Jun 16, 2001 - Martin Leon [1995], who reported maximum errors in the flash position ...... Hodanish, S., D. Sharp, W. Collins, C. Paxton, and R.E. Orville, A.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. Dll, PAGES 11,891-11,901, JUNE 16, 2001
Cloud-to-ground lightning activity in the Iberian Peninsula: 1992-1994
Luis Rivas Soriano,Femandode Pablo,andEulogioGarciaDiez Departamento de Fisicade la Atm6sfera,Universidadde Salamanca, Salamanca,Spain
Abstract. For the first time, the temporaland spatialdistributionof cloud-to-ground lightningactivityin the IberianPeninsulaarebeingstudied.The databasecoversthe years
1992-1994, andit wasrecorded over2.2x 106cloud-to-ground lightning flashes. The
monthlyvariationshowsa singlepeak in the warmermonths(May to September)with a strongyear-to-yearvariability.The diurnalcycleof lightningactivitypeaksat 1700 LT with a subsequent slow decreasetowardthe minimumin the morninghours.The percentageof positiveflashesis 8%, althoughthispercentageis higherin the winterthanin the summer. The lightningflashpolarityis foundto be very similarthroughoutthe day. The average multiplicityis foundto be 2.0 for the negativeflashesand 1.1 for thepositiveflashesand is very similarin thewinterandsummerseasons, while thepercentage of single-stroke flashes tendsto increasein the winter. The median(mean) intensityis found to be 22.5 kA (32.4 kA) for the negativeflashesand 52.3 kA (69.3 kA) for the positiveflashes.The excessivelyhigh intensityfor thepositiveflashescouldbe dueto the detectionsystemhaslow efficiencyin detectiongweak amplitudeflashes.The distributionof amplitudesis very similarin the winter and summerseasons.The decayat largeamplitudesis slowerfor the positiveflashes. The medianandmeanamplitudesare higherin the winter for bothpolarities.The spatial cloud-to-ground lightningdistributionconfirmsthe well-knownpreferencefor certainareas in the developmentof thunderstorms. The averagemaximumnegative,positive,and
combined flashdensities are3.1,0.2,and3.3flashes km'2yr'• respectively. Thecomparison
betweenindividualyearsshowsa considerable year-to-yearvariability.The average percentage of positiveflashesrangesfrom 0% to 41%. The grid blockmedianpeak current amplitudesvary from 12.5 to 47.4 kA for the negativeflashesandfrom 17.6 to 107.5 kA for thepositiveflashes.The resultsare discussed in the contextof othermeasurements takenat differentpartsof the world.
1. Introduction
In recent years, lightning location systems have been installedin many countries,and this has resultedin numerous researchworks. Hail, heavyprecipitation,flooding,wind, and lightning make thunderstormsone of the major cause of natural disasters. Lightning can be used to estimate convectiverainfall [i.e., Petersenand Rutledge, 1998]. The combination of lightning data with radar observations presentsgood perspectivesin the now-castingof convective storms,asdemonstrated by Sternet al. [1994]. Becauseof the sensivityof the global circuit to change in surface air temperature,lightning might be used as an indicator of changesin the global temperature[Williams, 1992, 1994; Reeve and Toumi, 1999]. There are further interests in lightningactivity,sinceit playsa fundamentalrole ascauseof naturalforestfires in midlatitudes[i.e., Krider et al., 1980], it is a sourceof nitrogenoxide and hencetropospheric ozone [Franzblauand Popp, 1989; Toumi et al., 1996], and it is a majorcauseof weather-related deaths,propertydamages,and powersystembreakdowns[FisherandKrider, 1982].
