Los Alamos Sferic Array Research Articles

Joint radio and optical observations of the most radio-powerful intracloud lightning discharges

Abstract. The most radio-powerful intracloud lightning emissions are associated with a phenomenon variously called "narrow bipolar events" or "compact intracloud discharges". This article examines in detail the coincidence and timing relationship between, on the one hand, the most radio-powerful intracloud lightning events and, on the other hand, optical outputs (or lack thereof) of the same discharge process. This is done, first, using coordinated very high frequency (VHF) and optical observations from the FORTE satellite and, second, using coordinated sferic and all-sky optical observations from the Los Alamos Sferic Array. In both cases, it is found that the sought coincidences are exceedingly rare. Moreover, in the handful of coincidences between optical and intense radio emissions that have been identified, the radio emissions differ from their usual behavior, by being accompanied by approximately simultaneous "conventional" lightning radio emissions. It is implied that the most radio-powerful intracloud emission process essentially differs from ordinary incandescent lightning.

Time domain waveform and azimuth variation of ionospherically reflected VLF/LF radio emissions from lightning

The Earth's geomagnetic field can, in principle, cause significant magnetic‐azimuth variations of the lower ionosphere's reflection of 2–150 kHz radio waves emitted by lightning. There has been little published work on this azimuth variation, either modeled or observational. We use broadband emissions from negative cloud‐to‐ground lightning strokes to study the azimuthal variations systematically. The data are from the Los Alamos Sferic Array, operating in the United States' southern Great Plains during 2005. We compare the observations to a model of lower‐ionosphere reflection of radio waves. The model recapitulates the basic features of the time domain reflection waveforms rather well, except at the lowest frequencies. The model transfer function describing the vertical electric field at the receiver is symmetric about 90° magnetic and about 270° magnetic. Two noteworthy features of the azimuth variation are both predicted by the model, and seen in the data: First, at the lowest frequencies (<30 kHz) there is enhanced reflection for eastward propagation, relative to westward propagation. Second, at the higher frequencies (>50 kHz) there is an opposite enhancement, of the reflection for westward propagation, relative to eastward propagation. The westward enhancement at >50 kHz depends sensitively on range and is most evident in nighttime conditions, while the eastward enhancement at <30 kHz occurs at all ranges studied. Range‐dependent frequency modulations of the transfer function are the least for magnetic northward propagation (duplicated by magnetic southward).

Revisiting "Narrow Bipolar Event" intracloud lightning using the FORTE satellite

Abstract. The lightning stroke called a "Narrow Bipolar Event", or NBE, is an intracloud discharge responsible for significant charge redistribution. The NBE occurs within 10–20 μs, and some associated process emits irregular bursts of intense radio noise, fading at shorter timescales, sporadically during the charge transfer. In previous reports, the NBE has been inferred to be quite different from other forms of lightning strokes, in two ways: First, the NBE has been inferred to be relatively dark (non-luminous) compared to other lightning strokes. Second, the NBE has been inferred to be isolated within the storm, usually not participating in flashes, but when it is in a flash, the NBE has been inferred to be the flash initiator. These two inferences have sufficiently stark implications for NBE physics that they should be subjected to further independent test, with improved statistics. We attempt such a test with both optical and radio data from the FORTE satellite, and with lightning-stroke data from the Los Alamos Sferic Array. We show rigorously that by the metric of triggering the PDD optical photometer aboard the FORTE satellite, NBE discharges are indeed less luminous than ordinary lightning. Referred to an effective isotropic emitter at the cloud top, NBE light output is inferred to be less than ~3 × 108 W. To address isolation of NBEs, we first expand the pool of geolocated intracloud radio recordings, by borrowing geolocations from either the same flash's or the same storm's other recordings. In this manner we generate a pool of ~2 × 105 unique and independent FORTE intracloud radio recordings, whose slant range from the satellite can be inferred. We then use this slant range to calculate the Effective Radiated Power (ERP) at the radio source, in the passband 26–49 MHz. Stratifying the radio recordings by ERP into eight bins, from a lowest bin (<5 kW) to a highest bin (>140 kW), we document a trend for the radio recordings to become more isolated in time as the ERP increases. The highest ERP bin corresponds to the intracloud emissions associated with NBEs. At the highest ERP, the only significant probability of temporal neighbors is during times following the high-ERP events. In other words, when participating in a flash, the high-ERP emissions occur at the apparent flash initiation.

