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Donald R. MacGorman and Donald W. Burgess

Abstract

Although negative ground flashes usually dominate cloud-to-ground lightning activity, positive ground flashes can dominate in some severe storms for periods ranging from 30 min to several hours. Unlike most other types of storms in which positive ground flashes occur, severe storms can have positive flash rates and densities of strike points comparable to those usually observed for negative ground flashes in active thunderstorms. Fifteen such storms are analyzed in this paper to examine relationships of positive ground flashes to various storm characteristics, especially reports of large hail and tornadoes.

In 4 of the 15 storms, ground flash activity was dominated by positive cloud-to-ground lightning throughout most of the life of the storm. In 11 storms, the dominant polarity of ground flashes switched from positive to negative sometime during the mature stage of the storm. In all cases observed by Doppler radar, storms dominated by positive flashes had at least some rotation, and in most cases they were low-precipitation or classic supercell storms. If negative ground flashes subsequently became frequent and dominated positive ground flashes in a storm, it usually remained strong and often became a classic or heavy-precipitation supercell storm.

In all cases for which hail verification efforts were vigorous, large hail was reported during the period when positive ground flashes dominated. Usually, the frequency and reported diameter of hail decreased after the dominant polarity of ground flashes switched to negative. In the 11 storms that were tornadic, tornadoes occurred either during or after the period when positive ground flashes dominated. The strongest tornado usually began after the positive ground flash rate decreased from its maximum value; this was always true when the maximum rate exceeded 1.5 min−1. Although many hailstorms and tornadic storms are dominated by negative flashes, tornadic storms and hailstorms constitute a small fraction of storms dominated by frequent negative flashes, but appear to constitute an overwhelming majority of storms dominated by frequent positive flashes.

The geographic region in which positive or negative flashes dominated on a given day appeared consistent from storm to storm; the dominant polarity switched in roughly the same region for sequential storms following similar tracks. It is inferred that the dominant polarity of lightning is strongly influenced by mesoscale properties of the atmosphere, possibly through systematic effects on other storm properties related to severe weather.

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Eric C. Bruning and Donald R. MacGorman

Abstract

Previous analyses of very high frequency (VHF) Lightning Mapping Array (LMA) observations relative to the location of deep convective updrafts have noted a systematic pattern in flash characteristics. In and near strong updrafts, flashes tend to be smaller and more frequent, while flashes far from strong vertical drafts exhibit the opposite tendency. This study quantitatively tests these past anecdotal observations using LMA data for two supercell storms that occurred in Oklahoma in 2004. The data support a prediction from electrostatics that frequent breakdown and large flash extents are opposed. An energetic scaling that combines flash rate and flash area exhibits a power-law scaling regime on scales of a few kilometers and a maximum in flash energy at about 10 km. The spectral shape is surprisingly consistent across a range of moderate to large flash rates. The shape of this lightning flash energy spectrum is similar to that expected of turbulent kinetic energy spectra in thunderstorms. In line with the hypothesized role of convective motions as the generator of thunderstorm electrical energy, the correspondence between kinematic and electrical energy spectra suggests that advection of charge-bearing precipitation by the storm’s flow, including in turbulent eddies, couples the electrical and kinematic properties of a thunderstorm.

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Donald R. MacGorman and Kurt E. Nielsen

Abstract

On 8 May 1986, the National Severe Storms Laboratory (NSSL) collected Doppler radar and lightning ground strike data on a supercell storm that produced three tornadoes, including an F3 tornado in Edmond, Oklahoma, approximately 40 km north of NSSL. The Edmond storm formed 30 km ahead of a storm complex and produced its first and most damaging tornado just as the storm complex began to overtake it from the west. In the mesocyclone that spawned the tornado, low-level cyclonic shear peaked as the first tornado dissipated and a second tornado began. As low-level cyclonic shear initially increased, negative cloud-to-ground lightning flash rates also increased, reaching a peak of 11 min−1 a few minutes after the peak in cyclonic shear. During this period lightning strike locations tended to concentrate just north of the mesocyclone, near and inside a 50-dBZ reflectivity core. As cyclonic shear decreased from its peak during and after the second tornado, negative ground flash rates also decreased, and strike locations became more scattered. Positive ground flashes began just before the storm became tornadic, and positive flash rates peaked during the tornadic stage of the storm.

