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47 would eventually move the airport to an optimum balance of lightning activity, lightning "cells" are composed of individ- safety and efficiency. ual, discrete lightning strokes and are more difficult to define With this sort of capability, the system operator could also and harder to track. While radar and lightning cells are clearly be provided with periodic summaries of system performance linked, it is important to remember that the radar echo bound- and daily reporting of all lightning events. With an additional aries do not always coincide with the limits for lightning option for manual entry of actual ramp closure times and strikes, as exemplified by the "bolt from the blue" phenom- durations, the system could provide a permanent record of ena discussed in Chapter 1. Nevertheless, a blended product lightning activity and ramp closures. identifying the boundaries of the most active lightning- These same capabilities, perhaps including a 24-hr tempo- producing areas through a combination of radar echoes and rary archive of nearby lightning strikes, would also be useful for direct lightning observations should provide a good estimate airport and airline accident/incident investigations, hard land- of the expected movement of active lightning areas. A better ings, or lightning-related injuries and damage. While much of delineation of the boundaries of the active lightning strike this information could be recovered or reconstructed from the areas should in turn allow a better estimate of the onset and national NLDN permanent data archive, the full set of local in- termination of the lightning threat. formation should also be available at airports that operate Knowledge of the advection direction of the lightning- lightning detection and warning systems. This capability would producing cells can also provide additional direct benefits. be particularly valuable in providing airport authorities, air- One of the most interesting results of our study generating lines, and other tenants with rapid access to recent lightning in- synthetic airport closure statistics based on archived NLDN formation and local statistics in response to emergencies. observations (discussed in Chapter 3) was the relatively high number of closures that resulted from a single lightning strike, or from an extremely short burst of lightning activity Integrating Technologies extending less than 1 min. While this phenomena needs for Improved Performance additional study, it is likely the result of lightning-producing Perhaps the most obvious way to improve the performance storms drifting past the airport, just barely within the dis- of existing lightning warning systems is to incorporate addi- tance criteria used for shutting down ramp operations. These tional weather information into the warning algorithms. storms would presumably be producing lightning as they Meteorological radars have traditionally been the observing sys- approach and move on past the airport area, but are only tem of choice for monitoring thunderstorms. The current na- within the warning range for a short time. In this case, it may tional U.S. network of high-quality Doppler radars (NexRad) is be possible to significantly reduce the total number of ramp a uniquely valuable resource for tracking the development and closures by adjusting the warning area boundaries based on movement of lightning-producing storms and should be able storm motion vectors. Adjustments of this sort are essentially to be used in conjunction with standard NLDN observations to equivalent to modifying the current distance-based lightning produce a new set of comprehensive warning products. proximity warning criteria to also consider the time before an approaching storm is likely to reach the airport. Warning boundaries in the direction of storms moving Radar Echo Properties and Tracking rapidly directly towards the airport, for example, may need Radar studies of storm structure have led to radar-based to be extended to provide adequate time to shut down oper- predictions of the likelihood of lightning (30, 31). Although ations before the storm reaches the airport. Shrinking the such second-order products are of little direct use when NLDN dimensions of the warning area in the directions perpendic- observations are available, they indicate that radar echo ular to the motion of the lightning cells, on the other hand, patterns and properties may be useful in helping to identify would reduce the number of storms that just brush along the specific meteorological situations that may be particularly side of the normal warning area and then move on without problematic or require additional safeguards. Studies of this becoming a real hazard. Once fast-moving storms have passed sort are currently underway and may lead to improved light- the airport, knowledge of the storm speed and direction of ning warning products (32). From an airport operational per- movement may also permit an earlier declaration of an "all spective, the most important potential contribution of radar clear" without compromising safety. data may be to provide a better estimate of the end of the lightning hazard as storms move away from the airport area. Total Lightning Systems Radar observations can also be used to track the movement of storm cells. Thunderstorm cells generally have great spa- The NLDN has been designed to provide high-quality, high- tial and time continuity and are relatively easy to track by collection-efficiency observations of CG lightning strikes. radar. While it is also possible to identify and track areas of While these ground strikes are the specific hazard that