AE41A-01
Statistics of the optical lightning radiation source derived from satellite observations
The next generation of geostationary satellites will carry an optical sensor for the operational detection of lightning on the full visible disk. For the development of the detection instrument a characterisation of the expected signal from natural lightning is necessary. A huge data set of optical lightning observation was gathered by the LIS and OTD lightning detection sensors both operating on board of low orbit satellites. These data are analysed in order to derive the statistics of lightning position, temporal characteristics and optical radiance. The statistics from the smallest to the largest possible scales give a good characterisation of lightning distribution on the storm scale for the different locations on Earth. The results of the statistics are extended from the small field of view of the LIS and OTD sensors to full disk as seen from a geostationary position. This gives an empirical probability distribution function of lightning radiance in space and time with the realistic correlation characteristics on the storm scales. This distribution function can be used in a random lightning signal generator providing an input for the simulation of the lightning source in instrument studies. The scattering of the lightning signal in the clouds is simulated by a Monte-Carlo method for various lightning source positions and channel orientations. The statistics of the simulation results are compared against the satellite observations.
AE41A-02
Transient Synchronicity and Coupling of Lightning Flashes
We analyzed sequences of lightning flashes in Mediterranean winter thunderstorm, based on data obtained during the 2004/5 and 2005/6 seasons from ground-based lightning location systems. Patterns of clustering and synchronicity of flashes in individual thunderstorm cells, separated by tens to hundreds of kilometers, were identified. This is similar to early findings of lightning synchronicity based on space shuttle images [Yair et al., 2006], hinting at a possible mutual electromagnetic coupling of remote thunderstorms. We developed a theoretical model that is based on the "leaky fire and integrate" approach, commonly used in models of neural activity, in order to simulate the flashing behavior of a coupled network of thunderstorm cells. In this type of network, the intensity of the electric field Ei within thunderstorm (i) grows with time until it reaches the critical breakdown value, generates a lightning flash while its electric field drops to zero, simultaneously adding a delta-E to the intensity of the internal electric field in all thunderclouds (Ej,k,l…) that are linked to it. The value of delta-E is inversely proportional to the distance between the "firing" cloud i and its neighbors j,k,l... We assumed that all thunderstorms are identical and occupy a grid with random spacing and organization. Several topologies of the thunderstorm network were tested with varying degrees of coupling, assuming a fixed probability of links between active cells. The results suggest that when the coupling in the network is higher than a certain critical value, all thunderstorm cells will eventually flash in a synchronized manner. The physical mechanisms that are possibly interacting in such a network of coupled thunderclouds will be discussed.
AE41A-03
Small, Continual Lightning Activity in the Overshooting Turret of Supercell Storms
Several supercell storms have occurred within the region in which the Oklahoma Lightning Mapping Array (OKLMA) maps all three spatial dimensions of lightning. These storms span much of the supercell spectrum -- from non-tornadic storms to storms that produced strong tornadoes and from low-precipitation to heavy- precipitation morphologies. As noted by several studies, supercell storms tend to have much larger flash rates than ordinary isolated thunderstorms; maximum rates are typically hundreds of flashes per minute, even when considering only flashes that produce at least ten mapped points per flash. However, the OKLMA indicates that most of the flashes occurring within the main body of the storm during periods of high flash rates have quite small spatial extents, many with a long dimension of 3-10 km. Not usually included in these flash rates are a large number that appear to be isolated points (sometimes called singletons), each failing criteria of distance or time for associating it with other points in a flash. Often determining whether these isolated points are artifacts of the OKLMA is difficult, but in the overshooting top, they present a coherent pattern that appears plausible. They are distributed throughout a cap having horizontal dimensions comparable to that of the overshooting top and sitting near or on the upper surface. They occur continually, though they are too far apart in time or space to be associated in a flash with each other. A comparison with high-resolution reflectivity data for one storm observed by the two mobile 5-cm wavelength SMART-R radars shows that these isolated points were most concentrated near the top of the 40 dBZ echo in the overshooting turret, but some occurred higher, in regions of small reflectivity or just above the overshooting top. These points may be similar to the continual lightning noted by Bill Taylor in the upper region of a severe storm in the early 1980s.
AE41A-04
Bolt-from-the-Blue Lightning Discharges
Bolt-from-the-blue (BFB) discharges are produced by normally electrified thunderstorms, and lower negative charge indirectly to ground from the storm mid-levels via the upper positive charge region of the storm. Because BFBs can strike well away from the storm they are a particularly dangerous type of lightning to humans. The discharges are also very interesting scientifically - for example concerning the relative magnitudes of the upper positive and mid-level negative charges in storms and the presence of positive screening charge around the lateral cloud boundary. In this paper we present photographic and VHF lightning mapping observations of BFB flashes, including some spectacular observations obtained at Langmuir Laboratory during the 2007 summer thunderstorm season.