Evolution of Eyewall Convective Events as Indicated by Intracloud and Cloud-to-Ground Lightning Activity during the Rapid Intensification of Hurricanes Rita and Katrina

Lightning data (cloud-to-ground plus intracloud) obtained from the Los Alamos Sferic Array (LASA) for 2005’s Hurricanes Rita and Katrina were analyzed to provide a first insight into the three-dimensional electrical activity of rapidly intensifying hurricanes. This information is crucial for modelers aiming at better forecasting hurricane intensity, because it is inherently related to key structural aspects of the storm often misrepresented in numerical models. Analysis of the intracloud narrow bipolar events (NBEs) for Rita revealed a general increase in discharge heights during the period of rapid intensification. The results also showed that for the case of Rita, NBEs were useful in tracking and mapping the evolution of individual strong convective elements embedded in the eyewall during rapid intensification. Those results are particularly revealing, and suggest that the general increase in height of the intracloud lightning is an aggregate consequence of numerous short-lived convective events rotating rapidly around the eyewall of Rita. A similar rise in discharge heights during periods of intensification was also observed for Katrina. However, the NBE lightning data show that for Katrina, the eyewall convection persisted for several hours at a fixed location instead of rotating cyclonically along the eyewall. This highlights the idea that NBE lightning data can also be used to identify different convective regimes attributed to possibly different internal or external forcing mechanism(s).

High-Resolution Simulation of the Electrification and Lightning of Hurricane Rita during the Period of Rapid Intensification

Abstract In this paper, a high-resolution simulation establishing relationships between lightning and eyewall convection during the rapid intensification phase of Rita will be highlighted. The simulation is an attempt to relate simulated lightning activity within strong convective events (CEs) found within the eyewall and general storm properties for a case from which high-fidelity lightning observations are available. Specifically, the analysis focuses on two electrically active eyewall CEs that had properties similar to events observed by the Los Alamos Sferic Array. The numerically simulated CEs were characterized by updraft speeds exceeding 10 m s−1, a relatively more frequent flash rate confined in a layer between 10 and 14 km, and a propagation speed that was about 10 m s−1 less than of the local azimuthal flow in the eyewall. Within an hour of the first CE, the simulated minimum surface pressure dropped by approximately 5 mb. Concurrent with the pulse of vertical motions was a large uptake in lightning activity. This modeled relationship between enhanced vertical motions, a noticeable pressure drop, and heightened lightning activity suggests the utility of using lightning to remotely diagnose future changes in intensity of some tropical cyclones. Furthermore, given that the model can relate lightning activity to latent heat release, this functional relationship, once validated against a derived field produced by dual-Doppler radar data, could be used in the future to initialize eyewall convection via the introduction of latent heat and/or water vapor into a hurricane model.

A closer examination of terrestrial gamma‐ray flash‐related lightning processes

This paper presents detailed comparisons between terrestrial gamma‐ray flashes (TGFs) detected by the RHESSI satellite and the lightning processes observed by the Los Alamos Sferic Array (LASA). It was found that all RHESSI‐detected TGFs were related to normal intracloud flashes that transported electrons upward in the cloud. Detailed temporal comparisons suggest that TGFs are more likely related to small, rather than large, lightning pulses, mostly at the level of 10−1−100 V/m when the pulses are normalized to an observation distance of 100 km. The corresponding peak current was estimated to be 3–19 kA with most of them below 10 kA, if the discharges were assumed to be individual steps in a stepped‐leader process. If the discharges were considered to be directly associated with runaway breakdown processes with a current propagation speed of c, the peak current would be 0.4–3.2 kA. Analysis of events that were close to the LASA stations showed that TGFs were most likely to occur during the initial milliseconds of IC flashes while the discharge developed vertically upward from the negative to the positive charge regions in the cloud. The heights of the TGF‐related lightning pulses were estimated to be in the range of 10.5–14.1 km along the upward development. An intense NBE with a peak current of 80 kA and a height of 16.1 km was found 2.9 ms after one TGF, and is believed to not be directly associated with the TGF but may be preconditioned by the TGF‐related discharge process.

