Acceptedby WeatherandForecastingManuscriptNo. 2003–75

A Derechoin Europe: Berlin, 10July 2002

ChristophGatzen

DLR–Institutfür PhysikderAtmosphäre,

Oberpfaffenhofen,82234Wessling,Germany

Received 25 June 2003, in revised form 14 November 2003

Correspondingauthor’s address:

ChristophGatzen

DLR–Institutfür PhysikderAtmosphäre,

Oberpfaffenhofen,82234Wessling,Germany.

eMail: christoph.gatzen@dlr.de,

Tel: +49–8153–28–2566,Fax: +49–8153–28–1841.

Abstract

TheEuropeanwindstormthataffectedpartsof Germany on 10 July 2002is classifiedasa

derecho.With respectto thesynopticsituation,it wasidentifiedasa dynamicor strongly–

forcedderecho.Examiningdifferentobservationaldata,thiscaseshows many similiarities

to thoseinvestigatedin theUnitedStates,giving evidencethatderechosdooccurin Europe

aswell: A well–definedsurfaceconvergenceline anda thermalboundaryaloft markedthe

derecho’spaththatwasassociatedwith astrongupper–level jet streakaswell asalow–level

jet at theleadingedgeof thestorm.

1

1 Intr oduction

On 10 July 2002,during the VERTIKATOR field experiment(Vertikaler Austausch und Oro-

graphie, vertical transportandorography)in southernGermany, a line of severeconvection

crossedthenortheasternpartof Germany, producingwidespreadwind gusts.Theline reached

Germany’s capital,Berlin, at 1815UTC, at which time a maximumgustof 42 ms��

wasob-

served.TheBerlin Tegelairporthadto beclosed,andhighwaysandrailroadswereblockedby

overturnedtrees.Theweatherservicesaswell asthepublicwerenot well prepared.In Berlin,

4 residentsdiedand39 wereinjured. Within thecity’s periphery, four additionalfatalitiesoc-

curred.Mostof themwerehit by uprootedtrees,asin themosttragiccase,wheretwo children

died at a youth camporganizedby Berlin’s fire departmentin Berlin–Wannsee.All over the

city, thousandsof treesbroke off or wereuprooted;about30000 treeshadto be felled in the

following weeks.Thepresentpapergivesananalysisof this event,classifyingit asa derecho

usingthedefinitionof JohnsandHirt (1987),who developedfour criteria to identify derecho

events:

� Theremustbe a concentratedareaof reportsconsistingof convectively-inducedwind

damageor convective gustsof morethan26 ms��

(50 kt). This areamusthave a major

axislengthof at least400km.

� Thereportswithin thisareamustalsoexhibit anonrandompatternof occurrence.Thatis,

the reportsmustshow a patternof chronologicalprogression,eitherasa singularswath

(progressive)or asaseriesof swaths(serial).

� Within theareatheremustbeat leastthreereports,separatedby 64km or more,of either

F1damageor convectivegustsof 33ms��

(65kt) or greater.

� No morethan3 h canelapsebetweensuccessivewind damage(gust)events.

Thisnomenclaturehasnotbeenusedin Germany before,leastof all thereis noclimatology

that dealswith suchevents,althoughprevious convective windstormsprobablyreachedthe

criteria. The21 July 1992CLEOPATRA squallline (www.pa.op.dlr.de/cleocd/start.htm) may

beoneexampleof a widespreadconvectively–inducedwindstormthatoccurredover Germany

in thepast(Haase–Straubet al., 1997;Meischneret al., 1993). However, theexperiencefrom

thosecaseswasignoredon10July, whenbackgroundknowledgeof organizedconvectionitself

2

wasnot usedto improve the forecasts.Accordingly, Germanmeteorologistsweresurprisedat

thestorm’ssizeandstrength.In fact,gustsupto25ms��hadbeenexpecteddueto therelatively

large synoptic–scalepressuregradientalongthe cold front that waspredictedby operational

modelruns.

This paperis structuredasfollows: section2 givesan overview of the synopticsituation,

while section3 treatsthe evolution of the convective system. In section4, the analysisof

observed wind gustsis usedto classify this severeconvective event asa derecho.Section5

presentstheconclusionsof thiscasestudy.

