Browsing by Subject "Lightning"
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Item An evaluation of lightning flash characteristics using LDAR and NLDN networks with warm season southeast Texas thunderstorms(Texas A&M University, 2008-10-10) Jurecka, Joseph WilliamA comparison of flash parameters from the National Lightning Detection Network (NLDN) is made with data obtained from the Houston Lightning Detection and Ranging II (LDAR) network. This research focuses on relating the peak current and number of strokes in a negative flash (multiplicity) of lightning with the spatial extent and mean altitude of three-dimensional lightning in 1407 flashes as mapped by the LDAR network. It is shown that increasing negative multiplicities over the range two through ten exhibit, on average, a higher flash extent with higher multiplicities. Singlestroke flashes have mean heights of nearly 2 km greater. Higher order multiplicities (2 to 10+) were correlated with mean source heights near 8 km. Increasing multiplicity tends to be associated with greater flash extents increasing more horizontally than vertically with a 50% to 70% increase in flash extent. No obvious relationship between peak current and flash extent was observed. Examining peak current and mean height shows that low current flashes (<10kA) exhibit higher mean heights. However, this may be due to intra-cloud only flashes being reported as cloud to ground events by the NLDN. Bipolar flashes do not show much variation with height and flash extent with the exception of negative-first bipolar flashes, which exhibited mean flash extents twice that of other types. Finally, the flash detection efficiency is 99.7% within 60 km of the network center.Item An examination of the relationship between cool season tornadoes and cloud-to-ground lightning flashes(Texas A&M University, 2007-04-25) Butts, Douglas Allen, Jr.The southeast United States is prone to severe weather throughout the year. Despite technological advances, some severe weather events occasionally remain unwarned in this part of the country. Past studies examined the relationship between cloud-to-ground (CG) lightning and warm season severe weather episodes. The present study examined the relationship between cool season tornadoes and CG lightning, with a focus over the southeastern United States, where most cool season tornadoes occur. Data from the Storm Prediction Center and National Lightning Detection Network (NLDN) were used to investigate CG lightning properties within 50 km and one hour before tornado touchdown. This was completed over a period of 13 cool seasons from October 1989 through March 2002. Of 3325 tornado events, 2358 contained at least one NLDN-detectable flash. CG lightning attributes of peak current, multiplicity, and flash density compared well with those of prior warm season lightning research. Overall event frequency appeared to be lower than in the warm season. Almost all Central Plains events were accompanied by at least one NLDN-detectable flash. Up to 70% of tornado events near the Gulf of Mexico and Atlantic coasts contained no NLDN-detectable lightning. Although it is not known why this trend was observed, it is speculated that NLDN detection efficiency and/or storm structure differences may play a role in these observations. Warm season studies have correlated tornadoes with predominantly positive (>50% positive CG lightning), or PPCG storms. Gridded maps showed the greatest percentage and highest frequency of cool season PPCG storms across Kansas and Nebraska, with up to 70% of events associated with PPCG lightning. A secondary, albeit lower, frequency maximum extended 1???? to 2???? inland across Louisiana into North Carolina. This study also subjectively defined a storm with ??????enhanced?????? positive cloud-to-ground (EPCG) lightning as one containing >25% positive cloud-to-ground lightning, which corresponds to approximately the 75th percentile of all cool season tornadoes. This has lead to speculation that EPCG criterion may be a better indicator of the possibility of severe weather than the traditional PPCG criterion.Item An investigation of lightning behavior during the Quasi-linar Convective System in northwest Texas on March 19, 2012(2013-08) Plourde, Camaron M.; Bruning, Eric; Weiss, Christopher C.; Schroeder, John L.Between late Sunday, March 18 and early Monday, March 19 a quasi-linear convective system (QLCS) developed as a cold front intersected a retreating dryline at the Texas/New Mexico border. Several vortices developed, one of which was a confirmed tornado, along the line as it grew and made its way