INTERNATIONAL JOURNAL OF CLIMATOLOGYInt. J. Climatol. (2012)Published online in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/joc.3533

Short Communication

Forecasting the onset of cloud-to-ground lightning usingradar and upper-air data in Romania

Bogdan Antonescu,* Sorin Burcea and Adrian TanaseNational Meteorological Administration, Sos. Bucuresti-Ploiesti 97, Bucharest, Romania

ABSTRACT: A method for forecasting the onset of cloud-to-ground (CG) lightning flashes is examined. Reflectivitydata and upper-air measurements were analysed for 49 convective storms observed in 36 different days over southernRomania during the convective season (May–September) from 2003 to 2005. The radar reflectivity data were associatedwith the CG lightning flash locations obtained from the Romanian National Lightning Detection Network. The CG lightninginitiation signature is based on reflectivity thresholds (35 and 40 dBZ) at a given environmental temperature aloft (−10and −15 °C). We have found that the best predictor is the 35 dBZ radar reflectivity at the height of the −10 °C isotherm,with a probability of detection of 95%, and mean lead time of 17 min before the first CG lightning. In previous studies, itwas found that the onset of CG lightning flashes had greater threshold value for reflectivity (40 dBZ) and shorter lead time(13 min). The results presented in this study are currently used in the nowcasting operational activities at the RomanianNational Meteorological Administration. Copyright 2012 Royal Meteorological Society

KEY WORDS radar; lightning; climatology; forecasting

Received 16 November 2009; Revised 9 May 2012; Accepted 16 May 2012

1. Introduction

During the warm season (May–September) cloud-to-ground (CG) lightning flashes are produced in Roma-nia mainly by airmass thunderstorms, or by thunder-storms initiated along or near surface airstreams bound-aries, especially in southern Romanian (Stan-Sion andAntonescu, 2006). The purpose of this study is to exam-ine a method for predicting the time of the first CGlightning flash associated with convective storms devel-oping in the southern part of Romania, by using radarreflectivity and upper-air temperatures. This forecastingmethod can be useful for issuing severe weather warn-ing, both for protecting human lives and for lightningsensitive operations, so that precaution may be taken.Because aircraft traffic and airport operations are affectedby thunderstorms, this information would also be usefulfor local aviation administrations. Although there are pre-vious studies on using the radar reflectivity to predict theonset of CG lightning (Table I), this is the first time, inauthor’s knowledge, when the combined data from S-band and C-band radars are used to forecast the onset ofCG lightning flashes over Eastern Europe.

The Romanian National Meteorological Administra-tion acquired in 2002 (Ioana et al., 2004) eight Doppler

∗ Correspondence to: B. Antonescu, Centre for Atmospheric Science,School of Earth, Atmospheric and Environmental Sciences, TheUniversity of Manchester, Simon Building, Oxford Road, Manchester,UK. E-mail: bogdan.antonescu@manchester.ac.uk

weather radars, five of which are Weather SurveillanceRadar-98 Doppler S-band systems, and three are C-bandunits. In this study we have used this radar network tocollect data from thunderstorms which developed in thesouthern Romanian Plain, during the convective season(May–September) from 2003 to 2005. This study wasdeveloped for the Bucharest city, an area with two inter-national airports, and a population of 2 million people,abundant with various outdoors activities. The study areais topographically homogeneous and the three Dopplerradars covering this region have no beam blockage atany of the elevation angles (Figure 1).

For understanding how radar reflectivity can be usedto forecast the first CG lightning flash, in Section 2 weexamine the main mechanism for storm electrification.Section 3 presents the data set and methodology andSection 4 contains the results. The conclusions arepresented in Section 5.

2. Previous studies

In their review chapter on charging mechanisms, Mac-Gorman and Rust (1998) stated that in order to pro-duce electrical charges inside the cloud, and thus elec-trical discharges, a strong updraft associated with vig-orous convection growth should occur in the lowerpart of the mixed-phased region of the cloud. Themixed-phase region of the cloud, situated approximately

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B. ANTONESCU et al.

Table I. Previous studies that have used the radar reflectivity threshold at a given environmental temperature aloft in order topredict the onset of cloud-to-ground lightning activity.