Full‐wave reflection of lightning long‐wave radio pulses from the ionospheric D region: Comparison with midday observations of broadband lightning signals

We are developing and testing a steep‐incidence D region sounding method for inferring profile information, principally regarding electron density. The method uses lightning emissions (in the band 5–500 kHz) as the probe signal. The data are interpreted by comparison against a newly developed single‐reflection model of the radio wave's encounter with the lower ionosphere. The ultimate application of the method will be to study transient, localized disturbances of the nocturnal D region, including those instigated by lightning itself. Prior to applying the method to study lightning‐induced perturbations of the nighttime D region, we have performed a validation test against more stable and predictable daytime observations, where the profile of electron density is largely determined by direct solar X‐ray illumination. This article reports on the validation test. Predictions from our recently developed full‐wave ionospheric‐reflection model are compared to statistical summaries of daytime lightning radiated waveforms, recorded by the Los Alamos Sferic Array. The comparison is used to retrieve best fit parameters for an exponential profile of electron density in the ionospheric D region. The optimum parameter values are compared to those found elsewhere using a narrowband beacon technique, which used totally different measurements, ranges, and modeling approaches from those of the work reported here.

Model Simulation of Very Low-Frequency and Low-Frequency Lightning Signal Propagation Over Intermediate Ranges

A model simulation for very low-frequency and low-frequency lightning signal propagation over ranges of hundreds to a couple of thousand kilometers is presented in this paper. The model is a composite of ground and ionospheric-reflected waves. The ground wave is modeled over a spherical earth that has a finite conductivity. The ionospheric-wave simulation is based on a recently developed full-wave model that treats the lower portion of the ionosphere as a magnetized, anisotropic, collisional, and cold-electron medium. In this paper, only the first-hop ionosphere reflection is presented, although higher order reflections can be readily implemented by the model. For the purpose of demonstration, the modeled results are compared to actual observations of negative cloud-to-ground strokes at various distances for day and nighttime. The model itself, however, can be used for any type of lightning discharge event, including the incloud events that occur above the ground. This model, together with multistation Los Alamos Sferic Array measurements of lightning discharges, might provide an alternate means for monitoring the temporal and spatial variations of the lower portion of the ionosphere.

Full‐wave reflection of lightning long‐wave radio pulses from the ionospheric D region: Numerical model

A model is developed for calculating ionospheric reflection of electromagnetic pulses emitted by lightning, with most energy in the long‐wave spectral region (f ∼ 3–100 kHz). The building block of the calculation is a differential equation full‐wave solution of Maxwell's equations for the complex reflection of individual plane waves incident from below, by the anisotropic, dissipative, diffuse dielectric profile of the lower ionosphere. This full‐wave solution is then put into a summation over plane waves in an angular direct Fourier transform to obtain the reflection properties of curved wavefronts. This step models also the diffraction effects of long‐wave ionospheric reflections observed at short or medium range (∼200–500 km). The calculation can be done with any arbitrary but smooth dielectric profile versus altitude. For an initial test, this article uses the classic D region exponential profiles of electron density and collision rate given by Volland. With even these simple profiles, our model of full‐wave reflection of curved wavefronts captures some of the basic attributes of observed reflected waveforms recorded with the Los Alamos Sferic Array. A follow‐on article will present a detailed comparison with data in order to retrieve ionospheric parameters.

Characteristics of cloud-to-ground lightning in warm-season thunderstorms in the Central Great Plains

