Positive Cloud-to-ground Flashes Research Articles

Monthly distribution of cloud‐to‐ground lightning flashes as observed by lightning location systems

This paper presents a comparison among the mean monthly distributions of the number of cloud‐to‐ground (CG) flashes, the percentage of positive CG flashes and the peak current of negative and positive CG flashes obtained in Brazil for the period from 1999 to 2004, with the same distributions observed by similar networks for long time periods in other countries. From a correlation analysis, it was found that the mean monthly distributions of the number of CG flashes are very similar, even though the period of larger lightning activity along the year in Brazil, a tropical country, is longer than in the other temperate countries. The mean monthly distributions of the percentage and the peak current of positive CG flashes are also very similar, while the mean monthly distribution of the peak current of negative CG flashes in Brazil differs from the other countries. This difference is related to a significant decrease in the mean negative peak current in Brazil in the months of August and September. Apparently, the decrease is related to the injection in the atmosphere of large amounts of smoke from fires in these months, since the results also show a significant correlation between the monthly distribution of the number of fires and the negative peak current.

A 3-year study of cloud-to-ground lightning flash characteristics of Mesoscale convective systems over the Western Mediterranean Sea

In this paper cloud-to-ground (CG) lightning flashes of 33 Mesoscale Convective Systems (MCSs) over the Western Mediterranean area are analyzed. Mean values of 22% for the positive CG ratio, 1.1 (2.4) for positive (negative) CG multiplicity, 17 kA (22.6 kA) of peak current for the positive (negative) CG flashes and a mean flash rate of around 13 min − 1 are obtained. The percentage of positive CGs and the multiplicity of negative CG are higher, while the mean peak currents are lower than in previous studies on MCSs. A more detailed case analysis reveals that there are great differences among the CG characteristics of MCSs. The life cycle of CG lightning associated with MCSs is also analyzed. The growing stages of MCSs are characterised by high CG lightning activity. The positive CG flash rate generally reaches a maximum before the negative CG flash rate does. In both cases the peak is recorded before or when MCSs show the largest area. Maximum flash rate and densities coincide with the area where the MCS shows the minimum cloud-top temperatures and therefore shows highest vertical development.

NO x ground concentration at a station at high altitude in relation to cloud-to-ground lightning flashes

Concentrations of both nitrogen oxides (NO x =NO+NO 2) observed at a station at high altitude (Puy de Dome (PDD), 1465 m) have been analyzed in relation with the cloud-to-ground (CG) lightning activity provided by the detection network Météorage. A first analysis of the time series of both types of data issued for 4 months of the 2003 summer shows a good correspondence between high values of NO x and stormy conditions in the area. During this 4-month period, NO x values exceeding 100 μg m −3 have been observed on 7 days and all of these days correspond to lightning activity in a 200×200 km 2 area around PDD. Thirty-three days of the period experience lightning activity in this area, and only 14 of these days (42%) correspond to NO x peak values exceeding 50 μg m −3. The presence of lightning activity is a necessary condition to have very large peak values of NO x at PDD, but this condition is not sufficient. Two days showed NO x exceeding 300 μg m −3 which represents a value between 176 and 270 ppbv according to the respective proportion of NO 2 and NO. Six days are analyzed in detail, using other information about the convective system and the wind conditions. The data of urban stations close to PDD have also been considered in the analysis for evaluating the possible transport by a plume from a city area. These days include the higher NO x cases, cases with high lightning activity, and cases with remote lightning activity. On each of these days, the location of the CG flashes, the time delays between lightning activity and NO x increase, and the air flow seem consistent with an interpretation of NO x produced by lightning. The higher values of NO x correspond with close and stationary thunderstorms which produced large proportions of positive CG flashes with low wind conditions. However, the characteristic increase seems to depend on several parameters of the CG flashes: their rate, location, and polarity. By comparing with the values given by other authors for aircraft measurements at altitude, our observations indicate that large quantities of NO x could be produced by the CG flash processes.

