Lightning Team Abstracts
Lightning data from the U.S. National Lightning Detection Network (NLDN) is used to perform preliminary validation of the satellite-based Optical Transient Detector (OTD). Sensor precision, accuracy, detection efficiency and biases of the deployed instrument are considered. We estimate the sensor to have, on average, better than 30 km spatial and 100 ms temporal accuracy. The detection efficiency for cloud-to-ground lightning is about 45-70%, slightly higher for intracloud lightning. There are only marginal day/night biases in the dataset, although 55 day averaging is required to remove sampling-based diurnal lighting cycle biases. The sensor detects statistically significant differences in the optical characteristics of intracloud, negative cloud-to-ground and positive cloud-to-ground lightning, although it cannot a priori determine flash types.
Stratiform precipitation regions accompany convective activity on many spatial scales. The electrification of these regions is anomalous in a number of ways. Surface and above-cloud fields are often "inverted'' from normal thunderstorm conditions. Unusually large, bright, horizontal "spider'' lightning and high current and charge transfer positive cloud-to-ground (CG) lightning dominate in these regions. Mesospheric "red sprite'' emissions have to date been observed exclusively over stratiform cloud shields. We postulate that a dominant "inverted dipole'' charge structure may account for this anomalous electrification. This is based upon laboratory observations of charge separation which show that in low liquid water content (LWC) environments, or dry but ice-supersaturated environments, precipitation ice tends to charge positively (instead of negatively) upon collision with smaller crystals. Under typical stratiform cloud conditions, liquid water should be depleted and this charging regime favored. An inverted dipole would be the natural consequence of large-scale charge separation (net flux divergence of charged ice), given typical hydrometeor profiles. The inverted dipole hypothesis is tested using radar and electrical observations of four weakly organized, late-stage systems in Orlando, Albuquerque and the Western Pacific. Time-evolving, area-average vertical velocity profiles are inferred from single Doppler radar data. These profiles provide the forcing for a 1-D steady state microphysical retrieval, which yields vertical hydrometeor profiles and ice/water saturation conditions. The retrieved microphysical parameters are then combined with laboratory charge transfer measurements to infer the instantaneous charging behavior of the systems. Despite limitations in the analysis technique, the retrievals yield useful results. Total charge transfer drops only modestly as the storm enters the late (stratiform) stage, suggesting a continued active generator is plausible. Generator currents show an enhanced lowermost inverted dipole charging structure, which we may infer will result in a comparable inverted dipole charge structure, consistent with surface, in-situ and remote observations. Fine-scale vertical variations in ice and liquid water content may yield multipolar generator current profiles, despite unipolar charge transfer regimes. This suggests that multipoles observed in balloon soundings may not necessarily conflict with the simple ice-ice collisional charge separation mechanism. Overall, the results are consistent with, but not proof of, the inverted dipole model.
Global measurements of large, optically bright lightning events from the Optical Transient Detector (OTD) satellite are used to validate estimates of lightning location from single-station Schumann resonance (SR) data. Bearing estimates are obtained through conventional magnetic direction-finding techniques, while source range is estimated from the range-dependent impedance spectrum of individual SR transients. An analysis of 40 such transients suggests that single-station techniques can locate lightning globally with an accuracy of 1-2 Mm. This is confirmed by further validation at close ranges from flashes detected by the National Lightning Detection Network (NLDN). Observations with both OTD and SR systems may be useful in globally locating lightning with necessary, if not sufficient, characteristics to trigger mesospheric "red sprites".
Observations of two summertime mesoscale convective systems (MCS') reveal that mesospheric optical "sprite" phenomena are often coincident with both large amplitude positive cloud-to-ground lightning and transient Schumann resonance excitations of the entire earth-ionosphere cavity ("Q-bursts"). These observations, together with previous studies of MCS electrification, are supportive of a simple electrostatic mechanism for the triggering of sprites, following concepts originally set forth by C.T.R. Wilson, in which the rapid removal of large quantities of positive charge from a really extensive charge layer stresses the mesosphere to dielectric breakdown.