Study Region Period of study Number of cases(number of days)

Criteria Lead time(min)

POD FAR CSI

Dye et al. (1989) New Mexico(USA)

July–August 1986 20 (18) 40 dBZ Immediately – – –

−10 °CBuechler andGoodman (1990)

Florida (USA) 1986 15 (–) 40 dBZ 4–33 1.00 0.20 0.80

−10 °CMichimoto (1990) Hokuriku (Japan) December 1987 – 30 dBZ 5 – – –

August 1988 −20 °CJanuary–March1988–1990

Hondl and Eilts(1994)

Florida (USA) August 1990 28 (8) 10 dBZ 20 – – –

0 °CGremillion andOrville (1999)

Florida (USA) May–September1992–1997

39 (38) 40 dBZ 7.5 0.84 0.70 0.79

−10 °CVincent et al.(2003)

North Carolina(USA)

2001–2002 50 (13) 40 dBZ 14.7 1.00 0.37 0.63

−10 °CPresent study Romania May–September

2003–200549 (36) 35 dBZ 17 0.95 0.10 0.85

−10 °C

Figure 1. The Romanian National Doppler Weather Radar Network – composed from five Weather Surveillance Radar-98 Doppler S-band systemslocated at Timisoara, Oradea, Barnova and Medgidia, and three C-band units operational at Bucharest, Craiova and Oradea (not shown on this

map). The figure also shows the Romanian National Detection Network (RNLDN) and the study area (black box).

between −10 and −40 °C isotherm, is the region wheresupercooled liquid water and ice coexist. This is afavourable environment for a non-inductive charging

mechanism that involves rebounding collisions betweencloud ice particles and riming graupel in the presence ofsupercooled liquid water. The non-inductive graupel-ice

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FORECASTING THE ONSET OF CLOUD-TO-GROUND LIGHTNING

charging mechanism is the only one that laboratory andmodeling studies have indicated it capable of causingclouds to be electrified enough to create CG lightningflashes (Saunders, 1993; MacGorman and Rust, 1998).The observed dependence of the non-inductive charg-ing mechanism on environmental parameters (Takahashi,1978; Saunders and Peck, 1998) appears to explainwhy some storms are thunderstorms and others are not.Through laboratory experiments Takahashi (1978) foundthat the graupel-ice mechanism depends on the temper-ature and liquid water content. For typical condition,low water content on the order of 1 g m−3 and temper-ature colder than −10 °C, the graupel tend to be nega-tively charged and the small ice crystals became posi-tive charged. The positively charged crystals are liftedin the upper regions of the storm, and the negativelycharged graupel is suspended in the updraft in the mid-levels of the storm. MacGorman et al. (1989) suggestedthat this mechanism operates at the 7–9 km level, wherethe temperature of the environment is −20 to −40 °Cand there are ice crystals in sufficient quantities (Michi-moto, 1990). Polarimetric techniques showed a correla-tion between radar reflectivity values and various pre-cipitations types (Doviak and Zrnic, 2006). Thus, theappearance of 30–40 dBZ radar echoes between −10and −15 °C indicates the possible presence of graupel orhail suspended in an updraft containing ice crystals.

Bright et al. (2005) indicated that three ingredientsare necessary for CG lightning flash production: (1) forensuring the presence of supercooled liquid water thelifted condensation level must be warmer than −10 °C,considering that in general ice begins to nucleate attemperature colder than −5 to −10 °C; (2) in order tohave ice nucleation the equilibrium level must be colderthan −20 °C; (3) convective available potential energy(CAPE) must be greater than 100–200 J kg−1 in the layerbetween 0 and −20 °C in order to have sufficient verticalmotion in the mixed-phase region. Similar ingredientsfor CG lightning flashes were used by van der Broekeet al. (2005). They agree with the first two ingredientsfor lightning production proposed by Bright et al. (2005),but they stated that CAPE should be present in thelower mixed-phase region of the cloud between −10 and−20 °C.

Previous studies, as far back to 1970s, have used thecapability of weather radar to forecast the onset of CGlightning flashes. One of the first studies on using radardata to forecast and detect the lightning flashes was thestudy by Larsen and Stansbury (1974). They generatedradar maps of precipitation with a time step of 10 min,at 7 km height, outlining the regions exceeding 43 dBZ.They showed that this region (Larsen area) was the sourceof lightning flashes throughout 90% of the life time ofthe thunderstorms, and that they could account for 75%of the flashes observed. In a subsequent study, Marshalland Radhakant (1978) used radar maps at the height of6 km (approximately −17 °C) and at 5-min intervals, with4 dBZ outlines intervals between 30 and 58 dBZ. Theyshowed that ‘Larsen area’ within the 38 dBZ outline is

a representation of the convectively active regions of thethunderstorms.

