The sting at the end of the tail: Damaging winds associated with extratropical cyclones

25 Pages·2006·2.35 MB·English

QUARTERLY JOURNAL
OF THE
ROYAL METEOROLOGICAL SOCIETY
Vol.130 JANUARY2004PartB No.597
Q.J.R.Meteorol.Soc.(2004), 130,pp.375–399 doi:10.1256/qj.02.143
Thesting at the endofthetail:
Damagingwindsassociated with extratropicalcyclones
ByK.A.BROWNING¤
UniversityofReading,UK
(Received3July2002;revised13March2003)
SUMMARY
Strong surface winds often accompany the low-level jetsthat occur along the cold fronts of extratropical
cyclones,butthereisevidencethatthestrongest surfacewindsoccurinadistinctlydifferent partofacertainclass
ofcyclone. Themost damaging extratropical cyclones gothrough anevolution thatinvolves theformation ofa
bent-back frontandcloudhead separated from themainpolar-front cloudbandbyadryslot.Whenthecyclone
attainsitsminimumcentral pressure, thetrailingtipofthecloudheadbounding thebent-back frontformsahook
whichgoesontoencircleaseclusion ofwarmair.Themostdamagingwindsoccurnearthetipofthishook—the
stingattheendofthetail.
Observations oftheGreatStormofOctober 1987 insouth-east England arere-examined insomedetail to
study this phenomenon. The cloud head is shown to have a banded structure consistent with the existence of
multiplemesoscale slantwise circulations. Airwithinthese circulations leaves thehooked tipofthecloud head
(andentersthedryslot)muchfasterthantherateoftravelofthecloud-head tip,implyingrapidevaporation and
diabatic cooling immediately upwindofthearea ofdamaging surface winds. Thecircumstantial evidence from
theobservational study leads onetohypothesize thatthemesoscale circulations and theassociated evaporative
heat sinks mayplay anactive role instrengthening thedamaging winds. Regardless ofhow important thisrole
maybe,theevolution ofthecloudpatternseeninsatelliteimageryisausefultoolfornowcasting theoccurrence
andlocationoftheworstwinds.
KEYWORDS: Bent-back front Cloud hook Evaporative cooling Mesoscale Satellite imagery
Slantwiseconvection
1. INTRODUCTION
Thispaperisabouttheobservedstructureofanextremeclassofrapidlydeepening
extratropical cyclone that produces a disproportionately large social and economic
impact as a result of severe surface winds. Robert Muir-Wood, in a lecture to the
Royal Meteorological Society in December 2001, stated that the total insurance loss
from such windstorms in Europe since 1985 was 24 billion Euro. Worldwide these
European windstorms are second only to US hurricanes as a traded catastrophe risk.
Hepointedoutthattwoextratropicalcyclones—LotharandMartin—occurringinnorth-
westEuropeoverChristmas1999(Pearceetal.2001;Ulbrichetal.2001),gaveforestry
losses that alone amounted to 3.4 billion Euro; they also left 3.5 million customers
without electricity and up to 1 million without telephone lines. Other extratropical
cycloneswellknownforthewinddamagetheyin(cid:143)icted includethe1979FastnetStorm
(Pedgley1997),the1990Burns’Daystorm (McCallum 1990)andthe1997Christmas
¤ Corresponding address: Joint Centre for Mesoscale Meteorology, Department of Meteorology, University of
Reading,POBox243,Reading,Berkshire RG66BB,UK.
c RoyalMeteorological Society,2004.
°
375
376 K.A.BROWNING
Eve storm (Young and Grahame 1999), as well as the Great Storm of October 1987
(see specialissuesofWeather(Vol.43,No.3)andMeteorologicalMagazine(Vol. 117,
No.1389)).
Coming from the perspective of global risk management, Muir-Wood derived a
worst-case scenario in which a ‘Lothar storm’ crosses London. He estimated a loss
potential of up to £8 billion and 50 to 300 fatalities according to time of day and day
ofweek.Hedrewattention tothepoorqualityofpresent-dayforecastsofsuchextreme
events: even 2 hours before Lothar hit Paris, wind speeds expected in the Paris area
were being underpredicted by 25%, equal to a 6- to 8-fold underestimate in damage.