A diagnostic analysis of the VVP (Volume Velocity Processing) retrieval method is presented, with emphasis on understanding the technique as a linear, multivariate regression. Similarities and differences to the VAD/EVAD retrieval techniques are discussed, using this framework. Conventional regression diagnostics are then employed to quantitatively determine situations in which the VVP technique is likely to fail. An algorithm for preparation and analysis of a robust VVP retrieval is developed, and applied to synthetic and actual datasets with high temporal and spatial resolution. A fundamental (but quantifiable) limitation to some forms of VVP analysis is inadequate sampling dispersion in the n-space of the multivariate regression, manifest as a collinearity between the basis functions of some fitted parameters. Such collinearity may be manifest either in the definition of these basis functions or in their realization in a given sampling configuration. This nonorthogonality may cause numerical instability, variance inflation (decrease in robustness) and increased sensitivity to bias from neglected wind components. It is shown that these effects prevent the application of VVP to small azimuthal sectors of data. The behaviour of the VVP regression is further diagnosed over a wide range of sampling constraints, and reasonable sector limits established.
This report describes a transient luminosity observed at the altitude of the airglow layer (about 95 km) in coincidence with a lightning flash in a tropical oceanic thunderstorm directory beneath it. This even provides new evidence of direct coupling between lightning and ionospheric events. This luminous event in the ionosphere was the only one of its kind observed during an examination of several thousand images of lightning recorded under suitable viewing conditions with Space Shuttle cameras. Several possible mechanisms and interpretations are discussed briefly.
An examination and analysis of video images of lightning, captured by the payload bay TV cameras of the space shuttle, provided a variety of examples of lightning in the stratosphere above thunderstorms. These images were obtained on several recent shuttle flights while conducting the mesoscale lightning experiment (MLE). The images of stratospheric lightning illustrate the variety of filamentary and broad vertical discharges in the stratosphere that may accompany a lightning flash. A typical event is imaged as a single or multiple filament extending 30 to 40 km above a thunderstorm that is illuminated by a series of lightning strokes. Examples are found in temperate and tropical areas, over the oceans and the land.
The sequence of video tape observations of the upper atmospheric optical flashes called sprites, jets, starters, and ELVES are described in the successive phases of search, discovery, confirmation, and exploration for the years before 1993. Although there were credible eyewitness accounts from ground observers and pilots, these reports did not inspire a systematic search for hard evidence of such phenomena. The science community would instead wait for serendipitous observations to move the leading edge of this science forward. The phenomenon, now known as a sprite, was first accidently documented on ground based videotape recordings on the night of July 6, 1989. Video observations from the space shuttle acquired from 1989 through 1991 provided 17 additional examples to confirm the existence of the sprites phenomenon. Successful video observations from a mountain ridge by Lyons, starting July 7, 1993, and night-time aircraft video observations by Sentman and Wescott on July 8, 1993 established the basic science of the sprite phenomena by acquiring and analyzing data based on hundreds of new events. The 1994 Sprites campaign and the video titled "Red Sprites and Blue Jets" popularized the name sprite and provided a vocabulary of terms to describe the visual attributes. Prior to this video, investigators used a variety of vague descriptive words to describe the individual events. Also, during the 1994 campaign, Wescott and coworkers obtained the first quantitative measurements of jets and provided the name "blue jets". A third phenomenon was discovered in video from the STS-41 mission (October 1990) in the lower ionosphere directly above an active thunderstorm. It consisted of a large horizontal brightening several hundred kilometers across at the altitude of the airglow layer. In 1995, Lyons and associates confirmed the existence of this type of very brief brightening which they named Emissions of Light and Very Low Frequency Perturbations From Electromagnetic Pulse Sources (ELVES). Because sprites, jets, and ELVES have appeared for millennia, their discovery was inevitable. The partial history related in this paper outlines the unsophisticated activities using shuttle video and the dissemination of the results by video presentations during the early phases of sprite research. This paper does not attempt to evaluate the advances in science based on the measurement campaigns of Lyons, Sentman and the many other investigators.
