Distinguishing the Cold Conveyor Belt and Sting Jet Airstreams in an Intense Extratropical Cyclone

25 Pages·2014·5.61 MB·English

Distinguishing the cold conveyor belt and 
sting jet air streams in an intense 
extratropical cyclone 
Article 
Published Version 
Martinez­Alvarado, O., Baker, L. H., Gray, S. L., Methven, J. 
and Plant, R. S. (2014) Distinguishing the cold conveyor belt 
and sting jet air streams in an intense extratropical cyclone. 
Monthly Weather Review, 142 (8). pp. 2571­2595. ISSN 1520­
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AUGUST2014 MART(cid:2)INEZ-ALVARADO ET AL. 2571
Distinguishing the Cold Conveyor Belt and Sting Jet Airstreams
in an Intense Extratropical Cyclone
OSCARMARTI´NEZ-ALVARADO,LAURAH.BAKER,SUZANNEL.GRAY,
JOHNMETHVEN,ANDROBERTS.PLANT
DepartmentofMeteorology,UniversityofReading,Reading,UnitedKingdom
(Manuscriptreceived1November2013,infinalform20March2014)
ABSTRACT
Strongwindsequatorwardandrearwardofacyclonecorehaveoftenbeenassociatedwithtwophenomena:
thecoldconveyorbelt(CCB)jetandstingjets.Here,detailedobservationsofthemesoscalestructureinthis
regionofanintensecycloneareanalyzed.Theinsituanddropsondeobservationswereobtainedduringtwo
researchflightsthroughthecycloneduringtheDiabaticInfluencesonMesoscaleStructuresinExtratropical
Storms(DIAMET)fieldcampaign.Anumericalweatherpredictionmodelisusedtolinkthestrongwind
regionswiththreetypesof‘‘airstreams’’orcoherentensemblesoftrajectories:twotypesareidentifiedwith
theCCB,hookingaroundthecyclonecenter,whilethethirdisidentifiedwithastingjet,descendingfromthe
cloudheadtothewestofthecyclone.ChemicaltracerobservationsshowforthefirsttimethattheCCBand
stingjetairstreamsaredistinctairmassesevenwhentheassociatedlow-levelwindmaximaarenotspatially
distinct.Inthemodel,theCCBexperiencesslowlatentheatingthroughweak-resolvedascentandconvection,
whilethestingjetexperiencesweakcoolingassociatedwithmicrophysicsduringitssubsaturateddescent.
DiagnosisofmesoscaleinstabilitiesinthemodelshowsthattheCCBpassesthroughlargelystableregions,
while the sting jet spends relatively long periods in locations characterized by conditional symmetric in-
stability(CSI).TherelationofCSItotheobservedmesoscalestructureofthebent-backfrontanditspossible
roleinthecloudbandingisdiscussed.
1. Introduction 4) warm-core seclusion. Frontal fracture describes the
break of a continuous thermal front as the cyclone in-
The potential to generate strong surface winds and
tensifies so that the cold front is dislocated eastward
gustsastheypassisoneofthemostimportantaspectsof
fromthewarmfrontwithaweakergradientinbetween.
extratropicalcyclonesduetothedirectimpactonsoci-
This region is termed the ‘‘frontal fracture zone’’ and
ety. The aim of this article is to analyze the three-
is associated with air descending cyclonically from the
dimensionalstructureoftheregionofstrongwindsnear
northwest to the south of the frontal cyclone. The de-
the center of an intense extratropical cyclone and de-
scendingairgivesrisetoapronounced‘‘dryslot’’insat-
terminetheoriginoftheairstreamswithinthatregion.
elliteimagery.Theextensivecloudwrappingaroundthe
Thestudyisfocusedonacyclonethatdevelopedaccording
poleward side of the cyclone core is described as the
to the Shapiro–Keyser conceptual model (Shapiro and
‘‘cloud head’’ (Bo€ttger et al. 1975), and its leading ex-
Keyser1990).Thismodelischaracterizedbyfourstages
tremityisdescribedasthe‘‘cloudheadtip’’(Browning
of development: 1) incipient frontal cyclone, 2) frontal
andRoberts1994).Figure1showsaschematicdiagram
fracture, 3) frontal T bone and bent-back front, and
of the structure of a Shapiro–Keyser cyclone during
developmentstage3.