across west Texas. An investigation of the lightning characteristics of the line in its initial development stage will be presented, including flash size and rate from the West-Texas Lightning Mapping Array (WTLMA) data. Previous studies suggest that lightning data will reveal more frequent occurrence of smaller flashes in and around the max dBZ values. The Lubbock WSR-88D radar reflectivity factor and storm relative velocity data sets were used to compare to the lightning data in an effort to gain a better understanding of lightning behavior. Other studies suggest that turbulence contributes to the electrical structure in thunderstorms so flash size spectra were made for the first seven hours of the storm’s life to provide evidence on this matter. A full analysis of the storm was done in hopes of gaining further understanding in the connection between meteorological characteristics and lightning behavior within QLCSs.Item Analysis of lightning transients on an aircraft(Texas Tech University, 1985-05) Wen, Chang-taiNot availableItem Houston LDAR II network: design, operation, and performance analysis(Texas A&M University, 2008-10-10) Ely, Brandon LeeThe Houston LDAR II network is an array of twelve VHF time-of-arrival (TOA) sensors operated by Texas A&M University. The goals of the network are to conduct indepth studies of thunderstorm electrification and provide timely lightning threats to the Houston area. Before analyses are conducted using data from the Houston LDAR network, it is necessary to understand the LDAR network? s performance and limitations, such as the LDAR source detection efficiency, network range, and location accuracy. Initial results from the 31 October 2005 Mesoscale Convective System (MCS) timing error analysis revealed an RMS timing error for the Houston LDAR network of 90 ns for 6 sensor solutions. This gives a three-dimensional location accuracy of 1 km at a distance of 150 km and 100 m over the center of the network. Reanalysis with updated sensor positions decreased the RMS timing error to 75 ns. This decrease in RMS timing error increased the median three-dimensional location accuracy by ~100 m at a 100 km range. The network has been operated at both 70 MHz and 40 MHz. Model results of detection efficiency suggest that the change to 40 MHz yields an increase of 9 - 10 dB in network sensitivity. Analysis of VHF source power distributions shows a similar shift from that expected from the model. These results show that the 40 MHz LDAR network detects ~3.3 times more VHF sources than the 70 MHz network. The analysis of the usable network range for research purposes is currently set by rough guidelines of location accuracy and detection efficiency. For location accuracy, a 1 km limit allows storm analysis out to a range of more than 150 km. For the detection efficiency analysis, results based on source power distributions suggest a gradual fall off with range. Examining the change in VHF source density by range reveals different results. VHF source density remained fairly constant out to a range of 100 km at which point a significant fall off was observed. Based on these results the usable network range for the Houston network is 100 km.Item Laboratory modeling and analysis of aircraft-lightning interactions(Texas Tech University, 1982-08) Turner, Clyde DavidNot availableItem Physical modeling of the electromagnetic resonances of an aircraft-lightning interaction(Texas Tech University, 1984-05) Wood, Gerald WayneNot availableItem Radar-Derived Forecasts of Cloud-to-Ground Lightning Over Houston, Texas(2011-02-22) Mosier, Richard MatthewTen years (1997 - 2006) of summer (June, July, August) daytime (14 - 00 Z) Weather Surveillance Radar - 1988 Doppler data for Houston, TX were examined to determine the best radar-derived lightning forecasting predictors. Convective cells were tracked using a modified version of the Storm Cell Identification and Tracking (SCIT) algorithm and then correlated to cloud-to-ground lightning data from the National Lightning Detection Network (NLDN). Combinations of three radar reflectivity values (30, 35, and 40 dBZ) at four isothermal levels (-10, -15, -20, and updraft -10 degrees C) and a new radar-derived product, vertically integrated ice (VII), were used to optimize a radar-based lightning forecast algorithm. Forecasts were also delineated by range and the number of times a cell was identified and tracked by the modified SCIT algorithm. This study objectively analyzed 65,399 unique cells, and 1,028,510 to find the best lightning forecast criteria. Results show that using 30 dBZ at the -20 degrees C isotherm on cells