Negative Ground Flashes Research Articles

Analysis of lightning activity characteristics and its relationship with precipitation in a strong convective weather process in Jiangsu Province

Based on multi-source observation data such as lightning locator, atmospheric electric field instrument and hourly precipitation data from automatic stations, the lightning characteristics of the strong convective weather process under the influence of the northeast cold vortex in Nanjing Jiangsu Province on 15 June 2020 was analyzed in depth, and the relationship between the lightning activity and the precipitation was obtained. The results show that this strong convective weather process is formed by the cold air from the back of the northeast cold vortex along the front of the high-pressure ridge and the back of the transverse through southward to the middle and lower reaches of the Yangtze River, which makes the cold and warm air currents converge along the south of the Yangtze River to form the heavy precipitation, and is accompanied by the strong lightning activity in the process of this heavy precipitation. The local thunderstorm electric field in Nanjing is mainly characterized by negative increasing type, positive and negative alternating type, and multi-single thunderstorm electric field, and the thunderstorm activity is mainly dominated by negative ground flashes. During this strong thunderstorm, the stronger precipitation periods corresponded to the stronger lightning frequency periods, and the lightning and precipitation fallout areas showed a consistent spatial distribution. The correlation between the lightning activity and precipitation was analyzed by linear fitting, and the correlation between them is 0.946.

Correlation analysis of preliminary breakdown and first return stroke process in negative ground flash as a function of Northeast and Southwest monsoon in tropic

The separation duration of Preliminary Breakdown to the First Return Stroke (PB-RS duration) and the ratio between the highest amplitude pulse of Preliminary Breakdown and the first Return Stroke pulse (PB/RS ratio) were successfully used as a tool by many researchers to determine the relationship between lightning ground flashes and physical changes (latitude, regional and thunderstorm). Malaysia is one of the countries with the highest lightning flash density globally, and the occurrences of ground flashes are mainly due to monsoon seasons. Monsoons in Malaysia are Northeast, Southwest, and two transition periods. However, there is no available information to relate the lightning activities in Malaysia under the influence of all monsoon seasons. The above statements motivated our team to present a new statistical analysis of lightning flashes under two seasons' effects (Northeast and Southwest). 83 out of 332 electric radiation field data (fast fields) from six thunderstorms were selected and analyzed. The data was recorded using a 12-bit high-speed transient recorder (HDO 4024) with a resolution of tens of nanoseconds and sampling of 12.5 MS/s. PB/RS ratio between the monsoons is similar to each other. Whereas, the PB-RS separation, has two factors longer during the Southwest monsoon than during the Northeast monsoon. We classified the PB/RS ratio into four categories (>100%, 50-100%, 20-50%, and < 20%) as an indication of the preliminary breakdown strength. It found that the weakest pulse train (PB/RS ratio is less than 20%) has shown a longer duration of PB-RS and produces higher lightning return strokes in a negative ground flash than the strong one. The results in this study could be evident that the lower strength of preliminary breakdown is correlated to a lower Low Positive Charge Region (LPCR), which means the initial breakdown process only requires low energy to break the LPCR. Hence, these features have shown the highest number of stroke counts compared to pronounced ones. However, it would be more remarkable if one could provide bulky data for this specific study.

Distribution of attempted leader with monsoon seasons and negative cloud-to-ground flashes in Melaka, Malaysia

<p>Ninety (90) waveforms recognized as attempted leader were identified with both positive (84 events) and negative (6 events) initial polarity observed from four consecutive years of data (N=10,206). The positive attempted leader shows no correlation with the number of thunderstorms producing it during monsoon. Meanwhile, the negative attempted leader during monsoon and both polarity of attempted leader (positive and negative) during inter-monsoon shows positive correlation with the number of thunderstorms producing it. In this study, the yearly statistical distribution of negative cloud-to-ground (CG) lightning flashes which were classified as positive preliminary breakdown pulses (214 events) and negative preliminary breakdown pulses (4982 events) in accordance of their preliminary polarity were also presented. In addition, there is no relationship of attempted leader and the initial breakdown of negative ground flash since both mechanisms performed as a negative correlation.</p>