Therearetwoseparateregionsusuallyassociatedwith
Correspondingauthoraddress:OscarMart(cid:2)ınez-Alvarado,De-
strong winds in Shapiro–Keyser cyclones. The first re-
partmentofMeteorology,UniversityofReading,EarleyGate,
gion is the low-level jet ahead of the cold front in the
P.O.Box243,ReadingRG66BB,UnitedKingdom.
E-mail:[email protected] warmsectorofthecyclone.Thislow-leveljetispartof
DOI:10.1175/MWR-D-13-00348.1
(cid:1)2014AmericanMeteorologicalSociety
2572 MONTHLY WEATHER REVIEW VOLUME142
FIG.1.ThestructureofaNorthernHemisphereShapiro–Keysercycloneindevelopment
stage 3: surface cold front (SCF); surface warm front (SWF); bent-back front (BBF); cold
conveyorbelt(CCB);stingjetairstream(SJ);dryintrusion(DI);warmconveyorbelt(WCB);
WCBanticyclonicbranch(WCB1);WCBcyclonicbranch(WCB2);andthelarge3represents
thecyclonecenteratthesurface,andthegrayshadingrepresentscloudtop(seealsoFig.2).
thebroaderairstreamknownasthewarmconveyorbelt, GreatOctoberstormof1987fromsatellite,precipitation
which transports heat and moisture northward and radar, and the surface wind network (Browning 2004).
eastwardwhileascendingfromtheboundarylayertothe The air associated with the sting jet descends from the
uppertroposphere(Browning1971;Harrold1973).The cloudheadtip,movingaheadofitaroundthecycloneinto
secondregionofstrongwindsdevelopstothesouthwest thedryslotbehindthecoldfront.Asthecyclonedevelops
and south of the cyclone center as a bent-back front into phase 3, the region of weak gradients between the
wraps around the cyclone. The strong winds in this re- bent-backfrontandcoldfrontexpandsandthestingjet
gionarethefocusofthiscontribution. airstreamdescendsintothisregion.Here,theboundary
Two different airstreams have been associated with layer has near-neutral stability or potential instability
strong winds in this region: the cold conveyor belt (Browning2004;Sinclairetal.2010);thesecharacteristics
(Carlson 1980; Schultz 2001) and sting jets (Browning havebeenhypothesizedtoenhanceturbulentmixingof
2004;Clarketal.2005).Thecoldconveyorbelt(CCB)is high-momentumairdowntothesurface.
a long-lived synoptic-scale airstream on the poleward Clark et al. (2005) analyzed simulations of the same
(cold)sideofthewarmfrontthatflowsrearwardrelative caseusingtheMetOfficeUnifiedModel(MetUM)and
to the cyclone motion in the lower troposphere. It ex- identified distinct clusters of trajectories calculated us-
tends round the poleward flank of the cyclone and in ing model winds with sting jet airstreams. A key char-
somematurecyclonesitwrapsaroundthewestandthen acteristicofstingjettrajectoriesisthattheydescendas
equatorwardflankwhereitprovidesawindcomponent theyaccelerate.Thereareseveralinfluencesonvertical
aligned with the system motion and therefore strong motioninthissectorofacyclone.Onthelargestscale,
ground-relativewinds.AkeyaspectoftheCCBisthat the cyclone forms as part of a baroclinic wave. On is-
thewindmaximumisnearthetopoftheboundarylayer entropic surfaces cutting through a baroclinic wave in
andslopesradiallyoutwardswithheightonthecoldside the midtroposphere the generic structure of motion
ofthebent-backfront,aswouldbeexpectedfromgra- givesrisetofourairmasses:airascendingpolewardand
dientthermalwindbalance. splittingintoacyclonicandanticyclonicbranchandair
The term ‘‘sting jet’’ was introduced by Browning descending equatorward and also splitting into a cy-
(2004)(seealsoClarketal.2005)todescribestronglow- clonicandanticyclonicbranch(Thorncroftetal.1993).
levelwindsinthecoldairbetweenthebent-backfront The two cyclonic branches wrap around the cyclone
and the cold front on the basis of observations of the core.Onhigher-isentropicsurfacestheyaredescribedas
AUGUST2014 MART(cid:2)INEZ-ALVARADO ET AL. 2573
the cyclonic branch of the warm conveyor belt (as- that evaporative cooling may also enhance the descent
cending) and the dry intrusion (descending). Both the rateofstingjetairstreams,althoughBakeretal.(2014)
CCB(ascendingorhorizontal)andstingjetairstreams foundlittleimpactinanidealizedcyclonesimulation.