High percentage of positive lightning along the USA west coast

[1] Analyses of the cloud-to-ground (CG) lightning characteristics recorded by the US National Lightning Detection Network (NLDN) along the west coast reveal an average annual percentage of positive CG flashes around 40%, while the average value for the USA is approximately 10%. The primary goal of the study was to document and suggest reasons for this positive CG anomaly. Through seasonal and monthly storm analysis, it was determined that the high annual percent positive along the coast is the result of the low variability in total CG flashes throughout the year coupled with a high number of positive CG flashes during the winter season which can be directly attributed to the climate of the Pacific Coast. The topography of the region was determined to affect the areal distribution of the percent positive anomaly by restricting the inward extent of the coastal climate. The secondary goal of the study was to determine whether the following meteorological variables were related to the dominant CG polarity in a storm: 1) the variation of charge region heights using the −10°C level as a proxy and 2) the tilting of the charge regions by strong windshear. Statistical analysis showed that the height of the −10°C temperature level is related to the dominant CG polarity in a storm, while the windshear did not show a significant relationship. In addition, analyses showed that the thunderstorms that produced few CG flashes (<6 flashes) contributed most to the total number of positive CG flashes.

Evolution of Cloud-to-Ground Lightning and Storm Structure in the Spencer, South Dakota, Tornadic Supercell of 30 May 1998

Abstract On 30 May 1998, a tornado devastated the town of Spencer, South Dakota. The Spencer tornado (rated F4 on the Fujita tornado intensity scale) was the third and most intense of five tornadoes produced by a single supercell storm during an approximate 1-h period. The supercell produced over 76% positive cloud-to-ground (CG) lightning and a peak positive CG flash rate of 18 flashes min−1 (5-min average) during a 2-h period surrounding the tornado damage. Earlier studies have reported anomalous positive CG lightning activity in some supercell storms producing violent tornadoes. However, what makes the CG lightning activity in this tornadic storm unique is the magnitude and timing of the positive ground flashes relative to the F4 tornado. In previous studies, supercells dominated by positive CG lightning produced their most violent tornado after they attained their maximum positive ground flash rate, whenever the rate exceeded 1.5 flashes min−1. Further, tornadogenesis often occurred during a lull in CG lightning activity and sometimes during a reversal from positive to negative polarity. Contrary to these findings, the positive CG lightning flash rate and percentage of positive CG lightning in the Spencer supercell increased dramatically while the storm was producing F4 damage on Spencer. These results have important implications for the use of CG lightning in the nowcasting of tornadoes and for the understanding of cloud electrification in these unique storms. In order to further explore these issues, the authors present detailed analyses of storm evolution and structure using Sioux Falls, South Dakota, (KFSD) Weather Surveillance Radar-1988 Doppler (WSR-88D) radar reflectivity and Doppler velocity and National Lightning Detection Network (NLDN) CG lightning data. The analyses suggest that a merger between the Spencer supercell and a squall line on its rear flank may have provided the impetus for both the F4 tornadic damage and the dramatic increase in positive CG lightning during the tornado, possibly explaining the difference in timing compared to past studies.

A comparative analysis of lightning data from lightning networks and LIS sensor in the North and Southeast of Brazil

Intracloud and cloud‐to‐ground (CG) lightning data in the North and Southeast regions of Brazil are compared based on observations from 01 October 1999 to 13 April 2000. The observations were made by ground networks and the Lightning Imaging Sensor (LIS) on board the TRMM satellite. For the whole period of observation, a detailed comparative analysis indicates that the cloud‐to‐ground lightning activity in the North region is about 20% higher than that in the Southeast for a similar size area, while the percentage of intracloud flashes is almost the same in both regions (∼60–65%). On a 49‐day basis (to avoid the local time bias in the LIS data), the same analysis indicates that the lightning activity recorded by LIS is representative of the CG lightning activity detected by the lightning networks, either in the same time and space seen by the satellite sensor, or in the area of the study. It was also found that the percentage of positive CG flashes in the North region is correlated to the percentage of intracloud flashes.

