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Thesis (M.S.)--University of Florida, 2004. / Title from title page of source document. Document formatted into pages; contains 137 pages. Includes vita. Includes bibliographical references.

Thesis (M.S.)--University of Florida, 2002. / Title from title page of source document. Document formatted into pages; contains viii, 184 p.; also contains graphics. Includes vita. Includes bibliographical references.

Thesis (Ph.D.)--Tennessee Technological University, 2006. / Bibliography: leaves 122-128.

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In concrete reinforced buildings, the steel framework is required to be bonded and is often used as a cost effective method of lightning protection. In defining lightning protection zones, it is essential to understand where the lightning current due to a direct strike will flow. A number of models exist to evaluate the current distribution, but are often applied to relatively simple structures. Using Maxwell’s equations, an approximate skin effect model is proposed and used to eval- uate the lightning current distribution in a complex structure. A reduced scale model of a structure, consisting of conductors arranged in rings, is developed to verify the model. Particular attention is given to the return path of the current, ensuring an even distrib- ution of the current in the structure. The equivalent circuit showed an even distribution of current across each conductor at dc and low frequencies and a distribution that concentrated in the outer conductors for higher frequencies. The measurements from the structure confirmed that the current con- centrates in the outer conductors at high frequencies. Applying a reduced scale lightning impulse, it is shown that the majority of the current flows on the outermost conductors. Any current on the inner conductors is not only greatly decreased in magnitude, but significantly slower in time than the applied impulse.

M.Sc. / Lightning data are used in various applications from risk management to weather forecasting providing valuable insight to everyday operations. In South Africa, the South African Weather Services (SAWS) commissioned a new lightning detection network in February 2006. The nineteen-sensor network spread across South Africa overcomes previous disadvantages to lightning detection by providing wider coverage, higher detection efficiencies and better spatial resolution. This study investigated the influence of terrain elevation on lightning density in South Africa using data from the SAWS lightning detection network. Data incorporating one year from February 2006 to January 2007 were analysed using Geoinformatic Information System (GIS) programmes. The study incorporated two phases of analysis. The first included an evaluation of lightning climatology and the influence of elevation on lightning density for the whole of South Africa. The second phase, using four sub-regions of South Africa, included evaluation of local influences of elevation, slope and aspect on lightning density, for a period restricted to one month, December, during the peak lightning season. Lightning density was found to be highest over high topographic regions of the Highveld and escarpment regions. Enhanced lightning activity occurs over the eastern coast and over the Indian Ocean adjacent to the KwaZulu Natal coast. Diurnal variations in lightning activity indicate the influence of solar radiation on convective thunderstorm development with peaks occurring during the late afternoon and early evening. Results of the influence of elevation on lightning density over the country of South Africa, indicate that lightning density increases with elevation up to a maximum of 1 800 m decreasing thereafter, in contrast to literature reports indicating maximum activity at 1 200 m elevation. This result was found throughout the year except for winter months where thunderstorms are dominated by frontal activity. Lightning density in all four sub-regions was found to increase with elevation up to approximately 1 500 m and decrease thereafter. Diurnal patterns of lightning activity in all regions indicate the dominance of convective thunderstorms. Slope and aspect was found to be influential in certain regions but mostly as a secondary factor influencing thunderstorm development. Lightning density in South Africa is influenced to a great degree by terrain elevation especially over the highly elevated regions of the Highveld and Escarpment. This work contains the first comprehensive analysis of lightning frequency distributions from the new (2006) SAWS lightning detection network, and the first significant update of lightning distribution over southern Africa since 1975.

Twenty-two isolated, non-severe, warm season thunderstorms (ordinary thunderstorms) were examined to test possible correlations between three-dimensional lightning flash characteristics and the complex evolution of the microphysical and kinematic processes involved in the electrical development of thunderstorms. Nine of the thunderstorm cases examined occurred within range of Vaisala Inc.'s Dallas-Fort Worth (DFW) Lightning Detection and Ranging (LDAR) network and the other thirteen cases occurred within range of the Texas A&M University Houston LDAR Network. Cloud-toground (CG) flash data were obtained from the National Lightning Detection Network (NLDN). The kinematic and microphysical properties of each convective cell were inferred from level II Weather Surveillance Radar 1988-Doppler data. Lightning properties were compared to radar reflectivity, Vertically Integrated Liquid, Severe Hail Index, and Vertically Integrated Ice (VII) (i.e. the measure of the precipitation ice water content in and above the mixed phase (-40°C < T < -10° C zone). In addition, total lightning (intra-cloud (IC) and CG) characteristics were compared against CG lightning characteristics to determine if total lightning data provide stronger correlations to convective intensity and state (i.e., developing, mature, dissipating) than CG lightning data alone. The results show that VII is well correlated to the total flash rate with r2 values of 0.45 and 0.81 for Houston and Dallas cases, respectively, whereas CG flashes show much weaker correlations to VII. The cases also follow the conventional model of lightning within ordinary storms with IC dominating over CG lightning in the initial stages of convective development. An average of 19 IC flashes occur before the first CG flash with an average lead-time between the first IC to the first CG of 12 minutes. Flash heights showed little correlation to VII, which is in disagreement with past studies suggesting that strong correlations exist between flash heights and storm intensity. Integration of the results from this study into an operational forecast setting could lead to improvements in the nowcasting of lightning threats using radar, numerical weather prediction via assimilation of total lightning data, and the nowcasting of severe weather and lightning hazards to aviation.

Total lightning mapping, along with radar and NLDN cloud-to-ground lightning data, can be used to diagnose the severity of a storm. Analysis of the 13 October 2001 supercell event (Dallas-Fort Worth, Texas), some supercells of which were tornadic, shows that LDAR II lightning source heights (quartile, median, and 95th percentile heights) increased as the storms intensified. Most of the total lightning occurred where reflectivity cores extended upwards and within regions of reflectivity gradient rather than in reflectivity cores. A total lightning hole was associated with an intense, nontornadic supercell on 6 April 2003. This feature was nonexistent from all supercells analyzed during the 13 October case. During tornadogenesis, the radar and LDAR II data indicated updraft weakening. The height of the 30 dBZ radar top began to descend approximately 10 minutes (2 volume scans) before tornado touchdown in one storm. Total lightning and CG flash rates decreased by up to a factor of 5 to a minimum during an F2 tornado touchdown associated with this storm. LDAR II source heights all showed descent by 2-4 km during a 25 minute period prior to and during this tornado touchdown. This drastic trend of decreasing source heights was observed in two tornadic storms prior to and during tornado touchdown, but did not occur in non-tornadic supercells, suggesting that these parameters can be useful to forecasters. These observations agree with tornadogenesis theory that an updraft weakens and the mesocyclone can become divided (composed of both updraft and downdraft) when a storm becomes tornadic. LDAR II source density contours were comma-shaped in association with severe wind events within mesoscale convective systems (MCSs) on 13 October 2001 and 27 May 2002. This signature is similar to the radar reflectivity bow echo. Consistent relationships between severe weather, radar and lightning storm characteristics (i.e., lightning heights) were not found for cells within MCSs as was the case for supercells. Cell interactions within MCSs are believed to weaken these relationships as reflectivity and lightning from nearby storms contaminate the cells of interest. It is also more difficult to clearly define a cell within an MCS.