(descending) also turn cyclonically and are found on The cyclone analyzed here produced very strong
lower-isentropic surfaces that can intersect the ground winds over the United Kingdom on 8 December 2011
inthewarmsector.Inadditiontotheprimarycirculation and was the focus of the intensive observing period
of the baroclinic wave, cross-frontal circulations con- 8(IOP8)duringthesecondfieldcampaignoftheDiabatic
tributetoverticalmotion.Forexample,frontogenesisat Influences on Mesoscale Structures in Extratropical
the cold front contributes to the ascent of the warm Storms (DIAMET) project. The storm has been the
conveyor belt and descent of the dry intrusion behind. subjectofextensiveinvestigationinvolvingnotonlythe
Semigeostrophic theory shows that the cross-frontal presentarticle.Bakeretal.(2013)describedtheflights
circulationsarenecessarytomaintainapproximatether- andsummarizedtheseveresocietalimpactsofthestorm.
malwindbalanceinatime-dependentflowandtherefore Vaughan et al. (2014, manuscript submitted to Bull.
depend on its rate of change (Hoskins and Bretherton Amer. Meteor. Soc.) give more details of the DIAMET
1972). Schultz and Sienkiewicz (2013) have used model experiment and present the results of research onhigh-
diagnosticstoshowthatdescentcanbeenhancedinthe resolution ensemble simulations and further in situ air-
regionbeyondthecloudheadtip,wherethestingjetair- craftobservations,aswellasobservationsfromautomatic
streamdescends,asaresultoffrontolysis.Theairstream weather stations across the north of the United King-
leavesthetightgradientofthebent-backfrontatthewest dom. The cyclone was named Friedhelm by the Free
ofthecyclone,andthereforethegradientmustdecrease University of Berlin’s adopt-a-vortex scheme (http://
with time in a Lagrangian frame. Similarly, ascent is ex- www.met.fu-berlin.de/adopt-a-vortex/).
pectedintheCCBwherethebent-backfrontstrengthens. With its aircraft field campaigns, DIAMET joins
Several studies have investigated the mechanisms worldwide efforts to sample weather systems through
leadingtostingjets.Browning(2004)proposedthatthe aircraft observations (e.g., Sch€afler et al. 2011; Sapp
sting jets (local wind maxima) occur beneath the de- etal.2013).Totheauthors’knowledgetherehaveonly
scendingbranchesofslantwisecirculationsgeneratedby been two previous research flights into an intense cy-
thereleaseofconditionalsymmetricinstability(CSI)in cloneofthistype,crossingthestrongwindregionsnear
the frontal fracture region between the cloud head tip the cyclone center. Shapiro and Keyser (1990) show
and the cold front. Numerical simulations represented dropsonde sections across a similar storm observed on
some form of slantwise motion in that region (Clark 16March1987duringtheAlaskanStormsProgramme.
etal.2005).Analysisofmodelhumidityandequivalent Asecondcyclonethatdevelopedextremelyrapidlywas
potential temperature along trajectories indicated that observedatthreestagesinitsevolutioninIOP4ofthe
theairstreamoriginatedfromasaturatedregionwithin ExperimentonRapidlyIntensifyingCyclonesoverthe
thecloudhead,butbecameunsaturatedondescent.This Atlantic (ERICA) experiment. Neiman et al. (1993)
would be consistent with the evaporation of cloud and present dropsonde sections through this storm and
bandinginthecloud.AnecessaryconditionforCSIto Wakimoto et al. (1992) present data from the aircraft
give rise to slantwise convection is that the air is satu- radar in more detail. Somecommon aspects of theob-
rated (at least initially). Further case studies of storms servedstructureswillbecomparedinthispaper.Friedhelm
withstrongwindsinthestingjetregionclearlyidentify also passed over Scotland where there is a high-density
regions meeting the CSI criterion that also exhibit automatic weather station network and radar network
bandinginthecloudhead(Grayetal.2011).Mart(cid:2)ınez- estimatingprecipitationratefromreflectivity(discussedby
Alvaradoetal.(2012)usedCSIdiagnosticstoconstruct Vaughanetal.2014,manuscriptsubmittedtoBull.Amer.
