Four Dimensional Lightning Surveillance System : Status and Plans (original) (raw)
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Lightning generated by convective storms, and identified as a formidable hazard to life and property, results from strong storms lofting liquid-phase hydrometeors to high altitudes where freezing occurs and collisions between drops, graupel, and ice crystals lead to electrification. Among the most widely accepted theories of convective storm electrification is the " charge transfer-separation process " by which interaction (esp. collision) between ice crystals and graupel particles result in the establishment and stratification of positive and negative charge centers within the convective cloud, respectively. Differing growth processes between ice crystals (i.e. deposition of water vapor) and graupel (accretion of supercooled water) result in differing molecular structure and electron arrangement, and subsequently, favor the transfer of electrons with negative charge from an ice crystal to a graupel particle. In general, negative charge accumulates in the middle levels of the convective storm cloud, referred to as the " main negative charge " charge center, while positive charge accumulates in the upper layer and anvil (if present) region and is referred to as the " upper positive charge " center. Other secondary charge centers may develop in a convective storm, such as the " lower positive charge " center that results from falling hail, however, the main electric field built within the storm results from the separation of the main negative and upper positive charge centers. When the electric field strength (E) between the storm cloud and the ground eventually increases to the threshold value of electric breakdown potential (3 x 10 9 V/km), a stepped leader, defined as a segmented ionized channel, develops and propagates downward toward the ground in steps of 50 to 100 m length. Upon contact of the stepped leader with the ground, a bright return stroke from the ground to the cloud occurs, in which electrons flow downward from progressively higher levels in the channel. The stepped leader and return stroke comprise the lightning strike. The strike with additional return strokes, triggered by a dart leader, then comprise the cloud-to-ground (CG) lightning flash. Intracloud (IC) lightning discharge between the main negative and upper positive charge centers also results from charge separation and electrical breakdown. Ground-based and satellite-based lightning detection systems often detect both IC and CG lightning flashes, with CG strokes a stronger emitter of low frequency (LF) radiation. Lightning detection systems (LDS) have a vital role in the real-time identification of the location of lightning strokes for the purpose of public safety and weather forecasting and warning operations. Archived LDS datasets also provide support to electric utility companies to identify lightning events associated with electric power grid faults and power outages, reduce frequency and duration of power outages, and make improvements to transmission line segments susceptible to lightning damage. More importantly, flash rate and density measurements are necessary for the inference of cloud thermodynamical and microphysical processes that favor severe thunderstorm hazard phenomena including hail, tornadoes, and damaging winds (downbursts).
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The spatial variations of lightning during small Florida thunderstorms
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Networks of field mills (FMs) and lightning direction finders (LDFs) have been used to locate lightning over the NASA Kennedy Space Center (KSC) on three storm days. Over 90 percent of all cloud-to-ground (CG) flashes that were detected by the LDFs in the study area were also detected by the FM network. 27 percent of the FM lightning events were correlated with CG flashes detected by the LDFs. About 17 percent of the FM CG events could be fitted to either a monopole or a dipole charge model. These projected FM charge locations are compared to LDF locations, i.e. the ground strike points. We find that 95% of the LDF points are within 12 km of the FM charge, 75% are within 8 km, and 50% are within 4 km. For a storm on July 22, 1988, there was a systematic 5.6 km shift between the FM charge centers and the LDF strike points that might have been caused by the meteorological structure of the storm.