Copyright2001by theAmericanGeophysical Union. Papernumber2001JD900055. 0148-0227/01/2001JD900055509.00
Timevariations of lightningactivityoverthedays,months, andyearshavebeenstudiedby severalauthors[e.g.,Lopez andHolle, 1986;PetersenandRutledge,1992;Orville, 1994; Reap, 1994; Watsonet al., 1994;Pinto et al., 1996;Finke and Haul, 1996; Tuomi, 1996; Orville et al., 1997; Hodanishet al., 1997; Orville and Silver, 1997;Hidayat and Ishii, 1998,
1999;I.R.C.A.Pintoet al.; 1999;OrvilleandHuffines,1999]. The hourly variability of the cloud-to-ground (CG) lightning activityoverlandiswellknownto havea largepeak associatedwith the maximum convectiveactivity in the afternoon. Variations can also exists
associated with local
meteorologicaland orographicaspects.Over the tropical westernPacific Ocean,Petersenet al. [1996] and Orville et al. [1997] havefoundthatCG lightningexhibita sharppeak between0200 and 0400 LT, a secondary maximumat 1500 LT
and a broad minimum
centered near 1200 LT.
The
monthly variability is also well known. In middle latitudes, lightning activity over land has a maximum in the summer seasonand a minimumin the winter season[e.g.,Finke and Hauf, 1996;Orville andSilver, 1997]while havingtwo peaks in tropicalregions[Orville et al., 1997; I.R.C.A. Pinto et al., 1999]. There is also evidenceindicatingthat not only the
numberof flashesbut also the flash characteristics change from one seasonto the another,as doesthe flash polarity [e.g., Finke and Hauf, 1996; Orville et al., 1997; I.R.C.A. Pinto et al., 1999]. The annualvariabilityis lessknown and 11,891
11,892
RIVAS SORIANO ET AL.: LIGHTNING IN THE IBERiAN PENINSULA
is restrictedto the contiguousUnites States(e.g., Orville
It should be noted that most studies on the characteristics
[1994],Orvilleand Silver[1997],andOrvilleandHuffines of lightning activity have been carried out in the United States,while the numberof studiesrelatingto othercountries
[ 1999] for a 9-yearperiod).
Thegeographical distribution of cloud-to-ground lightning is limited. In this paper we present for the first time an flasheshas been studiedby severalauthors.Orville [1991,
analysis of the temporal and geographicalvariations in
1994],Orville and Silver [1997], and Orville and Huffines lightningactivity in the Iberian Peninsula,over a periodof 3 [1999] have publishedresultsfor the contiguous United years, 1992 to 1994, and compareour findingswith results Statesfrom 1989 to 1997, ttodanish et al. [1997] for the State of Florida from 1986 to 1995, Finke and Hauf [ 1996]
reported by other authors. The information offered here constitutesone of the few contributionsabout lightning for southernGermanyfrom 1992to 1994,ttidayat and Ishii activity in Europe and may help to assessthe importanceof [1998]aroundJavafromDecember1994to January1996,O. thisgeographicalareain the globallightningactivity. The paper is organizedas follows. In section2 we give a Pinto et al. [1999] for Brazil in 1993. Orville and Silver [1997]reportedthatthe highestannual densities havebeen short descriptionof the lightning detectionsystemand the found tovarybetween 9 and13flashes km-2yr'•.Theannual data evaluation. In section 3 we present the temporal and percentage of positiveflasheswas alwaysbelow 10%, and distributionof lightningactivity and flash characteristics, the highestannualflash densitiesfor negativeand positive in section4 the spatialdistributionis considered.Conclusions flashes werefound tovarybetween 9 and13flashes km-2yr'• are givenin section5.