a regional sting jet climatology. They found that up to Meteor. Soc.). Also, numerical models have improved
a third of a set of 100 winter North Atlantic cyclones considerably in the last 20 years. Here, a state-of-the-art
overthepasttwodecades(1989–2009)satisfiedconditions numericalweatherpredictionmodelisevaluatedagainst
forstingjets(Mart(cid:2)ınez-Alvaradoetal.2012).However, in situ and dropsonde observations and then used to
in other studies the importance of CSI is not as clear analyze the history of air masses passing through the
(Bakeretal.2014;SmartandBrowning2014).Inaddi- regionsofstrongestlow-levelwinds.Thescientificques-
tion,therehavenotbeendetailedinsituobservationsin tionsaddressedareasfollows:
theappropriateregionofShapiro–Keyser-typecyclones (i) Howarethestrongwindregionssouthwestofthe
that could have established the existence of slantwise cyclonecorerelatedtothecharacteristicairstreams
rolls or connection to instability with respect to CSI. that have been proposed to exist there (CCB and
Finally,Browning(2004)andClarketal.(2005)proposed stingjet)?
2574 MONTHLY WEATHER REVIEW VOLUME142
TABLE1.Dropsondereleasetimes.
Leg1
DropsondeNo. 1 2 3 4 5 6 7 8 9 10
Releasetime(UTC) 1130 1146 1158 1203 1209 1212 1217 1223 1228 1234
Leg2
DropsondeNo. 11 12 13 14 15 16 17
Releasetime(UTC) 1243 1249 1255 1301 1306 1312 1318
Leg3
DropsondeNo. 18 19 20 21
Releasetime(UTC) 1754 1758 1802 1806
(ii) Where trajectory analysis identifies different air- Measurements(FAAM)BAe146researchaircraft.The
streams,aretheyobservedtohavedistinctairmass instruments allowed in situ measurements of pressure,
properties? wind components, temperature, specific humidity, and
(iii) Whatdynamical mechanismis responsiblefor the totalwater(allphases)aswellaschemicalconstituents
observed cloud banding in the cloud head and to suchascarbonmonoxide(CO)andozone.Theaircraft
thesouthofthecyclone? was equipped with comprehensive cloud physics in-
strumentation characterizing liquid droplet and ice
Dropsondeandinsitumeasurementsareusedtolinkthe
particle size and number distributions. A summary of
observedsystemtothestructuresimulatedintheMetUM.
the instruments, their sampling frequency, and un-
Themodelisthenusedtocalculatetheairstreamsandthe
certaintyonoutputparametersisgiveninVaughanetal.
evolution of their properties as they move into regions
(2014, manuscript submitted to Bull. Amer. Meteor.
of strongest winds. Throughout the paper, the term ‘‘air-
Soc.). The observations are shown here at 1Hz. In ad-
stream’’ is identified with a coherent ensemble of trajec-
dition 21 dropsondes (Vaisala AVAPS RD94) were
tories that describes the path of a particular air mass
launched from approximately 7km. The dropsondes
arriving in a region of strong winds. Wind speed is not
contributed measurements of temperature, pressure,
aLagrangiantracerandtypicallyregionsofstrongwinds
andspecifichumidityasafunctionoflatitude,longitude,
movewiththecycloneandchangestructureasitdevelops.
andtime(atasamplingfrequencyof2Hz).Horizontal
Therefore,airflowsthroughthestrongwindregions(local
wind profiles were obtained by GPS tracking of the
windmaximaorjets)andeachairstreammustbeidentified
dropsondes (logged at 4Hz). Two sondes could be
withthetimewhenitisintheassociatedstrongwindre-
loggedontheaircraftatanyonetime,andtheaverage
gion. Trajectory analysis is combined with potential tem-
timeforsondedescentwas10min,limitingtheaverage
peratureutracerstoinvestigatetheprocessesresponsible
sondespacingto5minalongtheflighttrackor30kmat
for the evolution of each identified air mass. Tracer ob- the aircraft science speed of 100ms21. The vertical
servationsfromtheaircraftareusedtoinvestigatewhether
resolutionisabout10m.Table1liststhesonderelease
theairstreamsidentifiedaredistinctincompositionornot.
times along the three dropsonde curtains across the
Thearticleisorganizedasfollows:Theaircraftobser-
cyclone.