In July 2005, a field campaign was conducted in the Central Great Plains to obtain 60-field/s video imagery of lightning in correlation with reports from the U.S. National Lightning Detection Network (NLDN) and broadband electric field waveforms from the Los Alamos Sferic Array (LASA). A total of 342 GPS time-stamped cloud-to-ground (CG) flashes were recorded in 17 different sessions, and 311 (91%) of these were correlated with reports from the NLDN. Only 6 of the 17 recording sessions were dominated by flashes that lowered negative charge to ground, and 11 were dominated by positive CG flashes. A total of 103 flashes recorded on video were correlated with at least one NLDN report of negative CG strokes, 204 video flashes were correlated with one or two positive stroke reports, and 4 had bipolar reports. In this paper, we will give distributions of the estimated peak current, I p, as reported by the NLDN, of negative and positive first strokes that were recorded on video, the multiplicity of strokes that were recorded on video, and the number of ground contacts per flash that were resolved on video. 41 (40%) of the negative flashes produced just a single-stroke on video, and 62 (60%) showed two or more strokes. The observed multiplicity of negative flashes averaged 2.83, which becomes about 3.14 after correcting for the finite time-resolution of the video camera. 195 (96%) of the positive flashes produced just a single-stroke on video, and 9 (4%) showed two strokes; therefore, the observed multiplicity of positive flashes averaged 1.04. Five out of 9 (56%) of the positive subsequent strokes re-illuminated a previous channel, and 4 out of 9 (44%) created a new ground contact. Simultaneous video, LASA, and NLDN measurements also allowed us to examine the classification of NLDN reports during 3 single-cell storms (one negative and two positive). Based on the LASA waveforms, a total of 204 out of 376 (54%) NLDN reports of CG strokes were determined to be for cloud pulses. The misclassified negative reports had | I p| values ranging from 3.8 kA to 29.7 kA, but only 58 (24%) of these had | I p| > 10 kA, and only one misclassified positive report had I p > 20 kA. Radar analyses showed that most of the negative and positive CG strokes that were recorded on video were produced within or near the convective cores of storms. The radar imagery also showed that single-cell storms tended to produce one polarity of CG flashes at a time, and that such storms could switch rapidly from negative to positive CG flashes when the reflectivity was near maximum. Multiple-cell storms produced both negative and positive flashes over a broad region, but each polarity tended to cluster near regions of high-reflectivity.

Relationships among Narrow Bipolar Events, “total” lightning, and radar‐inferred convective strength in Great Plains thunderstorms

Narrow Bipolar Events (NBEs) are a recently discovered distinct class of intracloud lightning discharges whose associated processes produce the most powerful very high frequency (VHF) radiation observed from lightning. NBEs are thus the prime candidate for proposed satellite‐based VHF global lightning mapping and storm tracking missions. In this study, we offer a detailed evaluation of the Great Plains Los Alamos Sferic Array (LASA). We then statistically compare NBE rates to non‐NBE lightning rates measured by both the LASA and the National Lightning Detection Network (NLDN) and to NEXRAD radar‐inferred metrics of convective strength for thunderstorms in the Great Plains from May to July 2005. We find strong correlations between total lightning rate and convective strength, especially in terms of the height of 30 dBZ radar echo. However, we find much weaker correlations between NBE rate and non‐NBE lightning rate and between NBE rate and radar‐inferred convective strength. Though NBEs occur in the same storms as other lightning, they cluster more closely in both space and time and may be indicative of specific types of storms and/or specific stages in convective development. Indeed, we find that NBEs are more prevalent in, and perhaps indicative of, the strongest convection. However, even the strongest convection (as inferred by radar) does not always produce NBEs. We compare these results to past studies of NBEs which were based in Florida. We also briefly discuss the implications of these results for satellite‐based VHF lightning detection.

Radar and Lightning Observations of Normal and Inverted Polarity Multicellular Storms from STEPS

Abstract This study discusses radar and lightning observations of two multicellular storms observed during the Severe Thunderstorm Electrification and Precipitation Study. The Lightning Mapping Array data indicated that the charge structure of the 19 June 2000 storm was consistent with a normal polarity tripole, while the 22 June 2000 storm exhibited an overall inverted tripolar charge structure. The 19 June storm consisted of weaker convection and produced little to no hail and moderate total flash rates peaking between 80 and 120 min−1. The cells in the 22 June 2000 storm were much more vigorous, exhibited strong, broad updrafts, and produced large quantities of hail, as well as extraordinary total flash rates as high as 500 min−1. The National Lightning Detection Network (NLDN) indicated that the 19 June storm produced mostly negative cloud-to-ground (CG) lightning, while the 22 June storm produced predominantly positive CG lightning, peaking at 10 min−1 just after two cells merged. However, the Los Alamos Sferic Array indicated that many of the positive CG strokes reported by the NLDN in the 22 June storm were intracloud discharges known as narrow bipolar events. Negative CG lightning was also observed in the 22 June storm, but typically came to ground beneath an inverted dipole in the storm anvil.