andbetween 0 and1.8flashes km-2yr-•,respectively. In the tropicalareaof PapuaNew Guinea,Orville½tal. [1997] have
found a highest flashdensity of2.0flashes km-2yr-•,anannual
2. Lightning Detection and Data Evaluation
The lightning detection system installed on the Iberian percentage of positiveflashesof 5.6% andan annualaverage peak currentof 25 kA for negativeflashesand 33 kA for Peninsula and belonging to the Spanish Meteorological positiveflashes.For Brazil, O. Pintoet al. [1999] havefound Service (INM) consistsof a network of 13 sensorsdistributed
a maximum flashdensity of 15.5flashes km-2yr-• witha throughoutthe Spanishterritory (Figure 1). Sincethere is not any sensorlocatedover Portugal,we expecta lower detection efficiency over this country than in Spain. The localization positive flashes. ForGermany, Finke andHauf[1796]have accuracyof flashesfor the Iberian Peninsulais analyzedby foundthatdensities largerthan6 flashes km-2yr- arerare. Martin Leon [1995], who reported maximum errors in the They have also found an averagepercentageof positive flashpositionlower than 8 km for Spainand errorsbetween 8 flashesof 5% and median (mean) peak currentsof 22 kA and 12 km for Portugal.This lightning detectionsystemuses (25.5 kA) for negative flasheswith 19 kA (32.6 kA) for ALDF model 141-T manufacturedby LightningLocationand positive flashes.For Finland, Tuomi [1996] has found an Protection Inc. and is similar to those used in other countries. averagepercentage of positiveflashesof 15% andmeanpeak This detectionsystemhas been describedin depthby several currentsof 42 kA for negativeflasheswith 77 kA for positive authors[Krider et al., 1976, 1980; Orville et al., 1983; Lopez flashes. and Holle, 1986] and basicallyconsistsof a widebandsystem
36.5% percentageof positive flashes and average peak currentsof 30.9 kA for negative flasheswith 17.8 kA for
iSLA
N
ß
200
.50.0 1000
BALEA
1500
>1500
Altitude
Figure1. Locations of theALDFsensors (indicated bystars) andmainorographic features in theIberianPeninsula.
RIVAS SORIANO ET AL.' LIGHTNING IN THE IBERIAN PENINSULA
11,893
1903
180000
0
0
1984
1992-94• 16O0O0
1600EO
Figure2. Monthlyvariationof thenumberof cloud-to-ground flashesin theIberianPeninsula for 1992-1994.
of orthogonalmagneticloop antennasand a flat planeelectric antenna.The magneticfield of a lightning flash producesa signal in the circuit of each loop. The directionof the flash can be determinedfrom the ratio of signalsfrom the two or three loop antennas.The 180ø ambiguityof the directionis solvedby using the flat plane electricantenna,which detects the polarityof the chargebroughtto ground.This lightning detectionsystem is designedto localize cloud-to-ground (CG) lightningflashesand ignoreintracloud(IC) discharges, thoughthe latter are more frequentthan CG discharges. The ratio betweenthe frequencyof both lightningflashesmay depend on the geographicallatitude. While Prentice and Mackerras [1977] give a factor 3 for the midlatitudes, Mackerrasand Darvaeniza [1994] indicatea lower value of
Number of flashesand flash densityvalues in this paper are the result of multiplying the measurementsby an arbitrary
between36øN and 43.8øN and that between3.2øEand 9.6øW,
near the edgesof the regionconsidered.However, in the lack
Detection efficiency depends on antenna and receiver characteristics, atmosphericpropagation,network geometry, and some other factors. The detection efficiency of this systemhas not been estimatedyet. However, for comparing our resultswith othermeasurements takenat differentpartsof the world, we must correct the number of flashes recorded.
constant factor of 1.4. We have used this value because an
overall detection efficiency of 70% is quoted for several detection systems (i.e., Orville [1994] for the National
LightningDetectionNetwork (NLDN) in the United States, Finke and Hauf [1996] for a Lightning PositionAnd TrackingSystem(LPATS) in Germany,and O. Pinto et al. 1.3. [1999] for a LPATS in Brazil). This assumptionof 70% In this study we have consideredthe region located uniformflashdetectionefficiencymay not be real, specially
which includesthe whole IberianPeninsula.To studythe of exactinformationon the detectionefficiencyit seemsto be temporaldistributionof lightningactivity,we haveconsidered reasonable. time integralsover3 years,1 year, 1 month,and 1 hourand integrated over the complete area. To analyze the 3. Temporal Distribution and Flash geographical distributionof flashes,the lightningdatawere Characteristics referenced to gridblocksof 0.2ølongitude x 0.2ølatitude(368
km2). Thissizewasadopted sothattheerrorinflashlocation
Within the Iberian Peninsula, the number of CG flashes
wouldnotsignificantlyaffecttheresults.A similarprocedure countedare 557,163 in 1992, 545,717 in 1993, and 437,859 in was adoptedby Reap [1986] or O. Pinto e! al. [1999]. It is 1994.After multiplyingby 1.4 it givesover780,000,764,000, worthmentioning herethatpossible variations in thelightning and 613,000 respectively.This gives a total of 2,157,000, a parametersfor distance< 20 km would not be observed in this analysis.
meannumber of-720,000flashes yr-I anda meanlightning frequency of 1.4 min-l.
