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Stephanie A. Weiss, W. David Rust, Donald R. MacGorman, Eric C. Bruning, and Paul R. Krehbiel

Abstract

Data from a three-dimensional lightning mapping array (LMA) and from two soundings by balloon-borne electric field meters (EFMs) were used to analyze the electrical structures of a multicell storm observed on 25 June 2000 during the Severe Thunderstorm Electrification and Precipitation Study (STEPS). This storm had a complex, multicell structure with four sections, each of whose electrical structure differed from that of the others during all or part of the analyzed period. The number of vertically stacked charge regions in any given section of the storm ranged from two to six. The most complex charge and lightning structures occurred in regions with the highest reflectivity values and the deepest reflectivity cores. Intracloud flashes tended to concentrate in areas with large radar reflectivity values, though some propagated across more than one core of high reflectivity or into the low-reflectivity anvil. Intracloud lightning flash rates decreased as the vertical extent and maximum value of reflectivity cores decreased. Cloud-to-ground flash rates increased as cores of high reflectivity descended to low altitudes. Most cloud-to-ground flashes were positive. All observed positive ground flashes initiated between the lowest-altitude negative charge region and a positive charge region just above it. The storm’s complexity makes it hard to classify the vertical polarity of its overall charge structure, but most of the storm had a different vertical polarity than what is typically observed outside the Great Plains. The electrical structure can vary considerably from storm to storm, or even within the same storm, as in the present case.

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Terry J. Schuur, W. David Rust, Bradley F. Smull, and Thomas C. Marshall

Abstract

An electric field sounding through the transition zone precipitation minimum that trailed an Oklahoma squall line on 18 June 1987 provides information about the electrical structure within a midlatitude trailing stratiform cloud. A single-Doppler radar analysis concurrent with the flight depicts a kinematic structure dominated by two mesoscale flow regimes previously identified in squall-line systems: a strong midlevel, front-to-rear flow coinciding with the stratiform cloud layer and a descending rear inflow that sloped from 6.5 km AGL at the stratiform cloud's trailing edge to 1.5 km AGL at the convective line. Electric field magnitudes as high as 113 kV m−1 were observed by the electric field sounding, which reveals an electric field structure comparable in magnitude and complexity to structures reported for convective cells of thunderstorms. The charge regions inferred with an approximation to Gauss' law have charge density magnitudes of 0.2–4.1 nC m−3 and vertical thicknesses of 130–1160 m; these values, too, are comparable to those reported for thunderstorm cells. In agreement with previous studies, an analysis of the lightning data revealed a “bipolar” cloud-to-ground lightning pattern with positive flashes being relatively more common in the stratiform region.

From the analysis, we conclude that the stratiform region electrical structure may have been advected from the squall line convective cells as the in-cloud charge regions were primarily found within the front-to-rear flow. Screening layers were found at the lower and upper cloud boundaries. In situ microphysical charging also seems to be a possible source of charge in the stratiform region. We hypothesize that the radar-derived similarities of this system to those previously documented suggests that the newly-documented stratiform electrical structure might also be representative of this type of mesoscale convective system.

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Douglas M. Mach, Donald R. MacGorman, W. David Rust, and Roy T. Arnold

Abstract

We have tested a network of magnetic direction-finders (DFs) that locate ground strikes in Oklahoma and surrounding states in order to determine detection efficiency for the network and systematic errors in azimuth (i.e., site errors) for each of four DF sites. Independent data on lightning strike locations were obtained with a television (TV) camera on a mobile laboratory and an all-azimuth TV system at the National Severe Storms Laboratory (NSSL). In two tests using these data, we found a location detection efficiency of about 70% for storms at about 70 and 300 km from the center of the network. Systematic errors in azimuth were determined by comparing locations from the lightning strike locating system with strikes located from the mobile laboratory system; also, for a single DF at NSSL, strike azimuths from the DF were compared with azimuths from the all-azimuth TV system for storms near NSSL. Furthermore, we developed a technique for using redundant DF data to determine systematic errors in azimuth measurements for each DF site. Azimuthal errors found by this analytic technique were consistent with errors found by using the two sets of direct measurements. The azimuthal errors are themselves a function of azimuth, with peak amplitudes ranging from less than 5° for DFs located at favorable sites to about 11° for one DF located at an unfavorable site.