Estimating lightning channel characteristics of positive narrow bipolar events using intrachannel current reflection signatures

A set of 133 positive narrow bipolar lightning events located by the Los Alamos Sferic Array (LASA) is examined. The events were selected because their fast electric field change (EFC) records show a secondary positive peak, following the primary positive peak, which is consistent with what would be expected for a propagating current pulse reflecting off an impedance discontinuity along a transmission line. With an assumed propagation speed, the time difference between the primary and secondary (reflected) peak in the field change record offers a direct technique for estimating the length of the discharge channel. If the signal amplitude is assumed to decay exponentially with time and that the reflection coefficient is near unity, the time difference between peaks and the ratio of their amplitudes yields an estimate of the temporal decay rate constant. Statistics on the 133 located events examined show an average peak‐to‐reflected‐peak time of 6.7 μs, a mean amplitude ratio estimate of 6.1, and an amplitude decay rate constant of 4.0 μs. Bound by the speed of light, the upper limit on channel length for the typical positive narrow bipolar event (NBE) in this study is shown to be approximately 2 km.

Comparison of Narrow Bipolar Events with Ordinary Lightning as Proxies for the Microwave-Radiometry Ice-Scattering Signature

Abstract The narrow bipolar event (NBE) is a unique lightning discharge that has a short (∼10 μs) overall duration, lacks a prior leader phase, and produces too little light output to be visible by optical lightning detectors on satellites. NBEs thus have basic differences from ordinary lightning discharges, which occur in flashes lasting up to a fraction of a second, carry significant current in a “stroke” only after a leader stage that prepares the conductive channel, and produce copious light that is recordable from space. Thus, the authors are motivated to determine whether the meteorological setting of NBEs differs from, or is similar to, that of ordinary lightning. A previous paper started this project of comparing NBEs with ordinary lightning by comparing the placement of either type of lightning within spatial structures of cloud depth, as revealed by infrared cloud-top temperature. That previous study employed lightning data from the Los Alamos Sferic Array (LASA) in Florida. The present paper extends this approach to comparing LASA NBEs with ordinary lightning events’ spatial relationships to radiometric cloud imagery from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) 85-GHz channels. This form of radiometric imagery reveals the location of deep, active convective cores with more acute spatial selectivity than does the infrared cloud-top temperature. It was found that the behaviors of NBEs and ordinary lighting are, once again, indistinguishable, but with regard this time to proximity to deep convective cores as revealed by TMI.

Total Lightning Observations with the New and Improved Los Alamos Sferic Array (LASA)

Abstract Since 1998, Los Alamos National Laboratory (LANL) has deployed an array of fast electric field change sensors in New Mexico and Florida in support of LANL’s satellite lightning observations. In April 2004, all the sensors were significantly upgraded and improved, and a new array was deployed in north-central Florida. This paper describes the operations of the new array and reports the first 12 months of lightning observations. The new array is about 10 times more sensitive than the previous one and can capture millions of discharge events during a stormy day in Florida. In this paper, the array’s lightning location accuracy, minimum detectable peak current, and ratio of intracloud-to-cloud-to-ground flashes are analyzed. Some case studies that illustrate the storm evolution, lightning classification, and radar comparisons are presented. A new three-dimensional capability of the array is demonstrated.

Performance Assessment of the World Wide Lightning Location Network (WWLLN), Using the Los Alamos Sferic Array (LASA) as Ground Truth

Abstract The World Wide Lighting Location Network (WWLLN) locates lightning globally, using sparsely distributed very low frequency (VLF) detection stations. Due to WWLLN’s detection at VLF (in this case ∼10 kHz), the lightning signals from strong strokes can propagate up to ∼104 km to WWLLN sensors and still be suitable for triggering a station. A systematic evaluation of the performance of WWLLN is undertaken, using a higher-frequency (0–500 kHz) detection array [the Los Alamos Sferic Array (LASA)] as a ground truth during an entire thunderstorm season in a geographically confined case study in Florida. It is found that (a) WWLLN stroke-detection efficiency rises sharply to several percent as the estimated lightning current amplitude surpasses ∼30 kA; (b) WWLLN spatial accuracy is around 15 km, good enough to resolve convective-storm cells within a larger storm complex; (c) WWLLN is able to detect intracloud and cloud-to-ground discharges with comparable efficiency, as long as the current is comparable; (d) WWLLN detects lightning-producing storms with high efficiency in every 3-h epoch; thus, WWLLN can be useful for locating deep convection for weather forecasting on 3-h update cycles; and (e) WWLLN detects a stroke count in each storm that is weakly proportional to the stroke count detected by LASA. Thus, to the extent that lightning rate can serve as a statistical proxy for rainfall, WWLLN may eventually provide rainfall-proxy data to be assimilated in 3-h forecast update cycles.

