The VHF Broadband Digital Interferometer developed by Osaka University has been improved to allow continuous sampling over the entire duration of a lightning flash and to utilize a generalized ...cross‐correlation technique for determining the lightning source directions. Time series waveforms of 20–80 MHz signals received at three orthogonally located antennas are continuously digitized over multisecond intervals, as opposed to sequences of short‐duration triggers. Because of the coherent nature of the measurements, radiation sources are located down into the ambient receiver and environmental noise levels, providing a quantum leap in the ability to study lightning discharge processes. When postprocessed using cross correlation, the measurements provide angular uncertainties less than 1° and time resolution better than 1 μs. Special techniques have been developed to distinguish between actual lightning sources and noise events, with the result being that on the order of 50,000–80,000 radiation sources are located for a typical lightning flash. In this study, two‐dimensional interferometer observations of a classic bilevel intracloud flash are presented and combined with three‐dimensional Lightning Mapping Array observations to produce a quasi 3‐D map of lightning activity with the time resolution of the interferometer. As an example of the scientific utility of the observations, results are presented for the 3‐D progression speed of negative leaders associated with intracloud K‐leaders.
Key Points
Cross‐correlation algorithm for continuously sampled DITF data
Quasi 3‐D conversion of 2‐D interferometric maps of lightning
Measured 3‐D velocities of K‐events as a function of time
The dissonant development of positive and negative lightning leaders is a central question in atmospheric electricity. It is also the likely root cause of other reported asymmetries between positive ...and negative lightning flashes, including the ones regarding: stroke multiplicity, recoil activity, leader velocities, and emission of energetic radiation. In an effort to contrast lightning leaders of different polarities, we highlight the staggering differences between two rocket‐triggered lightning flashes. The flash beginning with upward positive leaders exhibits an initial continuous current stage followed by multiple sequences of dart leaders and return strokes. On the other, in its opposite‐polarity counterpart, the upward development of negative leaders is by itself the entire flash. As a result, the flash with negative leaders is faster, briefer, transfers less charge to the ground, has lower currents, and smaller spatial extent. We conclude by presenting a discussion on the three fundamental leader propagation modes.
Plain Language Summary
Lightning flashes that carry positive and negative charges are completely different. In this article, we report on lightning triggered by launching a rocket tethered to the ground toward an electrified cloud. The staggering differences between positive and negative flashes are exposed by a three‐dimensional radio location system and by the current transferred to ground via the trailing wire.
Key Points
Triggered flashes with positive and negative leaders are contrastingly different with the latter being faster, briefer, and more compact
The channel behind triggered positive leaders decays engendering dart leaders and return strokes, which is unparalleled in the negative case
Average conductivity is higher in the negative leader channel despite the lack of return strokes and the lower charge transferred to ground
Six Lightning Mapping Array (LMA) stations were deployed in April 2010 around Eyjafjallajökull volcano in southern Iceland. Single‐station LMA observations were made during the first explosive period ...(14–18 April), and three‐dimensional LMA observations were made during the second explosive period (5–22 May). The single‐station observations revealed that continuous RF electrical activity caused by high rates of small vent discharges occurred during the first explosive period, but not the second, indicating that the strength of vent charging varied between the first and second explosive periods. During the second explosive period, very little lightning was detected between 5 and 10 May, while moderate rates of lightning were detected between 11 and 21 May, signaling that another change occurred on 11 May that affected plume electrification. The data do not make clear if it was changing eruptive activity or changing meteorological activity that resulted in the sudden onset of lightning. The plume charge structure during the second explosive period was inferred from the three‐dimensional lightning data, showing that the dominant charge structure varied between a positive monopole and a negative‐over‐positive dipole. The predominance of a low‐altitude region of positive charge and the observation that electrical activity was concentrated near the vent indicate that net positive vent charging was dominating the electrification.
