2.4 THE POTENTIAL OF HIGH PERFORMANCE, REGIONAL TOTAL LIGHTNING NETWORKS ANDENHANCED DISPLAY PRODUCTS FOR PUBLIC SAFETY AND BROADCAST METEOROLOGY APPLICATIONS

Nicholas W. S. Demetriades and Jean-Yves LojouVaisala, Inc.

Tucson, Arizona and Meyreuil, France

1. INTRODUCTION

Vaisala has been operating a regional total(cloud and cloud-to-ground) lightning demonstrationnetwork in the Dallas-Fort Worth, TX area since 2001.This Lightning Detection and Ranging (LDAR II)network builds upon the Very High Frequency (VHF)lightning detection technology first developed atNASA Kennedy Space Center called LDAR (Maier etal., 1995). This same technology has also beenmodified by New Mexico Tech Institute of Mining andTechnology (NMT) into the Lightning Mapping Array(LMA) (Rison et al., 1999). The Dallas-Fort Worth(DFW) LDAR II network detects over 90% of all cloudand cloud-to-ground (CG) lightning within 120 km ofDFW International Airport. LDAR II’s ability to mapthese lightning flashes in three dimensions, coupledwith its high flash detection efficiency, allow acomplete three-dimensional reconstruction of thelightning channels in a thunderstorm. Data from thisnetwork are currently being used for real-timethunderstorm monitoring at the Fort Worth NationalWeather Service Forecast Office. Vaisala alsoprovides LS8000 VHF total lightning mappingnetworks that provide similar capabilities to LDAR II,except only in two-dimensions. LS8000 sensorsemploy interferometric lightning detection techniques.An example of data from this system will also bediscussed in this paper.

Current media and safety display products showCG lightning. This information only conveys part ofthe CG lightning threat that exists to the public duringthunderstorm activity. The DFW LDAR II network hasregularly detected lightning flashes that extend over50-100 km in length. These flashes pose a significantsafety risk to the general public because, at any time,they are capable of producing a CG flash along theirpath. Many examples of these horizontally extensivelightning flashes will be shown using unique displaytools developed by Vaisala. CG-only lightningdisplays are also not as useful as total lightningmapping displays for monitoring thunderstorm growthand dissipation and severe weather trends.Approximately 70% of all lightning stays in the cloudand never reaches the ground. In fact, somethunderstorms produce greater than 10 cloud flashesfor every CG flash and a few may only produce cloudlightning for the first 60 minutes of a severethunderstorm. Total lightning detection provides a

*Corresponding author address: Nicholas W.S.Demetriades, Vaisala, Inc., Tucson, AZ 85706; email;nick.demetriades@vaisala.com

rich dataset that can be used with radar data toimprove severe weather warnings and potentiallyincrease the lead time for these warnings. Since thetotal lightning data is continuous, it can providevaluable information on thunderstorm growth anddissipation trends and severe weather developmentbetween radar volume scans.

Several unique storm animation tools will bepresented. These tools provide easily-interpretedsimultaneous displays of storm/cell location, totallightning rate, and/or CG lightning rate. These fieldscan be accurately superimposed on base maps ofgeo-coded information such as terrain height.

2. DFW LDAR II PERFORMANCE

The DFW LDAR II network consists of 7 sensorswith 20 to 30 km baselines (Fig. 1). These sensorsdetect pulses of radiation produced by the electricalbreakdown processes of lightning in 5 MHz VHFbands that currently have center frequencies rangingfrom 61 to 64 MHz. These pulses of radiation areused to reconstruct the path of individual cloud andCG lightning flashes in three dimensions. The DFWLDAR II network can map lightning flashes in threedimensions within approximately 150 km of the centerof the network, degrading in performance withincreasing range. Lightning flash detection efficiencyis expected to be greater than 95% within the interiorof the network (a range of 30 km from DFW

Figure 1. DFW LDAR II sensor sites. Each site isrepresented by a star, with its site identifier and sitename located to the right of the star. Countyboundaries and names are also shown.