2 SynopticOverview

In thesecondweekof July, theupperandmid troposphericflow overEuropewasdominatedby

an intensetroughacrossthenorthwesternpartof thecontinent,yielding a deepsouthwesterly

flow over thewesternregions(Fig. 1a).Oversouth–centralEurope,theflow wassplit into two

branches,a northernbranchwhich stretchedfrom FranceandBeneluxto the North Sea,and

a secondbranch,which deviatedtowardsthe centralMediterranean,whereit formeda weak

ridge. Along theperipheryof thenorthwesternmaintrough,severalvorticity maximaaffected

westernEurope.

In associationwith an intenseshort–wave trough, anothervorticity maximumtravelled

aroundthe main troughreachingthe northernborderof the IberianPeninsulalate in the day

of 9 July. At 1500UTC on10July, theaxisof thisnegatively tilted short–wavetroughextended

from southernGreatBritain into southerncentralGermany. At the300hPalevel, themaximum

wind speedreachedmorethan50 ms��

over France. The synoptic–scaleforcing associated

with suchintensejet streakscanbequitestrong,helpingconvectionto develop. Derechosthat

occurunderthosesynopticconditionsarecalleddynamic(JohnsandHirt, 1987)or strongly–

forcedderechos(EvansandDoswell,2001).

At the850hPalevel,anair masscharacterizedby highequivalentpotentialtemperature(���

)

up to 340K wasadvectedfrom southeasternCentralEuropeinto easternGermany (Fig. 1b). In

contrast,intensecoolingdueto coldair advectionandevaporationof stratiformrain tookplace

westof aneastwardmoving cold front in westernCentralEurope.By the afternoon,the 850

hPa���

–gradientreached30K overadistanceof 500km.

3

3 DerechoEvolution

Thelocationsof orographicfeaturesmentionedin thetext areshown in Fig. 2. Thesatelliteim-

age(Fig. 3a)indicatesthefront’spositionat1449UTC, threeandahalf hoursbeforethedere-

chohit Berlin. A persistentline of thunderstormscanbeseenadvancingeastwardoversouthern

Germany. Stratiformprecipitationwasobservedwestof the front underneaththebroadcloud

shieldoverwesternGermany. Eastof thecold front, new thunderstormsarevisible thatformed

rapidly parallelto the BöhmerWald Mountains,OberpfälzerWald Mountains,andThüringer

WaldMountainsbetween1400and1500UTC.As thecoldfront’sconvectiondissipatedduring

thenext hour, thenew line of convectivecellsmergedinto aMCSthatis portrayedat1628UTC

in Fig. 3b. Downstreamof this feature,thenortheasternpartof Germany waspartly clear. At

thesurface,windswereweakwith a northerlydirectionto thewestof Berlin, andsoutherlyto

easterlydirectionseastof it (seeFig. 4). A convergenceline reachingfrom theBaltic Seaeast

of RügenIslandto thewesternErzgebirgeMountainsseparatedbothwind regimesandmarked

thepathof thederecho.Justabove this surfaceconvergenceline, theoperational850hPa���

–

fields of the BolognaLimited Area Model (BOLAM, Buzzi et al., 2003)indicateda thermal

boundaryat the leadingedgeof thesouthernCentralEuropewarmair advectionregime(Fig.

1b). Observationalstudiesin the United Statespoint out that suchpre–existing, line–normal

thermalboundariescanplay a role in the formationandpropagationof bow echoes.Along

thoseboundaries,enhancedlow–level convergencecould leadto moreintenseconvectionand

associatedwind gusts(Davis etal., 2001).

OvereasternGermany, this thermalboundaryis well portrayedby thesurfaceobservations

of theGermanweatherservice:Eastof this boundary, temperatureswereabout3–6K higher,

while thedewpointswereabout3–7K lower. This indicatesthat theboundarylayerwasmore

turbulently mixedeastof the convergenceline thanwestof it, which is confirmedby a 1800

UTC soundingfrom Lindenberg about40km southeastof Berlin (Fig. 5). Thissounding,being

a proximity soundingin termsof the definition given by Evansand Doswell (2001), shows

morepropertiesof the pre–stormenvironment. Relatively dry air nearthe groundandsteep

lapseratesup to 750hPawerepresentwhichenhancedthepotentialfor strongdowndraftgusts

andcold pool formationdueto evaporationinsidethe downdrafts. For the samereason,the

mid–level dry airmassabove 700 hPa that wasalsodetectedby soundingswestandsouthof

Berlin could had beenimportantfor the cold pool formation. Anotherpoint that shouldbe

4

discussedis the instability of the pre–stormenvironment. At Lindenberg, a 50 hPa mean–

layerCAPEof around1000J/kgwasobserved. However, this observationdid not reflectthe

CAPE–valueswestof the convergenceline, whereshallow low–level moisturewasobserved

that wasnot reproducedby availablenumericalmodels.Finally, the verticalwind profile did

not indicatevery stronglow level shearby the time the rawinsondewasstartedat 1645UTC.