This underlines the need for improved prediction, and better prediction may require
both better targeted observational input and better representation of processes within
numerical weather-prediction models. This can be achieved in part by gaining better
understanding.Sincetheextentofthedamagingwindsisoftenquitelocalizedtheremay
be a particular need for improved understandingof the small and mesoscale structures
andmechanisms—hencethepresentstudy.
Thereis a large bodyof literature addressing the synoptic-scale structure of extra-
tropical cyclones,henceforth cyclones for short—see the monographsedited by New-
ton andHolopainen(1990)and Shapiro and Grønaƒs (1999).Rather fewer studies have
addressed the observed mesoscale structure of these cyclones, particularly those that
are extremely damaging or potentially so. Notable studies of this kind include those
of cyclones observed in ERICA (Experiment on Rapidly Intensifying Cyclones over
the Atlantic) for which Neiman et al. (1998, 1993), Neiman and Shapiro (1993) and
Wakimoto et al. (1995)presented detailed observationsfrom special aircraft and other
sources. Some of these observations formed the basis of the cyclone life-cycle model
of Shapiro and Keyser (1990) which applies to many cyclones and particularly to the
rapidlydevelopingcyclonesthatproducethemostdamagingsurfacewinds.Thepresent
author, in two review articles (1999, 2003), used the Shapiro–Keyser model as the
frameworkwithinwhichtointerpretvariousmesoscaleandsmaller-scale structuresand
processes, but the emphasis in those reviews was not on the damaging winds per se.
Thepurposeofthepresentstudyistoaddressthesmaller-scale structuresandprocesses
speci(cid:142)cally with aview to ascertaining their possible relationship to the damagingsur-
facewinds.
Itiswellknownthatmuchofthedevelopmentinrapidlydeepeningcyclonescanbe
dueto thereleaseoflatentheatduringcondensation(KuoandReed1988;Shutts1990;
Grønaƒs 1995;Wernliet al. 2002).An extra dimensionidenti(cid:142)ed in the presentpaperis
the possible role of mesoscale circulations and evaporativelatent-heat sinks in locally
intensifying the surface winds. In high-wind situations with surface gusts exceeding
40ms 1, further relatively small increases in wind speed can havea disproportionate
¡
impactontheamountofwinddamage.Insection2someearlierresultswillbereviewed
to provide the dynamical context for the ensuing observational case-study. Section 3
then revisits the Great Storm of October 1987. Although this cyclone has been much
studied on the larger scale, it is shown that there is more that can be learned from a
detailed meso-analysisofsatellite andradarimageryandthemanysurfaceobservations
including autographiccharts collected by the Met Of(cid:142)ce andkindly made available by
M. J. Bader. Mechanisms are hypothesizedin section 4, and the (cid:142)nal section, 5, gives
theconclusions.Thepurposeofthepresentpaperistoestablishstructuralcharacteristics
solely by means of observational data and to use these to derive hypothesesas to the
mesoscale mechanisms that may be important in locally enhancingthe surface winds.
Thefocusis ona speci(cid:142)cpartofthecyclone,closeto andto therightofits centre,just
downwindofthecharacteristic cloudsignaturereferred to asthehookedcloudhead.
EXTRATROPICALCYCLONEWINDDAMAGE 377
Figure1. Conceptualmodelofthelifecycleofanextratropical cyclone:(I)incipientfrontalcyclone,(II)frontal
fracture, (III)bent-back frontandfrontalT-bone,and(IV)warm-corefrontalseclusion. Upperdiagram:sea-level
pressure, fulllines; fronts, boldlines; cloud signature, shaded. Lowerdiagram: temperature, fulllines;andcold
andwarmaircurrents, fullanddashedarrows,respectively.(FromShapiroandKeyser1990.)