within 75 km of the radar that have been tracked for at least 2 consecutive scan produces the best forecasts with a critical success index (CSI) of 0.71. The best VII predictor was 0.734 kg m-2 on cells within 75 km of the radar that have been tracked for at least 2 consecutive scans producing a CSI of 0.68. Results of this study further suggest that combining the radar reflectivity and VII methods can result in a more accurate lightning forecast than either method alone.Item The utility of total lightning observations in severe weather forecasting(2010-12) Burling, Christopher D.; Leary, Colleen; Wiens, Kyle C.; Wiess, Christopher C.A key aspect of short term weather forecasting is the ability to provide the public with adequate lead time in the event of severe weather. During severe weather events, it is vital that real time information such as radar data and spotter reports are available to aid National Weather Service (NWS) forecasters in the decision making process. An additional source of information that may prove useful in this regard is lightning data. Until very recently, forecasters have had access only to cloud-to-ground (CG) lightning data from the National Lightning Detection Network (NLDN). However, the utility of CG data as a severe weather forecasting tool is limited. Total (CG plus intracloud) lightning observations from very high frequency systems such as the Lightning Mapping Array (LMA; Rison et al. 1999) may be more useful. This thesis utilizes total lightning data from the Oklahoma LMA to assess the effectiveness of total lightning as an indicator of a given thunderstorm’s potential to produce severe weather. Specifically, a dataset of 52 thunderstorms (30 severe, 22 non-severe) within the domain of the Oklahoma LMA is analyzed to determine if severe weather is preceded by two features: a threshold total flash rate value which distinguishes severe thunderstorms from non-severe thunderstorms, and the presence of lightning jumps. A lightning jump algorithm was applied to each thunderstorm in the dataset in consideration of this second objective. Additionally, five thunderstorms are analyzed in greater detail to investigate these trends as they pertain to individual thunderstorms. A threshold flash rate upon which to determine thunderstorm severity is not apparent in the Oklahoma dataset. This is contrary to the results of a study of Florida thunderstorms by Williams et al. (1999), in which a clear threshold value was demonstrated. Lightning jumps are found to often precede the occurrence of severe weather, in good agreement with previous work.Item Total lightning characteristics of ordinary convection(2009-06-02) Motley, Shane MichaelTwenty-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.Item Using EFM and soundings to examine variations in thunderstorm thermodynamics(2012-08) Gusack, Natalie; Bruning, Eric; Weiss, Christopher C.; Leary, ColleenMany different thermodynamic effects can alter thunderstorm charging processes and therefore the overall charge structure. The objective of this research is to understand the thermodynamic processes involved in growth of ice in the mixed-phase region where supersaturated conditions with respect to ice are observed. One goal is to determine whether this leads to changes in the thunderstorm characteristics, most notably, the release of latent heat in the mid to upper levels of the storm and an alteration of the thunderstorm charge structure, including changes in cloud-to-ground lightning and intra-cloud lightning polarity. Data from two different field programs, the Severe Thunderstorm Electrification and Precipitation Study (STEPS) and the Thunderstorm Electrification and Lightning Experiment (TELEX), were used in this study. Three plots were created for analysis. One focused on the electric charge and the relative humidity with respect to liquid water and ice, another focused on the in-storm thermodynamics, and the final focused on the variation in water content and equivalent potential temperature. With these three diagrams it was shown that liquid water is saved for cold cloud processes by some combination of mesoscale environmental factors. The results led to the conclusion that storms which produce mainly positive CGs, and therefore may have regions of inverted thunderstorm charge structure, generally had the characteristics of supersaturation with respect to ice and values near saturation with respect to water in the mixed-phase region. These storms did not follow parcel theory ascent due to large areas of warming which are presumed to be caused by the latent heat of freezing and deposition.