vations,numericalmodel,andtrajectoryandtracertools
aredescribedinsection2.Asynopticoverviewofthecase
b. Numericalmodel
study and a detailed account of the evolution of strong
windregionsnearthecyclonecenteraregiveninsection3. ThecasestudyhasbeensimulatedusingtheMetUM
In section 4, the air masses constituting strong wind re- version7.3.TheMetUMisafinite-differencemodelthat
gions are identified and classified as CCBs or sting jets, solves the nonhydrostatic deep atmosphere dynamical
according to their evolution and properties. The condi- equations with a semi-implicit, semi-Lagrangian in-
tionsformesoscaleatmosphericinstabilitiesinthevicinity tegration scheme (Davies et al. 2005). It uses Arakawa
oftheidentifiedairstreamsareinvestigatedinsection5. Cstaggeringinthehorizontal(ArakawaandLamb1977)
Finally,discussionandconclusionsaregiveninsection6. and is terrain following with a hybrid height Charney–
Phillips (Charney and Phillips 1953) vertical coordinate.
2. Methodology Parameterizationofphysicalprocessesincludeslongwave
and shortwave radiation (Edwards and Slingo 1996),
a. Availableaircraftobservations
boundary layer mixing (Lock et al. 2000), cloud micro-
Cyclone Friedhelm was observed with the instru- physics,andlarge-scaleprecipitation(WilsonandBallard
mentsonboardtheFacilityforAirborneAtmospheric 1999)andconvection(GregoryandRowntree1990).
AUGUST2014 MART(cid:2)INEZ-ALVARADO ET AL. 2575
Thesimulationhasbeenperformedonalimited-area model and the trajectory model used here perform
domaincorresponding totheMet Office’srecently op- similarly even though there are differences in inter-
erational North Atlantic–Europe (NAE) domain with polation (Mart(cid:2)ınez-Alvarado et al. 2014). Atmospheric
6003 300grid points. The horizontalgrid spacing was fields,suchasuandspecifichumidity,wereinterpolated
0.118 (;12km) in both longitude and latitude on a ro- ontotheparcelpositionstoobtaintheevolutionofthose
tated grid centered around 52.58N, 2.58W. The NAE fields along trajectories. The material rate of change of
domainextendsapproximatelyfrom308to708Ninlat- the fields along trajectories was computed using a cen-
itudeandfrom608Wto408Einlongitude.Thevertical tered differenceformulaalong the temporalaxis. Thus,
coordinate is discretized in 70 vertical levels with lid rather than being interpreted as instantaneous values,
around 80km. The initial and lateral boundary condi- ratesofchangealongtrajectoriesshouldbeinterpretedas
tionsweregivenbytheMetOfficeoperationalanalysis anestimateofhourlymeanvalues.
validat0000UTC8December2011and3-hourlylateral d. Potentialtemperaturetracers
boundary conditions (LBCs) valid from 2100 UTC 7
Theutracersusedinthisworkhavebeenpreviously
December2011for72h.
described elsewhere (Mart(cid:2)ınez-Alvarado and Plant
Severalpreviousstudieshaveusedresolutionsofthis
2014). Theyare basedontracer methods developedto
ordertostudythistypeofstorm(e.g.,Clarketal.2005;
Partonetal.2009;Mart(cid:2)ınez-Alvaradoetal.2010),mo- studythecreationanddestructionofpotentialvorticity
(Stoelinga 1996; Gray 2006). Potential temperature is
tivated on the basis that the fastest growing mode of
decomposedinaseriesoftracerssothatu5u 1(cid:1) Du .
slantwise instabilityshould beresolvable atthese hori- 0 P P
EachtracerDu accumulatesthechangesinuthatcanbe
zontal and vertical resolutions (Persson and Warner P
attributedtotheparameterizedprocessP.Theparam-
1993; Clark et al. 2005). Vaughan et al. (2014, manu-
eterizedprocessesconsideredinthisworkare(i)surface
scriptsubmittedtoBull.Amer.Meteor.Soc.)providean
fluxes and turbulent mixing in the boundary layer, (ii)
analysisofanensembleat2.2-kmgridspacing,including
convection,(iii)radiation,and(iv)large-scalecloudand
thelow-levelwindstructure,butthedomaininthatcase
precipitation.Thetraceru matchesuattheinitialtime.
isrestrictedtotheUnitedKingdom;theuseofthe12-km 0
Bydefinition,thistracerisnotmodifiedbyanyparam-
grid spacing allows the simulation of a larger domain
eterizationbutitis,nevertheless,subjecttoadvection.