11,894
RIVAS SORIANO
ET AL.' LIGHTNING
IN THE IBERIAN
3.1. Annual and Diurnal Cycles
PENINSULA
1992-94Average 70000
To characterize the annual cycle, we calculated the monthlynumberof CG flashes(combinednumberof negative andpositiveflashes).Resultsare shownin Figure2 for each of the 3 years,togetherwith the averageoverthe 3 years.The monthly variation showsa single peak: about 79% of all lightningeventswere observedbetweenthe monthsMay and September.The lightning number increasesfrom April to June,and the sharpdecreaseafter Octobermarksthe end of the stormperiod.This single-peakdistributionis relatedto the annualcyclefor the surfaceair temperature.The high surface air temperature during the warm season supplies an appropriate thermodynamic atmospheric background for 1oooo convection.The same single-peakannual distributionwas found in other middle latitude zones: Germany [Finke and 0 Hauf, 1996], Finland[Tuomi, 1996], Florida[Hodanishet al, 1 3 5 7 9 11 13 15 17 19 21 23 1997], and the contiguousUnited States[Orville and Silver, Local Time 1997]. This annual cycle is different from the distribution found in tropical regions, where two peaks exist: global Figure3. Diurnalcycleof thenumberof thec]oud-to-•round [Williams, 1994], Papua New Guinea [Orville et al., 1997], flashesin the Iberianpeninsulafor the 1992-1994average. and Brazil [I.R.C.A.Pinto et al., 1999]. Figure 2 showsthat lightning activity exhibits a strong year-to-yearvariability. This is causedby the variability of stormevolution,includingthe dissipation,showsconsiderable synoptic conditions favorable for the development of scatter of timescales. convection.For instance,situationsof easterlywarm and
humid air flows from the Mediterraneansea cause high thunderstormactivity in the easternIberian Peninsula.This strongyear-to-yearvariabilityhasalsobeenfoundby Orville [1994] for the contiguousUnited Statesand by Finke and Haul[ 1996] for Germany.
3.2. Polarity
Figure 4a showsthe percentageof positiveand negative flashesfor each month in the Iberian Peninsula.Only 8% of all detected flashes were positive. This value is slightly To studythe diurnalcycleof lightningactivity,'we higherthanthe valuesof 3.1-4.0% reportedby Orville [1994] calculatedthe numberof CG flashes(combinednegativeand for the United States, 5.1% reported by Finke and Hauf positiveflashes)in eachhour.Resultsare shownin Figure3 [1996] for Germany,and 3-4% foundby Orville et al. [1997] for the 3-year average. Lightning activity increasesfrom a in PapuaNew Guinea,but it is lower than 15% reportedby minimum in the morning hoursto a maximum at 1700 LT, Tuomi[1996] in Finland and-20% foundby I.R.C.A. Pinto et followed by a slow decreaseto the early minimum. This al. [1999] in Brazil. During the warmer monthsof the year minimum is observed between 0900 and 1100 local time. (May to September),positive flashescontributeonly 5.9%, Only-•6% of all lightning flashesis observedin this time whereas this percentageincreasesto 13.1% in the colder interval. This diurnal cycle is similar to diurnal cycles months(Januaryto March and Octoberto December).This reported by other authors in middle latitudes(e.g., Reap annual variation in the percentage of positive flashes [ 1986] in Florida; Watsonet al. [ 1994] in Arizona;Finke and compareswell with the resultsfound in other studiesoutside Haul [1996] in Germany) and in tropical regions (e.g., the tropics:Orville and Huffines [1999] found-6% in the I.R.C.A. Pinto et al. [1999] in Brazil) and is causedby the summerand 24% in the winter in the United States,and Finke response of convective activity to the diurnal cycle of and Hauf [1996] reported 5% for Germany from May to insolation.Only 11% of all lightning activity is observed Augustwith a significantlyhighervalue in the colderseasons between0700 and 1100 LT, but it is a higherpercentagethan of the year. Percentagesof positive flasheshigher in the in tropicalregions[e.g., I.R.C.A. Pinto et al., 1999]; it winter than in the summerhave alsobeenobservedin Japan