Another approach to forecast the CG lightning flashesis based on the outputs of the numerical weather predic-tion (NWP) models (Mazany et al., 2002; Burrows et al.,2005). Burrows et al. (2005) developed statistical mod-els based on tree-structured regression for predicting theprobability of lightning, using lightning data from theNorth American Lightning Detection Network and pre-dictors (e.g. CAPE, convective inhibition, Showalter, pre-cipitable water, 700 hPa vertical motion, 500–1000 hPalayer thickness) derived from Global Environmental Mul-tiscale model output at the Canadian Meteorological Cen-ter. Real-time forecast in 3 h intervals for lightning prob-ability was made in 2003 and 2004, and results demon-strate that tree-structured regression is a viable methodfor building statistical probability forecast models. Themain issues with their approach is that this can be usedfor finding regions favourable for thunderstorm forma-tion, and can be used just as guideline for nowcastingand short range forecast. Shafer and Fuelberg (2008) alsouse statistical schemes to forecast warm-season lightningover Florida (USA). Based on analysis data from RapidUpdate Cycle and lightning data form National LightningDetection Network, and using a perfect prognosis tech-nique, Shafer and Fuelberg (2008) developed a griddedforecast guidance product for CG lightning activity. Theyalso classified the amounts of CG lightning in two cate-gories, with one or more CG flashes, using binary logisticregression. The scheme is applied to output from threemesoscale models during an independent test period,and the results showed that although the temporal andspatial forecast lightning was not perfect, there was a‘generally good agreement between the forecasts andtheir verification, with most of the observed lightningsoccurring within the higher forecast probability contours’(Shafer and Fuelberg, 2008). McCaul et al. (2009) haveproposed and developed two methods for quantitativeshort-range forecast of lightning threat based in the ice-phase hydrometeor fields from regional cloud-resolvingnumerical simulations. The first method is based on theupward fluxes of precipitating hydrometeors at −15 °Clevel. The second method is based on the verticallyintegrated amount of hydrometeors for each model gridcolumn. Because the first method is able to representbetter the temporal variability of lightning threat, and thesecond method is representing more accurate the arealcoverage of the threat, McCaul et al. (2009) have pro-posed a ‘blended’ forecast method. This ‘blended’ fore-cast method was designed to retain most of the temporalsensitivity of the first method, while adding the improvedspatial coverage of the second. As was noted by McCaulet al. (2009), the main issues associated with their fore-cast methods are represented by: (1) imperfect locationand timing of convective storms from the individual sim-ulations and (2) NWP output needs to be recalibratedagainst observed lightning data for every change of themodel grid mesh or parameterization schemes.

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More recent studies (Table I) show that an evidenceexists concerning the correlation between radar reflec-tivity factor at certain altitude where the temperatureassumes typical values and time lapse before the occur-rence of the first CG lightning flash.

These methods, besides the fact that are straightforwardand easy to implement in operational environments,represent an alternative to the use of complex schemesof NWP models. Also, it can be shown that thesenowcasting methods for CG lightning forecasting basedon radar data, eliminate some problems associated withthe forecast based on NWP data, e.g. location and timingof the lightning activity. But, similar with NWP forecastmethods based on sets of convective parameters thathave different values for different regions, the methodsbased on radar data can be regional dependent. Withone exception, Michimoto (1990), all the previous studiesfrom Table I have been carried out in the United States.The majority of the previous studies have found similarresults for criteria for the onset of CG lightning activity,but with different lead times ranging from immediatelyup to 33 min.

3. Data and methods

3.1. Radar data

The Romanian National Radar Network (RNRN) includesfive S-band radars and three C-band radars. The S-band radars from RNRN are based on the technologyand meteorological algorithms developed over more than30 years in the USA NEXRAD network. The radardata used in this study were obtained from Bucharest,Medgidia and Craiova radars (Figure 1) using the VCP21(Volume Coverage Pattern) scan strategy.