AE41A-05
Comparison of Lightning Channels Recorded by Video and Lightning Mapping Array
The Oklahoma Lightning Mapping Array (OKLMA)provides maps of lightning channel geometry by successive determinations of the locations of sources of VHF radio emissions from segments of discharge channels. During the Summer of 2007 we obtained a high-definintion video image of a lightning discharge that was also mapped by the OKLMA. We overlay and compare the two views of the same flash.
AE41A-06
Evolution of radar structure, total lightning, and sprite production in an Oklahoma mesoscale convective system on 20 June 2007
On 20 June 2007 a classic leading-line/trailing-stratiform mesoscale convective system (MCS) passed through central Oklahoma. This MCS consisted of two bowed convective lines that merged around 0600 UTC. Between 0000 and 0800 UTC, the storm produced 13,450 positive cloud-to-ground flashes (+CGs; > 10 kA) and 205,736 negative CGs (> 10 kA), and over 250 sprites, halos, and elves were captured on camera at ranges out to 814 km from Yucca Ridge, Colorado. Approximately 50 of these transient luminous events (TLEs) were within 2-D range (~200 km) of the Oklahoma Lightning Mapping Array (LMA). While negative CGs dominated the storm, the census of CGs with peak currents > 75 kA was nearly even, 712 positive to 725 negative. The large positives were mostly confined to the trailing stratiform and were associated with large impulse charge moment changes and TLE production. During the peak TLE production time the cloud canopy area of -50 deg C or colder was 560,000 km2. The evolution of the radar structure of this storm is being studied using 3-D composites developed from multiple radars covering Oklahoma, while 2-D and 3-D total lightning production is being examined using Oklahoma LMA data. In addition, a database of optically detected sprites linked to their parent positive CG discharges and impulse charge moment changes has been developed. Using these data, the possible dependence of sprite generation on the vertical structure of the stratiform region, as well as MCS organization, is being examined. In addition, the relationship between MCS electrical morphology (e.g., arrangement of charge layers in the stratiform region) and sprite production is being examined. In particular, one question being addressed is: How does the probability of a stratiform +CG to produce a sprite depend on the initiation location of the +CG, as well as the vertical location of the positive charge layer it is tapping? Initial results of this study will be presented.
AE41A-07
Comparison of Sprite Locations With Lightning Channel Structure
On June 25, 2000, sprites were produced above a decaying supercell storm in northwestern Kansas. A few of the sprites in the 05:29:06 UT sprite event were simultaneously recorded by light-intensified video cameras at the center of the New Mexico Tech Lightning Mapping Array (LMA) north of Burlington, Colorado and at FMA Research, Inc. in Fort Collins, Colorado. A detailed three dimensional VHF map of the parent discharge was obtained by the LMA. The triangulated sprite locations are correlated with the horizontal extent of the discharge and exhibit a tendency to favor the discharge periphery. A similar pattern was noted previously by Stanley [2000] for sprites above lightning discharges mapped by the Kennedy Space Center Lightning Detection and Ranging (LDAR) system. These observations will be compared with theoretical models of sprite initiation in order to gain important insights into the initiation process.
AE41A-08 [WITHDRAWN]
Early VLF perturbations observed in Crete, Greece in relation with TLEs over southwest Europe
In support of the summer EuroSprite campaigns in southwest Europe, and in the framework of an EU-RTN (European Union research training network) project (e.g., http://www.dsri.dk/cal/), a Stanford narrow band VLF receiver was installed in Crete, Greece (35.31 deg. N; 25.08 deg. E) and started operation in the summer of 2003. The Crete VLF station was capable of monitoring several transmitters, some of them chosen in order to provide VLF links that traverse subionospheric regions in the proximity of the areas viewed for transient luminous event detection (sprites and elves) by the EuroSprite cameras. In this presentation we summarize several observational findings regarding the so called "early" VLF events of ionospheric perturbations which are found to occur in relation with sprites and elves. In particular, we focus on a new category of early type perturbations, which, contrary to the so called "early/fast" events whose onset duration is less than ~20 ms, are characterized by a gradual growth and thus a "slow" onset duration ranging from about 0.5 to 2.5 s. These long growths are indicative of a new physical process at work which, following a sprite-causative cloud-to-ground discharge, leads to a gradual ionization build up in the lower ionosphere which can be responsible for the long onset duration of the observed perturbations. We postulate that such long onset durations are due to secondary ionization build-up in the upper D region below the nighttime VLF reflection heights, caused mainly by the impact on sprite-produced electrons of sequential electromagnetic pulses radiated upwards from in-cloud discharges. Also, the measured early event recoveries are modeled to obtain estimates of sprite-related electron density in the upper D region.