Simulated three‐dimensional branched lightning in a numerical thunderstorm model

Lightning discharges are simulated by using a stochastic dielectric breakdown model within a numerical thunderstorm model with extensive parameterizations of electrification mechanisms. The lightning model simulates the macroscopic bidirectional extension of discharges as a step‐by‐step stochastic process. Discharge channels are propagated on a uniform grid, and the direction of propagation (including diagonals) for a particular step is chosen randomly, with the probability for choosing a particular direction depending on the net electric field. After each propagation step the electric fields are recomputed via Poisson's equation to account for the effect of the conducting channel. The lightning parameterization produces realistic looking, three‐dimensional, branched lightning discharges. A variety of lightning types have been produced, including intracloud discharges, negative cloud‐to‐ground (CG) lightning, and positive CG lightning. The model simulations support the hypothesis that negative CG flashes occur only when a region of positive charge exists below the main negative charge region. Similarly, simulated positive CG flashes were found to occur only in regions of storms where the two significant charge layers closest to ground had roughly a “normal dipole” structure (i.e., positive charge above negative).

Characteristics of Cloud-to-Ground Lightning in Chinese Inland Plateau.

Characteristics of cloud-to-ground (CG) lightning flashes in the Chinese inland plateau have been analyzed by using the data obtained from a broadband slow antenna system, with a time resolution of 1 microsecond in 1996, and a CG lightning location network from 1997 to 1998. It has been found that lightning discharges in a plateau summer thunderstorm usually present some special characteristics compared with the typical summer thunderstorm. On the average, positive CG flashes comprised 16% of total CG flashes in the Chinese inland plateau. After comparison with radar echo, it is found that some weak thunderstorms are conducive to positive CG flashes. The first return strokes are usually preceded by a long duration preliminary breakdown process which is characterized by discrete bipolar pulses for both positive and negative CG flashes. The mean duration of electric field change prior to first return stroke is about 126.1 ms, in which 23% exceeds 200 ms for negative CG flashes, and the corresponding value is 165.1 ms for positive CG flashes. The mean interval between 152 successive analyzed pulses is 211.0±148.0 ms, and the total duration of the pulses is 32.0±20.0 ms for negative CG flashes, while the mean time interval between the 50 analyzed pulses is 165.0±120.0 ms and the total duration of pulses is 27.0±19.0 ms for positive CG flashes. Other discharge features, such as interstroke intervals, amplitude distribution of the subsequent return strokes and etc., are also analyzed in the paper.

Lightning and precipitation relationship in coastal thunderstorms

Some differences between lightning and precipitation characteristics of thunderstorms over sea and over land are analyzed. The rain field is observed by a meteorological radar. The cloud‐to‐ground (CG) lightning activity is observed by the French network METEORAGE, and the total lightning activity is observed by a Surveillance et Alerte Foudre par Interférométrie Radioélectrique (SAFIR) interferometric system. Several parameters are compared for 21 stormy days over land and over the ocean in different meteorological conditions. The parameters considered for precipitation are cumulative rainfall and rain rate. Those for the lightning activity are the density, rate, proportion, peak current, and multiplicity of CG flashes, and the density and rate of the total flashes and of the VHF sources. The whole data set is statistically analyzed, and some individual convective systems are discussed. The storm activity and some parameters like the CG peak current clearly seem to be affected by the surface characteristics (sea or land). Others parameters, like the positive CG flash percentage, are not affected. The land‐sea ratio is found to be 1.8 for cumulative rainfall and 2.36 for CG flash number. Most of the thunderstorms observed over sea develop close to the coastline. A substantial decrease of lightning activity is observed during the lifetime of a hail‐bearing thunder cell. The rain volume per CG flash, averaged for all situations and for the whole domain, is 68 × 103 m3 per CG flash. For individual cases this volume increases with the positive CG proportion. Moreover, the low values of the rain volume per CG flash correspond to a relatively strong lightning activity and are often associated with a large cloud vertical development. Assuming that a high probability of lightning flash occurrence corresponds to particularly efficient charging processes, this last observation suggests that the role of strong vertical velocities in these electrical processes is essential.

