Positive CG Flashes Research Articles

Analysis of Lightning and Precipitation Activities in Three Severe Convective Events Based on Doppler Radar and Microwave Radiometer over the Central China Region

Hubei Province Region (HPR), located in Central China, is a concentrated area of severe convective weather. Three severe convective processes occurred in HPR were selected, namely 14–15 May 2015 (Case 1), 6–7 July 2013 (Case 2), and 11–12 September 2014 (Case 3). In order to investigate the differences between the three cases, the temporal and spatial distribution characteristics of cloud–ground lightning (CG) flashes and precipitation, the distribution of radar parameters, and the evolution of cloud environment characteristics (including water vapor (VD), liquid water content (LWC), relative humidity (RH), and temperature) were compared and analyzed by using the data of lightning locator, S-band Doppler radar, ground-based microwave radiometer (MWR), and automatic weather stations (AWS) in this study. The results showed that 80% of the CG flashes had an inverse correlation with the spatial distribution of heavy rainfall, 28.6% of positive CG (+CG) flashes occurred at the center of precipitation (>30 mm), and the percentage was higher than that of negative CG (−CG) flashes (13%). Moreover, the quantity of thunderstorm cells in Case 1 was more than other cases, the peak time of +CG flashes was prior to that of total CG flashes in Case 2 and Case 3, and the time of +CG flashes’ peak in Case 2 was prior to that of precipitation at about 2 h. Based on the analysis of the cloud environment, there are three main reasons for the differences of CG flashes and precipitation. Firstly, the structure of the LWC vertical profile and the height of the LWC peak are different, and high LWC makes it difficult for the collision of ice particles to generate electricity. Secondly, the differences between convective available potential energy (CAPE), precipitation, and CG flashes is caused by the sudden increase of VD from 1.5 km to 3 km, and thirdly, the production of CG flashes is very sensitive to RH at the surface layer and the total CG flashes increase as the RH increasing.

Expanding on the relationship between continuing current and in‐cloud leader growth

AbstractWhen lightning connects to the ground, there is a large surge of current, called the return stroke, which is occasionally followed by a longer‐lasting steady current, called continuing current (CC). In a previous study of negative cloud‐to‐ground (−CG) flashes, we observed the growth rate of in‐cloud positive leaders in an attempt to identify occurrences of CC. However, there was no observed change in positive leader growth rate during CC of negative CG flashes. In this study, we use the Langmuir Electric Field Array, Lightning Mapping Array, and Flash‐Continuous Broadband Digital Interferometer data to extend the previous study to the growth of the negative leader during positive CG flashes. We have found that in contrast with previous results, negative leader growth during positive CG flashes does show increases in growth rates coincident with CC. Finally, we find that the growth rate magnitudes for positive and negative leaders are typically ∼2–4 km/10 ms and ∼25–40 km/10 ms, respectively. These contrasting results highlight the differences between positive and negative leaders and provide strong evidence as to why −CC and +CC behave differently. Negative leaders inject higher amounts of current and allow the channel to remain conductive throughout the duration of CC. Whereas for positive leaders, the channel becomes nonconductive relatively quickly. It is therefore disconnected from the channel to the ground, and, due to the positive leader's continued growth, an electric potential is built up until a K event is produced that re‐ionizes the channel.

Upward lightning flashes characteristics from high‐speed videos

AbstractOne hundred high‐speed video recordings (72 cases in Brazil and 28 cases in USA) of negative upward lightning flashes were analyzed. All upward flashes were triggered by another discharge, most of them positive CG flashes. A negative leader passing over the tower(s) was frequently seen in the high‐speed video recordings before the initiation of the upward leader. One triggering component can sometimes initiate upward leader in several towers. Characteristics of leader branching, ICC pulses, recoil leader incidence, and interpulse interval are presented in this work. A comparison of the results is done for data obtained in Brazil and USA. The duration of ICC and the total flash duration are on average longer in Brazil than in USA. Only one fourth of all upward leaders are followed by any return strokes both in Brazil and USA, and the average number of return strokes following each upward leader is very low. The presence and duration of CC following return strokes in Brazil is more than two times larger than in USA. Several parameters of upward flashes were compared with similar ones from cloud‐to‐ground flashes.