that includes the full cyclone without dominant effects
Theutracersandtrajectoryanalysisprovidedifferent
from the LBCs. Moreover, the trajectory analysis (see
approximations to the Lagrangian description of the
sections2cand2d)requiresalargedomaintoallowlong
flow field. The u tracers are computed online whereas
trajectories to be calculated without the majority of
trajectories are computed offline from hourly velocity
themleavingthedomain.
dataonthemodelgrid.Traceru experiencestransport
0
c. Trajectoryanalysis
only, without diabatic modification. Therefore, in the
absenceofsubgridmixingornumericaladvectionerrors
Twotrajectorymodelsareusedinthepaper.Thefirst
(inthetracerortrajectoryschemes),itisexpectedthat
model is the Reading Offline Trajectory Model
u conservesthesamevaluewhensampledalongatra-
(ROTRAJ) as developed by Methven (1997). Its appli- 0
jectory. To focus on results where the u tracers and
cation to aircraft flights is detailed in Methven et al.
trajectoriesareconsistent,thecriterion
(2003). It calculates trajectories using European Centre
for Medium-Range Weather Forecasts (ECMWF) anal- ju [x(t )]2u [x(t )]j,Du
ysisdata.Inthispaper,theECMWFInterimRe-Analysis 0 i arr 0 i origin 0
(ERA-Interim) dataset has been used in its native con-
is applied where the tolerance on nonconservation is
figuration (T255L60 in hybrid sigma-pressure vertical Du 53.5K.Here,x referstoapointalongtrajectoryi,t
0 i arr
coordinates every 6 h). A fourth-order Runge–Kutta
isthearrivaltimeofthetrajectoryinthestrongwindre-
schemeisusedforthetrajectoryintegration(withatime
gion(thereleasetimeofthebacktrajectories),andt is
origin
step of 15min). The boundary condition on vertical ve-
acommonreferencetime(0100UTC8December2011)
locityisusedduringtheinterpolationtoensurethattra-
describedasthetrajectoryorigin.Approximately20%of
jectoriescannotintercepttheground.
trajectories are rejected by this criterion, although the
Thesecondmodel,basedontheLagrangianAnalysis
identificationofairstreamsisinsensitivetothisfilter.
Tool(LAGRANTO)modelofWernliandDavies(1997),
e. Diagnosticstoidentifyregionsofatmospheric
calculated trajectories using hourly output from the
instability
MetUM(inthemodel’snativeverticalcoordinate).The
time-steppingschemeisalsofourth-orderRunge–Kutta. Previous studies on sting jets have shown that the
Previous comparison has shown the LAGRANTO necessaryconditionsforCSIaresatisfiedintheregions
2576 MONTHLY WEATHER REVIEW VOLUME142
TABLE2.Cyclonedevelopmentbasedon6-hourlyMetOfficeanalysischartsbetween1200UTC6Decand1800UTC9Dec2011
(archivedbywww.wetter3.de).ThedevelopmentstagecolumnreferstothestagesintheShapiro–Keysermodelofcyclogenesis(Shapiro
andKeyser1990).ThetermDp isthepressurechangeintheprevious6h,whereasDp isthepressurechangeintheprevious24h.
6h 24h
Time Latitude Longitude Pressure Development Dp Dp Deepening
6h 24h
Day (UTC) (8N) (8E) (hPa) stage (hPa) (hPa) (Bergeron)
6Dec 1200 50 256 1019 1
1800 49 255 1014 1 25
7Dec 0000 51 250 1014 1 0
0600 53 242 1008 1 26
1200 54 235 1001 2 27 218 0.824
1800 55 226 992 2 29 222 1.007
8Dec 0000 57 220 977 3 215 237 1.650
0600 58 215 964 3 213 244 1.927
1200 59 27 957 4 27 244 1.904
1800 59 0 956 4 21 236 1.549
9Dec 0000 59 2 957 4 11 220 0.851
0600 59 8 964 4 17 0 0.000
1200 59 11 966 4 12 19 20.379
1800 60 15 971 4 15 115 20.628
thatstingjetairstreamspassthrough(Grayetal.2011; All these diagnostics indicate necessary, but not suf-
Mart(cid:2)ınez-Alvaradoetal.2013;Bakeretal.2014).Here, ficient,conditionsforinstability.Themostbasictheories
weidentifyregionsthatsatisfynecessaryconditionsfor for each of these instabilities rely ondifferent assump-
instabilityintheanalyzedcaseandtheirlocationsrela- tions regarding the background state upon which per-
tivetotheairstreams. turbations grow, namely, uniform flow for CI, uniform
Conditional instability (CI) with respect to upright PVforCSI,anduniformpressureinthehorizontalfor
convectionisidentifiedinregionswherethemoiststatic inertialinstability.Theseconditionsarefarfrombeing
stability[N2,definedasinDurranandKlemp(1982)]is met in an intense cyclone where there are strong pres-
m
negative. A necessary condition for inertial instability sure gradients, wind shears, and PV gradients. Shear
(II)isthattheverticalcomponentofabsolutevorticity instability is also present on all scales and grows as
z is negative. Inertial instability can be regarded as aresultofopposingPVgradientsinshearflows.