Key Points
Monopolar and dipolar charge structures were inferred
Vent charging was the dominant mechanism
Volcanic ejecta carried net positive charge
On 3 August 2010 an extensive lightning flash was triggered over Langmuir Laboratory in New Mexico. The upward positive leader propagated into the storm's midlevel negative charge region, extending ...over a horizontal area of 13 × 13 km and 7.5 km altitude. The storm had a normal‐polarity tripolar charge structure with upper positive charge over midlevel negative charge. Lightning Mapping Array (LMA) observations were used to estimate positive leader velocities along various branches, which were in the range of 1–3 × 104 m s−1, slower than in other studies. The upward positive leader initiated at 3.4 km altitude, but was mapped only above 4.0 km altitude after the onset of retrograde negative breakdown, indicating a change in leader propagation and VHF emissions. The observations suggest that both positive and negative breakdown produce VHF emissions that can be located by time‐of‐arrival systems, and that not all VHF emissions occurring along positive leader channels are associated with retrograde negative breakdown.
Key Points
Positive breakdown can produce weak, impulsive VHF emissions
Positive leaders can be mapped using VHF time‐of‐arrival techniques
During the night of 22–23 October 2012, together with the Hydrology cycle in the Mediterranean eXperiment (HyMeX) Special Observation Period 1 campaign, optical observations of sprite events were ...performed above a leading stratiform Mesoscale Convective System in southeastern France. The total lightning activity of the storm was monitored in three dimensions with the HyMeX Lightning Mapping Array. Broadband Extremely Low Frequency/Very Low Frequency records and radar observations allowed characterizing the flashes and the regions of the cloud where they propagated. Twelve sprite events occurred over the stratiform region, during the last third of the lightning activity period, and well after the coldest satellite‐based cloud top temperature (−62°C) and the maximum total lightning flash rate (11 min−1). The sprite‐producing positive cloud‐to‐ground (SP + CG) strokes exhibit peak current from 14 to 247 kA, Charge Moment Changes (CMC) from 625 to 3086 C km, and Impulsive CMC (iCMC) between 242 and 1525 C km. The +CG flashes that do not trigger sprites are initiated outside the main convective core, have much lower CMC values, and in average, shorter durations, lower peak currents, and shorter distances of propagation. The CMC appears to be the best sprite predictor. The delay between the parent stroke and the sprite allows classifying the events as short delayed (<20 ms) and long delayed (>20 ms). All long‐delayed sprites, i.e., most of the time carrot sprites, are produced by SP + CG strokes with low iCMC values. All SP + CG flashes initiate close to the convective core and generate leaders in opposite directions. Negative leaders finally propagate toward lower altitudes, within the stratiform region that coincides with the projected location of the sprite elements.
Key Points
The sprite‐producing lightning flashes have a long propagation within the stratiform region
The CMC of the stroke is confirmed as a good predictor of the sprite production
Long‐delayed sprites are associated with current moment waveforms of low amplitude and long duration
By using a combination of radio frequency time-of-arrival and interferometer measurements, we observed a sequence of lightning and electrical activity during one of Mount St. Augustine's eruptions. ...The observations indicate that the electrical activity had two modes or phases. First, there was an explosive phase in which the ejecta from the explosion appeared to be highly charged upon exiting the volcano, resulting in numerous apparently disorganized discharges and some simple lightning. The net charge exiting the volcano appears to have been positive. The second phase, which followed the most energetic explosion, produced conventional-type discharges that occurred within plume. Although the plume cloud was undoubtedly charged as a result of the explosion itself, the fact that the lightning onset was delayed and continued after and well downwind of the eruption indicates that in situ charging of some kind was occurring, presumably similar in some respects to that which occurs in normal thunderstorms.
An intracloud lightning flash in central New Mexico began with the initiation of a negative stepped leader at an altitude of 8.2 km above sea level. As this leader propagated eastward and upward, at ...9.1 km above sea level it passed about 200 m to the north of a balloon‐borne, electric field‐change instrument (Esonde). After the first leader stopped, a second negative stepped leader began near the point of origin of the first leader, but it propagated away from the Esonde. From the changes in the electric vectors and the locations of impulsive radio frequency sources detected by a lightning‐mapping array (LMA), we conclude the following: (1) The first negative stepped leader was not preceded by any significant charge rearrangement due to positive leaders. (2) Each step of the first negative leader had both a forward‐going wave and a step recoil wave that propagated simultaneously backwards away from the leader tip along the existing channel. The presence of a step recoil wave during each step leads to an explanation for the existence of stepping. (3) After the first (nearby) leader stopped, step recoil waves from the second (distant) leader may have found their way onto the channel formed by the first leader. (4) After the second leader stopped, waves carrying negative charge propagated along the channel of the first leader, producing strong K changes in the electric field at the Esonde and providing a good record of the wavefront shapes.