But 1290MHz wind profilerandsodarmeasurementsat thesameplaceshow thedevelopment

of low–level shearover Lindenberg during the next hours(Fig. 6) with increasinglow–level

southeasterlywinds. In therangeof the850hPa level, thewind changedfrom about10 ms��

southeastto morethan20ms��

south–southeastbetween1600and1900UTC.Theoperational

1200UTC run of theLocal Model (LM, DomsandSchaettler, 1999)of theGermanweather

serviceshowedthis low–level jet, just downstreamof thenegatively tilted 850hPa troughaxis

extendingto thenorthwith Berlin at its westernflank. At 300hPa,LM outputsuggeststhatthe

strongCentralEuropeanjet streakhadreachedsoutheasternGermany, while its left exit region

affectednortheasternGermany. This regional relationshipof upper–level andlow–level flow

canalsobeidentifiedfrom theoperationalBOLAM 0000UTC + 15h forecastfieldsin Fig. 1a

and1b (cf. www.cmirl.ge.infn.it/map/bolam/bolamin.htm).

Theobservedlow–level jet mayhave formedasa resultof the indirect thermalcirculation

underneaththeexit region of this jet streak(Uccellini andJohnson,1979). Accordingly, isal-

lobaricsurfaceanalysis(not shown) indicatesa synoptic–scaleincreaseof thesurfacepressure

of 4 hPa within 3 h over the CzechRepublicandsoutheasternGermany anda decreaseof 3

hPawithin 3hovernortheasternGermany. Thelow–level jet wasdirectedalongthis isallobaric

gradient.

Theconvectiveline reachedBerlin at1815UTC.TheC–bandDopplerradaratBerlin Tem-

pelhof airport documentedthe mesoscalestructureof the convective line (Fig. 7). The re-

flectivity over Berlin at 1845UTC shows signaturesdescribedin conjunctionwith high wind

eventsin theUnitedStates(Cannonetal.,1998).OvernorthernBerlin, abow–shapedsegment

of the propagatingderechois visible. This featureoriginatedfrom an intenseconvective cell

thatmergedwith theconvective line, andmostlikely producedthestrongwind gustsobserved

between1815and1845UTC. New pre-stormconvective cellsaredetectedto thenortheastof

thecity thatmergedwith theMCS lateron. By this means,a coupleof bow–shapedsegments

developedalongtheleadingedgeof theconvective line asit movednorthward,classifyingthe

derechoasaserialone.Southof thebow echothatis displayedin Fig. 7, thesouthwesternpart

5

of Berlin is coveredby aweakreflectivity regionknown asarearinflow notch(RIN, Przybylin-

ski andGery,1983;BurgessandSmull,1990).Descendingair originatingfrom therearinflow

jet andevaporationis thoughtto leadto this weakradarreflectivity (Burke andSchultz,2003).

Therear–inflow jet pointsfrom therearof theconvectivesystemto theleadingedgeof thebow

echo. It transportsrelatively dry air into thedowndraft,enhancingevaporationandassociated

cooling(Weisman,2001). In this case,therearinflow jet reachedthegroundonly closeto the

bow echo,whereasit waselevatedsomekilometersbehindit, asindicatedby wind profilerdata

from Lindenberg southeastof Berlin (seeFig. 6). It shouldbenotedthatLindenberg wasnot

hit by thederechothatpassedthelocationto thenorthwest.Nonetheless,astrongwesterlyflow

wasmeasured300metersaboveLindenberg after1900UTC thatremainedduringthefollowing

hours.

Without losing intensity, the derechoreachedthe northeasterncoastof Germany shortly

after2000UTC, producingwind gustsup to 35 ms��

at RügenIsland,andthendecayedover

theBaltic Sea.

