Thomas C. Marshall

Electric field magnitudes and lightning initiation in thunderstorms

Lightning may be initiated via an electron avalanche that may occur when energetic electrons (∼1 MeV) are accelerated by thunderstorm electric fields to velocities sufficient to produce new energetic electrons during ionizing collisions with nitrogen or oxygen molecules. For the avalanche to occur, the thunderstorm electric field must exceed a critical value called the breakeven field. At any altitude the breakeven field is substantially less than the field usually thought necessary either for dielectric breakdown or for streamer propagation. We show that 23 electric field soundings through thunderstorms seem to confirm that lightning occurs when the electric field exceeds the breakeven field. The soundings also show that the electric field inside storms tends to be limited to magnitudes less than or equal to the breakeven field. This breakeven mechanism may explain why electric field magnitudes greater than 150 kV m−1 are rarely found inside thunderstorms. It may also help explain the initiation of lightning and other types of discharges that either propagate upward from the tops of thunderstorms or occur above them.

Electrical structure in thunderstorm convective regions: 3. Synthesis

In this paper, results from nearly 50 electric field soundings through convective regions of mesoscale convective systems (MCSs), isolated supercells, and isolated New Mexican mountain storms are compared and synthesized. These three types of thunderstorm convection are found to have a common, basic electrical structure. Within convective updrafts the basic charge structure has four charge regions, alternating in polarity, and the lowest is positive. Outside updrafts of convection there are typically at least six charge regions, alternating in polarity, and the lowest is again positive. Among the three storm types, there are differences in the heights and temperatures at which the basic four charge regions are found in updrafts. The height (temperature) of the center of the main negative charge region averages 6.93 km (-16°C) in MCS convective region updrafts, 9.12 km (-22°C) in supercell updrafts, and 6.05 km (-7°C) in New Mexican mountain storm updrafts. In updraft soundings through all three storm types, the center height of the main negative charge region increases with increasing average balloon ascent rate and updraft speed at a rate of about 0.3 km per 1 m s -1 , with a correlation coefficient of 0.94. A schematic illustrates the basic four- and six-charge structure for thunderstorm convective regions, and it is offered as an improved model for thunderstorm charge structure.

Initial results from simultaneous observation of X-rays and electric fields in a thunderstorm

With an X ray detector designed for flight on a free balloon, we obtained a sounding of X ray intensity and electric-field strength in a mesoscale convective system (MCS) near Norman, Oklahoma, in the spring of 1995. The balloon passed through a region of high electric field strength, at which time an increase in X ray intensity of about 2 orders of magnitude occurred, lasting for approximately 1 min. The X ray intensity returned to background levels at the time of a lightning flash that reduced the electric field strength measured at the balloon. This observation suggests that the production mechanism for the X rays we observed is related to the storm electric field and not necessarily to lightning discharge processes.

Electrical structure in thunderstorm convective regions: 1. Mesoscale convective systems

Electric field (E) soundings through convective regions of mesoscale convective systems (MCSs) are examined in this paper. Ten E soundings through updrafts in MCS convective regions and five soundings in MCS convective regions outside updrafts are used to show that a typical electrical structure exists in this region. These 15 E soundings plus one other previously published sounding, which is included in this analysis, comprise all known soundings in the convective region of MCSs. The basic charge structure identified in MCS updrafts consists of four charge regions, alternating in polarity, with the lowest region positive and the highest region negative. The basic charge structure outside updrafts in MCS convective regions has six charge regions, alternating in polarity, with a positive charge region lowest. Maximum E magnitudes of both polarities are larger and are located at lower heights in soundings outside updrafts compared to those within updrafts. Excepting the upper positive charge, inferred charge regions are shallower and have larger charge densities outside updrafts. The center of the main negative charge is lower and warmer outside updrafts (5.5 km, -6.2°C) than within updrafts (6.9 km, -15.7°C). These features inside and outside updrafts are incorporated in a new conceptual model of MCS convective region charge structure. Also, an expanded conceptual model is developed, using previously published data from other parts of MCSs. This more complete conceptual model shows the typical electrical charge, airflow, and reflectivity features in the stratiform region, the transition zone, and the convective region of midlatitude mesoscale convective systems.

Horizontal Distribution of Electrical and Meteorological Conditions across the Stratiform Region of a Mesoscale Convective System

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 storms 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 struct...