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Steven M. Hunter, Terry J. Schuur, Thomas C. Mapshall, and W. David Rust

Abstract

Balloon soundings of electric field in Oklahoma mesoscale convective systems (MCS's) were made by the National Severe Storms Laboratory (NSSL) during the spring of 1989. A sounding made in the rearward edge of an MCS stratiform rain area on 7 June 1989 is the centerpiece of this study. We used data from Doppler radars, a lightning ground-strike location system, satellite, and other sources to relate the mesoscale attributes of the MCS to the observed electric-field profile.

The stratiform area did not lag an organized convective line, but was apparently initiated by anvil precipitation downwind of convective cells that were at the west and southwest flanks of the MCS. The mature system produced a bipolar cloud-to-ground lightning pattern, in which most flashes of positive polarity came to ground in the downshear anvil or stratiform region. The bipole was oriented from 220°, parrallel to upper-level geostrophic flow, and its length was 190 km. We visually observed lightning flashes in the stratiform region that had long segments along cloud base.

At least two cells separated from the main convective zone and moved near the region of stratiform cloud penetrated by the balloon. These convective cells may have influenced the intricate charge structure, which was manifested by 11 distinct charge layers from the electric-field sounding. Ten of the 11 charge layers were found between 3 and 8 km MSL (+10° to −24°C), a depth coinciding with rear inflow into the stratiform region. Charge-density magnitudes (up to 3.9 nC m−3) are comparable to those reported for convective cells. The brightband level had negative charge, and just above that was the maximum total electric-field magnitude (120 kV m−1). The horizontal component of the field was 20–40 kV m−1 through much of the flight and attained a peak value of 70 kV m−1, supporting the presence of charge distribution inhomogeneities in the stratiform region. Despite the complexity of the vertical electric-field profile, its coarse structure is similar to others reported for the stratiform region and transition zone. Charged particle advection from the main convective zone may have produced this structure, prior to its complication by convection.

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Maribeth Stolzenburg, Thomas C. Marshall, W. David Rust, and Bradley F. Smull

Abstract

Five soundings of the electric field and thermodynamic properties were made in a mesoscale convective system (MCS) that occurred in Oklahoma and Texas on 2–3 June 1991. Airborne Doppler radar data were obtained from three passes through the stratiform echo. From these electrical, kinematical, and reflectivity measurements, a conceptual model of the electrical structure of an MCS is developed.

Low-level reflectivity data from the storm's mature and dissipating stages show typical MCS characteristics. The leading convective region is convex forward, and the back edge of the stratiform echo is notched inward. The maximum areal extent of the low-level echo is about 250 km × 550 km, and the radar bright band is intense (reflectivity 45–50 dBZ) through an area of at least 50 km × 100 km. The reflectivity above the bright band is horizontally stratified with decreasing intensity and echo-top height toward the rear of the system. Analyses of the velocity data reveal a convective-line-relative flow structure of front-to-rear flow and mesoscale ascent aloft, and weak rear inflow and descent below about 5 km.

The electric field soundings are similar over a period of 3 h and a horizontal scale of 100 km across the stratiform region, suggesting that the charge structure is nearly steady state and the charge regions are horizontally extensive and layered. The basic charge structure consists of four layers: a 1–3-km-deep region of positive charge (density ρ ≈ +0.2 nC m−3) between 6 and 10 km, negative charge (ρ ≈ −1.0–2.5 nC m−3) between 5 and 6 km, positive charge (ρ ≈ +1.0–3.0 nC m−3) near 0°C, and negative charge (ρ ≈ −0.5 nC m−3) near cloud base. The upper positive and densest negative charge layers could result from advection of charge from the convective region. The negative charge layer may be augmented by noninductive collisional charging in the stratiform region. The positive charge near 0°C is probably caused by one or more in situ charging mechanisms. The negative charge near cloud base is likely the result of screening layer formation. In addition to the basic four charge layers, positive charge is found below the cloud in each sounding, and in the two soundings closest to the convection (70–100 km distant) there is a low-density negative charge region near echo top.