International Airport – sensor A) and greater than90% out to a range of 120 km from DFW InternationalAirport. Expected 3-dimensional location accuracy forindividual pulses of radiation is between 100 and 200m within the network interior and better than 2 km to arange of 150 km from the center of the network.

3. VHF CLOUD FLASH MAPPING

Figure 2 shows a cloud flash that was detectedover the southeast part of the DFW LDAR II networkstarting at 1004:31 UTC 11 April 2001. At about1004:31.2 the initial upward electrical breakdown ofthis cloud flash occurs from about 5 to 9 km altitude,as indicated by the slight rise in elevation of sourcesin the altitude versus time panel of Figure 2 (top).The electrical breakdown along the whole path of thiscloud flash and the charge reorganization within theclouds continue for the next 700 ms.

The vertical cross-section and plan-view panelsclearly demonstrate the detailed three-dimensionalmapping ability of the DFW LDAR II network (Fig. 2).The bi-level structure of the cloud flash can be seenby the sideways “H-type” pattern to the sources inboth vertical cross sections. Many intricate branches

and the true horizontal extent of the cloud flash areshown in the plan-view panel. This cloud flashcovered an area of about 600 km2.

As Figure 2 has shown, the presentation ofLDAR II source data, while fascinating, could bedifficult to interpret for today’s television viewer. Thisis due to the knowledge required to mentally connecthundreds to thousands of dots on a display in order tovisualize the branches associated with one lightningflash. “Connect-the-dots” algorithms have beendeveloped at Vaisala in order to help solve thisproblem. Figures 3 and 4 show a horizontallyextensive cloud lightning flash detected by the DFWLDAR II network. Figure 3 shows the flash asrepresented by hundreds of dots (sources), similar towhat is seen in Figure 2. Figure 4 shows this sameflash, except the sources are connected with linesaccording to the lightning’s path of propagation. Theinterpretation of the intricate branching and extent ofthis lightning flash is much simpler because the TVviewer no longer has to mentally connect theindividual points. This should make it easier for on-airmeteorologists to show fascinating replays ofhorizontally extensive lightning flashes and numerouslightning flashes produced by interesting storms overspecific time intervals.

4. CG LIGHTNING HAZARD

4.1 Stratiform precipitation lightning

Assessing the CG lightning threat region iscritical for many applications that include, but are notlimited to: (1) ground crew safety at airports, (2)outdoor events attended by large numbers of people,such as sporting events, (3) mission criticaloperations, including space shuttle launches, (4)operations involving explosives or highly flammablematerial, (5) golf courses and (6) recreation facilities.

CG lightning produced within stratiform rainregions that are attached to active thunderstorms are

Figure 2. A cloud lightning flash detected by the DFWLDAR II network on 11 April 2001 between 1004:31-32UTC. The top panel is an altitude (y) versus time (x)plot. The lower left is a plan view showing latitude (y)versus longitude (x). The lower right is a latitude (y)versus altitude (x) cross-section. The panel just abovethe plan view is an altitude (y) versus longitude (x)cross-section and the panel just to its right is analtitude (y) versus source frequency (x) plot. LDAR IIsources are shown by dots.

Figure 3. DFW LDAR II lightning sources detectedwithin a horizontally extensive cloud flash 15 June2001.

an important cause of lightning related injuries andfatalities (Holle et al., 1993). In these thunderstormcomplexes, horizontally extensive cloud lightningflashes typically initiate in the convectivethunderstorm cores and propagate through attachedstratiform rain regions for distances sometimesexceeding 100 km. These horizontally extensivecloud flashes are particularly dangerous because theyhelp produce isolated CG lightning discharges.Figure 5 shows the intricate 3-dimensional structureof one of these lightning flashes as detected by LDARII. This lightning flash initiated approximately 40 kmto the east-southeast of DFW Airport and propagatedin a westward arc, terminating 25 km to the south ofDFW Airport (plan view). A total of 337 LDAR IIradiation sources were detected along its ~100 kmpath. According to Vaisala’s National LightningDetection Network (NLDN), this flash helped producefour isolated CG lightning strokes along its path. Thefinal CG stroke injured a person at DFW Airport.Another impressive cloud flash was detected on 13October 2001 (Fig. 6). It initiated just to the northwestof Waco and propagated north toward Fort Worth,before turning eastward and terminating near Dallas.This flash covered a path length of ~190 km andlasted over two seconds. To date, it is the longestflash found in the DFW data. NLDN CG lightning dataindicated that this spider flash helped initiate two CGflashes with a separation of ~70 km (Fig. 6). TheDFW LDAR II network can help address the problemof properly identifying the CG lightning hazard withinthe DFW area because it maps the horizontal extentof both cloud and CG flashes.