Key Points
Negative lightning leaders generate recoil waves
We characterize the geometrical and electrical characteristics of the initial stages of nine Florida triggered lightning discharges using a Lightning Mapping Array (LMA) and measured channel‐base ...currents. We determine initial channel and subsequent branch lengths, average initial channel and branch propagation speeds, and channel‐base current at the time of each branch initiation. The channel‐base current is found to not change significantly when branching occurs, an unexpected result. The initial stage of Florida triggered lightning typically transitions from vertical to horizontal propagation at altitudes of 3–6 km, near the typical freezing level of 4 km and several kilometers below the expected center of the negative cloud‐charge region at 7–8 km. The data presented potentially provide information on thunderstorm electrical and hydrometeor structure and discharge propagation physics. LMA source locations were obtained from VHF sources of positive impulsive currents as small as 10 A, in contrast to expectations found in the literature.
Key Points
First Lightning Mapping Array VHF images of Florida triggered lightning
Primary negative charge source for FL triggered lightning may be at 3‐6 km
VHF sources obtained from positive impulsive currents less than 10 A
A GPS‐based system has been developed that accurately locates the sources of VHF radiation from lightning discharges in three spatial dimensions and time. The observations are found to reflect the ...basic charge structure of electrified storms. Observations have also been obtained of a distinct type of energetic discharge referred to as positive bipolar breakdown, recently identified as the source of trans‐ionospheric pulse pairs (TIPPs) observed by satellites from space. The bipolar breakdown has been confirmed to occur between the main negative and upper positive charge regions of a storm and found to be the initial event of otherwise normal intracloud discharges. The latter is contrary to previous findings that the breakdown appeared to be temporally isolated from other lightning in a storm. Peak VHF radiation from the energetic discharges is observed to be typically 30 dB stronger than that from other lightning processes and to correspond to source power in excess of 100 kW over a 6 MHz bandwidth centered at 63 MHz.
Angus x Hereford heifers (15 mo and artificially inseminated to a single sire) were used to evaluate the effect of prenatal nutritional restriction on postnatal growth and development. At d 32 of ...gestation, dams were stratified by BW and BCS and allotted to a low-nutrition 55% of NRC (1996) requirements, n = 10 or moderate-nutrition 100% of NRC (1996) requirements, n = 10 diet. After 83 d of feeding, dams were commingled and received a diet in excess of requirements. Dams were allowed to calve naturally, and birth weights and growth of calves were recorded. Bulls were castrated at birth. Steers (16 mo of age, 5 per treatment) received a high-concentrate diet ad libitum to a constant age (88 ± 1 wk). Steers were slaughtered and weights of the empty body and organs were recorded. Samples of organs, muscle (complexus), and perirenal and subcutaneous adipose tissue were stored at -80°C, and then DNA and protein concentrations were quantified and expression of genes associated with fatty acid metabolism and glucose uptake were measured in adipose and muscle tissue. Dams had similar (P > 0.33) BW and BCS at the beginning of the experiment. At the end of restriction, dams on the low-nutrition diet weighed less (P less-than or equal to 0.01) and had less BCS (P < 0.001) than those on the moderate-nutrition diet. Length of gestation was 274 ± 2 d for dams in the low-nutrition treatment and 278 ± 2 d (P = 0.05) for dams in the moderate-nutrition treatment. Nutrient restriction during gestation did not influence birth weight or postnatal growth of calves. Lungs and trachea of steers whose dams were fed the low-nutrition diet weighed less (P = 0.05) at slaughter than those of steers whose dams were fed the moderate-nutrition diet; weights of other organs were not influenced by treatment. Complexus muscle from steers whose dams were fed the low-nutrition diet had a greater (P = 0.04) concentration of DNA and larger muscle fiber area compared with steers whose dams were fed the moderate-nutrition diet. Abundance of mRNA for fatty acid binding protein 4, fatty acid translocase, and glucose transporter 4 was less in perirenal adipose tissue of steers whose dams were fed the low-nutrition diet compared with those whose dams were fed the moderate-nutrition diet. Nutritional restriction of dams during early gestation did not alter postnatal calf growth. However, concentrations of DNA in muscle tissue and muscle fiber area were greater in steers from dams exposed to restricted nutrient intake during early gestation.
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