Latitude and Topographical Dependence of Lightning Return Stroke Peak Current in Natural and Tower-Initiated Negative Ground Flashes

Experimental data show that the peak currents of first and subsequent lightning return strokes in negative ground flashes increase with decreasing latitude. In this paper, the reason for this dependence of peak return stroke current on latitude is explained using the fact that the height of the charge centers increases with decreasing latitude. Results show that in tropical regions where the height to the negative charge center is about 8 km, the median values of the first and the subsequent return stroke peak currents are about 42 kA and 15 kA, respectively. If the height to the charge center is larger than 8 km, the peak currents will also become larger. For example, if the location of the charge center is increased to about 9 km, the median values of the first and subsequent return stroke peak currents will increase to about 45 kA and 16 kA respectively. The same reasoning shows that, even in the same geographical region, the peak return stroke current may decrease as the elevation of the ground where the lightning strikes take place increases. The results also indicate that the peak subsequent return stroke current in tower-initiated negative lightning flashes decreases as the height of the tower increases. These theoretical predictions are in general agreement with the available experimental data.

Classification of VLF/LF Lightning Signals Using Sensors and Deep Learning Methods.

Lightning waveform plays an important role in lightning observation, location, and lightning disaster investigation. Based on a large amount of lightning waveform data provided by existing real-time very low frequency/low frequency (VLF/LF) lightning waveform acquisition equipment, an automatic and accurate lightning waveform classification method becomes extremely important. With the widespread application of deep learning in image and speech recognition, it becomes possible to use deep learning to classify lightning waveforms. In this study, 50,000 lightning waveform samples were collected. The data was divided into the following categories: positive cloud ground flash, negative cloud ground flash, cloud ground flash with ionosphere reflection signal, positive narrow bipolar event, negative narrow bipolar event, positive pre-breakdown process, negative pre-breakdown process, continuous multi-pulse cloud flash, bipolar pulse, skywave. A multi-layer one-dimensional convolutional neural network (1D-CNN) was designed to automatically extract VLF/LF lightning waveform features and distinguish lightning waveforms. The model achieved an overall accuracy of 99.11% in the lightning dataset and overall accuracy of 97.55% in a thunderstorm process. Considering its excellent performance, this model could be used in lightning sensors to assist in lightning monitoring and positioning.

Variation of Atmospheric Temperature with Height in the Phenomena of Lightning Waveforms

The temperature at different height at different places is different, it may depend on various factors. The variation of atmospheric temperature varies as a function of height taken from the sea level. The temperature decreases with height in the troposphere. The location of the charge centre appears to be determined by temperature and not by the height of the cloud above the ground. The main negative charge centre is generally located between the -10°C and -25°C isotherms with the main positive charges some km higher up. Due to the variation of temperature, the ice crystal, super cooled water droplet etc. move updraft and downdraft, forms the charge in the cloud due to which the lightning phenomena occurs. In the cloud, the negative charge lies in the central part of the cloud, positive charge remains on above the negative charge centre in the cloud and small amount of positive charge as a pocket charge remains on the bottom part of the cloud. The positive ground flashes are initiated from the upper positive charge region and the negative ground flashes are initiated from the negative charge region in the thunder-cloud. The lower positive charge also play an important role for determining the different types of lightning.

Lightning Characteristics and Electric Charge Structure of a Hail-Producing Thunderstorm on the Eastern Qinghai–Tibetan Plateau

An isolated hail-producing thunderstorm that occurred on 20 August 2012, from the Composite Observing Experiment for Lightning project carried out in the Da Tong Region of Qinghai Province, China, was studied. In most stages, the charge structure in the storm showed a positive tripole structure, which resulted in a low positive ground lightning rate for the whole event. In the thunderstorm process, 202 lightning flashes were detected. The peak lightning rate of lightning was 28 times/5 min. The average number of return strokes of negative ground flash was 2.4, and only one return stroke of positive ground lightning occurred. The duration of the in-cloud process prior to the first return stroke was relatively long. Dynamic transport had a considerable influence on the charge structure of the cloud. Before and after hail landing, the ground electric field of several stations showed a consistent trend of reversal.