VCP21 is a standard precipitation scan and it is usedwhen the storms are situated at a certain distance fromradar to avoid sampling errors when the storms are nearthe radar. The radar performs a volume scan using ninesweeps at different tilts (0.5°, 1.45°, 2.4°, 3.35°, 4.3°,6.0°, 9.9°, 14.6°, 19.5°) every 6 min, and has low verticalcoverage for areas close to the radar. Although this scanstrategy could potentially miss radar observation in themixed-phase region close to the radar site, the operationalforecasters use VCP21 due to the short time betweenobservation and the availability of data. One applicationthat integrates the data from the two types of radars(S-band and C-band) is called Principal User Processorand was developed for real-time interpretation of radardata in nowcasting and forecasting environments. Eachstorm has been observed either with the C-band radarsfrom Bucharest and Craiova or with the S-band radaroperational at Medgidia according to the distance criteria(i.e. the closest radar).

3.2. CG lightning data

One of the new networks installed in 2002 in theframe of National Integrated Meteorological System(SIMIN, Romania) (Ioana et al., 2004) is the Romanian

National Lightning Detection Network (RNLDN). TheRNLDN consist of eight SAFIR3000 Total LightningAutomatic Detection Stations manufactured by VaisalaOyj (Helsinki, Finland), and provides national coverage,with a horizontal accuracy lower than 1 km for theCG lightning flashes. The detection stations performtwo types of detection, one being based on the veryhigh frequency interferometry, to obtain accurate angularlocalization of intra-cloud and CG lightning flashes. Thesecond type of detection is provided by a wide-bandlow frequency electric-field sensor, and the data fromthis sensor are used to obtain the characteristics of CGlightning flashes.

In this paper, we use only the CG lightning data,which are detected with a detection efficiency estimatedby the manufacturer at 90%. During the study period(2003–2005) the working parameters of the RNLDNwere constant, therefore no corrections were applied forthe detection efficiency of the CG lightning data. TheCG lightning data used in this study were recordedby the RNLDN within the box 43.58° – 45.00 °N and22.83° – 28.08 °E (Figure 1), situated in a plain regionin the southern part of Romania. The main reason forchoosing this area was to develop a forecast techniqueto monitor the thunderstorms developing near Bucharestcity.

3.3. Forecasting the onset of CG lightning flashes

On the basis of the non-inductive charging mechanismproposed by Takahashi (1978) several criteria have beenproposed in previous studies to examine the onset ofCG lightning activity. In this study, four criteria for theonset CG lightning activity were tested. These criteria arebased on the existence of radar reflectivity factor withvalues of 35–40 dBZ, showing the possible presence ofgraupel suspended in the updraft, in a region situatedbetween −10 and −15 °C. The analysed sample consistsof 49 isolated storm events from 36 lightning days, stormsdetected at more than 30 km from the radar.

In order to identify the individual storm cells, theStorm Cell Identification and Tracking algorithm (John-son et al., 1998) was used. After the storm identification,the lightning data from RNLDN were overlaid with thestorm track. This has been done to verify if a stormproduced lightning and to identify the first CG light-ning flash. The storms were post-analyzed using radarsoftware environments and a similar procedure with thatdescribed by Vincent et al. (2003). This procedure wasused because it can be easily applied during real-timeoperational context. A four-panel plan position indica-tor (PPI) display of the reflectivity factor at the lowestfour elevation angles (0.5°, 1.45°, 2.4°, 3.35°) was usedto visualize the radar data. For a specific sample, the val-ues of radar reflectivity factor and echo height are readdirectly from the radar display. The values of isothermheights were obtained from the Bucharest soundings (12UTC). Only the storms being within 185 km from thesounding release point, and during 6 h period centred at12 UTC sounding were analysed.

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FORECASTING THE ONSET OF CLOUD-TO-GROUND LIGHTNING

For the cases in which the height of −10 or −15 °Cisotherm was not directly observed on the PPI elevationangle, cubic spline interpolation was used to obtainthe elevation. The spline interpolation is a form ofinterpolation that allows each segment to have a uniqueequation while still constraining the curve to fit thedata properties (De Boor, 1978). This interpolation ispreferred over polynomial interpolation because of thesmall interpolation errors. In this study, the cubic splineinterpolation was performed between the closest elevationangles at which the beam centres are found above andbelow the level of the −10 or −15 °C isotherm.

