Xuan-Min Shao

A distinct class of isolated intracloud lightning discharges and their associated radio emissions

Observations of radio emissions from thunderstorms were made during the summer of 1996 using two arrays of sensors located in northern New Mexico. The first array consisted of three fast electric field change meters separated by distances of 30 to 230 km. The second array consisted of three broadband (3 to 30 MHz) HF data acquisition systems separated by distances of 6 to 13 km. Differences in signal times of arrival at multiple stations were used to locate the sources of received signals. Relative times of arrival of signal reflections from the ionosphere and Earth were used to determine source heights. A distinct class of short-duration electric field change emissions was identified and characterized. The emissions have previously been termed narrow positive bipolar pulses (NPBPs). NPBPs were emitted from singular intracloud discharges that occurred in the most active regions of three thunderstorms located in New Mexico and west Texas. The discharges occurred at altitudes between 8 and 11 km above mean sea level. NEXRAD radar images show that the NPBP sources were located in close proximity to high reflectivity storm cores where reflectivity values were in excess of 40 dBZ. NPBP electric field change waveforms were isolated, bipolar, initially positive pulses with peak amplitudes comparable to those of return stroke field change waveforms. The mean FWHM (full width at half maximum) of initial NPBP field change pulses was 4.7 μs. The HF emissions associated with NPBPs were broadband noise-like radiation bursts with a mean duration of 2.8 μs and amplitudes 10 times larger than emissions from typical intracloud and cloud-to-ground lightning processes. Calculations indicate that the events represent a distinct class of singular, isolated lightning discharges that have limited spatial extents of 300 to 1000 m and occur in high electric field regions. The unique radio emissions produced by these discharges, in combination with their unprecedented physical characteristics, clearly distinguish the events from other types of previously observed thunderstorm electrical processes.

Radio interferometric observations of cloud-to-ground lightning phenomena in Florida

Radio frequency observations of cloud-to-ground lightning in Florida have been analyzed to document a number of features of the lightning. The observations have been made using an interferometer system which determined the direction to the lightning radiation as a function of time during close lightning discharges. Observations are presented for about 50 radiating events during five multiple-stroke, normal-polarity flashes to ground. The results confirm and extend results of a similar study of New Mexico lightning by Rhodes et al. (1994). Dart leaders, in-cloud K events, and attempted leaders are found to be the same phenomena, namely, a negative-polarity streamer that propagates horizontally and/or vertically to ground at estimated speeds of about 10 7 ms −1 down to 10 6 ms −1 . The step or impulsive fast electric field change produced by some K events is found to be caused by positive breakdown back at the starting point of the negative streamer, after the streamer has progressed over some or all of its distance. This breakdown appears to initiate a forward stroke along the streamer channel which can renew the breakdown at the front end of the channel. K breakdown which happens to occur during a continuing current discharge to ground produces an M event (channel brightening) when it connects with the conducting channel to ground. The step fast field change of the M events is found to be produced at the time of connection inside the cloud ; no fast field change is detected when this breakdown reaches ground that would be associated with a return stroke. M events can also be initiated by fast (10 7 ms −1 ) positive streamers which often propagate away from the leader source region within a few milliseconds of the arrival of a return stroke back in the source region. These streamers often generate an even faster negative recoil event back along their extent which propagates down the channel to ground, as the M event. In all cases the current increase or channel brightening produced by K and M events is predicted to occur in a forward direction, that is, toward or down the channel to ground, in the same direction as the initiating negative streamer. Positive breakdown is detected only immediately following return strokes or during some K streamers. In both cases the electric stresses are expected to be very large, suggesting that positive breakdown which produces a conducting channel is difficult to initiate. Results for one flash suggest that negative charge was left at low altitude by the initial stroke to ground ; this forced the subsequent leaders to develop a new path to ground, first as series of attempted leaders and then as a dart-stepped leader.

Observations of lightning phenomena using radio interferometry

A radio interferometer system is described which utilizes multiple baselines to determine the direction of lightning radiation sources with an angular resolution of a few degrees and with microsecond time resolution. An interactive graphics analysis procedure is used to remove fringe ambiguities from the data and to reveal the structure and development of lightning discharges inside the storm. Radiation source directions and electric field waveforms have been analyzed for different types of breakdown events for two lightning flashes. These include the initial breakdown and K type events of in-cloud activity, the leaders of initial and subsequent strokes to ground, and activity during and following return strokes. Radiation during the initial breakdown of one flash was found to consist of intermittent, localized bursts of radiation that were slow moving. Source motion within a given burst was unresolved by the interferometer but was detected from burst to burst, with negative charge being transported in the direction of the breakdown progression. Radiation during initial leaders to ground was similar but more intense and continuous and had a characteristic intensity waveform. Radiation from in-cloud K type events is essentially the same as for dart leaders; in both cases it is produced at the leading edge of a fast-moving negative streamer that propagates along a well-defined, often extensive, path. K type events are sometimes terminated by a fast field change that appears analogous to the field change of a return stroke. Dart leaders are sometimes observed to die out before reaching ground; these are termed “attempted leaders” and, except for their greater extent, are no different than K type events. Several modes of breakdown during and after return strokes have been documented and analyzed. One mode corresponds to the launching of a positive streamer away from the upper end of the leader channel, apparently as the return stroke reaches the leader start point. In another mode, the quenching of the dart leader radiation upon reaching ground reveals concurrent breakdown in the vicinity of the source region for the leader. In both instances the breakdown appears to establish channel extensions or branches that are followed by later activity of the flash. Finally, a new type of breakdown event has been identified whose electric field change and source development resemble those of an initial negative leader but which progresses horizontally through the storm. An example is shown which spawned a dart leader to ground.