Figure 7 gives a dramatic example of the CGlightning risk within the trailing stratiform region of anMCS that passed through the DFW area on 15 June2001. A comparison with Figure 8 shows that theNLDN flashes are mainly clustered along the leadingconvective line that is >50 km southeast of the centerof the DFW LDAR II network (sensor A). However,the LDAR II sources indicate that cloud lightning ispropagating over 100 km to the northwest of the

Figure 4. Same as Figure 3, except DFW LDAR IIsources are connected using one of Vaisala’s flashalgorithms.

Figure 5. Same as Figure 2, except for a horizontallyextensive cloud lightning flash detected by the DFWLDAR II network on 17 August 2001 between 1515:38-39 UTC. LDAR II sources are shown by dots andNLDN CG lightning stroke locations are indicated bythe white lightning symbols.

Figure 6. Same as Figure 2, except for a horizontallyextensive cloud lightning flash detected by the DFWLDAR II network on 13 October 2001 between 0416:33-36 UTC. LDAR II sources are shown by dots and NLDNCG lightning stroke locations are indicated by the whitelightning symbols.

leading convective line, through the trailing stratiformregion. The apparently random pattern of isolated CGflashes (sometimes located over 100 km from theleading convective line) produced by these trailingstratiform cloud flashes demonstrates theunpredictable nature of CG lightning in stratiformregions. Figure 7 shows that the whole DFW area iscovered by trailing stratiform cloud lightning andtherefore is at risk for isolated CG lightningdischarges.

LDAR II and radar reflectivity observations fromseveral MCSs that passed through the DFW areahave shown that trailing stratiform regions do notnecessarily have cloud lightning propagatingthroughout their entire area of coverage (Murphy andHolle, 2005). Therefore radar data alone can notdefine the isolated CG lightning threat throughidentification of the boundaries of the trailing

stratiform region. It is critical to be able to map thehorizontal extent of cloud lightning through the trailingstratiform region in order to properly define theisolated CG lightning threat.

4.2 Thunderstorm anvil lightning

CG lightning produced within anvil cloudsattached to active thunderstorms are anotherimportant cause of lightning related injuries andfatalities (Holle et al., 1993). Depending on themagnitude of the vertical wind shear on a given day,anvil lightning can sometimes extend over 60 km infront of active thunderstorm cores. As with thestratiform region cloud lightning, anvil cloud lightningcan produce isolated CG lightning flashes anywherealong its path.

Flash extent density (FED) is a product used byVaisala to display VHF total lightning data. FED isdefined as the number of lightning branches thatpasses through a 1 km2 grid box per minute. Figure 9shows a two-minute FED plot of supercellthunderstorms in the DFW area on 25 April 2005.Satellite and radar reflectivity images from this timeperiod showed that strong southwesterly upper levelwinds were blowing thunderstorm anvils distancesover 60 km toward the east-northeast. The highesttotal lightning activity is identified near the center ofFigure 9 in shades of red, pink and white. Anvillightning is shown in shades of purple, extendingclose to 60 km toward the east of the southernmostcell and 60 km toward the northeast of the cell locatedjust northeast of the southernmost cell. Anvil lightningproduced isolated CG lightning flashes during thelifetime of these supercells.

4.3 “Bolt from the blue” flashes

Observations from the NMT LMA have helpeddefine another important application involving themapping of total lightning flashes. Rison et al. (2003)

Figure 7. DFW LDAR II lightning sources (red) andNLDN CG lightning flashes (black) detected between0312:30 and 0317:30 UTC 15 June 2001.