Wavelet analysis of the first electric field pulse of lightning flashes in Sweden

In this paper, we investigate the wavelet characteristics of the first electric field pulse of four different flash types namely negative cloud-to-ground flash (−CG), positive cloud-to-ground flash (+CG), cloud flash (IC) and isolated breakdown flash (IB). A total of 110 fine waveforms out of 885 were selected which have been recorded during summer between May and August 2010 in Uppsala, Sweden. The measurement was done by using an electric field broadband antenna system with the time resolution of 20 Megasample per second. Evidence from our wavelet analysis suggests that the ionization process during the initial leader development (coincide with the detection of the first electric field pulse) of at least three flashes (−CG, IC, and IB) in Sweden was dictated directly by the existence and magnitude of localized free ions region and lower positive charge center (p region). In general, we found that both single peak and multiple peaks pulses of the negative CG and IC flashes radiated energy at higher frequencies and gain larger bandwidth when compared to positive CG and IB flashes. We suggest that the initial leader development of both IC and negative CG flashes underwent very rapid and much more extensive ionization process when compared to the positive CG and IB flashes. In specific comparison between negative CG and IC flashes, both temporal and wavelet analyses suggest that the first electric field pulses of IC flash radiated energy at higher frequency in both single spread categories and radiated energy at lower frequency in both multiple spread categories when compared to the negative CG flash. This finding may explain the observation of much slower and less bright type α leader (ionization process not so extensive) compared to much faster and very bright type β leader (ionization process very rapid and more extensive). As for positive CG and IB flashes, the single peak pulses of positive CG radiated energy at much higher frequency while the multiple peaks pulses radiated energy at much lower frequency than IB flash. We suggest that the initiation mechanism of positive CG was dictated directly by the background electric field between the main positive charge center and the ground, which is distinct from the initiation mechanism of IB flash. The presence of small magnitude of localized free ions region facilitated the rapidity of the ionization process in the initial leader development of IB flash.

Using lightning locating system based on time-of-arrival technique to study three-dimensional lightning discharge processes

A time-of-arrival (TOA) system based on GPS technology for locating VHF radiation sources from lightning has been developed and used in observation sites in the northern Shandong Province, China. The 3D images of the lightning progression have been obtained successfully for the first time in China. The 3D-channel evolutions of typical negative CG, positive CG and IC lightning flashes have been discussed together with the data of fast electric field change. It was found that significant differences existed between the negative and positive CG lightning flashes in terms of the initiation and propagation of the radiation sources. The preliminary breakdown of a negative CG lightning flash propagated at a speed about 5.2×104 m/s. The stepped leader of negative CG lightning flashes was trigged by negative initial breakdown. Thereafter, it propagated downward at a speed of 1.3×105 m/s. The initial process of the positive CG lightning flashes was also a propagation process of negative streamer. These streamers propagated dominantly horizontally in the positive charge region and accumulated positive charges at the origin of the lightning, and as a consequence, initiated downward positive streamers. A new type of lightning discharge that was triggered by a narrow bipolar pulse (NBP) is discussed in this study. The NBP was originated at altitude of about 10.5 km in the upper positive charge region. As a distinct difference from normal IC flash, its channels extended horizontally all around and produced a lot of radiation sources. The source power of the NBP could approach 16.7 kW, which is much greater than that of normal lightning discharge ranging between 100 mW and 500 W. The 3D propagation of this new type of lightning discharge was observed and obtained for the first time in China. The possible initiation mechanism of this new type of lightning is discussed here.

The lower positive charge center and its effect on lightning discharges on the Tibetan Plateau

Electrical characteristics of thunderstorms on the central Tibetan Plateau at an altitude of 4508 m have been studied. The evolution of surface electric (E) field and the E field changes produced by lightning flashes under a representative thunderstorm revealed a tripole charge structure with a larger‐than‐usual lower positive charge center (LPCC). The storms appear to begin with the lower dipole of a normal tripole structure, rather than with the upper dipole followed by the development of a weaker lower positive charge. The flash rate is quite low and the average value is usually 1 fl/min. The IC flashes were usually polarity‐inverted and occurred in the lower dipole. The large LPCC did not cause positive CG flashes to occur during the whole storm lifetime, and only negative CG flashes were observed in the late stage of the storm.