representsthe contributionof lightning from frontal [Takeuti et al., 1978; Hojo et al., 1989]. Finke and Hauf occurringin the colder thunderstorms. The latter are approximatelyhomogeneously [1996] suggestedthat thunderstorms seasons mostly take place in sheared environment and do not distributedover the day. Their contributionto the diurnal extend to large altitudes. Under these conditions, more positivestreamers proceedingfrom the upper,positivecharge centerof the cloud can reachthe ground[e.g., Brook et al., 1982]. However, in the tropics the percentageof positive flashesseemsto be the samethroughoutthe year, in spiteof the differentvalues:around3-4% in both seasonsin Papua New Guinea[Orville et al., 1997] and-20% in both seasons thediurnal radiatively forced heating cycleoftheatmospheric boundarylayer [Stull, 1988]. The initiation of deep in Brazil [I.R.C.A. Pinto et al., 1999]. convection requires a well-mixedstateof thefullydeveloped Figure 4b showsthe diurnalvariationof the positiveand boundary layer,whichusuallyis thecaseat about1100-1200 negativeflash percentagesin the Iberian Peninsula.Different
cycleis therefore moreor lessconstant throughout thedaybut appears asthesolelightningsource in theabsence of storms forcedby heating.The risingto the diurnalmaximumis steeper thanthefallingfromit (Figure3). Thisfactmaybe explained following the workby FinkeandHaul [1996]. Theysuggested thattheriseto thediurnalmaximum follows
LT in the morning. While this storm initiation is from the annual variation, the diurnal variation does not show approximately the samefor all locations in the space,later a great difference in the percentageof positive flashes,
RIVAS SORIANO ET AL.' LIGHTNING 1992-94
(a)
P
IN THE IBERIAN
PENINSULA
11,895
and/orin discriminating betweenstrokesthatoccurvery close in time, as suggestedby I.R.C.A. Pinto et al. [1999]. The valuesfor the summerand the winter are very similar, but with respectto the percentageof single-stroke flashes,it was found that it has a clear tendencyto increasein the winter (68.5% in the winter and 51.9% in the summer),mainly in negativeflashes(63.5% in the winter and 49.8% in the summer).The sameresultwas obtainedby Orville [1987] in the United StatesandI.R.C.A. Pinto et al. [1999] in Brazil.
_ __DNegative Flashes
10
Positive Flashes
3.4. Intensity
The distributionof peak currentfor negativeand positive flashesin the IberianPeninsulais depictedin Figure6 for the year, the summer,and the winter. The differentshapeof the
Month
distribution for the negative and the positive flashes is remarkablesincethe decayto large amplitudesis slower for
1992-94
(b) lOO
1992-94 lOO 90 80 70
[]Negative Flashes] .,., I Positive Flashes J
60
o
50
a-
40
ßPositive Flashes
LrnNe._g at•iv_e•F I_a_s_h e•J
30 20 lO
11
13
15
17
19
21
23 o
Local
Time
1
2
3
4
Number
Figure4. (a) Monthlyand(b) diurnalvariations of the proportions of negative andpositive flashes in theIberian
5
1992-94
Peninsulafor 1992-1994.Note the logarithmicscaling.
>5
of strokes
Summer
lOO 9o
80
althoughthereseemsto exista higherpercentage of positive flashesduringthe morningminimum(-9% from 0700 to 1100 LT) than duringthe afternoonmaximum(-7% from
7o
60
ß Positive Flashes LD Negative Flashes I
50
1500 to 1900 LT).
