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19 The Weather Decisions Technologies (WDT) real-time panel indicating the current alert or alarm status (defined in lightning display (www.wdtinc.com/pages/home_page/ terms of lightning strikes within a user-specified warning and lightningDSS/realtimeLD/web_page.xml) provides a simple, advisory area, indicated by a circle centered on the airport). direct view of nearby lightning strikes (see Figure 11). WDT's The WeatherSentry system (www.meteorlogix.com/industry/ main focus, however, is on a more comprehensive Lightning aviation.cfm) is provided as an online web application (see Decision Support System (LDSS) that can be augmented to Figure 13). include lightning range alerting and a lightning prediction The Integrated Terminal Weather System (ITWS) is a U.S. algorithm. governmentsponsored development of a comprehensive Weather Services International (WSI) has a number of terminal area weather system (www.ll.mit.edu/Aviation systems designed to provide general-purpose weather Weather/sitdisplay.html). It is intended for installation at information for airports and weather-sensitive applica- large airports that have been provided with Terminal Doppler tions (www.wsi.com/aviation/solutions). Their systems are Weather Radar (TDWR). The basic ITWS display is relatively based on a dedicated workstation at the airport with a satel- complex, but includes a simple "lightning within 20 nautical lite data feed that provides general weather information, miles" display light based on real-time access to NLDN data including radar and satellite imagery, augmented by real- (see Figure 14). It would be relatively easy to enhance the time NLDN or USPLN lightning observations. Their Weather lightning warning features of this system in an environment Workstation product is used by Delta, UPS, and FedEx. that has the capacity to integrate lighting observations with Figure 12 shows a detail from the WSI Fusion Display, other meteorological data sets. combining radar imagery, flight tracks, and real-time light- ning observations. Lightning Prediction Technologies DTN/Meteorlogix offers a general airport weather infor- mation system, featuring real-time NEXRAD radar data from Lightning warning systems often make a distinction be- NOAA. As one component of this system, NLDN lightning tween lightning detection and prediction. Detection systems, data are overlaid on top of the radar display, with an on-screen as the name implies, simply detect and report lightning Figure 11. An example of the WDT real-time lightning display. The age of the displayed lightning strokes is indicated by their color on a zoomable map display (figure courtesy of WDT).

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20 Figure 12. Detail of a screen image from the WSI Fusion Display, showing a combination of aircraft track, flight plans, and radar imagery (in shades of green) as a background for CG lightning strikes that are color-coded, with the most recent strikes plotted as white "plus" signs (figure courtesy of WSI). strikes after they occur. Prediction systems, on the other The timing and path of an individual lightning stroke are, for hand, provide warnings that a lightning strike is likely to all practical purposes, unpredictable. occur. Most of the time there is not much of a difference be- There are two distinctly different approaches to predicting tween the two approaches. If an active thunderstorm moves lightning hazards. The first, based on monitoring the buildup towards an airport, lightning detection technology will con- of the atmospheric electric field in response to nearby charged tinually monitor the locations of the CG lighting strikes. clouds, represents a true prediction. Electric field measure- When the activity reaches a specified distance from the air- ments will not, however, necessarily predict all nearby light- port, the system will generate an alert or warning--essentially ning strikes, and they can be expected to produce occasional a prediction, based on the proximity of the lightning, that the false alarms (19, 20). storm presents an imminent threat and hazard. In some The other approach to lightning prediction is to monitor cases, however, a lightning storm may develop directly over the growth and movement of the systems that develop into an airport, and the very first strikes can put airport workers thunderstorms using techniques that have been developed at risk. In this case, a prediction system may be able to pro- for short-term weather forecasts ("nowcasting"), using vide a uniquely valuable warning. general storm properties that can be monitored by radars Even the best predictions only give a general indication that or satellites as a proxy for lightning activity. This approach a lightning strike is likely to occur in the immediate vicinity. can provide significantly longer advance warnings of pos-

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21 Figure 13. A screen image of the DTN/Meteorlogix online Lightning Manager, showing a combination of radar and lightning data, with a user-configured warning and advisory pop-up window (figure courtesy of DTN/Meteorlogix). sible lightning activity than other approaches, but with less ria for shortening the duration of ramp closures after a warn- accuracy. ing is sounded. The convective nowcasting approach is well-suited for haz- ardous operations such as missile ranges and weapons test- ing, which require a long lead time to shut down or reschedule Monitoring the Local Atmospheric operations and a high probability of detection of a potential Electric Field hazard. These systems will, however, normally have a corre- As already discussed, electric field measurements can de- spondingly high false alarm rate. With respect to commercial tect the presence of high levels of charge separation in nearby airport operations, most long-range predictions of this sort clouds that suggests a strong likelihood of current or future would be considered only as an advisory forecast; they would lightning activity. These systems have the unique potential to not mandate that activities be rescheduled or that operations provide advance warning of the first lightning strike from a be shut down. developing storm. Shutting down ramp operations at a busy airport is a major This ability to offer an advance prediction of the first light- decision that cannot be taken lightly; there is thus a low ning strike makes these systems particularly attractive for tolerance for false alarms. In most cases, there will likely be applications where response time is critical, such as athletic a natural hesitancy to clear the ramp on the basis of a "pre- fields and golf courses with limited access to sheltered areas and diction" without some additional evidence of nearby lightning stadiums that would take a long time to clear of spectators. strikes. While most studies of lightning prediction have naturally concentrated on forecasting the initial onset of lightning Monitoring and Predicting activity, the same observing systems may also be able to pro- Overall Storm Evolution vide valuable information about the cessation of a lightning hazard as storms are dying down and moving out. In those Lightning activity is an integral part of the life-cycle of a cases, the technologies may be able to provide objective crite- thunderstorm. For example, Figure 15 shows a summary of