A three-dimensional downward leader model incorporating geometric and physical characteristics

In this paper, a three-dimensional, stochastic model of a lightning downward leader is proposed. The background electric field in which the downward leader propagates is used to determine the direction of propagation. The critical background electric field for the development of a downward leader was obtained by extrapolating the results of negative long air gap discharge experiments. The distribution of charge in the downward leader was modelled with the charge simulation method. The calculation took into account branching of the leader channel. By choosing a suitable value of the probability exponent, the simulation results of fractal dimension and charge in the downward leader are shown to closely match observational data of lightning. Finally, the model was used to reproduce a negative ground flash. The ground electric field calculated by the model was found to agree well with measurement results from the field.

Unusual lightning electric field waveforms observed in Kathmandu, Nepal, and Uppsala, Sweden

Unusual lightning events have been observed in Uppsala, Sweden, and Kathmandu, Nepal, using essentially the same electric field measuring system developed at Uppsala University. They occurred in the storms that also generated “normal” lightning events. The unusual events recorded in Uppsala occurred on one thunderstorm day. Similar events were observed in Kathmandu on multiple thunderstorm days. The unusual events were analyzed in this study assuming them to be positive ground flashes (+CGs), although we cannot rule out the possibility that some or most of them were actually cloud discharges (ICs). The unusual events were each characterized by a relatively slow, negative (atmospheric electricity sign convention) electric field waveform preceded by a pronounced opposite-polarity pulse whose duration was some tens of microseconds. To the best of our knowledge, such unusual events have not been reported in the literature. The average amplitudes of the opposite-polarity pulses with respect to those of the following main waveform were found to be about 33% in Uppsala (N = 31) and about 38% in Kathmandu (N = 327). The average durations of the main waveform and the preceding opposite-polarity pulse in Uppsala were 8.24 ms and 57.1 μs, respectively, and their counterparts in Kathmandu were 421 μs and 39.7 μs. Electric field waveforms characteristic of negative ground flashes (-CGs) were also observed, and none of them exhibited an opposite-polarity pulse prior to the main waveform. Possible origins of the unusual field waveforms are discussed.

Electrification and Lightning in Idealized Simulations of a Hurricane-Like Vortex Subject to Wind Shear and Sea Surface Temperature Cooling

Abstract Relationships between intensity fluctuations, cloud microphysics, lightning variations, and electrical structures within idealized tropical cyclones are investigated with the cloud-resolving Collaborative Model for Multiscale Atmospheric Simulation (COMMAS). An initial strong tropical cyclone is subjected to either steady-state control conditions (CTRL), increased wind shear (SHEAR), or a reduction in sea surface temperature (SST). In CTRL, nearly all the lightning (>95%) occurred in the outer region (100 < r ≤ 300 km) and was overall very episodic in the inner core (r ≤ 100 km), consistent with observations. The inner-core updrafts were weaker and experienced greater depletion of cloud water by warm rain processes, which, in contrast to the deeper updrafts in the rainband convection, reduced the mixed-phase cloud depth and confined the bulk of the charging and lightning initiations to lower levels. Notably, larger flash rates were produced in the asymmetric inner core of the SHEAR case, with the majority of the flashes located in the downshear left quadrant, consistent with prior observational works. In contrast to CTRL, the more vigorous inner-core convection in SHEAR resulted in the formation of a prominent negative charge region and enhanced production of negative ground flashes. With a nearly identical filling rate as SHEAR, the introduction of cooler sea surface temperature in the SST case caused lightning activity to fade rapidly in both the inner core and rainbands.