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.

Observations of narrow bipolar events reveal how lightning is initiated in thunderstorms

A long-standing but fundamental question in lightning studies concerns how lightning is initiated inside storms, given the absence of physical conductors. The issue has revolved around the question of whether the discharges are initiated solely by conventional dielectric breakdown or involve relativistic runaway electron processes. Here we report observations of a relatively unknown type of discharge, called fast positive breakdown, that is the cause of high-power discharges known as narrow bipolar events. The breakdown is found to have a wide range of strengths and is the initiating event of numerous lightning discharges. It appears to be purely dielectric in nature and to consist of a system of positive streamers in a locally intense electric field region. It initiates negative breakdown at the starting location of the streamers, which leads to the ensuing flash. The observations show that many or possibly all lightning flashes are initiated by fast positive breakdown.

RF radiation observations of positive cloud‐to‐ground flashes

During the summers of 1995 and 1996 we conducted broadband HF-UHF and narrowband VHF radio frequency (RF) observations of positive cloud-to-ground (+CG) flashes at Langmuir and Los Alamos laboratories, New Mexico. These observations indicate that positive leaders to ground produce no or very weak radiation from HF to UHF. The broadband system was able to record 2 ms data each time it was triggered. For a +CG the system was usually triggered by the return stroke, and a 1 ms pretrigger period was coincident with the positive leader process. It was commonly observed that no or little radiation was associated with the leader process in the 1 ms pretrigger period. The narrowband VHF system employed a logarithmic power amplifier and recorded one 1 s data each time it was triggered. The narrowband observations show that strong and often continuous radiation occurs at the beginning of the +CGs, but the radiation usually becomes intermittent and impulsive during the last few tens of milliseconds preceding the return strokes. The observations for most of the +CGs also show complete lack of radiation a few ms before the beginning of the return strokes, suggesting that the ongoing downward positive leaders were quiet at VHF, at least during the final few ms. The results of this study for natural positive leaders are in agreement with the results obtained from laboratory gap discharges and rocket-triggered lightning.

Reduction of electron density in the night-time lower ionosphere in response to a thunderstorm

Tropospheric thunderstorms have been reported to disturb the lower ionosphere, at altitudes of 65–90 km. The use of lightning signals from a distant mesoscale storm to probe the lower ionosphere above a small tropospheric thunderstorm reveals a reduction in ionospheric electron density in response to lightning discharges in the small storm. Tropospheric thunderstorms have been reported to disturb the lower ionosphere, at altitudes of 65–90 km, by convective atmospheric gravity waves1,2,3,4,5 and by electric field changes produced by lightning discharges6,7,8,9,10,11,12,13,14,15. Theoretical simulations suggest that lightning electric fields enhance electron attachment to O2 and reduce electron density in the lower ionosphere7,8. Owing to the low electron density in the lower ionosphere, active probing of its electron distribution is difficult16,17, and the various perturbative effects are poorly understood. However, it is now possible to probe the lower ionosphere in a spatially and temporally resolved manner by using remotely detected time waveforms of lightning radio signals4,5,18,19. Here we report such observations of the night-time ionosphere above a small thunderstorm. We find that electron density in the lower ionosphere decreased in response to lightning discharges. The extent of the reduction is closely related in time and space to the rate of lightning discharges, supporting the idea that the enhanced electron attachment is responsible for the reduction. We conclude that ionospheric electron density variations corresponding to lightning discharges should be considered in future simulations of the ionosphere and the initiation of sprite discharges.

Broad band radio interferometry for lightning observations

A broad band radio interferometer for locating lightning emissions has been designed, constructed and tested. For a broad band interferometer, a single fixed pair of antennas is equivalent to having many baselines in a narrow band interferometer. So, a broad band system requires fewer antennas than a narrow band system to achieve equivalent angular resolution. In addition, frequency dependent locations of the radio emissions can be extracted for a more detailed look at the lightning breakdown processes. Such a system has been tested by a computer simulation and by measuring a man-made broad band radiation source. The system consists of two antennas which are separated vertically. Bandwidth of the system is from 40 to 350 MHz. Measurements of the man-made source indicate that with a signal-to-noise ratio (SNR) above 10 dB, the system is able to locate the source with an accuracy of about 2° over the detectable frequencies. Preliminary observations of lightning discharges with this new technique appear to indicate that breakdown processes of a dart leader to ground radiate solely at its descending tip.

Katrina and Rita were lit up with lightning

Hurricanes generally produce very little lightning activity compared to other noncyclonic storms, and lightning is especially sparse in the eye wall and inner regions within tens of kilometers surrounding the eye [Molinari et al., 1994, 1999]. (The eye wall is the wall of clouds that encircles the eye of the hurricane.) Lightning can sometimes be detected in the outer, spiral rainbands, but the lightning occurrence rate varies significantly from hurricane to hurricane as well as within an individual hurricanes lifetime. Hurricanes Katrina and Rita hit the U.S. Gulf coasts of Louisiana, Mississippi, and Texas, and their distinctions were not just limited to their tremendous intensity and damage caused. They also differed from typical hurricanes in their lightning production rate.

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

AbstractLightning 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...