A link between terrestrial gamma‐ray flashes and intracloud lightning discharges

Atmospheric electric field change (sferic) waveforms were detected at Los Alamos Sferic Array stations in association with terrestrial gamma‐ray flashes (TGFs). Five TGF sferic waveforms detected at sufficiently close range were all found to be consistent with a positive‐polarity intracloud (+IC) discharge process which transported electrons upward. The amplitudes of the events were among the top 5% of IC discharge flashes. Altitudes obtained from ionosphere reflections for two of the closer events were found to be 13.6 km and 11.5 km. These altitudes are lower than expected if one assumes that the sferic was near the source of the gamma‐rays. One of the sferics was an energetic narrow bipolar event which occurred near the inferred onset of a flash, suggesting that the preceding TGF may correspond to the actual onset.

A method for determining intracloud lightning and ionospheric heights from VLF/LF electric field records

The Los Alamos Sferic Array (LASA) recorded VLF/LF electric‐field‐change signals from over ten million lightning discharges during the period from 1998 to 2001. Using the differential‐times‐of‐arrival of lightning sferics recorded by three or more stations, the latitudes and longitudes of the source discharges were determined. Under conditions of favorable geometry and ionospheric propagation, sensors obtained ionospherically reflected skywave signals from the lightning discharges in addition to the standard groundwave sferics. In approximately 1% of all waveforms, automated processing identified two 1‐hop skywave reflection paths with delays indicative of an intracloud (height greater than 5 km) lightning source origin. For these events it was possible to determine both the height of the source above ground and the virtual reflection height of the ionosphere. Ionosphere heights agreed well with published values of 60 to 95 km with an expected diurnal variation. Source height determinations for 100,000+ intracloud lightning events ranged from 7 to 20 km AGL with negative‐polarity events occurring above ∼15 km and positive‐polarity events occurring below ∼15 km. The negative‐polarity events are at a suprisingly high altitude and may be associated with discharges between the upper charge layer of a storm and a screening layer of charge above the storm. Approximately 100 of the intracloud events with LASA height determinations were also recorded by VHF receivers on the FORTE satellite. Independent FORTE source height estimates based on delays between direct and ground‐reflected radio emissions showed excellent correlation with the VLF/LF estimates, but with a +1 km bias for the VLF/LF height determinations.

The Los Alamos Sferic Array: A research tool for lightning investigations

Since 1998 the Los Alamos Sferic Array (LASA) has recorded electric field change signals from lightning in support of radio frequency (RF) and optical observations by the Fast On‐orbit Recording of Transient Events (FORTE) satellite. By “sferic” (a colloquial abbreviation for “atmospheric”), we refer to a remote measurement of the transient electric field produced by a lightning flash. LASA consisted of five stations in New Mexico in 1998 and was expanded to 11 stations in New Mexico, Texas, Florida, and Nebraska in 1999. During the 2 years of operation described in this paper, the remote stations acquired triggered 8‐ or 16‐ms duration, 12‐bit waveforms and GPS‐based sferic time tags 24 hours per day year‐round. Source locations were determined daily using differential time of arrival techniques, and the waveforms from all geolocated events were transferred to Los Alamos National Laboratory (LANL), where they have been archived for further analysis, including event classification and characterization. We evaluated LASA location accuracy by comparing temporally coincident (occurring within 100 μs) LASA and National Lightning Detection Network (NLDN) event locations. Approximately one half of the locations agreed to within 2 km, with better agreement for events that occurred within the confines of LASA subarrays in New Mexico and Florida. Of the ∼900,000 events located by the sferic array in 1998 and 1999, nearly 13,000 produced distinctive narrow bipolar field change pulses resembling those previously identified as intracloud discharges.