An examination and analysis of video images of lightning, captured by the payload bay TV cameras of the space shuttle, provided a variety of examples of lightning in the stratosphere above thunderstorms. These images were obtained on several recent shuttle flights while conducting the mesoscale lightning experiment (MLE). The images of stratospheric lightning illustrate the variety of filamentary and broad vertical discharges in the stratosphere that may accompany a lightning flash. A typical event is imaged as a single or multiple filament extending 30 to 40 km above a thunderstorm that is illuminated by a series of lightning strokes. Examples are found in temperate and tropical areas, over the oceans and the land.
Lightning observations from satellites in low Earth orbit have been made over the past 25 years, producing estimates of global flash frequency by season and latitude as well as information on diurnal variations. However, these measurements have suffered from low detection efficiencies, poor spatial resolutions, and the inability to continuously monitor specific storms or storm systems. Using results of investigations with a high-altitude NASA U-2 aircraft and other research, a space sensor capable of mapping both intracloud and cloud-to-ground lightning discharges from geostationary orbit during day and night with a spatial resolution of 10 km and a detection efficiency of 90% is currently being developed. In addition, this sensor, which is called the Lightning Mapper Sensor (LMS), will monitor storms on a continual basis. The combination of modern solid state mosaic focal planes with extensive on-board signal processing in the LMS provides a powerful technique for the detection of weak background-contaminated signals and permits the detection of lightning during the day. The LMS has a 10.5\deg\ field of view that covers all of the continental United States, large oceanic areas, all of Central America, much of South America including the Andes and the Amazon Basin, and large regions of the inter-tropical convergence zone. It is anticipated that the LMS will be flown on a GOES satellite in the mid-1990s. The characteristics and design of the LMS are presented as well as a discussion of the scientific research that will be possible with this instrument.
Vertical motion profiles can be diagnosed with the mass continuity equation using horizontal divergence fields derived from various single-Doppler radar techniques such as EVAD (extended velocity-azimuth display), CEVAD (concurrent extended velocity-azimuth display), and VVP (volume velocity processing). These methods allow for the retrieval of mesoscale air motions in precipitating regions when the wind field is relatively homogeneous. In contrast, VHF wind profiler data can provide a direct measurement of vertical motion, albeith across a much smaller domain compared to the single-Doppler radar techniques. In this study, we compare horizontal divergence and vertical motion patterns derived from the various single-Doppler methods with those obtained from VHF profiler data. The diagnosed profiles of horizontal divergence and vertical velocity from the single-Doppler (scanning radar) techniques are in qualitative agreement in the lower troposphere but often exhibit large variability at higher levels. Because of less stringent radar echo requirements, the VVP technique often analyzed data above the top of the EVAD-CEVAD analysis domain, resulting in a deeper layer of upper-level divergence. The CEVAD technique often produced a deeper and larger region of upward motion despite similar profiles of divergence, probably due to the CEVAD top boundary condition specification of particle terminal fall speed as opposed to the vertical air motion, as well as to the adjustment procedure employed during the regression solutions. The wind profiler data showed much larger vertical gradients and magnitudes of divergence and vertical velocity when averaged over the same time interval required to collect data for a single-Doppler retrieval. However, when all the available data were composited, the high-frequency variability in the wind profiler retrievals was reduced resulting in relatively good agreement between all analysis methods. The wind profiler usually sampled vertical motion (divergence) several kilometers above the single-Doppler retrievals, which the authors attribute to the stringent precipitation echo coverage requirements imposed by the scanning radar analysis techniques, thus limiting their vertical extent near echo top.
A thorough examination of the results of a time-dependent computer model of a dipole thunderstorm revealed that there are numerous similarities between the time-averaged electrical properties and the steady state properties of an active thunderstorm. Thus, the electrical behavior of the atmosphere in the vicinity of a thunderstorm can be determined with a formulation similar to what was first described by Holzer and Saxon in 1952. From the Maxwell continuity equation of electric current, a simple analytical equation was derived that expresses a thunderstorm's average current contribution to the global electric circuit in terms of the generator current within the thundercloud, the intracloud lightning current, the cloud-to-ground lightning current, the altitudes of the charge centers, and the conductivity profile of the atmosphere. This equation was found to be nearly as accurate as the more computationally expensive numerical model, even when it is applied to a thunderstorm with a reduced conductivity thundercloud, a time-varying generator current, a varying flash rate, and a changing lightning mix.
