have found that the best predictor is the 35 dBZ radar reflectivity at the height of the − 10 °C isotherm, with a probability of detection of 95%, and mean lead time of 17 min before the first CG lightning. In previous studies, it was found that the onset of CG lightning flashes had greater threshold value for reflectivity (40 dBZ) and shorter lead time (13 min). The results presented in this study are currently used in the nowcasting operational activities at the Romanian National Meteorological Administration. Copyright © 2012 Royal Meteorological Society

Positive lightning: An overview, new observations, and inferences

We examine the various conceptual cloud charge configurations and scenarios leading to production of positive lightning with a view toward an explanation of its observed properties. Data for 52 positive cloud‐to‐ground flashes containing 63 strokes recorded in Gainesville, Florida, in 2007–2008 are presented and discussed. The U.S. National Lightning Detection Network (NLDN) located 51 (96%) of the positive strokes at distances of 7.8 to 157 km from the field‐measuring station. In 42 (81%) of the 52 flashes, the first (or the only) leader‐return stroke sequence was not preceded by in‐cloud discharge activity. The average number of strokes per flash is 1.2. Single‐stroke flashes constitute 81%. In three (38%) of eight two‐stroke flashes, the second stroke likely followed the channel of the first stroke, and in five (62%) flashes it likely created a new termination on ground. Eight (15%) of the 52 positive flashes had detectable preliminary breakdown pulse trains in their electric field records. Electric field waveforms of 14 (27%) first strokes (also of one third stroke) are preceded by pronounced pulses, apparently indicative of leader stepping. NLDN‐reported peak currents for 48 positive strokes in 40 flashes ranged from 20 to 234 kA with a geometric mean value of 75 kA. The median charge transfers in 1, 2, and 40 ms after the beginning of return stroke electric field change for an assumed height of 12 km were estimated to be 13, 18, and 34 C, respectively. Two bipolar lightning discharges are additionally examined.

Radar Nowcasting of Total Lightning over the Kennedy Space Center

Abstract A long-term radar dataset over Melbourne, Florida, was matched with three-dimensional lightning data to optimize radar-derived predictors of total lightning over the Kennedy Space Center (KSC). Four years (2006–09) of summer (June–August) daytime (about 1400–0000 UTC) Weather Surveillance Radar-1988 Doppler data were analyzed. Convective cells were tracked using a modified version of the Storm Cell Identification and Tracking (SCIT) algorithm, and correlated to cloud-to-ground (CG) lightning data from the National Lightning Detection Network (NLDN) and grouped intracloud (IC) flash data from the KSC Lightning Detection and Ranging (LDAR) I and II networks. Reflectivity values at isothermal levels and a vertically integrated ice (VII) product were used to optimize radar-based forecasting of both IC and CG lightning. Results indicate the best reflectivity threshold predictors of CG and IC lightning according to the critical success index (CSI) were 25 dBZ at −20°C and 25 dBZ at −15°C, respectively. The best VII predictors of CG and IC were the 30th (0.840 kg m−2) and 5th percentiles (0.143 kg m−2), respectively, suggesting less ice mass is needed in the main mixed-phase region for IC flashes to occur. In addition, VII at lightning initiation (both CG and IC) was higher than at cessation. Seventy-six percent of cells had IC initiation before CG initiation. Using the first IC flash as a predictor of CG occurrence also statistically outperformed other CG predictors, but yielded a 2.4-min average lead time. However, this lead time is comparable to the reflectivity threshold and VII methods when accounting for radar scan and processing time.