2. CONTEXT FOR THE DAMAGING WINDS
(a) Frontalfractureandthedevelopmentofthecloudheadanddryslot
A cloud head and dry slot are characteristic features of cyclones that produce
damagingwinds, and the most damaging winds are closely associated with them. It is
therefore appropriate to start by reviewing some of the attributes of these features to
provide a context for analysing and interpreting the strong-wind events. The cyclone
life-cyclemodelofShapiroandKeyser(1990),reproducedinFig.1,showstheevolution
of the cloud head as the cyclone develops. The cloud head becomes well de(cid:142)ned at
Stage III where it can be seen extending west- and north-westwards from the bent-
backfront.Atthis stagethemaincoldfrontandassociated polar-frontcloudbandhave
advanced ahead of the cyclone centre leaving a region relatively free from cloud in
between. This is called the dry slot. As the cyclonematures (Stage IV) the cold air in
thedryslotencirclesitscentre,leadingtoaseclusionofwarmairsurroundedbythetail
ofthebent-backfrontandtheassociatedtipofthecloudhead.Thepresentstudyfocuses
ontheregionneartheendofthetailofthebent-backfront,knowntobeapreferredsite
fordamagingwinds.Grønaƒs(1995)recalls that‘asayoungforecasterinthelate1960s,
[he]wasinformedthatthestrongestwindseverrecordedin[Norway]havebeenlinked
378 K.A.BROWNING
Figure 2. Conceptual model of the principal air(cid:143)ows in an extratropical cyclone undergoing transition from
StageIIItoStageIVinFig.1.The(cid:143)owsaredrawnrelativetothecyclonesystemwhichistravellingtowardsthe
topright-hand corner ofthepage.Thetwomaincloudfeatures areshownstippled:(i)thepolar-front cloudband,
whichis associated withtheascent oftheprimary warm conveyor belt (broad arrow labelled W1)asit travels
paralleltotheprimarycoldfront(CF1);and(ii)thecloudhead(totheleftofthebent-back frontWF2),whichis
associated withtheascentofthesecondary warmconveyor belt(W2,represented bythreebroaddif(cid:143)uentarrows
originating totherightofWF2/CF2)andthecoldconveyor belt(broad dashed arrowlabelled CCBrepresenting
a (cid:143)ow beneath W1and W2). Notice the hooked end of the cloud head where the cold conveyor belt intrudes
intothedryslotbehind thesecondary coldfront (CF2)beneath anintrusion ofrecently-descended very dryair
(thindashedline).(Redrafted fromBaderetal.1995.)
toa[bent-backfront].Suchastructure hasbeencalled “thepoisonoustail”ofthebent-
back front, after F. Spinnangr’. In the presentpaperthe damaging winds in this region
will bereferredtoasthestingattheendofthetail.
(b) Thehookedtail-endofthecloudheadandassociatedcoldconveyorbelt
Figure 2, from Bader et al. (1995), shows the cyclone structure in terms of the
conveyor-belt paradigm. The primary warm conveyor belt (W1) extends as a cloudy
airstream alongthemaincoldfront(CFI)beforerisingabovethewarmfront.Warmair
in the secondary warm conveyor belt (W2) crosses the dry-slot region at low levels
before rising above the bent-back front (WF2). The W2 (cid:143)ow then fans out and forms
the upperpartof the cloudhead.Thecold conveyorbelt occupiesthe lowerpart of the
cloudhead,includingitssoutherntipwhereitbeginsto hookaroundthecyclonecentre
behindasecondarycoldfront(CF2).Inotherwords,thecoldconveyorbeltisdominated
bya(cid:143)owthatcirculatescyclonicallyasistobeexpectedwhenthecyclonecirculationis
strong(Schultz2001).Dry-intrusionair,recentlydescendedfromupperlevels,overruns
theW2air aheadofthesecondarycoldfrontwithin thedry-slotregion.Figure1shows
thetypicalevolutionofthehookedcloudhead.Itstartsasa(cid:142)ngerofcloudasinStageIII
in Fig. 1, and evolves through the hooked stage shown in Fig. 2, to the (cid:142)nal spiral
structure whichis characteristic ofStageIV inFig.1.Accordingto Baderetal.(1995),
by the time the hookhas rotated to the forward side of the cyclonecentre, the cyclone
EXTRATROPICALCYCLONEWINDDAMAGE 379
willhavereachedits maximumdevelopment.Atthis stagethecyclonevortexis almost
vertical.