4. Results

To study the potential use of these criteria for forecast-ing thunderstorms in the densely populated region ofBucharest, each criterion was tested using a 2 × 2 contin-gency table, to obtain the probability of detection (POD),the false alarm ratio (FAR) and the critical success index(CSI; Wilks, 2006). The CSI, unlike POD and FAR, takesinto account both false alarms and missed events, thusbeing a more balanced score. Its range is 0–1, where1 represents a perfect forecast. The best criterion mustbe valuable from an operational forecast perspective (i.e.long lead time) and in the same time accurate (i.e. highvalue for POD and CSI, and low values for FAR).

The results show that the best prediction criterionwas when the 35 dBZ echo was detected at the levelof −10 °C isotherm (Figure 2), with a POD of 95%and a CSI of 85%. Recent studies (Dye et al., 1989;Buechler and Goodman, 1990; Gremillion and Orville,1999; Vincent et al., 2003) showed that the best criterionfor the onset of CG lightning flashes for United States was40 dBZ at −10 °C level. For Japan, Michimoto (1990)showed that the 30 dBZ at −20 °C level as the bestcriteria for the CG lightning initiation.

Figure 2. Results of the statistical analysis for the onset of cloud-to-ground lightning activity in this study; the probability of detection(POD), the false alarm rate (FAR) and the critical success index (CSI)

are represented.

The results for southern Romania showed that increas-ing the reflectivity criteria from 35 to 40 dBZ at −10 °Clevel results in an increase of the FAR (12.5%) and adecrease of the POD (83.3%) and CSI (79%) (Figure 2).Increasing the isotherm level from −10 to −15 °C hasless impact on the statistical scores. Thus, the 35 dBZ at−15 °C was the second best criterion (Figure 2) based onPOD (90%) and CSI (80%).

Vincent et al. (2003) used a third variable, namely thepersistence of criteria for two consecutive volume scans,resulting eight sets of criteria. The use of a third criterioncould result in a decrease of the lead time betweendetection of the thunderstorm initiation signature and thefirst CG lightning flash, but it may also help reduce FAR.Two consecutive volume scans criteria were also tested inthe present study. Applying this criterion for the 35 dBZ

at −10 °C level results in lower values of POD (88.6%)and higher values of FAR (11.3%) in comparison withthe single scan criterion, and a lead time of 10 min.

In this study the mean lead time obtained was 17 minfor 35 dBZ at −10 °C criteria and 13 min for 40 dBZ

at −10 °C (Figure 3). For 35 dBZ at −15 °C, the secondbest criterion based on FAR and CSI, the lead time was12 min. These findings are consistent with the resultsobtained by Vincent et al. (2003) for North Carolina(USA).

5. Conclusion

The analysis of radar data for 49 thunderstorms showsthat the radar reflectivity and the sounding data canbe used to forecast the onset of CG lightning activity.In this study, the best predictor for the onset of CGlightning activity was the 35 dBZ radar reflectivity atthe level of −10 °C isotherm, with a POD equal to95%, a FAR of 10% and CSI of 85%. The mean leadtime was 17 min. Gremillion and Orville (1999) foundthat the best criterion for CG was the 40 dBZ echodetected at the −10 °C level, with an average lead time

Figure 3. Results of the statistical analysis for the mean time intervalbetween the lightning initiation signature and the time of the first CG

lightning flash.

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of 7.5 min (Table I). They also noted that the 35 dBZ

radar echo at the −10 °C isothermal level was the secondbest predictor, with POD of 88% and FAR of 20%. Inthis study was found that the best criterion has a lowerradar reflectivity threshold than those found in previousstudies, and this would act by increasing the lead time.Contrary to the expectation, the increase of lead timeshas not increased FAR. Vincent et al. (2003) found thatthe best predictor of CG lightning, based on CSI, wasthe presence of a 40 dBZ echo at the −10 °C, with 37%FAR, 100% POD, 63% CSI, and an average lead timeof 14.7 min (Table I). Despite several potential sourcesof error (e.g. detection efficiency of lightning detectionnetwork, radar detection errors, and interpolation betweendifferent radar elevation angles to find the proper heightof isotherms) the results from this study compares wellwith previous studies.

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