AE41A-01 INVITED 08:00h
Do Extensive Air Showers of Cosmic-Ray Secondaries Initiate Lightning, and If So, How Would We Know?
The primary source of ionization in the atmosphere below approximately 50 km altitude is energetic penetrating radiation from extraterrestrial sources, known as cosmic rays. Runaway electron theory suggests that the electron population produced in this process can serve as seeds for initiation of electrical discharges when strong electric fields such as those generated in and above thunderstorms are present. We undertake an examination of the possibility, suggested explicitly by a few and accepted implicitly by many, that cosmic rays initiate lightning. The development of a strong measurable discharge depends not only on the availability of seed electrons but also on the fundamental nature of the discharge mechanism itself. Recent theoretical work, such as that of Gurevich, Milikh, and Roussel-Dupre (1992) and recent observations of X-ray emissions during times of observed elevated electric fields in thunderstorms, such as those by Eack et al. (1996) suggest that the combination of ionization by cosmic-rays and runaway breakdown may well be responsible for initiating electrical discharges in thunderstorms as suggested by Gurevich, Zybin, and Roussel-Dupre (1999). We set out to see if existing cosmic-ray observations in conjunction with lightning location data could provide suitable tests of that hypothesis. It turns out that co-located arrays of cosmic-ray detectors and lightning location systems are not necessarily sufficient because of the variability of the spatial distribution and orientation of extensive air showers with respect to active thunderstorm regions. However, we now have an idea of the requirements for observations that could test runaway breakdown hypotheses for initiation of lightning by cosmic rays.
AE41A-02 08:16h
Observed Electric Fields Associated with Lightning Initiation
The main goal of this presentation is to determine if conditions exist in a thunderstorm such that a runaway breakdown avalanche could occur. We compare in situ electric field (E) measurements and inferred lightning initiation locations of cloud-to-ground and intracloud flashes to several proposed thresholds for runaway breakdown. For these flashes we estimate the vertical depth and the volume in which E exceeds one or more of the runaway thresholds. The presentation focuses on three cloud-to-ground flashes that initiated in the same thunderstorm region within a few minutes of each other. The maximum measured E in the region was 186 kV m$^{-1}$ at 5.77 km altitude, which for runaway electrons is equivalent to 370 kV m$^{-1}$ at sea level; this E value is 130-183% of the various runaway breakdown thresholds. In addition, the volume where E exceeded the runaway thresholds was estimated to be 1-4 km$^{3}$, with a vertical depth of 1000 - 1200 m. At least within part of this volume (and perhaps in most of it) the vertical scale height for exponential growth of runaway electrons was 100 m or less. Thus for these three flashes the conditions necessary for runaway breakdown existed, and runaway breakdown could have initiated the flashes.
AE41A-03 INVITED 08:28h
X-ray emission from natural and triggered lightning
We report on x-ray observations of lightning made at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, FL. Using NaI(Tl)/photomultiplier tube detectors housed inside heavy aluminum boxes designed to keep out light, moisture and RF noise, we have measured x-ray emission from nearby natural lightning and rocket-triggered lightning. Electric fields and channel-base currents (for triggered lightning only) were also recorded. The x-ray emission most often occurs during the stepped and dart leader phases with possibly some emission occurring during the very beginning of the return strokes. The energy spectra for both natural and triggered lightning typically extend up to a few hundred keV, and the x-rays arrive in a sequence of discrete bursts, less than 1 microsecond in duration, with the stepped leader emission usually starting approximately 1 millisecond before the return stroke and the dart leader emission usually starting within about 100 microseconds of the return stroke. In this presentation, we will review observations made during the 2002 and 2003 seasons and present new results from the 2004 season, including new x-ray data from several natural lightning strikes. In addition, we will discuss the Thunderstorm Energetic Radiation Array (TERA), currently under construction at the ICLRT.
AE41A-04 08:44h
The dE/dt and E Waveforms Radiated by Leader Steps Just Before the First Return Stroke in Cloud-to-Ocean Lightning
We have analyzed the shapes and other characteristics of the electric field, E, and dE/dt waveforms that were radiated by leader steps just before the first return stroke in cloud-to-ocean lightning. dE/dt waveforms were recorded using an 8-bit digitizer sampling at 100 MHz, and an integrated waveform, Eint, was computed by numerically integrating dE/dt and comparing the result with an analog E waveform digitized at 10 MHz. All signals were recorded under conditions where the lightning locations were known and there was minimal distortion in the fields due to the effects of ground-wave propagation. The dE/dt waveforms radiated by leader steps tend to fall into three categories: (1) "simple" - an isolated negative peak that is immediately followed by a positive overshoot (where negative polarity follows the normal physics convention), (2) "double" - two simple waveforms that occur at almost the same time, and (3) "burst" - a complex cluster of pulses with a total duration of about one microsecond. In this paper, we will give examples of each of these waveform types, and we will summarize their characteristics on a submicrosecond time-scale. For example, in an interval starting 9 $\mu$s before to 4 $\mu$s before the largest, negative (dominant) peak in dE/dt peak in the return stroke, 131 first strokes produced a total of 296 impulses with a peak amplitude greater than 10% of the dominant peak, and the average amplitude of these pulses was 0.21 of the dominant peak. The last leader step in a 12 $\mu$s interval before the dominant peak was a simple waveform in 51 first strokes, and in these cases, the average time-interval between the peak dE/dt of the step and the dominant peak of the stroke was 5.8 $\pm$ 1.7 $\mu$s, a value that is in good agreement with prior measurements. The median full-width-at-half-maximum (FWHM) of 274 simple Eint signatures was 141 ns, and the associated mean and standard deviation were 187 $\pm$ 131 ns.