The amount of upward current provided to the ionosphere by a thunderstorm that appeared over the Kennedy Space Center (KSC) in July 11, 1978, is reexamined using an analytic equation that describes a bipolar thunderstorm's current contribution to the global circuit in terms of its generator current, lightning currents, the altitudes of its charge centers, and the conductivity profile of the atmosphere. Ground-based measurements, which were obtained from a network of electric field mills positioned at various distances from the thunderstorm, were used to characterize the electrical activity inside the thundercloud. The location of the lightning discharges, the type of lightning, and the amount of charge neutralized during this thunderstorm were computed through a least-squares inversion of the measured changes in the electric fields following each lightning discharge. These measurements provided the information necessary to implement the analytic equation, and consequently, a time-averaged estimate of this thunderstorm's current contribution to the global circuit was calculated. From these results the amount of conduction current supplied to the atmosphere by this small thunderstorm was computed to be less than 25% of the time-averaged generator current that flowed between the two vertically displaced charge centers.
A one-speed Boltzmann transport theory, with diffusion approximations, is applied to study the radiative transfer properties of lightning in optically thick thunderclouds. Near-infrared ($\lambda = 0.7774 \mu$m) photons associated with a prominent oxygen emission triplet in the lighting spectrum are considered. Transient and spatially complex lightning radiation sources are placed inside a rectangular parallelepiped thundercloud geometry and the effects of multiple scattering are studied. The cloud is assumed to be composed of a homogeneous collection of identical spherical water droplets, each droplet a nearly conservative, anisotropic scatterer. Conceptually, we treat the thundercloud like a nuclear reactor, with photons replaced by neutrons, and utilize standard one-speed neutron diffusion techniques common in nuclear reactor analyses. Valid analytic results for the intensity distribution (expanded in spherical harmonics) are obtained for regions sufficiently far from sources. Model estimates of the arrival-time delay and pulse width broadening of lightning signals radiated from within the cloud are determined and the results are in good agreement with both experimental data and previous Monte Carlo estimates. Additional model studies of this kind will be used to study the general information content of cloud top lightning radiation signatures.
Recent measurements show that the Maxwell current densities, $\vec J_m$, produced by thunderclouds are quasi-steady at the ground even in the presence of lightning. These measurements also show that $\vec J_m$ is usually dominated by the displacement current component when the electric field is close to zero; therefore, $\vec J_m$ can often be estimated from just an electric field measurement. Maps that show the time-development of the pattern of the average $\vec J_m$ over a relatively large area are given for one Florida thunderstorm. These maps are in good agreement with radar echoes and the locations of lightning charges, and can now be used in further studies of thunderstorm electricity.
In the fall of 1992 a lightning direction finder network was deployed in the western Pacific Ocean in the area of Papua, New Guinea. The instruments were modified to detect cloud-to-ground lightning out to a distance of 900 km. Data were collected from cloud-to-ground lightning flashes for the period 26 Nov 1992-15 Jan 1994. The analyses are presented for the period 1 Jan 1993-31 Dec 1993. In addition, a waveform recorder was located at Kavieng to record both cloud-to-ground lightning and intracloud lightning in order to provide an estimate of the complete lightning activity. The data from these instruments are to be analyzed in conjunction with the data from ship and airborne radars, in-cloud microphysics, and electrical measurements from both the ER-2 and DC-8. The waveform instrumentation operated from approximately mid-January through February 1993. Over 150,000 waveforms were recorded. During the year, Jan--Dec 1993, the cloud-to-ground lightning location network recorded 857,000 first strokes of which 5.6% were of positive polarity. During the same period, 437,000 subsequent strokes were recorded. The peak annual flash density was measured to be 2.0 flashes/km-2 centered on the western coastline of the island of New Britain, just southwest of Rabaul. The annual peak lightning flash density over the Intensive Flux Array of Tropical Oceans Global Atmosphere Coupled Ocean-Atmosphere Response Experiment was 0.1 flashes/km-2, or more than an order of magnitude less than that measured near land. The diurnal lightning frequency peaked at 1600 UTC (0200 LT), perhaps in coincidence with the nighttime land-breeze convergence along the coast of New Britain. Median monthly negative peak currents are in the 20-30 kA range, with first stroke peak currents typically exceeding subsequent peak currents. Median monthly positive peak currents are typically 30 kA with one month (June) having a value of 60 kA. Positive polar conductivity was measured by an ER-2 flight from 40\deg N geomagnetic latitude to 28\deg S geomagnetic latitude. The measurements show that the air conductivity is about a factor of 0.6 lower in the Tropics than in the midlatitudes. Consequently, a tropical storm will produce higher field values aloft for the same rate of electrical current generation. An ER-2 overflight of tropical cyclone Oliver on 7 Feb 1993 measured electric fields and 85-GHz brightness temperatures. The measurements reveal electrification in the eye wall cloud region with ice, but no lightning was observed.