Two Types of Vertical Electrical Structures in Stratiform Precipitation Regions of Mesoscale Convective Systems

Abstract Electric field (E) soundings in the stratiform regions and transition zones of mesoscale convective systems (MCSs) are reported. Most of the E soundings were made during the 1991 Cooperative Oklahoma Profiler Studies (COPS-91). Multiple E soundings were made in several MCSs. All of the E soundings collected here can be grouped into one of two types. Within each type the soundings and the inferred charge structures are remarkably similar from one place in an MCS to another and from one MCS to another. The charge regions inferred from the E soundings are hundreds of meters thick and have charged densities up to 5.3 nCm−3. Typically, the maximum E in the soundings is about 100 kV m−1. Here, E soundings from three classes of MCSs are discussed. The bow-echo MCSs have simpler vertical charge structures with tour main charge regions, while squall-line MCSs and predominantly stratiform MCSs have five main charge regions. In all of the E soundings there is a substantial region of charge and a large E at ...

X‐ray pulses observed above a mesoscale convective system

During a balloon flight into and above the stratiform region of a mesoscale convective system, we observed three x-ray pulses while the balloon was at an altitude of approximately 15 km MSL (atmospheric pressure of 130 mb). These pulses were one to two orders of magnitude above the background x-ray count rate with peak fluxes between 37 and 270 (cm²-s-sr)−1 and durations of about one second. No significant electric field was measured at the time of these pulses.

Electrical structure in thunderstorm convective regions: 2. Isolated storms

Electric field (E) soundings through the convective regions of two types of isolated thunderstorms are examined. Analysis of seven soundings through strong updrafts of isolated supercell storms show that the basic E structure there has three |E| peaks: a positive peak near 8 km height, a negative peak near 10.5 km, and a positive peak near 12 km. Strong updraft soundings are those with average balloon ascent rates in excess of 10 m s -1 . The basic charge structure in strong updrafts of supercells has four charge regions of alternating polarity. The lowermost charge is positive, between about 4 and 8 km, and the uppermost region is negative. All the supercell updraft soundings are incomplete due to balloon burst or lightning-induced instrument destruction below cloud top. Six supercell soundings that ascended outside strong updrafts are used to show that the E and charge structures there are more complex than within the strong updrafts. Analysis of 15 new or previously published soundings through small, New Mexican mountain thunderstorms indicates that the basic E structure in or near their convective cores consists of three |E| peaks: a lower positive peak at about 5 km height, a midlevel negative peak near 6.5 km, and an upper positive peak near 9.5 km. The basic charge structure near the center of New Mexican storm convection has four charge regions, alternating in polarity, with a positive charge region lowest. Soundings in New Mexican convection that did not ascend near the convective cores show more complex E profiles and charge structures.

Electric fields and charges near 0°C in stratiform clouds

Abstract Earlier studies of mesoscale convective system stratiform regions have shown that large electric fields and charge densities are found near the 0°C level. Here 12 soundings of the electric field were analyzed through the 0°C level in various types of electrified stratiform clouds. For each electric field sounding, the thermodynamic sounding and supporting radar data were also studied. For comparison, five soundings not from stratiform clouds were included. Charge densities were found at or near 0°C in the stratiform clouds of at least 1 nC m−3 in eight of the soundings, and four of those had charge densities of at least 2 nC m−3. Of the stratiform soundings, 11 had an electric field magnitude of greater than 50 kV m−1 near 0°C, and 7 of those had an electric field magnitude of at least 75 kV m−1. The evidence suggests that melting may be the primary cause of the charge density found at and below 0°C in electrified stratiform clouds. In all 12 of the stratiform soundings, positive charge density w...

Electric field measurements above mesoscale convective systems

We show that electric field discontinuities occur above the stratiform clouds associated with mesoscale convective systems. Above cloud top, 12 discontinuities were observed at altitudes between 10 and 16 km. The field changes of the discontinuities ranged from −1.1 to −4.0 kV m−1. The data suggest that the electric field discontinuities were caused by coincident, positive, cloud-to-ground lightning flashes. The coincident ground flashes included both single and multiple return stroke flashes, with first-stroke peak currents between 20 and 154 kA. We modeled the electric field change that would occur if lightning discharged a horizontally extensive positive charge layer within the stratiform cloud. In the model, disks with charge densities of 1 and 3 nC m−3, a thickness of 400 m, and diameters ranging from 20 to 200 km were discharged and produced field changes similar to the observed above-cloud field discontinuities. Our results support the idea that sprites may be initiated by above-cloud field changes caused by positive cloud-to-ground lightning flashes that discharge a horizontally extensive charge region in the stratiform cloud of a mesoscale convective system. During the time between the electric field discontinuities the electric field above the stratiform clouds was −0.5 to −1.0 kV m−1; this field may be important in the global electrical circuit because the stratiform clouds have large horizontal extents (∼104 km2).