AE41A-05 08:56h
Leader/Return-Stroke-Like Processes in the Initial Stage of Rocket-Triggered Lightning
The process of cutoff and re-establishment of current during the initial stage of rocket-triggered lightning is examined and discussed. Linear streak film, video, current, and electric field records from 9 triggered-lightning flashes are analyzed. All of the data were acquired at Camp Blanding, FL, in 2002 and 2003. It is shown that in some rocket-triggered events, the process of cutoff and re-establishment during the initial stage is similar to a leader/return-stroke process, although the currents in this process are typically one to two orders of magnitude smaller than those in a natural or triggered subsequent stroke. In some cases, this process occurs two or three times prior to the successful resumption of current in the channel; these failed attempts at current re-establishment are typically an order of magnitude smaller than the process which finally reestablishes the current. The processes described are very similar to those reported by Rakov et al. [2003]. It is notable that not every rocket-triggered event contains these leader/returnstroke- like processes during the initial stage. References: Rakov, V. A., D. E. Crawford, V. Kodali, V. P. Idone, M. A. Uman, G. H. Schnetzer, and K. J. Rambo, Cutoff and reestablishment of current in rocket-triggered lightning, J. Geophys. Res., 108(D23), 4747, doi:10.1029/2003JD003694, 2003.
AE41A-06 09:08h
Upward Lightning Flashes Observed at the 200-m Fukui Chimney in Winter
The study of upward lightning flashes is very important for the lightning protection methods of tall structures. For over twenty years we have been observing the lightning discharge at the 200-m Fukui chimney in winter in Japan for clarifying winter lightning characteristics. About thirty to forty events of the lightning flashes are observed in a winter season. About ninety percent of all the recorded events were the upward lightning flashes initiated by an upward moving positively charged leader. About ten percent was the lightning initiated by an upward moving negatively charged leader. Some of the lightning produced the subsequent discharge composed of downward leader and upward return stroke sequences following the upward leader development. Two-dimensional average speed of upward moving positive leader was 3.6x105 m/sec with a range from 0.6x105 to 14x105 m/sec. Most positive leader progressed upward without any appreciable stepped motion. In the lightning initiated by the upward positive leader, the lightning discharge processes are categorized in four types. Type a: upward positive leader only. Type b: upward positive leader and downward return stroke sequences. Type c: downward negative leader and upward return stroke sequences following the upward positive leader development. Type d: downward positive leader and upward return stroke sequences following the upward positive leader development. Most upward-initiated lightning was observed without return strokes, as type a. Type b is a rare case. After the upward positive leader developed at a speed of 2.3x106 m/sec, downward return stroke was observed at average speed of 1.6x108 m/sec. The downward return-stroke decreased the speed from 2.5x108 m/sec to 2.5x107 m/sec as it developed towards the ground. Type c is similar to the lightning from the Empire State Building and the _gclassical triggered lightning_h of the rocket-triggered lightning. Type d is a new type of lightning discharge process from the tall structure in winter. The lightning flash initiated by the upward negative leader is rare case and it is very hard to observe it. One example was observed with a good condition. The average speed of the upward leader was 6x106 m/sec and the mean value of each step speeds was 3x107 m/sec. The upward leader moved with stepped motion like a downward stepped-leader of natural lightning in summer. Interestingly, the step length, the speed, and the peak current increased as the upward leader development and the measured upward leader current is big compared to the estimated downward leader current.