Electric Field Signatures in Wideband, 3 MHz and 30 MHz of Negative Ground Flashes Pertinent to Swedish Thunderstorms

In this study, the electric field signatures of negative ground flashes pertinent to the Swedish thunderstorms were recorded simultaneously during the summer of 2014 using wide (up to 100 MHz) and narrow (at 3 MHz and 30 MHz as central frequencies) bandwidth antenna systems. The electric field signatures were recorded for a time duration of 250 ms. In the analysis, the whole flash was considered and a total of 98 flashes were chosen where electric field signatures of all wideband, 3 MHz and 30 MHz signals were present. It is observed that preliminary breakdown pulses are stronger radiators at 3 and 30 MHz compared to the return strokes. A comparison of our results with those of the previous studies obtained from different geographical regions clearly shows that the strength of preliminary breakdown pulses is higher in the temperate region (Sweden for instance) and is a function of latitude.

Analyzing a Heavy Rainfall Process Based on Dual-Doppler Radar and Lightning Data

In order tofurther understand the application of Doppler radar data in mesoscale weather system as well as thedynamical structureof precipitationsystem,a large-scaledand more accurate inversion resultis aimed to get,basing on the new ly-proposed fuzzy logical algorithm of cloud classification and the two-step variational method which is applied to dual-Doppler radar. The data come from Dual-Doppler Radar' s(S-band) observation in Hefei and Fuyang,Anhui province. A heavy rainfall processoccurred in Anhui Province on 7 ~ 10 June in 2010 is analyzed by utilizing vertical accumulative total liquid water content(VIL),rainfalland distribution of lightning. The results show that this persistentrainfall is associated with the wind fieldsw hichcantains a convergence in low er layers and a divergence in higher layers,a cyclonic wind rotation and a wind shear line. This rainfall process which is based on stratiformis show n to be a hybridcumuluscloudprecipitation. Besides,the stratiform lightning is few during this rainfall and mainly focuses on the negative ground flashes. The heavy rain occurred20 minutes or so after lightning. The value of VIL in this heavy rainfall is small,which is less than 3 kg ·m-2.There are strong relationships among cloudclassification,the area of rain,VIL and Radar's echo.

Lightning-initiator type of narrow bipolar events and their subsequent pulse trains

Previous observations show that some narrow bipolar events (NBEs) can initiate intracloud discharges, but the role of NBE as lightning initiation is still unclear. During the summer of 2013, 827 NBEs were detected with a 3-D LF lightning location system in Osaka, Japan. Out of 638 positive NBEs, 103 occurred as the initial events of lightning flashes. These initiator-type NBEs, called “INBEs” in this paper, are always followed by positive pulse trains whose locations show upward propagations probably from the main negative charge region to the upper positive charge region. Most of INBEs develop into intracloud flashes. Only two INBEs develop into positive ground flashes and five INBEs develop into negative ground flashes. Pulse widths and peak amplitudes of electric field change waveforms of INBEs are almost the same as those of normal NBEs. A major difference is that INBEs have much lower discharge heights. Most of INBEs are lower than 10 km while normal NBEs are mainly higher than 10 km. Characteristics of positive pulse trains following INBEs are closely related with discharge heights of INBEs. Higher INBEs are usually followed by weaker, fewer, and less frequent positive pulses with slower upward propagations. As the height increases to above 10 km, NBEs are usually no longer followed by such positive pulses.

Analyzing the transmission structures of long continuing current processes from negative ground flashes on the Qinghai‐Tibetan Plateau

AbstractThis paper presents a new overlap and progressive method using either a one‐point charge model or a point dipole model for studying the electric field change data of lightning and tests the suitability of the proposed method by analyzing the slow electric field change of the long continuing current data from two negative ground flashes. The current measurements of the ground flashes are from the Composite Observing Experiment for Lightning project carried out in the Datong region of Qinghai Province, China, in 2009, obtained from field observations synchronized through high‐precision GPS clocks at seven observation stations. Analysis shows that the time‐averaged current can reach 800 A, and the current intensity relates to the height of the top of the return stroke before the continuing current process. The two current processes studied in this work neutralized a large amount of negative charge, 39.5C and 60.8C, respectively. The neutralized charges from the negative charge layer are typically at 2.5–4.7 km above ground. The intracloud transition of the positive leader is a complex process, and the horizontal propagation of the positive leader plays an important role in current duration as well as horizontally influencing the center of the electrical dipole moment with an obvious change of up to 3.0 km. The proposed analysis method is very useful compared to previous methods in terms of determining infinitesimal changes in long continuing currents. This work's new analysis method will help increase understanding of the fine physical processes of long continuing current processes.