4 Analysisof observedwind gusts

Fig. 8 showsobservedwind gustsof thederecho,andits position,asindicatedby thewind shift

detectedby two ground-observationnetworks,oneof the Germanweatherservice, the other

of Meteomedia,a privateweathercompany. Combiningthesetwo networks, morethan100

surfacewind observationsdenotethepositionof thewind shift of thecold front andassociated

derechoovernortheasternGermany, which is displayedby solidanddashedlines.As the1200

and1500UTC dataindicate,thecold front propagationwasrestrictedto southernandcentral

Germany while it was stalling over northernGermany. At this time, broadconvection that

marked the cold front positionwas observableon satelliteimages(Fig. 3a). To the east,a

new line of convectionwasforming, leadingto a new outflow boundaryat 1600UTC that is

displayedby adashedline in Fig. 8. Thenew convective line reachedits maturestagebetween

1600and1700UTC asindicatedby thesatelliteimagein Fig. 3bandthesubjectiveanalysisin

Fig. 4: Justbetweentheleadingedgeof theoutflow boundaryandtheremainingsurfacetrough

of thecold front a sharpmesoscaleridgewasanalysed,yielding to theassumptionthata cold

poolhadformedincreasingthepressuregradientaswell astheisallobaricgradientto thesouth

of Berlin. RasmussenandRutledge(1993)foundthatthepropagationspeedof convectivelines

6

increasedby thetime of thetransitionbetweentheintensifyingandmaturestage;thiswasalso

observedon10JulyovereasternGermany, whentheconvectiveline suddenlyacceleratedafter

1600UTC. This wasassociatedwith a strenghteningof observed wind gusts,asdepictedby

crosses,dotsandgrey circles,markingwherewind gustsof morethan22, 26, and34 ms��

occured,respectively (seeFig. 8).

Over southernGermany, no gustsof morethan26 ms��

wereobservedexcepton moun-

tains.In contrast,widespreadmaximumwind gustsupto 34ms��did occuroverbroadpartsof

northeasternGermany, wherefront propagationwasfastest.Nearly two-thirdsof all observed

gustsof more than26 ms��

occurredwithin a strip extendingfrom the westernErzgebirge

Mountainsto Berlin andfurther to the Baltic SeanearRügenIsland. The pathof strongest

wind gustswasabout500 kilometerslong and100 kilometerswide. Insidethis region, five

measuredwind gustsexceeded34ms��

, six moreobservationsattained33ms��. Four of the

gustreportsof morethan34 ms��

wereseparatedby at least100kilometers.Therefore,the

windstormunderconsiderationsatisfiedthecriteria for a derechoproposedby JohnsandHirt

(1987).

5 Conclusions

This casestudy of a derechoover Germany shows that severe convection can attain a size

andintensitycomparableto that in theUnitedStates.EventhoughtheEuropeanlandmassis

smallerandmoreheterogeneous,organizedsevereconvectionover CentralEuropehasshown

to beanappreciablehazardto life andproperty. More precisely, theBerlin derechoof 10 July

2002sharedsomewell establishedsynoptic-andmesoscalefeaturesof derechosin theUnited

States:

� Thesynoptic–scaleflow patternwascharacterizedby an intensenegatively tilted upper

short–wave troughassociatedwith a strongupper–level jet streak.A thermallyindirect

circulationmayhave provideda low–level jet, enhancingwarmair advectionandmois-

turepoolingjustbelow theleft exit regionof theupper–level jet.

� Along theleadingedgeof the low–level jet, a pre–existing frontal boundarymarkedthe

derechopathover northeasternGermany. At the surface,it waspossibleto analyzea

well–definedpre–existingconvergenceline.

7

� The increasein thespeedof observedwind gustsover easternGermany wasassociated

with an accelerationof the leadingedgepropagationof a convective line after it had

reachedits maturestage.

� Radarreflectivity imagesshow signaturesoftendescribedin conjunctionwith high wind

events,includingbow echoes,rearinflow notchesandcell mergersat theleadingedgeof

theline of convection.

� Wind profiler datafrom Lindenberg indicatestronglow–level wind shearaheadof the

derechoandsuggestthepresenceof anelevatedrearinflow jet.