Cloud‐to‐ground lightning activity in hail‐bearing storms

The cloud‐to‐ground lightning flash (CG) produced by several apparent hailstorms are analyzed in different locations of southern Europe. The hail detection is performed via four different analysis techniques: radar reflectivity factor estimation, radar echo shape analysis, polarimetric hydrometeor identification, and reflectivity difference between radars with different wavelengths. The CG parameters considered are the flash rate and density, the flash polarity, the multiplicity, and the peak current. Some of the observed storms exhibit hail‐bearing cell features, while others exhibit heavy precipitation rates without any apparent hail production. One of the hailstorms can be classified as severe because reported hailstorm diameters reach 39 mm. The CG lightning characteristics of both types of storms are compared. The CG rates corresponding to all hail‐bearing storms are singularly lower than those of rain‐only storms, typically by a factor 5. So, the CG rate of the hail‐bearing storms considered does not exceed 2 min−1 while it can reach about 12 min−1 for heavy rain‐bearing storms. Moreover, some of them can produce especially high positive CG proportions associated with negative CGs with low values of peak current and multiplicity. This last observation suggests that the negative charge available for CG flashes is weak within the cloud when positive CG flashes are dominant. In order to interpret the low CG rates observed in the case of hail‐bearing storms several interpretations are discussed, but it would be necessary to know the intracloud flash activity in such cases of storms in order to enrich the discussion.

Some aspects of the correlation between lightning and rain activities in thunderstorms

The lightning activity of convective systems is a sometimes fruitful indicator of their precipitation production. The present study compares rain volume with different types of lightning activity in several convective systems. The study uses data obtained in the Paris area where two lightning detection systems coexist. The Météorage network provides the location and the polarity of cloud-to-ground flashes, while the SAFIR system detects the total flash activity: cloud-to-ground, intra-cloud flashes and VHF individual sources within a given time window. The overall spatial correlation between rain and lightning appears to be very consistent for all lightning types. A pixel-to-pixel study shows that positive CG flashes are associated with higher rainwater volume than negative flashes. Introducing a weight coefficient for positive CG flashes considerably improves the correlation between rain amount and lightning production. Taking into account the specific contribution of each type of lightning flash, the amount of rain can be estimated from the total electrical activity of each system. Comparison with the amount derived from radar measurement shows reasonable agreement. Finally, the parallel time evolutions of rain and lightning rates display quite similar characteristics.

VLF signatures of ionospheric perturbations caused by lightning discharges in an underlying and moving thunderstorm

A sequence of VLF events (first of early/fast type, then of LEP type), observed at Poitiers on the amplitude of the signal from the GQD/19.02 kHz transmitter in England, is analysed. VLF data, sampled at 100 ms intervals, are combined with sferics and METEORAGE data on lightning ground strikes detected within a ±50 km belt centered on the VLF path. Both the slowly and rapidly recovering early/fast events are observed in association with intense positive CG flashes, located in the Baie de Seine (L=2.25), at a distance less than 35 km from the VLF path. The LEP events are observed when the storm, initially overlapping the path, moves southeastwards. Some early/fast events are also observed on the very short path HWU/18.3 kHzPoitiers.

Observations of ‘Columniform’ sprites

This paper reports observations of a distinctive form of sprites associated with positive CG flashes carrying currents of 23 or less to about 100 kA in mesoscale thunderstorms. The sprites are characterized by long vertical columns about 10 km long, less than 1 km in diameter, and show virtually no variation in brightness along their length. Three dimensional triangulation of what we define as a ‘columniform’ sprite (c-sprite) event on the evening of 19 June 1995 showed that the individual elements had an average terminal altitude of 86.7 km and an average bottom of 76.2 km. Some show faint diffuse ‘hair’ or tendrils extending above and below the column. The sprite columns are nearly vertical, in video imagery. On some evenings, c-sprites are the dominant form of sprite activity above thunderstorms but, on other nights with many sprites, they may not be observed at all. Comparison of c-sprite forms vs National Lightning Detection Network (NLDN) positive cloud-to-ground current, shows a progression from simple thin vertical forms to brighter and more complicated forms. Theoretical explanations which predict the form and vertical structure of the classical sprites do not at present account for these different forms.