Negative cloud-to-ground lightning flashes in Malaysia

The characteristics of the negative cloud-to-ground lightning flashes in Malaysia are studied by analyzing the electric fields generated by the whole flash in nanosecond resolution. A total of 405 strokes obtained from 100 successive negative cloud-to-ground lightning flashes were analyzed, which were recorded from seven convective thunderstorms during the southwest monsoon period, i.e. from April to June 2009. It was found that the total number of interstroke intervals has an arithmetic mean value of 86ms, a geometric mean value of 67ms and does not depend on the return stroke order. Of the 100 negative ground flashes, 38 flashes (38%) have at least one subsequent return-stroke (SRS) whose electric field peak was greater than that of the first return-stroke (RS). Furthermore, 58 (19%) out of 305 SRS have electric field peak larger than those of the first RS. The arithmetic and geometric mean ratio between the peak electric field of the SRS and the peak electric field of the first RS are 0.7 and 0.6, respectively. The percentage of single-stroke flashes was 16% while the mean number of strokes per flash and maximum number of stroke per flash were 4 and 14, respectively.

Fine Structure Signatures in Ground Flashes as a Source for HF Radiation

Lighting radiation fields below 10 MHz are of considerable interest since these frequencies correspond to the natural resonance of structures with dimensions of a few meters to tens of meters. In this paper we present the fine structure signatures of sub-microsecond range pulses appeared at the leader phase and after the return stroke in negative ground flashes which act as a source for HF radiations at 10 MHz, 5 MHz and 3 MHz observed in Sri Lanka, in the tropics. Of the total sub-microsecond range pulses analyzed, 298 were due to positive field changes and 228 were due to negative field changes. The average rise time of those pulses for both polarities is 127 ns and it was found to be varying from 110-160 ns. The peak amplitude is in the range of 0.65-2.19 V/m. For the total 526 pulses analyzed for this study, the FWHM was between 190-310 ns with an arithmetic mean of 238 ns. Signatures of these pulses are similar to the leader like electric field pulses which acted as a strong source for HF radiations at 10 MHz, 5 MHz and 3 MHz. The initiation process of pulses reported in this study could be similar to the initiation process of leader like pulses.

Fine Structure Signatures in Ground Flashes as a Source for HF Radiation

Lighting radiation fields below 10 MHz are of considerable interest since these frequencies correspond to the natural resonance of structures with dimensions of a few meters to tens of meters. In this paper we present the fine structure signatures of sub-microsecond range pulses appeared at the leader phase and after the return stroke in negative ground flashes which act as a source for HF radiations at 10 MHz, 5 MHz and 3 MHz observed in Sri Lanka, in the tropics. Of the total sub-microsecond range pulses analyzed, 298 were due to positive field changes and 228 were due to negative field changes. The average rise time of those pulses for both polarities is 127 ns and it was found to be varying from 110-160 ns. The peak amplitude is in the range of 0.65-2.19 V/m. For the total 526 pulses analyzed for this study, the FWHM was between 190-310 ns with an arithmetic mean of 238 ns. Signatures of these pulses are similar to the leader like electric field pulses which acted as a strong source for HF radiations at 10 MHz, 5 MHz and 3 MHz. The initiation process of pulses reported in this study could be similar to the initiation process of leader like pulses.