z
aspecialcaseof(dry)symmetricinstability(SI);inthe
limit that u surfaces are horizontal, SI reduces to II. A
3. Synopticoverviewandidentificationofregions
necessaryconditionforCSIisthatthesaturationmoist
ofstrongwinds
potential vorticity (MPV*) is negative (Bennetts and
Hoskins1979).MPV*isgivenby a. Synopticoverview
1 On 6 December 2011, extratropical cyclone Fried-
MPV*5 z(cid:2)$u*,
r e helm started developing over Newfoundland (508N,
568W).Itsdevelopmentwaspartofabaroclinicwave,in
wherer isdensity,z istheabsolutevorticity,andu*is tandemwithanotherstrongcyclonetothewest(named
e
the saturated equivalent potential temperature. Note Gu€nther) that, as Friedhelm, reached maturity on 8
thatu*isafunctionoftemperatureandpressure,butnot December 2011, but near Newfoundland. Traveling to
e
humidity(sincesaturation isassumed initsdefinition). the northeast, Friedhelm continued its development
Following Schultz and Schumacher (1999), a point is according to the Shapiro–Keyser cyclogenesis model
only defined as having CSI if inertial and conditional (Shapiro and Keyser 1990), as shown in Table 2. The
instabilitiesareabsent.IfthenecessaryconditionsforCI cyclone satisfied the criterion to be classified as an at-
orCSIaremetthentheycanonlybereleasediftheairis mospheric ‘‘bomb’’ by consistently deepening by more
saturated,soweapplyanadditionalcriteriononrelative than1Bergeron(SandersandGyakum1980).At1200
humiditywithrespecttoice:RH .90%.AsinBaker UTC8December2011,thecyclonecenterwaslocated
ice
et al. (2014), we use the full winds rather than geo- around 598N, 78W, just northwest of Scotland. The
strophic winds in these CSI and II diagnostics. The di- FAAM aircraft reached the cyclone center at 1234
agnosticsfortheconditionsforinstabilityareappliedat UTCwhensatelliteimagery(Fig.2a)showsaverywell-
each grid point; a grid point is labeled as stable (S) if defined cloud head hooking around the cyclone center
noneofthethreeinstabilitiesareidentified. (earlystage4).Thisimagealsoshowsprominentcloud
AUGUST2014 MART(cid:2)INEZ-ALVARADO ET AL. 2577
FIG.2.(a)High-resolutionvisiblesatelliteimageat1215UTC8Dec2011(figurecourtesyofNERCSatellite
ReceivingStation,DundeeUniversity,Scotland);(b)MetOfficeanalysisvalidat1200UTC8Dec2011(figure
courtesy of Crown); and (c) model-derived mean sea level pressure (black contours) every 4-hPa, and 850-hPa
equivalentpotentialtemperature(boldredcontours)every5Kat1200UTC8Dec2011(T112).The3in(c)marks
thepositionofthecyclonecenterinthesimulation.
banding, especially southeast of the cloud head tip, to wind field onthe 850-hPaisobaric level every3h from
thesouthwestandsouthofthecyclonecenter. 0900to1800UTC.At0900UTC(Fig.3a),theregionof
Thefrontalsystemandtheintensityofthecycloneare maximumwindswasabout50kmwidewithwindsupto
depictedintheMetOfficeanalysisvalidat1200UTC8 49ms21spanning600–800hPa.By1200UTC(Fig.3b),
December2011(Fig.2b).Figure2cshowsthesynoptic theregionofmaximumwindshadmovedoverScotland
situation in the 12h forecast using the MetUM. The and orographic effects might have influenced its struc-
similaritywiththeMetOfficeanalysischartatthistime ture. The first dropsonde curtain (D–C) crosses just to
isremarkablygoodintermsofthedepthofthecyclone thewestofthelow-levelwindmaximum.