The evolution of cloud-to-ground lightning in the Edmond storm differed considerably from the evolution of lightning in the Binger tornadic storm of 22 May 1981 that was studied previously. In the Binger storm, ground flash rates were negatively correlated with cyclonic shear and peaked 15–20 min later than low-level shear and intracloud lightning. It is suggested that the very strong mesocyclone and updraft in the Binger storm enhanced intracloud flash production and delayed ground flashes by causing the initial height of negative charge to be higher than in most storms. It is also suggested that weaker updrafts and a weaker, shallower mesocyclone in the Edmond storm resulted in higher negative ground flash rates when the Edmond mesocyclone was still strong, because negative charge near the mesocyclone was at the lower heights common to most thunderstorms.

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Conrad L. Ziegler and Donald R. MacGorman

Abstract

This study uses a kinematic numerical cloud model that includes electrification and lightning discharge processes to investigate hypotheses concerning intracloud lightning flash rates in the Binger, Oklahoma, tornadic storm of 22 May 1981. MacGorman et al. have observed that intracloud (IC) flash rates in this storm's mesocyclone region peak when overall storm intensity is greatest and cloud-to-ground flash rates are low. They hypothesize that precipitation interactions involved in reflectivity growth at the 7–9-km level of the updraft are involved in precipitation charging and electrification. They also hypothesize that the intense convection in the mesocyclone region elevates the lower negative charge of the storm closer to upper positive charge, thereby enhancing IC flash rates.

These hypotheses are tested by examining the charge and electric field distributions and charging rates produced by the kinematic model for the Binger storm. The model produces maximum electric field and net space charge magnitudes of around 200 kV m−1 and 1 nC m−3 in runs where the threshold for activating simulated lightning discharges was set at 200 kV m−1. The noninductive mechanism, driven by charge separation during rebounding collisions between ice particles and riming graupel, generally dominates the inductive mechanism in the model. Computed precipitation charging rates of up to −5 × 10−11 C m−3 s−1 are partially compensated by cloud particle charging from discharges in middle levels of the updraft.

Simulated discharges add positive charge to cloud particles in the main negative precipitation charge region and negative charge to cloud particles in the upper positive precipitation charge region. The principal effect of lightning in the model is not to neutralize the charge on individual particles, but to partially mask the net charge carried by precipitation. The simulated discharges occur at a rate of 12 min−1, comparable to the peak observed IC flash rate of 13 min−1 within 10 km of the mesocyclone. The model results also suggest that lightning, combined with subsequent particle motions, creates new regions of charge comparable to those created by particle collisions.

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Howard B. Bluestein and Donald R. MacGorman

Abstract

On 31 May 1990, four tornadic supercell storms formed sequentially near the intersection of a dryline and an outflow boundary in the northern Texas panhandle. “Staccato” lightning flashes, which have been hypothesized to be positive ground flashes, were observed beneath the anvil of one storm during the most violent tornado that the storm produced. Evidence was found from a lightning mapping system that at least some of the staccato flashes were negative ground flashes.

Although the four supercell storms on this day formed in approximately the same area, traveled over roughly the same region, and produced tornadoes and large hail, the relationship between the genesis and evolution of tornadoes and the polarity and flash rates of ground flashes varied widely, as in previous studies. The second of the supercell storms had low-precipitation supercell characteristics; the third and fourth did not. In previously studied storms, ground flash activity in low-precipitation supercell storms has always been dominated by positive ground flashes. However, all ground flashes detected in the second, low-precipitation storm were negative ground flashes.