Not only small–scale,but alsolarge–scalesevereconvectionoverEuropebearsmany simi-

laritiesto convectivephenomenain theUnitedStates.As aconsequenceof theBerlin derecho,

theGermanweatherserviceandMeteomediahave developednew conceptsfor severeweather

warnings.In addition,the"EuropeanStormForecastExperiment"(www.estofex.org) wasini-

tiatedby agroupof Europeanmeteorologiststo forecastsevereconvectionoverEurope.

Acknowledgments

Theauthorisgratefultoall whocollecteddatausedfor thiscasestudy:JohannesDahl,Manfred

Wegener, andSebastianUngerfrom theFreeUniversityof Berlin,SusanneDanßmannandSte-

fanKreibohmfrom Meteomedia,andDirk HeizenrederandVolkerLehmannfrom theGerman

weatherservice.Theradarimagesshown in this paperwereprovidedby DennisSchulze,MC

WetterBerlin. George Craig,Nikolai Dotzek,JohannesDahl, andJanHoffmanncontributed

valuablesuggestions.This work waspartly fundedby theGermanMinistry for Educationand

ResearchBMBF undercontract07ATF 45 in theprojectVERTIKATOR (Vertikaler Austausch

und Orographie, vertical transportand orography)within the atmosphericresearchprogram

AFO 2000.

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10

Figurecaptions

Figure1: OperationalBOLAM forecastfor 10July, 1500UTC. (a)300hPageopotential(gpm)

andwind speed(ms��). Geopotentialheightsareshown by blacklines,wind speedis indicated

by grayshading.(b) 850hPa���

–field in K andwind vectorsscaledby the10 ms��

vectorat

thebottomof thefigure.

Figure2: Orographicmapof theaffectedregion. Terrainheightsareshadedin greyscale.

Figure3: NOAA satellitevisiblechannelimagesof Germany at1449(a)and1628UTC (b).

Figure4: Subjectiveanalysisof surfacepressurefield at1700UTC basedonobservationsby

theGermanweatherservice.Temperaturesanddewpointsin � C, wind barbsin kt.

Figure5: 10July, 1800UTC soundingatLindenberg in thepre–stormenvironment40km

southeastof Berlin. Wind barbsaregivenin knots.

Figure6: Wind profiler andsodardatafrom theGermanweatherserviceat Lindenberg. Wind

barbsaregivenin knots.

Figure7: Radarreflectivity in theBerlin region at 1845UTC. Arrows point at therearinflow

notchsouthof thebow echooverBerlin anddevelopingpre–stormconvection.

Figure8: Measuredmaximumwind gusts(symbols)andcold front position(lines)over Ger-

many. Wind gustsabove 22 ms��

aredepictedby crosses,above 26 ms��

by dots,andabove

34ms��

by grey circles.TheUTC timeof cold front positionis shown at theendof eachline.

Solid linesareplottedevery threehours,with dashedlinesof hourly intervals.

11

Figure1: OperationalOperationalBOLAM forecastfor 10July, 1500UTC. (a)300hPa

geopotential(gpm)andwind speed(ms��). Geopotentialheightsareshown by blacklines,

wind speedis indicatedby grayshading.(b) 850hPa���

–field in K andwind vectorsscaledby

the10ms��

vectorat thebottomof thefigure.

12

Figure2: Orographicmapof theaffectedregion. Terrainheightsareshadedin greyscale.

13

Figure3: NOAA satellitevisiblechannelimagesof Germany at1449(a)and1628UTC (b).

14

Figure4: Subjectiveanalysisof surfacepressurefield at1700UTC basedonobservationsby

theGermanweatherservice.Temperaturesanddewpointsin � C, wind barbsin kt.

15

Figure5: 10July, 1800UTC soundingatLindenberg in thepre–stormenvironment40km

southeastof Berlin. Wind barbsaregivenin knots.

16

Figure6: Wind profilerandsodardataof theGermanweatherserviceatLindenberg. Winds

aregivenin knots.

17

Figure7: Radarreflectivity in theregionBerlin at1845UTC. Arrowspointat therearinflow

notchsouthof thebow echooverBerlin anddevelopingpre–stormconvection.

18

Figure8: Measuredmaximumwind gusts(symbols)andcold front position(lines)over

Germany. Wind gustsabove22ms��

aredepictedby crosses,above26ms��

by dots,and

above34ms��

by grey circles.TheUTC timeof cold front positionis shown at theendof

eachline. Solid linesareplottedevery threehours,with dashedlinesof hourly intervals.

19