Stepped‐to‐dart leaders preceding lightning return strokes

AbstractUsing time‐correlated high‐speed video and electric field change data, three cases are described in which dart leaders toward ground are immediately preceded by stepped leaders that find and use previously used channels. These “stepped‐to‐dart leaders” occur in three natural negative ground flashes. Prior to the stepped‐to‐dart connection, the leaders have characteristics of stepped leaders, including average two‐dimensional speeds of 1.6–2.7 × 105 m s−1 (visible from 5.5, 3.4, and 0.9 km altitude). After the connection, they behave as dart (or dart‐stepped) leaders, with larger amplitude E‐change pulses and faster average speeds of 3.4–7.8 × 106 m s−1. Connection altitudes are 3.32, 1.57, and 0.75 km. Immediately after the connection, there is a brief lighting in a short part of the prior return stroke channel. Luminosity travels up the stepped leader path after the connection, while the dart leader proceeds toward ground. In two cases, all the strokes subsequent to the stepped‐to‐dart stroke follow the visible portion of its channel. The other case has two subsequent strokes which do not reuse any portion of the stepped‐to‐dart channel. For the other 12 strokes in these flashes, stepped leader average speeds range from 1.7 to 3.0 × 105 m s−1, and dart leader average speeds are 0.82 to 16.67 × 106 m s−1. Overall, the return stroke waveforms give reasonable indication of the type of leader that preceded the stroke. Stepped‐to‐dart leaders are apparently rare in optical data, occurring in about 1% of subsequent strokes and 2.5% of flashes.

Measured current and close electric field changes associated with the initiation of upward lightning from a tall tower

We examine in detail the simultaneous lightning current waveforms, close electric field changes, and lightning location system data for upward lightning discharges initiated from the Gaisberg Tower (GBT) from 2005 to 2009. Out of 205 upward flashes, most of them (87% or 179/205) were initiated from the tower top without any nearby preceding lightning activity (called “self‐initiated”), whereas 26 upward flashes (13%) were initiated from the tower top with immediately preceding nearby lightning activity (called “nearby‐lightning‐triggered”), including 15 positive ground flashes, one negative ground flashes, and 10 cloud discharges. The possible reasons for self‐initiated upward flashes dominating at the GBT could be the field enhancement due to the Gaisberg Mountain above the surrounding terrain and low altitude of charge region during non‐convective season (September to March), since we note that self‐initiated lightning at the GBT occurred predominantly (79% or 142/179) during non‐convective season. On the other hand the majority (85% or 22/26) of nearby‐lightning‐triggered upward flashes at the GBT occurring during convective season (April to August) and 80 nearby‐lightning‐triggered upward flashes out of 81 upward flashes observed at the ten tall towers in Rapid City in South Dakota of USA occurring during summer seasons, could be due to the result of high altitude of charge region. The triggering flashes were detected to be within 1 and 18 km distance and the time intervals between them and upward lightning initiation are in the range of 0.3 to 90.7 ms.

Occurrence of positive and negative polarity cloud-to-ground lightning flashes: case study of CGR4 and GPATS data for Brisbane, Australia

The occurrences of positive and negative polarity cloud-to-ground lightning flashes were detected by ground-based lightning flash counters CGR4 in Brisbane,Australia. Positive (negative) flashes constituted 2–5 per cent (95–98 percent) of total cloud-to-ground flashes. These results were compared with data obtained by a GPATS lightning location system over the period 2005–2008. It was found that prior to January 2007 the GPATS lightning location system was reporting an excessive proportion of ground flashes as being positive, with a correspondingly low proportion as negative. Over the same period, the CGR4 lightning flash counter reported positive to negative ground flash ratios consistent with those obtained by other researchers both in Australia and elsewhere using a range of instrumentation. The high proportion of positive ground flashes (up to 50 per cent) recorded by the GPATS lightning location system in Australia prior to 2007 may be attributed to one or more possible factors,including (i) changes in the processing of raw GPATS sensor data implemented in January 2007, (ii) upgrades to sensor firmware, and (iii) the installation of more GPATS sensors.