(c) Thecreationofpotentialinstability in thedryslot
AsshownbyBrowningetal.(1996,1997)andGrif(cid:142)thsetal.(2000),differentialro-
tationwithheightaroundthecyclonevortexcausesthethree-dimensionalcon(cid:142)guration
ofthe wet-bulbpotential temperature(µ )-frontalsurfacesto changein acharacteristic
w
way as the cyclone evolves(see Fig. 8 of Browning et al. 1997).Initially the µ -front
w
slopes backwardseverywhere(i.e. it is an ana-front) but, as time progresses, a portion
justsouthofthecyclonevortexcorrespondingto thedry-slotregionacquiresaforward
slope,i.e.akata-frontalstructure.Thatistosay,low-µ air overrunshigher-µ air asin
w w
the split-front model of Browning and Monk (1982).By the time the cyclone reaches
thestageshowninFig.2,theentiredry-slotregiontendstobecharacterizedbythiskind
of overrunning.Such overrunninghas been demonstrated by Mass and Schultz (1993)
using air-parcel trajectories derived from a mesoscale model. The resulting potential
instability canleadto afewoutbreaksofuprightconvectionandshowersin thedryslot
(Carr and Millard 1985). A recent example is the series of arc rainbands analysed by
Browning and Roberts (1999).Dry-slot convectiveshowers were observed also in the
case-study in the presentpaper(section 3(c)).Theseshowersproducedsome(but only
asmallproportion)oftheobserveddamagingwindgusts.
(d) Theexistenceofmultiplestackedslantwise convectivecirculationsin the
cloudhead
Incontrasttotheuprightconvectionthatoccurswithinthedryslot,themainbodyof
acloudheadischaracterizedbyslantwiseconvectionandstrati(cid:142)edprecipitation.Intheir
study of cloud heads, Dixon et al. (2002) found large values of slantwise convective
availablepotentialenergy(SCAPE)butnegligibleconvectiveavailablepotentialenergy
(CAPE). There may be some upright convection in the form of boundary-layer line
convectionalongtheinneredgeofthecloudhead,andintheformofconvectivecells at
upperlevelstowardsthetopof thecloudhead(e.g.BrowningandWang2002),butthe
primary circulationsinthecloudheadareslantwise.
At ana-cold fronts the slantwise circulation is sometimes subdivided to form
multiple stacked slantwise convective circulations as discussed by Browning et al.
(2001).Cloud heads are structured like ana-cold fronts and so it is not surprising that
similar stackedcirculationsarefoundincloudheads.Figure3,adaptedfromBrowning
et al. (1995 and 1997), portrays the structure of one particularly well observed cloud
head.Figure3(b)showsthebandedstructure ofthe cloudtopatastage in its evolution
correspondingtothatinFig.2.AccordingtoHewson(1993)thestrongestobservedgust
at this time occurred close to the tip of this cloud head. During the previous 24 hours
the cloudheadhadevolvedas shownin Fig. 3(a).Seventeendropsondesweredropped
from an aircraft (cid:143)ying along AB between 1623 and 1748 UTC as the tip of the cloud
headpassedbelowthe(cid:143)ightpath.Figure3(c),depictingthepatternofrelativehumidity
withintheverticalsectionalongAB,showstheslantwiseorientationofmultipledryand
moist layers associated with the stackedslantwise circulations nearthetip ofthe cloud
head.