Positive ground flashes dominated ground flash activity in the third and fourth supercell storms for roughly their first hour, after which the dominant polarity switched to negative. In the third storm, the maximum positive ground flash rate before this polarity reversal was 1 min−1 and the most intense tornado produced by the storm occurred before the maximum positive ground flash rate. In the fourth storm, positive ground flash rates increased to 7.4 min−1 over a period of 30 min early in the storm, followed by a rapid decrease to 0 min−1 over the next 10 min; the most intense tornado produced by the fourth storm occurred during the lull in ground flash rates following the large maximum. These observations are consistent with a previously reported tendency for a storm dominated by positive ground flashes to produce its most violent tornado after it attains its maximum positive ground flash rate, whenever the rate is in excess of 1.5 min−1.

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Steven A. Rutledge and Donald R. MacGorman

Abstract

As part of the field program for the Oklahoma–Kansas PRE-STORM Project conducted in May–June 1985, a network of electromagnetic direction-finders was deployed to locate and detect the polarity of cloud-to-ground (CG) lighting flashes associated with Mesoscale Convective Systems (MCSs). We present an analysis of such data for the 10–11 June MCS. This storm consisted of a line of convective cells trailed by an 80 km wide stratiform precipitation region. Data from the lightning strike locating network, along with both conventional and Doppler radar data, are analyzed over a significant portion of the storm's lifetime to examine the relationship between the storm precipitation structure and the position and polarity of the lighting activity. The majority of the negative CG activity is located in the convective precipitation region. The frequency of negative CG activity is highest around the period of most intense convective rainfall. Positive CG activity is mainly confined to the trailing stratiform region, and there is a correlation between the areally integrated stratiform precipitation and the frequency of positive CG flashes. We propose that the occurrence of positive CG flashes in the trailing stratiform region is a result of the reward advection of positive charge on small ice particles from the upper levels of the convective cells by the storm relative winds. However, charging of hydrometeors may occur within the stratiform region and contribute to the positive space charge. Candidate charging mechanism are discussed.

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Ronald M. Reap and Donald R. MacGorman

Abstract

Data for nearly 2 million lightning flashes recorded during the 1985–86 warm seasons by the National Severe Storm Laboratory's (NSSL's) lightning strike locating network were evaluated to determine some of the climatological characteristics of cloud-to-ground lightning. Among the characteristics studied were the seasonal, diurnal, and spatial variations Of Positive and negative lightning strike activity, including flush rates, signal strength, and flash multiplicity. The lightning data were also compared to manually digitized radar data, reports of tornadoes, large hail, and damaging winds, and to analyzed 0000 UTC fields obtained from operational numerical models.

An examination of the diurnal distribution of lightning revealed that peak rates occurred later than in other sections of the country, reflecting the prevalence of nocturnal convection within much of the NSSL network. An analysis of the spatial variations in lightning activity also confirmed the existence of distinct climatological regimes within the network. A study of the diurnal variations in signal strength revealed that first return strokes lowering negative charge have higher signal strengths at night and in the early morning hours, when flash rates are normally decreasing. In addition, positive flashes were found to exhibit three distinct peaks in signal strength, two of which are associated with late afternoon and nocturnal maxima in fish activity.

A good correspondence between lightning frequency and radar echo intensity was found for the region within the effective range of the Oklahoma City WSR-57 radar. Both positive and negative flashes were found to be strongly correlated with the low-level moisture flux and circulation, as characterized by favorable moisture convergence, cyclonic relative vorticity, and strong upward vertical motions in the boundary layer. Contrary to expectations, freezing level height and wind shear were not nearly as important as the boundary layer fields in determining thunderstorm formation and subsequent positive lightning activity. A significant correlation was also found between the occurrence of severe local storm and elevated rates of 30 or more positive flashes per hour within 48 km grid blocks.