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Donald R. MacGorman, W. David Rust, Paul Krehbiel, William Rison, Eric Bruning, and Kyle Wiens

Abstract

Balloon soundings were made through two supercell storms during the Severe Thunderstorm Electrification and Precipitation Study (STEPS) in summer 2000. Instruments measured the vector electric field, temperature, pressure, relative humidity, and balloon location. For the first time, soundings penetrated both the strong updraft and the rainy downdraft region of the same supercell storm. In both storms, the strong updraft had fewer vertically separated charge regions than found near the rainy downdraft, and the updraft’s lowest charge was elevated higher, its bottom being near the 40-dBZ boundary of the weak-echo vault. The simpler, elevated charge structure is consistent with the noninductive graupel–ice mechanism dominating charge generation in updrafts. In the weak-echo vault, the amount of frozen precipitation and the time for particle interactions are too small for significant charging. Inductive charging mechanisms and lightning may contribute to the additional charge regions found at lower altitudes outside the updraft. Lightning mapping showed that the in-cloud channels of a positive ground flash could be in any one of the three vertically separated positive charge regions found outside the updraft, but were in the middle region, at 6–8 km MSL, for most positive ground flashes. The observations are consistent with the electrical structure of these storms having been inverted in polarity from that of most storms elsewhere. It is hypothesized that the observed inverted-polarity cloud flashes and positive ground flashes were caused by inverted-polarity storm structure, possibly due to a larger than usual rime accretion rate for graupel in a strong updraft.

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Eric C. Bruning, W. David Rust, Donald R. MacGorman, Michael I. Biggerstaff, and Terry J. Schuur

Abstract

Lightning mapping, electric field, and radar data from the 26 May 2004 supercell in central Oklahoma are used to examine the storm’s charge structure. An initial arc-shaped maximum in lightning activity on the right flank of the storm’s bounded weak echo region was composed of an elevated normal polarity tripole in the region of precipitation lofted above the storm’s weak echo region. Later in the storm, two charge structures were associated with precipitation that reached the ground. To the left of the weak echo region, six charge regions were inferred, with positive charge carried on hail at the bottom of the stack. Farther forward in the storm’s precipitation region, four charge regions were inferred, with negative charge at the bottom of the stack. There were different charge structures in adjacent regions of the storm at the same time, and regions of opposite polarity charge were horizontally adjacent at the same altitude. Flashes occasionally lowered positive charge to ground from the forward charge region. A conceptual model is presented that ties charge structure in different regions of the storm to storm structure inferred from radar reflectivity.

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W. David Rust, William L. Taylor, Donald R. MacGorman, and Roy T. Arnold

In 1978 we began a coordinated effort to study the electrical behavior of large and severe thunderstorms that form over the Great Plains of the central United States. Methods of approach include the study of characteristics of individual phenomena and storm case studies. Our goal is to understand the interrelationships between electrical phenomena and the dynamics and precipitation of storms. Evidence that interrelationships do exist can be seen in the results to date. In one squall-line storm we have studied, 44% of all observed lightning flashes were cloud-to-ground (CG); the total flashing rate averaged 12 min−1 and coarsely followed the changes in Doppler-derived maximum updraft speed. Most of the intracloud (IC) discharge processes in a supercell severe storm were located predominately around the region of the intense updraft of the mesocyclone and near large gradients in reflectivity and horizontal velocity.

Both 10 cm and 23 cm wavelength radars have been used to detect lightning radar echoes. The lightning echoes from the 10 cm radar generally had peak signals 10–25 dB greater than the largest precipitation echo in the storm, and they usually were observed where precipitation reflectivities were less than maximum. Comparison of lightning echoes and electric field changes shows that abrupt increases in radar reflectivity often are associated with return strokes and K-type field changes.

CG flashes that lower positive charge to earth have been observed to emanate from the wall cloud, high on the main storm tower, and well out in the downwind anvil of severe storms. The percentage of CG flashes that lower positive charge is apparently small.