A recent example of a hooked cloud head with multiple bands of cloud and
precipitation,indicativeof multiple slantwise circulations, occurredin association with
a rapidly developingcyclone that crossed the British Isles on 30 October 2000.As in
the case-study presented in section 3, this was an extremely severe event.It deepened
380 K.A.BROWNING
Figure3. Evidenceofmultipleslantwisecirculations withinadevelopingcloudheadobserved intheFRONTS-
92experiment. (a)Planoutlines ofcloudhead (< 15BC)at12-hour intervals. Thelongarrow showsthetrack
ofcyclone centre(L).ABshowsthelocationofth¡esection in(c).(b)Planviewofcloudheadinmoredetail,at
thelatesttimein(a),showingthemultiplebanded cloudtops(thicklystippledshading 6 30BC,thinlystippled
shading 20BC,outer wavy line 15BC).(c) Vertical section along ABin(a), derive¡d from 17 dropsondes
(arrows a·t¡topof diagram), showing¡sloping layers ofmoist anddry airassociated withthe multiple slantwise
circulations (shading represents relativehumidity>80%).(Simpli(cid:142)edfromBrowningetal.1995and1997.)
explosively by60mb in 24hours,attaining an estimated depthof941mbin theNorth
Sea where it produced sustained winds of hurricane force. The satellite image at this
time (Fig.4(a)) showsthemultiple cloudbandsterminating atthetip ofthecloudhead
betweenlatitudes 54and55BNnearlongitude212BE.Similar bandednesscanbeseenin
theradarimageryabout3hoursearlierasthecyclonecrossedtheeastcoastofEngland
(Fig. 4(b)). A surface gust of 41ms 1 was recorded at Grimsby near the hooked tip
¡
of the cloud head between 0900 and 1000 UTC, roughly corresponding to the time of
Fig.4(b).
Thetipofanotherhookedcloudhead,resemblingthatinFig.4,passeddirectlyover
theMST(Mesosphere–Stratosphere–Troposphere)wind-pro(cid:142)lingradaratAberystwyth
on 26 February 2002.The time–height section of returned power shown in Fig. 5 can
be interpreted in terms of the vertical structure of the cloud head and dry intrusion.
Themostintense(white)radarechocorrespondstotheboundaryofanintrusionofdry,
possiblystratospheric,air slopingdownfromtheleft(west), asin thecasedocumented
by Browning and Dicks (2001). The dry intrusion itself is characterized by very low
returned power. The cloud head was situated beneath the dry intrusion. Between 0245
and0415UTC, duringthepassageofthetip ofthecloudhead,thereareseveralsloping
echo layers which, according to Ottersten (1969), can be interpreted as sloping stable
layers and are probablyrelated to multiple stacked circulations as identi(cid:142)ed in Figs. 3
and4.Asintheothercases,thereweredamagingsurfacewindsnearthetipofthiscloud
EXTRATROPICALCYCLONEWINDDAMAGE 381
(b)
Figure4. Satelliteandradar-network pictures showingthe(cid:142)nely banded structure ofthetipofthecloudhead
foranintensecycloneon30October2000.(a)Meteosatinfraredimagefor1200UTCwhenthecyclonewasover
theNorthSeaandclosetoitsminimumcentralpressure. Multiplebandingcanjustberesolved bythegrey-scale
shading at the southern end ofthecloud head near thecoast of north-east England. (b) Radar-network picture
showing the distribution of near-surface precipitation at 0915 UTC as the cyclone left the English mainland.
Severalcurvedbandsofprecipitation canbeseenterminating nearLincolnshire, inassociation withthetipofthe
cloudhead.
Figure5. Time–height section ofreceived power (dB)from0000 to0800 UTC26February 2002, asobtained
fromtheMSTradaratAberystwyth,overwhichthetipofacloudheadpassedshortlyafter0300UTC.Thesloping
maximainreceivedpower,highlightedbythethreelinesdrawndownto1km,arebelievedtobeindicativeofthe
presence ofmultipleslantwisecirculations inthecloud-head region.
382 K.A.BROWNING
head and indeed the Aberystwyth MST radar measured a low-level jet of 45ms 1 at
¡
2.2kmat0320UTC inthisregion(notshown).