An instrumented free balloon measured electric fields and field changes as it rose through a thundercloud above Langmuir Laboratory, New Mexico. The variation of the electric field with altitude implied that the cloud contained negative space charge of density -0.6 to -4 nC m-3 between 5.5 and 8.0 km MSL. The environmental temperature at these levels ranged from -5°C to -20°C. Our measurements imply that the aerial extent of this negative charge center was significantly greater than that of the cloud's intense precipitation shafts. At altitudes greater than 8 km, the instrument ascended past net positive charge. We also inferred from our measurements positive space charge adjacent to the Earth's surface (concentration 0.6 nC/m-3) and in the lower portion of the cloud (1.0 nC/m-3). Electric field changes from intracloud lightning were interpreted by using a simple model for the developing streamer of the initial phase. Thunder source reconstructions provided estimates for the orientation of lightning channels. Seven 'streamers' so analyzed propagated on the average, at 5x104 m/s-1 and carried a current of 390 A. The mean charge dissipated during a flash was 30°C.
Cloud temperature, liquid water content, and vertical air velocity are all considered in evaluating the microphysical growth state of ice phase precipitation particles in the atmosphere. The large-scale observations taken together with in situ measurements indicate that the most prevalent growth condition for large ice particles in active convection is sublimation during rimming, whereas the most prevalent growth condition in stratiform precipitation is vapor deposition. The large-scale electrical observations lend further support to the idea that particles warmed by rimming into sublimation charge negatively and particles in vapor deposition charge positively in collisions with small ice particles.
A multiple Doppler radar network can be constructed using only one, traditional, transmitting pencil-beam radara and one or more passive, low gain, non-transmitting receiving sites. Radiation scattered from the pencil beam of the transmitting radar as it penetrates weather targets can be detected at the receive-only sites as well as at the active transmitter. The Doppler shifts of the radiation received at all the sites can be used to construct two- and three-dimensional windfields in a manner similar to that used with traditional Doppler radar networks. There are unique scientific advantages to a bistatic multiple Doppler network: 1.All radial velocity measurements from individual pulse volumes are collected simultaneously since there is only one source of radiation. 2.The intensity of the obliquely scattered radiation can be compared to Rayleigh scattering predictions and used for hail detection. 3.Rapid scanning of localized weather phenomena can be aided by elimination of the need to scan with multiple radars. This type of radar network also has significant economic advantages. Passive sites contain no high voltage transmitting equipment or large rotating antennas. They require no operators and much less maintenance. We estimate initial investment costs and subsequent operational and maintenance costs are less than 1/30 that of conventional radars. There are shortcomings particular to these types of networks: 1.Passive, low gain receiving sites are more sensitive to contamination from transmitter side-lobes and to secondary scattering from weather echoes. 2.Low gain receiving sites are less sensitive to weak weather echoes. 3.Cartesian (u,v,w) windfields derived from bistatic network data exhibit about twice the expected error as those constructed from data from traditional active networks containing equal numbers of radars. Multiple scattering and side-lobe contamination levels are acceptable in most situations and can be reduced with the use of higher gain receiveing antennas. The low sensitivity and higher erros of the bistatic networks can be ameliorated with the use of multiple passive sites, a practical solution due to their very low cost.
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