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Donald R. MacGorman, Donald W. Burgess, Vladislav Mazur, W. David Rust, William L. Taylor, and Brenda C. Johnson

Abstract

On 22 May 1981, we acquired lightning and Doppler radar data on two tornadic storms in Oklahoma. Cloud-to-ground lightning flash rates were measured with a magnetic direction-finder network, and total flash rates in the vicinity of the mesocyclone were measured with an L-band radar. In both storms, there was no clear relationship between tornado occurrence and ground flash rates of the storm as a whole, but the stroke rate of each storm was highest after it stopped producing tornadoes. For the second storm, we examined both intracloud and cloud-t-ground lightning rates relative to mesocyclone evolution, analyzing the region within 10 km of the mesocyclone core. Our analysis began during initial stages of the mesocyclone core associated with the fourth and strongest of five tornadoes in the storm and continued until all mesocyclone cores in the storm dissipated. During this period, intracloud lightning flash rates reached a peak of almost 14 min−1 approximately 10 min after the peak in cyclonic shear at the 6 km level and at the same time as the peak in cyclonic shear at the 1.5 km level. The peak in intracloud rates also occurred 5–10 min after the peak in the area within 40 and 45 dBZ contours at the 8 km level and at about the same time as the peak in the area within 50 dBZ contours at 8 km and within 40 dBZ at 6 km. However, ground flash rates in the mesocyclone region were usually less than 1 min−1 during periods when intracloud rates were high and were negatively correlated with cyclonic shear at both 1.5 and 6 km. The ground flash rate was the last parameter to peak, approximately 15 min after intracloud lightning and a few minutes after the latest reflectivity area (the area having >55 dBZ at the 1 km level).

We suggest that intracloud rates were governed, in part, by particle interactions during the growth in reflectivity at 7–9 km and, in part, by some process associated with the evolution of cyclonic shear at low altitudes. Earlier studies of tornado storms indicate that the evolution of updrafts and downdrafts affects the evolution of both reflectivity and low-altitude cyclonic shear and so, as in previous storm studies, updraft evolution will affect intracloud rates. We suggest that the peaks in ground flash rates resulted from increasing the distance between the main positive and negative charge centers, from the sedimentation of negative charge to lower altitudes, or from the generation or advection of positive charge below the main negative charge. Although these data are from only a single day, consideration of sferics data from previous studies suggests that 1) most tornadic storms (80% or more) have an increase in total flash rates near the time of the tornado, and 2) the increase in total flash rates is often dominated by intracloud flashes.

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W. David Rust, Donald W. Burgess, Robert A. Maddox, Lester C. Showell, Thomas C. Marshall, and Dean K. Lauritsen

We have tested the NCAR Cross-Chain LORAN Atmospheric Sounding System (CLASS) in a fully mobile configuration, which we call M-CLASS. The sondes use LORAN-C navigation signals to allow calculation of balloon position and horizontal winds. In non-stormy environments, thermodynamics and wind data were almost always of high quality. Besides providing special soundings for operational forecasts and research programs, a major feature of mobile ballooning with M-CLASS is the ability to obtain additional data by flying other instruments on the balloons. We flew an electric field meter, along with a sonde, into storms on 8 of the initial 47 test flights in the spring of 1987. In storms, pressure, temperature, humidity, and wind data were of good quality about 80%, 75%, 60%, and 40% of the time, respectively. In a flight into a mesocyclone, we measured electric fields as high as −135 kV/m (at 10 km MSL) in a region of negative charge. The electric field data from several storms allow a quantitative assessment of conditions that accompany loss of LORAN data. LORAN tracking was lost at a median field of about 16 kV/m, and it returned at a median field of about 7 kV/m. Corona discharge from the LORAN antenna on the sonde was a cause of the loss of LORAN. We provided our early-afternoon M-CLASS test soundings to the National Weather Service Forecast Office in Norman, Oklahoma, in near real-time via amateur packet radio and also to the National Severe Storms Forecast Center. These soundings illustrate the potential for improving operational forecasts. Other test flights showed that M-CLASS data can provide high-resolution information on evolution of the Great Plains low-level jet stream. Our intercept of Hurricane Gilbert provided M-CLASS soundings in the right quadrant of the storm. We observed substantial wind shear in the lowest levels of the soundings around the time tornadoes were reported in south Texas. This intercept demonstrated the feasibility of taking M-CLASS data during the landfall phase of hurricanes and tropical storms.

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