Figure 8. Fort Worth WSR-88D base reflectivity imagefrom 0315 UTC 15 June 2001. The color key forreflectivity values is located on the left. (Image savedfrom The Weather Underground, 2001,www.wunderground.com)

Figure 9. Total lightning FED (number of lightningbranches that pass through a 1 km2 area per minute)image from collected over a two-minute interval on 25April 2005. Shades of red, pink and white identifiesareas with high total lightning flash rates.

discuss numerous observations of the mapping of“bolts from the blue” within NMT LMA data (notshown). These flashes can sometimes extend overten kilometers beyond the cloud boundary ofthunderstorms and always go to ground. Rison et al.(2003) found that these flashes are quite commonwithin storms observed with the NMT LMA and canrepresent the majority of CG flashes producedthroughout the lifetime of a storm. A storm observedby the NMT LMA on 2 August 1999 produced 24 CGflashes throughout its lifetime, 18 of which wereconsidered “bolts from the blue.” These observationsimply that using radar reflectivity data in combinationwith the mapping of total lightning flashes will identifystorms that are producing dangerous flashes at somedistance from the radar echo.

4.4 Total lightning demonstration display products forCG lightning safety

Vaisala has created two demonstration displayproducts that integrate VHF total lightning data withLF CG lightning ground locations in order to give acomplete representation of the areas at risk for CGlightning. The first demonstration product uses ablue/red color scheme to identify areas at risk for CGlightning. Figure 10 shows a 15-minute FED plot ofcloud and CG lightning in a mesoscale convectivesystem (MCS) that passed through the DFW area on16 June 2002. Areas containing cloud flash activitywithout CG lightning activity are shown in shades ofblue. The darker the blue shade, the higher the cloudlightning rate. Areas containing both cloud and CGlightning data are shown in shades of red. The lighterthe red shade, the higher the CG lightning rate as

detected by Vaisala’s NLDN. The high CG lightningrisk region is highlighted in red along the leadingconvective line of the MCS as it moved from west-northwest to east-southeast through the DFW area.Areas shaded in blue show that there remains anisolated CG threat from continuing cloud lightningactivity in the trailing stratiform region located west ofthe leading convective line. Notice a number ofscattered red areas showing isolated CG lightningflashes within the trailing stratiform region.

The second demonstration product combinesCG lightning point locations with the cloud lightningconnect the dots representation. Figure 11 shows 30minutes of Vaisala NLDN CG lightning data plottedwith VHF total lightning mapping data from an MCSthat passed through the DFW area on 15 June 2001.The latest minute of CG (total) lightning data is shownin white (green) and turns to red (blue) when the datais 1 to 30 minutes old. Similar to the firstdemonstration product, the high CG lightning risk areais located in the leading convective line as this MCSmoved from northwest to southeast through the DFWarea. The continued isolated CG lightning risk isshown by the cloud and isolated CG lightning activityextending over 100 km to the northwest of the leadingconvective line, through the trailing stratiform region.

5. THUNDERSTORM GROWTH ANDDISSIPATION

Total lightning rates detected at VHF are a muchbetter indicator of thunderstorm updraft growth anddissipation than CG lightning detected at LowFrequencies (LF) used in Vaisala’s NLDN. Sincehundreds to thousands of VHF lightning sources are

Figure 10. CG lightning hazard demonstration productshowing 15 minutes of VHF total lightning mappingand CG lightning activity from 16 June 2002. Areascontaining cloud flash activity without CG lightningactivity are shown in shades of blue. The darker theblue shade, the higher the cloud lightning rate. Areascontaining both cloud and CG lightning activity areshown in shades of red. The lighter the red shade, thehigher the CG lightning rate as detected by Vaisala’sNLDN.

