2. Description of Lightning Systems
2.1 NLDN Lightning System
2.2 LDAR Lightning System
3. Operational Applications Using Both Lightning Systems.
3.1 Examples Of Operational Applications
Public
"By 2 pm, a line of strong thunderstorms will approach the greater Daytona Beach area. Lightning detection systems indicated that deadly cloud to ground lightning was increasing with these storms. Some of these dangerous lightning flashes where striking the ground well ahead of the heavy rain. Beach-goers are urged to clear the area as the beach is a dangerous location to be at during a thunderstorm. Remember, an open beach shack offers no protection from a direct lightning strike. However, a closed automobile offers excellent protection during a lightning storm".
Marine
Site Specific
Aviation
Local benefit
4. Additional Findings
4.1. Long Range Capacity Using LDAR
By memorandum of understanding, NWS/MLB is responsible in evaluating the LDAR unit with regard to the following:
The first point above was discussed in section 3.1. The second and third points are discussed below.
. As discussed in section 2.2, the LDAR system is very accurate close to the antennae site at the Kennedy Space Center. Farther out from this location, however, the overall accuracy of the LDAR unit is unknown. In the limited cases which have been collected and analyzed, the LDAR system has identified lightning discharges at significant distances from the LDAR site. Some locations in which lightning discharges have been detected include Jacksonville, the northwestern Florida peninsula, the Gulf of Mexico, southwest Florida, south Florida near and over Lake Okeechobee and over the Atlantic.
Overall, the LDAR can detect lightning discharges at distances beyond those found by Forbes (1993). However, the farther from the lightning is from the LDAR site the less amount of data is displayed. For significant distances, that is distances more than 100 miles away, only 2 or 4 LDAR points may be detected per lightning strike.
It was found that the LDAR signature becomes deformed with increasing distance from the central site. The deformation is most noticeable in the upper parts of the lightning flash, as this part of the flash will "azmuthally shear" away from the lower part of the flash.
Although exact values are not known at this time, it appears that for storms at significant distances, the LDAR system (when it detects the flash) will display data points very close to where the actual lightning flash (according to the NLDN data) occurred.
4.2 Examination of an Unique LDAR Lightning Signature
In addition to the operational applications of the LDAR system, the LDAR permits the forecasters to observe and analyze all types of lightning discharges. One type of discharge which is rather unique is a "bolt from the blue". To avoid confusion, a bolt from the blue is defined here as:
Lightning which comes out of the side of the updraft, travels a significant distance in the horizontal away from the parent updraft from which it originated, then turns down towards the ground and strikes the ground.
A bolt from the blue is different from "anvil lightning" in that anvil lightning originates from the anvil and typically travels straight down towards the ground.
A total of three cases of bolts from the blue have been either seen or documented using the LDAR unit. One of these cases was unknowingly documented by Forbes (1995 - Fig. 2). The second was documented by the author, using a 35 mm camera. The third case was witnessed (on the LDAR display) by the author during the summer of 1995, but was not documented on film. Of the two cases which were documented, the bolt from the blue originated from 7 to 9 kilometers AGL. The bolt then travelled horizontally on a slight downward angle for 7 to 8 km and then angled down and struck the ground. Of the two cases which were witnessed by the author, the strikes emanated from the opposite direction in which the storm was moving.
Holle et al. (1993), found people in Florida were more likely to be victims of lightning strikes at the end of the storm, rather then at the beginning, or during the storm. It was surmised that this was likely due to people resuming outdoor activities too soon after the storm had ended. Bolts from the blue can be very dangerous in this regard since they appear, from the publics point of view, to strike well after the storm has passed.
Intense Cloud to Ground Lightning Activity
All thunderstorms, by definition, produce lightning. However, There are days when the lightning activity over the peninsula is significantly more intense than others. Cloud to ground lightning flash rates exceeding 500 flashes in 30 minute over a relatively small area (such as part of a county) have been documented with the NLDN system (Fig. 3). This poses a question to what is the cause of this intense lightning activity.
Preliminary analysis indicate that the cause of the intense lightning activity is twofold. First, the amount of Convective Available Potential Energy (CAPE) over the peninsula needs to be higher than normal. Typically in summer over the Florida peninsula CAPE values range between 1100 to 1300 J/Kg (Hagemeyer et. al, 1991). Higher than normal CAPE is believed to be caused by temperatures aloft being cooler than normal. The second necessary ingredient is boundary interactions. Thunderstorms which form when either the two sea breezes collide (west coast sea breeze collides with east coast sea breeze) or when an outflow boundary collides with one of the sea breezes are very favorable for intense lightning activity, as long as the first condition above is meant.
5. Summary
The LDAR and NLDN systems have shown promise for improving short term forecasting. The LDAR system has also shown evidence for improving aviation forecasting, especially during times when weak convection is embedded stratiform rain events and only in cloud lightning is occurring. In the future, NWS/MLB will continue to analyze LDAR to define unique signatures which could possibly indicate severe storm potential. It has been proposed intense LDAR signatures which form farther aloft are conducive to producing microburst.
In addition to continuing to evaluate the LDAR system, we plan on continuing our overall lightning research. This Includes understanding the social factors regarding lightning casualties, such as time of day, location, tourists vs residents, etc. We also plan to quantify the thermodynamic and mesoscale factors to get a better understanding of intense lightning activity.
6. References
Forbes, G.S.: 1993. Lightning studies using LDAR and LLP data and applications to weather
Forbes, G.S.: 1994. Lightning studies using LDAR and LLP data and
companion data sets.
Forbes, G.S., Hoffert, S.G., and M Pearce: 1995. Thunderstorm studies using LDAR, LLP, field
Hagemeyer, B. C., Schmocker, G. K. and K. E. Hileman: 1991. Mean atmosphere over Cape
Holle, R. L.:1993. Overview of real time lightning detection systems and their
Holle, R. L., Lopez, R.E., Ortiz, R., Paxton, C.H., Decker, D.M. and D. Smith: 1993. Preprints
Lennon, C. and L. Maier: Lightning mapping system. NASA CP-3106, Vol II, 1991. International
Orville, R. E.: 1994. Using and interpreting lightning in the Southeastern United States.