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Edward R. Mansell, Conrad L. Ziegler, and Donald R. MacGorman

Abstract

Lightning observations have been assimilated into a mesoscale model for improvement of forecast initial conditions. Data are used from the National Lightning Detection Network (cloud-to-ground lightning detection) and a Lightning Mapping Array (total lightning detection) that was installed in western Kansas–eastern Colorado. The assimilation method uses lightning as a proxy for the presence or absence of deep convection. During assimilation, lightning data are used to control the Kain–Fritsch (KF) convection parameterization scheme. The KF scheme can be forced to try to produce convection where lightning indicated storms, and, conversely, can optionally be prevented from producing spurious convection where no lightning was observed. Up to 1 g kg−1 of water vapor may be added to the boundary layer when the KF convection is too weak. The method does not employ any lightning–rainfall relationships, but rather allows the KF scheme to generate heating and cooling rates from its modeled convection. The method could therefore easily be used for real-time assimilation of any source of lightning observations. For the case study, the lightning assimilation was successful in generating cold pools that were present in the surface observations at initialization of the forecast. The resulting forecast showed considerably more skill than the control forecast, especially in the first few hours as convection was triggered by the propagation of the cold pool boundary.

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Mark A. Shafer, Donald R. MacGorman, and Frederick H. Carr

Abstract

Cloud-to-ground (CG) lightning data are examined relative to digitized radar data for a storm system that occurred in Oklahoma on 26 May 1985. This system evolved through three stages: 1) two lines of cells, one near the dryline and the other 60 km ahead of it; 2) a supercell storm; and 3) a mesoscale convective system (MCS). The behavior of lightning in each stage was different. Initially no ground flashes were observed in either line until reflectivity increased to ≥46 dBZ and vertically integrated liquid (VIL) increased to ≥10 kg m−2; then ground flash rates remained <1.2 min−1 for >1 h. Most ground flashes in the line of storms near the dryline were negative (18 −CG, 3 +CG), while most in the leading line were positive (11 +CG, 3 −CG), a pattern of polarity opposite to what usually has been observed. Approximately 3 h after radar detected the first storm, ground flash rates increased to >5 min−1 and remained so for 6 h. A mesocyclone formed approximately 30 min after flash rates exceeded 5 min−1, and a few positive ground flashes occurred near it. Ground flash rates increased briefly to >20 min−1 as the mesocyclone dissipated and then remained >10 min−1 as a squall line formed along the outflow boundary from the dissipating supercell and produced a stratiform region. Most ground flashes in this MCS occurred in the convective line and had negative polarity. The few ground flashes in the stratiform region tended to be positive (42 +CG, 32 −CG during 3 h). During 1 h of the MCS, ground flash rates decreased and then increased again simultaneously in both the convective and stratiform regions, a previously undocumented behavior. It is possible that this was caused by updrafts in both the convective line and stratiform region changing at roughly the same time. It is also possible that most ground flashes in the stratiform region originated near the convective line, and so were influenced by the line. Overall trends in ground flash density, flash relative frequency, reflectivity, VIL, and severe hail reports appeared similar as the storm system evolved.

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Conrad L. Ziegler, Peter S. Ray, and Donald R. MacGorman

Abstract

This paper addresses aspects of the airflow, microphysics, and electrification in a mountain thunderstorm which occurred on 7 August 1979 over the Langmuir Laboratory new Socorro, New Mexico, site of the Thunderstorm Research International Program (TRIP). Single Doppler observations are used to form a conceptual model of the essentially one-dimensional storm updraft which is expressed in simple analytical form. A one-dimensional kinematic numerical cloud model is employed with the analytic updraft profile to diagnose the evolution of temperature, war substance, radar reflectivity, space charge density and axial electric field in the main updraft region. Retrieved thermal, microphysical, and electrical variables are verified with in situ aircraft and balloon observations and measured radar reflectivity. The calculated rate of noninductive charge transfer accompanying collision and separation of ice crystals and riming graupel particles is in direct proportion to cloud and precipitation content, and attains a peak value of about 10 C km−3 min−1 between −30° and −40°C. Agreement between calculations and balloon measurements of space charge density and vertical electric field imply that the noninductive graupel-ice charge separation mechanism accounts for a substantial portion of the storm's total separated charge. The peak noninductive charging rate appears to balance the discharge rate implied by the observed flash rate.

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