Evaluation of LPATS data using VHF interferometric observations of lightning flashes during the Eulinox experiment

During the Eulinox (European Lightning Nitrogen Oxides Project) campaign, two lightning sensors were available: a Lightning Position and Tracking System (LPATS), and the ONERA VHF interferometric (ITF) mapper. We looked for the typical VHF signature of positive and negative cloud-to-ground flashes in ITF records in order to separate intra-cloud (IC) flashes from cloud-to-ground (CG) flashes during five periods from 2 days. Then we used this set of data to check the efficiency of the South Germany LPATS. We concluded that this LPATS detected nearly all the negative CG flashes, but one-fourth of LPATS negative strokes are in fact IC flashes. The ratio of misinterpretation is much smaller (15%) for multi-stroke negative CG flashes. The LPATS recorded 62% of the positive strokes, but only 43% with the right polarity, and only 25% of LPATS positive strokes actually belonged to a positive CG flash. This reliability ratio is 100% if we consider only high-intensity LPATS strokes.

Diurnal Variations in the Characteristics of Cloud-to-Ground Lightning Activity in the Great Lakes Region of the United States

Cloud-to-ground (CG) lightning observations for the warm seasons (May-September) of 1989 and 1990 were analyzed for the Great Lakes region in order to assess the diurnal variations in the lightning characteristics. Several parameters, including flash rate, the spatial extent of lightning activity, first stroke peak current, and the percentage of positive flashes, varied markedly over the course of the day. In contrast, other parameters, such as the frequency of lightning periods and the multiplicity of both negative and positive CG flashes, displayed little diurnal variation. A large degree of intraseasonal (i.e., month-to-month) variation was observed in the diurnal patterns for several of the lightning parameters.

Analysis of a Small, Vigorous Mesoscale Convective System in a Low-Shear Environment. Part I: Formation, Radar Echo Structure, and Lightning Behavior

The evolution of a small, vigorous mesoscale convective system (MCS) over northern Alabama is described using Doppler radar, GOES satellite, surface mesonet, lightning, and sounding data. The MCS formed near noon in a relatively unstable environment having weak synoptic forcing and weak shear. The initiation of separate lines and clusters of deep convection occurred in regions exhibiting cumulus cloud streets, horizontal variations in stratocumulus cloud cover, and variations in inferred soil moisture. MCS growth via merger of storms within clusters and lines, and among the clusters, was accomplished largely through intersection of storm-scale and mesoscale outflow boundaries. The MCS maximum anvil area (∼60000 km2 at 220 K) and lifetime (8 h) were about 50% that of the typical Great Plains mesoscale convective complex (MCC). Despite its smaller size, this MCS displayed many aspects that typify the mostly nocturnal Great Plains MCS. The precipitation output was highly variable due to the transient nature of the intense convective elements, many of which produced microbursts. The radar measurements documented the formation of a stratiform region along the trailing side of an intense convective line. This stratiform region formed as decaying convective cores coalesced, rather than through advection of precipitation particles directly from the convective region. Combined GOES IR imagery and radar reflectivity analyses within the stratiform region show a sinking anvil cloud top in the presence of increases in the vertical radar reflectivity gradient within the cloud during the maturation of the stratiform region. During its intense developing stages, the MCS generated a peak cloud-to-ground (CG) flash rate of 2400 h−1, comparable to rates produced by larger MCCs. Early on, positive CG flashes were most prevalent around intense convective core regions exhibiting strong divergence at anvil level. During the latter stages, the emergence of positive CG was coincident with the formation of a prominent radar bright band within the stratiform region. Thus, a bipole was established, but its length was quite short at approximately 50 km, 25%–50% of the distance documented in other MCSs.