40
3.3. Multiplicity
2o
30
Figure5 showsthe multiplicityfor the year,the summer, andthewinterfor negativeandpositiveflashesin theIberian
lO
o 1
2
3
4
5
>5
Peninsula. As expected, negative flashes show a larger Number of Strokes multiplicity than the positive flashes.The mean number of strokesper flash is found to be 2.0 for the negativeflashes 1992-94 Winter and 1.1 for the positive flashes.Thesevalues are lower than lOO the values reported by other authors using the LPATS 9o technique[e.g., Montandonet al., 1992;I.R.C.A. Pinto et al., 8o 1999]. The percentageof single-strokeflasheswas found to 70 be 51% for the negativeflashesand 93% for the positive 60 flashes.Thesevaluesare also lower than the valuesreported 50 [•egative Flashes by other authors.From an analysisof 26 negativecloud-to40 groundflashes,Dinitz et al. [1996], for example,reporteda 3O percentageof single-strokeflashesof 63%. However, Tuomi [1996], using the ALDF (Advanced Lightning Direction 10 -Finder), reportedfor Finland a percentageof single-stroke 0 --J---• F'-I 1 2 3 4 5 >5 flashesover 50% for the negative flashes and 88% for the Number of strokes positive flashes. These facts seem to indicate a possible influence of the ALDF on the multiplicity results,perhaps Figure 5. Distributionof the numberof strokesper flash in owing to the systemhaving a lower efficiency than other the Iberian Peninsulafor 1992-1994 for the year, the summer, techniquesin detecting low-intensity subsequentstrokes and the winter.
IiPositive Flashes
11,896
RIVAS
SORIANO
ET AL.' LIGHTNING
IN THE IBERIAN
PENINSULA
1992-94 35 30 25
•
20
ß Positive Flashes
•
15
[] Negative Flashes
10 5
0
Peak current (kA)
1992-94
Winter
25 20
=
15
ß Positive Flashes
ß
10
[] NegativeFlashes
5
'
--•
O i
O i
O
O
O
O
O
O
O
O
O
O
O
i
i
i
O i
q• O
O
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O
-•
--•
.-•
.-•
.-•
O
O
O
O
O
i
.-•
'
'
•0
'
(,O
'
.•
.-•
'
O
Peak current (kA)
1992-94
Summer
40 35
30
= 25 o
'-
ßPositive Flashes
20
[] NegativeFlashes
a. 15 10 5 0
, o, -.-,. •O O O
o,
o,
O
O
O0
.•
o, (,• O
o •
O
,o
,o
o,
o,
o o
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•,o
c0 o
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--•
O
O
O
O
O
"4
O0
q3
--•
,
,
'
.•
..-,. o', O
Peak current (kA)
Figure 6. Distributionoœfirst strokepeakcurrentin the IberianPeninsulaœor1992-1994for the year,the winter, and the summer.
thepositiveflashes.Thisfeaturewasalsofoundby Lopezet reportedby otherauthors:35 kA for Switzerland[Bergeret al. [ 1991] in Colorado,Finke and Hauf [ 1996] in Germany, al., 1975], 19 kA for Germany[Finkeand Hauf, 1996] and and Tuomi [1996] in Finland.Median (mean)peak current amplitudesare found at 22.5 kA (32.4 kA) for negative flashes and 52.3 kA (69.3 kA) for positive flashes.The median peak current for negative flashes in the Iberian Peninsulais thereforecloseto the resultof Finke and Hauf [1996] for Germany(22 kA) andthe valuereportedby Orville and Huffines [1999] for the contiguousUnited States(-20 kA), and it is lower than the value reportedby Berger et al. [1975] for Switzerland(30 kA). The meanpeak currentfor negativeflashesin theIberianPeninsula is closeto thevalues reportedby Finke and Hauf [1996] for Germany(25.5 kA) and Tuomi [1996] for Finland (30 kA), and it is lower than thevaluefoundby PetersenandRutledge[1992] for northern Australia (39 kA). The median peak current for positive flashesin the Iberian Peninsulais muchhigherthanthe values
between 15 and 25 kA for the contiguousUnited States [Orville and Huffines, 1999]. The mean peak currentfor positiveflashesis alsomuchhigherthanthevaluesfoundby mostotherauthors:32.6 kA for Germany[Finkeand Hauf, 1996] and 39 kA for northern Australia [Petersenand Rutledge,1992] In contrast,Tuomi[1996], usingthe ALDF, reportedfor Finlandmeanpeak currentsfor positiveflashes higherthanthevaluesfoundfor theIberianPeninsula (77 kA in 1996 and 72 kA for the period 1995-1996).It mustbe notedthat we have also foundthat the percentage of weak (