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22 Figure 14. A screen image of the main ITWS weather display, including a simple lightning alert button. the evolution of an intensely studied, microburst-producing short-term forecasting of thunderstorm activity, including pre- thunderstorm. The bottom two panels show vertical profiles dicting areas of new growth and explosive development (22). of the time evolution of the storm radar reflectivity and Using standard meteorological data sets, including output from updraft strength, while the top panel shows the IC and CG numerical models, radar, and satellite observations, storm lightning activity. nowcasting has proved to be a valuable tool for understand- In this storm, the initial mid-level strengthening of the ing and predicting storm behavior and evolution. Given the radar echo preceded an intensive growth period, with the importance of timely predictions of hazardous weather, it is highest lightning flash rates well correlated with the period of natural that storm forecasters are now beginning to generate the maximum updrafts. This storm's ratio of IC to CG light- short-term, high-resolution lightning forecasts (23). ning strikes was unusually high, but follows the normal pattern Figure 16 shows a graphical depiction of the results of a of IC lightning developing several minutes before the first CG lightning prediction algorithm included in WDT's Lightning stroke. Decision Support System (LDSS). This algorithm combines Storm studies such as shown in Figure 15 indicate that real-time lightning observations with storm-cell motion lightning data, particularly IC lightning data, are a valuable tracks to identify separate moderate and high threat areas indicator of the updraft strength and can play an important out to 30 min in advance. A similar system, which combines role in short-term prediction of storm behavior. At the same radar and lightning observations to provide lightning warn- time, observations of storm strength and evolution can be ings for a variety of public service applications, including used as an approximate indicator of lighting activity. In recent airport ground operations, is currently under development years, there have been a number of significant advances in the in Australia (24).

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23 Total Lightning as a Predictor for Ground Strikes The CG lightning events that are the focus of the NLDN and the USPLN lightning detection networks make up only a small fraction of all lightning activity. Studies, such as the one depicted in Figure 15, show that systems that can monitor all lightning strokes will be able to perform detailed monitoring of the time-resolved flash rates and the areal extent of a storm's lightning activity. Since IC lightning is normally a precursor to the first CG strokes, total lightning systems can be used to identify storms that are entering an active lightning-generation period and act as a predictor for subsequent CG lightning strikes (25). While it is important to remember that it is the CG lightning flashes that represent the actual hazard to ground operations, IC lightning activity is a direct indicator of a given storm's lightning potential and thus should be a uniquely valuable short-term predictor of the CG hazard. As a predictor, total lightning is also attractive since it is based on an observed event, rather than dependent on extrapolated storm behaviors. While current lightning detection networks can detect some IC lightning flashes, high-efficiency detection of IC lightning events will require the network sensors to be enhanced, com- bined with a significant increase in the number of network sensors. As an attractive alternative to a nationwide enhance- ment of the current lightning detection networks, a number of regional total lightning networks could well be embedded within the national CG networks. The regional total light- ning systems could provide improved storm monitoring and prediction capabilities as well as enhanced lightning de- tection capabilities for a wide variety of community-based applications. Sometime after 2014, the next generation of U.S. geosta- tionary meteorological satellites is expected to include a large- area optical lightning mapper. From geostationary orbit, this Figure 15. Lightning and precipitation history instrument will provide total-lightning observations at a spatial of a severe thunderstorm (21). The bottom panel resolution of about 10 km. While this is much lower resolu- shows the updraft strength in the main cell tion than would be provided by a dedicated regional sensor as a function of time and height. The middle panel network, the 10-km resolution would still provide a valuable shows the corresponding evolution of the radar measurement of the extent of the IC lightning and help define reflectivity, while the top panel shows the the areal extent of the lightning hazard. Perhaps more impor- lightning activity correlated with significant tant, data from the satellite-based system would be available features in the storm's evolution. via free, real-time broadcasts from space.

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24 Figure 16. An example of the WDT lightning prediction algorithm running within the WDT LDSS. The algorithm is based on the current lightning observations, coupled with the expected evolution of the storm, as reflected by its radar signature and indicates the location and magnitude of the expected lightning threat 30 min into the future (figure courtesy of WDT).