Electrical and multiparameter radar observations of a severe hailstorm

Using data from the 11 cm, CSU‐CHILL multiparameter radar, the simultaneous evolution of the radar‐inferred precipitation structure and electrical characteristics of a severe hailstorm is investigated. We compare the substorm point discharge current, intracloud (IC) lightning flash rate, cloud‐to‐ground (CG) lightning flash rate, ground strike location, and flash polarity to the progression of precipitation types and amounts. This study is unique in that it presents multiparameter radar observations of a thunderstorm complex which exhibited an extremely high IC‐to‐CG ratio (IC/CG, 20–70) and predominantly positive CG lightning (over 74%) after it became severe, producing large hail and weak tornadoes. In particular, we investigate the reported relationship between large hail and positive CG lightning. Although a direct correlation is found between a rapid increase in IC/CG, the first positive CG lightning flashes, and the initial production of hail aloft, the temporal and spatial behavior of large hail and positive CG lightning appear to be anticorrelated, as broad peaks in the positive CG flash rate lag relative maxima in the fallout of large hail by up to 30 min. The majority of positive ground strikes were adjacent to the main precipitation core in a region of light rain and little or no hail at the surface. Aloft, radar data indicated that ice crystals were aligned vertically in a strong electric field. Corona point observations at the surface indicate that these regions adjacent to the convective core were characterized by net positive charge aloft. Possible mechanisms to explain these observations are discussed.

Blue starters∷ Brief upward discharges from an intense Arkansas thunderstorm

This paper documents the first observations of a new stratospheric electrical phenomenon associated with thunderstorms. On the night of 30 June (UT 1 July) 1994, 30 examples of these events, which we have called “blue starters,” were observed in a 6 m 44 s interval above the very energetic Arkansas thunderstorm where blue jets were first observed. The blue starters are distinguished from blue jets by a much lower terminal altitude. They are bright and blue in color, and protrude upward from the cloud top (17–18 km) to a maximum 25.5 km (83,655 ft.) in altitude. All blue starters events were recorded from two small areas near Texarkana, Texas/Arkansas where hail 7.0 cm in diameter was falling. Comparison to cloud‐to‐ground (CG) lightning flashes revealed: 1. Blue starters were not observed to be coincident with either positive or negative CG flashes, but they do occur in the same general area as negative CG flashes; 2. Cumulative distributions of the negative CG flashes in ±5 s before and after the starter and within a radius of 50 km shows a significant reduction for about 3 s following the event in the two cells where starters and jets were observed. The energy deficit is approximately 109 J. It is possible that blue starters are a short‐lived streamer phenomenon.

Anomalous Cloud-to-Ground Lightning in an F5-Tornado-Producing Supercell Thunderstorm on 28 August 1990

An F5 tornado that devastated Plainfield, Illinois, and environs on 28 August 1990, killing 29 people, is shown to be produced by a thunderstorm characterized by highly anomalous cloud-to-ground (CG) lightning activity. Unlike typical summertime convection in which the majority of CG flashes lower negative charge to ground, the Plainfield storm produced predominantly positive-polarity CG flashes during development. Changes in storm structure revealed by radar imagery appear tied to distinct patterns in the CG flash parameters of polarity, flash frequency, first stroke peak current, flash multiplicity, and flash location relative to the parent cumulonimbus. The primary findings are 1) the anomalous predominance (91%) of positive-polarity CG flashes during development; 2) positive CG flashes anomalously occurring mainly within the region of the storm's radar reflectivity core; 3) the onset of a major downburst coinciding with a sudden increase in CG flash rate, from 4 to 17 flashes min-1, and positive perce...