AE41A-07 INVITED 09:20h
Environmental control of cloud-to-ground lightning polarity in severe storms
The overwhelming majority of severe storms throughout the contiguous U.S. generate primarily ($>$75%) negative ground flashes (so-called negative storms). However, a certain subset of severe storms produces an anomalously high ($>$ 25%) percentage of positive ground flashes (so-called positive storms). The frequency of these "anomalous" positive storms varies regionally and seasonally. In some regions (e.g., central and northern plains) and months, these positive storms are common, representing 30% or more of all severe storms. Several past studies have noted that severe storms passing through similar mesoscale regions on a given day exhibit similar cloud-to-ground (CG) lightning behavior. This repeated observation led to the idea that the local mesoscale environment indirectly influences CG lightning polarity by directly controlling storm structure, dynamics, and microphysics, which in turn control storm electrification. Although a few studies have explored this relationship, the exact conditions favoring positive storms are poorly understood. The purpose of this study is to conduct a systematic comparison of the mesoscale environments for positive and negative storms, set in the framework of a testable hypothesis. According to our hypothesis, intense updrafts and associated high liquid water contents in positive storms lead to positive charging of graupel and hail via the non-inductive charging mechanism, an enhanced lower positive charge (or inverted-polarity), and increased frequency of positive CG lightning. We have utilized abundant environmental soundings taken during the International H$_{2}$O project (IHOP, May-June 2002) to document the relationship between mesoscale environment and dominant CG lightning polarity in the central plains. From hundreds of IHOP soundings, we carefully selected roughly fifty inflow proximity soundings that best represented the mesoscale environment of five (four) negative (positive) storm systems. Mean convective available potential energy (CAPE) estimated in the electrically important mixed phase zone (-10 to -40 degrees Celsius) was significantly higher in positive storms (1210 J kg$^{-1}$) than in negative storms (957 J kg$^{-1}$). Positive storms (14.7 m s$^{-1}$) had noticeably higher mean low-level (0-3 km) shear than negative storms (10.7 m s$^{-1}$). Interestingly, the mean lifting condensation level (LCL) for positive storms (2079 m) was 1.9 times higher than for negative storms (1121 m). The environmental freezing level (FL) was also lower in positive storms (3777 m) than in negative storms (4070 m). As a result, the mean warm cloud depth (FL-LCL) was dramatically larger in negative storms (2949 m) than in positive storms (1699 m). According to parcel theory, higher mixed phase CAPE directly leads to stronger updrafts and higher liquid water contents in positive storms. Larger low-level shear in positive storms aids in the development of intense low-to-mid level updrafts and enhanced liquid water contents through dynamic forcing. Higher LCL or cloud base height, which is associated with increased parcel size and decreased entrainment of dry air, in positive storms results in more efficient conversion of CAPE into kinetic energy and hence enhanced updraft strength and liquid water content. Reduced warm cloud depth in positive storms may decrease the amount of liquid water that is lost through the collision-coalescence and rainout process in a rising air parcel below the mixed phase zone, effectively increasing the amount of supercooled cloud water that is available for cloud electrification.
AE41A-08 09:36h
Overview of the 2003 and 2004 Field Program Phases of the Thunderstorm Electrification and Lightning Experiment (TELEX)
The scientific purpose of TELEX is to test and revise hypotheses concerning the inter-relationships among the wind field, microphysical characteristics, electrical structure, and lightning of isolated nonsevere and severe storms and mesoscale convective systems (MCSs). We conducted the field program of TELEX in central Oklahoma, 11 May-6 June 2003 and 9 May-20 June 2004. At the beginning of the 2003 field program, several new and upgraded observing systems were operating in central Oklahoma: the polarimetric part of the KOUN 11-cm wavelength Doppler radar, the Oklahoma three-dimensional Lightning Mapping Array (OK-LMA), and a mobile laboratory for storm intercept and mobile ballooning with up to four balloon soundings being possible simultaneously. Furthermore, the balloon-borne electric field meter was substantially upgraded the second year (both mechanically and electronically) to provide higher resolution data, including more accurate determination of instrument orientation to increase the resolution of three-dimensional electric field vectors in context of the three-dimensional structures of storm parameters and lightning. Presented in this paper are examples from both years in which instrumented balloons carrying a radiosonde and electric field meter obtained soundings. Other sensors were sometimes added to the instrument train by visiting researchers. In 2003, fourteen flights were made during seven missions. Owing to a scarcity of isolated deep convection in central Oklahoma during the 2003 program, the flights were mostly in nighttime multicellular storms and MCSs. In 2004, thirty-six flights were made during 13 ballooning missions. Soundings were made through nonsevere and severe storms and mesoscale convective systems. Several flights recorded data on both ascent and descent through the storm. Electric fields ranging above 150 kV/m were measured.
AE41A-09 09:48h
Polarimetric Radar and Electric Field Observations of a Multicell Storm
Much prior thunderstorm electrification research uses one-dimensional analyses of vertical profiles of the thunderstorm electric field, often incorporating cloud-to-ground lighting strike data and radar reflectivity observations. New instrumentation has provided the opportunity to investigate thunderstorm electrification and lightning in greater spatial detail. We present data from the late stages of a multicellular storm occurring on 28-29 June 2004 during the Thunderstorm Electrification and Lightning Experiment (TELEX) field program in central Oklahoma. Three-dimensional (3-D) vector electric field (measured by balloon sounding), total lighting mapping, and polarimetric radar are utilized. The maximum measured electric field exceeded -150 kV m$^{-1}$. Preliminary charge analysis using the electric field vectors indicates a positive layer below 0\deg C, followed by a large negative layer just above the melting level. Another positive and negative layer follow this. Polarimetric radar signatures within the melting layer are examined in the context of the electric field observations. Mapped lightning flashes are used to clarify and support the inferred charge structure. An interactive 3-D display is used to combine these data sources. Temporal evolution of the storm is also considered.