Figure 11. CG lightning hazard demonstration productshowing 30 minutes of VHF total lightning mappingand CG lightning activity from 15 June 2001. Thelatest minute of CG (total) lightning data is shown inwhite (green) and turns to red (blue) when the data is1 to 30 minutes old. (Patent pending)

detected within each flash by VHF total lightningmapping networks, flash initiation points (FIP) areused to help determine total lightning flash rateswithin thunderstorm cells. An FIP is defined as thefirst VHF lightning source detected within a lightningflash after VHF lightning sources are grouped into aflash by Vaisala’s algorithms. Figure 12 shows a timeseries of radar and lightning parameters from anordinary thunderstorm in the DFW area. Totallightning rates go through two periods of rapidincrease and then rapid decrease. The first periodcovers about 20 minutes from 2115 to 2135 UTC andthe second period covers only 5 minutes from 2140 to2145 UTC. These increases in total lightning ratesappear to be approximately correlated with increasesin the maximum Vertically Integrated Liquid (VIL) fromthe WSR-88D radar. However, the negative CGlightning rates are very low and do not show anymarked increases and decreases. These rapidincreases (decreases) in total lightning rate areassociated with a strengthening (weakening) updraftin the storm.

Vaisala has created a demonstration productthat helps identify thunderstorm growth anddissipation using a three-dimensional representationof the total lightning rates within thunderstorms.Figures 13 and 14 show a number of thunderstormsthat were occurring over Poland on 4 July 2002. Totallightning produced by these storms were detected byan older version of the LS8000 networks currentlyproduced by Vaisala. Total lightning flash densitiesare converted from two to three dimensions byincreasing the altitude of cloud-like features as totallightning flash rates increase. The color of the cloud-like feature also changes with increasing totallightning flash rates. At 0100 UTC, the total lightningflash rates of a number of storms are high andindicated by tall cloud-like features and shades ofyellow and orange (Fig. 13). Sixty minutes later the

total lightning flash rates in all active thunderstormsare low to moderate and are shown by lower altitude“cloud-like” features and shades of light blue andgreen (Fig. 14). This display is also useful for otherapplications. A shadow is placed over all areaswhere cloud and CG lightning activity is observed.This helps address the CG lightning hazardapplication. Also, yellow lines are drawn from the“cloud-like” feature base to the exact positions at theground where a LF CG lightning detection networklocated CG lightning flashes. Another useful elementof this product is that lightning can be plotted on highresolution base maps that include such features asterrain altitude, cities, highways and even radar data.

05

10152025303540

22:01 22:21 22:41 23:01

time (UT)

flash

es/ra

dar

para

met

ers

max VIL avg Z max ET FIP neg CG

Figure 12. LDAR II total lightning rates calculated fromflash initiation points (FIP), Vaisala NLDN negative CGlightning rates (neg CG), maximum VIL (max VIL),average low level reflectivity (avg Z) and maximumecho tops (max ET) from an ordinary thunderstorm inthe DFW area.

Figure 13. A thunderstorm growth and dissipationdemonstration product showing VHF total lightningmapping and CG lightning activity from Poland on 4July 2002. “Cloud-like” features represent totallightning flash rates. The higher the altitude of the“cloud-like” feature and the brighter the color (frompurple to red), the higher the total lightning flash rate.CG flashes in yellow show the exact location of theCG flash on the terrain map. The area covered bytotal lightning is shown by a shadow-effect on theterrain map. (Patent pending)

Figure 14. Same as Figure 13, except 60 minuteslater. (Patent pending)

6. SEVERE WEATHER APPLICATIONS

Total lightning data is very sensitive to changesin the strength of the updrafts within thunderstorms.Similar to radar reflectivity, increases in total lightningflash rates and the areas over which these increasesare occurring are very useful indicators of stormstrength. Patterns in VHF total lightning mappingdata as identified in FED images can be very similarto radar reflectivity signatures. Often, one can identifybow echoes and echo free regions with FED data.Similarities to radar reflectivity signatures andsensitivity to changes in updraft strength make totallightning a tool forecasters could use to increase leadtime for severe weather warnings, because totallightning data can be provided in a continuous datastream. The Fort Worth NWS Forecast Office iscurrently receiving two-minute FED plots. This meansthat for every 10-minute time period they receive fiveupdates of information for thunderstorms in the DFWarea, as opposed to two updates of radar informationdue to the five minutes it takes to complete a volumescan.