(957hPainbothcharts)andthelocationofthesurface Bythetime(1500UTC)oftheinsituaircraftlegswest
fronts.Thepositionerrorofthelowpressurecenterin of Scotland (F–G in Fig. 3c), the wind maximum had
thesimulationislessthan50km. reached the eastern side of Scotland. However, it was
important for the aircraft to remain upstream of the
b. Developmentofregionsofstrongwinds
mountains to reduce the orographic influence on the
Thestructureofregionsofstrongwindstothesouthof observed winds, cloud, and precipitation. The second
thecyclonecentervariedthroughouttheintervalunder flight dropped sondes across the low-level wind maxi-
study.Before0500UTC,theonlyairstreamassociated mumat1800UTC(J–HinFig.3d),andthesubsequent
withstrongwindswasthewarmconveyorbeltaheadof in situ legs (continuing until 2000 UTC) were at the
thesurfacecoldfront(notshown).Althoughthisregion longitudeofthejetmaximumbutonitsnorthernflank.
of strong winds continued to exist throughout the in-
c. Identificationofregionsofstrongwinds
terval under study, it was excluded from the airstream
analysis to focus on the strong low-level winds behind The structure of strong wind regions, and associated
thesurfacecoldfronttothesouthofthecyclonecenter. temperatureandhumidityfieldsnearthecyclonecenter,
These winds first exceeded 40ms21 at 0500 UTC when weremeasuredusingdropsondeobservationsforthree
a distinct jet developed at 600hPa. By 0600 UTC, the sections across the storm during the two FAAM re-
maximum winds (47ms21) had descended to 700hPa. search flights (see Table 1). The dropsonde data were
Figures3a–dshowthedevelopmentoftheground-relative relayedtotheGlobalTransmissionSystem(GTS)from
2578 MONTHLY WEATHER REVIEW VOLUME142
FIG.3.Ground-relativewindspeed(ms21)onthe850-hPaisobaricsurfaceat(a)0900,(b)1200,(c)1500,and(d)1800UTC.Gray
contoursshowu every5K.Panels(b)–(d)showthetrackfollowedbytheFAAMresearchaircraft,highlightingthehourcenteredatthe
e
timeshown(purpleline).Crossesindicatethepositionofthecyclonecenter.Endpointsofverticalsectionsdiscussedinlaterfiguresare
indicatedbydotsandlabeledbyletters(exceptforA,whichcoincideswiththecyclonecenter).
theaircraftandwasassimilatedbytheglobalforecasting The southern arm of the bent-back front was crossed
centers.All17sondesfromthefirsttwolegsmadeitinto between 578 and 57.38N and divides two distinct air
theassimilationwindowforthe1200UTCglobalanal- masses: the cyclone’s warm seclusion to the north and
ysisofboththeMetOfficeandECMWFandwouldhave the frontal fracture zone to the south. The strongest
influenced subsequent operational forecasts. However, winds are confined below 720hPa near the bent-back
the simulation shown here starts from the global Met front with a maximum at 866hPa, just above the
Office analysis for 0000 UTC 8 December 2011 and boundarylayer(51ms21).Atthislevel,thestrongwinds
thereforeisindependentofthedropsondedata. extendsouthwardtoabout55.58Nintoaregionofnear
The first dropsonde leg (1130–1234 UTC) was from saturation and moist neutrality (›u /›z ’ 0). At about
e
southtonorthtowardthelowpressurecenter(D–Cin 56.58N, the strong winds extend upward to meet the
Fig.3b).Duringthislegtheaircraftflewfromjustnorth upper-leveljet.Between800and600hPa,thereissub-
ofthesurfacecoldfront,crossingabovethecloudbands saturated air on the southern flank of this wind maxi-
into the cyclone center. Surface pressure measured by mumandsaturatedairtothenorthofit.Insection4b,it
thetenthsondewas959hPa,justabovetheminimumin will be shown that this humidity structure indicates an
theanalysisat1200UTC.Figure4ashowsthestructure airmass boundary. At 600hPa, there is a second wind
ofwindspeed,u ,andRH obtainedfromthesondes. speedmaximumtothesouth(558N)associatedwiththe
e ice