Figure 15a-c shows an example of how usefultotal lightning information can be when severethunderstorms are evolving on time scales shorterthan radar volume scans. Figure 15a shows FEDdata from a number of supercells that passed throughthe DFW area on 25 April 2005. The highest rates ofFED data are shown in shades of red. There was alarge supercell thunderstorm producing high totallightning rates near the center of the image. Hightotal lightning rates were occurring along the whiteline oriented west-northwest to east-southeast whichindicates the storm’s path and projected future path,according to the Storm Cell Identification andTracking (SCIT) algorithm used on radar reflectivitydata. Two minutes later, moderate total lightningflash rates start to develop due to a strengtheningupdraft located south of the main storm track (Fig.15b). Four minutes later, the new updraft thatdeveloped south of the storm track became thedominant updraft and total lightning rates increaseddramatically (Fig. 15c). This is an example of a right-moving supercell thunderstorm. In this case, the totallightning information would allow a forecaster toidentify strengthening updrafts in the storm andanticipate the right turn of the storm before the nextradar volume scan arrived.

7. CONCLUSIONS

VHF total lightning mapping provides aninvaluable data set for the thunderstorm nowcastingprocess. A significant safety risk currently exists tothe public due to the lack of information regarding thespatial extent of cloud lightning activity withinthunderstorms. Any cloud lightning flash, regardlessof whether it is moving through a stratiform rainregion, anvil cloud or clear air, can produce anisolated CG lightning flash anywhere along its path.

Figure 15. Total lightning FED (number oflightning branches that pass through a 1 km2 areaper minute) images from (a) 2114, (b) 2116, and(c) 2118 UTC 25 April 2005. Shades of redidentify areas with high total lightning flash rates.Past and future projected storm tracks are shownas white lines. Future projected storm tracks arecreated by the SCIT algorithm.

a

b

c

Currently, no other data source can be used forcontinuously monitoring the true extent of all lightningactivity in thunderstorms besides VHF total lightningmapping. Forecasters have radar reflectivity andvelocity data, satellite imagery, CG lightning data andhuman reports that are all used to help identify severeweather. However, none of those data sources canshow the forecaster the whole region at risk for CGlightning. Therefore, total lightning mapping fills thebiggest gap that currently exists in thunderstormhazard nowcasting.

VHF total lightning mapping is useful foridentifying thunderstorm growth and dissipation, andsevere weather trends that complement radar dataand are updated more frequently than radar. Totallightning data could help add minutes of valuable leadtime for severe weather events in situations whereevery minute may mean the difference between lifeand death.

In this paper, a number of unique demonstrationproducts have been shown that should appeal to theviewer as well as convey important informationregarding many hazards produced by thunderstorms.

8. REFERENCES

Holle, R.L., R.E. Lopez, R. Ortiz, C.H. Paxton, D.M.Decker, and D.L. Smith, 1993: The localmeteorological environment of lightningcasualties in central Florida. Preprints, 17th

Conference on Severe Local Storms andConference on Atmospheric Electricity,October, St. Louis, MO, AmericanMeteorological Society, Boston, MA, 779-84.

Maier, L., C. Lennon, T. Britt, and S. Schaefer, LDARsystem performance and analysis, inProceedings of the International Conference onCloud Physics, Am. Meteorol. Soc., Boston,MA, Dallas, TX, Jan 1995.

Murphy and Holle, 2005: A warning method for therisk of cloud-to-ground lightning based on totallightning and radar information, in Proceedingsof the First Conference on MeteorologicalApplications of Lightning Data, Am. Meteorol.Soc., Boston, MA, San Diego, CA, Jan 2005.

Rison, W., P.R. Krehbiel, R. Thomas, T. Hamlin andJ. Harlin, 2003: Lightning mapping and radarobservations of bolts from the blue.Proceedings of the 12th InternationalConference on Atmospheric Electricity,Versailles, France, June 9-13, 467-470.

Rison, W., R.J. Thomas, P.R. Krehbiel, T. Hamlin,and J. Harlin, 1999: A GPS-Based three-dimensional lightning mapping system: Initialobservations in central New Mexico, Geophys.Res. Lett., 26, 3573-3576.