The detailed case-study of the exceptionally severeOctober1987storm presented
in the next section will again show that the damaging winds occurred in the dry slot
closetothetipofamultiple banded,hookedcloudhead.Thecloudheadwaswrapping
aroundthetailofthebent-backfrontanditwasairfromthetipofthecloudheadthatwas
entering the region of damaging winds. The study suggests that upright and slantwise
convectionofthekindsdiscussedabove,togetherwith theassociatedlatent-heateffects
(evaporationaswellascondensation),mayhaveplayedaroleinthelocalenhancement
ofthealreadystrongsurfacewinds.
3. THE GREAT OCTOBER ’87 STORM REVISITED
(a) Surfaceanalyses
The cyclone of 15/16 October 1987 travelled north-eastwards from west of the
Iberian peninsula at 1200 UTC on the 15th to the North Sea close to the England/
Scotlandborderby1200UTC onthe16th.Thepathofthecyclonecentretookitacross
Englandwhere the greatest wind damage, to the east of the track, was oversouth-east
England.Thisis a regionwith ahighdensity ofautographicrecordingsurfaceweather
stations.Theavailabilityofchartsfromalargenumberofthesestationsandthedetailed
cloudandprecipitation information providedbyMeteosatandweatherradarmade this
anexcellentcaseforexaminingthemesoscalestructureandthepossiblerolesofupright
andslantwise convectioninincreasingthesurfacewinds.
According to Woodroffe (1988), the cyclone reached its lowest central pressure
of 952 mb at 0000/16th just north of Brest. This followed a pressure fall of 26 mb
over the previous 12 hours, in excess of the 24 mb in 24 hours (at latitude 60 ) that
B
correspondsto explosive development,or a ‘bomb’ in the terminology of Sanders and
Gyakum(1980).OverthesameperioditwentthroughthelifecycleportrayedinFig.1.
By0000/16thithadjustaboutdevelopedawarmseclusionandthetipofits cloudhead
was beginningto hookaroundit as in Fig. 2. A goodtime for a detailed mesoanalysis
of the surface wind (cid:142)elds was centred around 0130/16thas this is when the region of
damagingwindsbeganto come inland across southernEngland.Thisanalysis is given
in section 3(b). To provide a context for this, surface analyses are shown (cid:142)rst for the
earlierandlatertimes,0000and0300/16th(Fig.6).Thesimilarity tothestructureinthe
ERICA cycloneof4–5 January1989is noteworthy(see Figs. 7(c) and9(c) ofNeiman
andShapiro(1993)).
Figures 6(a) and (c) show the surface pressure and frontal analyses at 0000 and
0300 UTC, respectively (the frontal analyses are based on the analyses in Figs. 6(b)
and (d)). Theyshow the formation of the warm seclusion as the bent-backfront wraps
aroundthecyclonecentreandcoldair advancesfarheadofit (behindasecondarycold
front that corresponds to CF2 in Fig. 2). The precise shape of the bent-back front is
a little uncertain over the sea but the analysis has been aided by the satellite imagery
and knowledgeof how the inner boundaryof the cloud head related to the bent-back
front when it was over land. It is shown later (Fig. 11) that this was the time when
the hooked tip of the cloud head was beginningto rotate aroundthe southern (cid:143)ank of
the cyclone. The frontal analyses show that the axis of the elongated warm seclusion
wasalso rotating duringthis period.Pressure gradientsare seen to beespecially strong
around its southern and south-eastern (cid:143)anks. This and the high speed of travel of the
cyclone (22 to 25ms 1) account for the exceptional rate of pressure rise (23 mb
¡
EXTRATROPICALCYCLONEWINDDAMAGE 383
Figure6. Meso-analyses oftheintense cyclone on16October1987:(a)and(b)arefor0000 UTC,and(c)and
(d) are for 0300 UTC.Mean-sea-level pressure (isobars every 4mb) and frontal analyses are shown in (a) and
(c).Surfacewet-bulbtemperature (isolinesevery1degC)areshownin(b)and(d).Thelongdashedarrowshows
theaxisofthecold(cid:143)owwhich(relativetothesystem)isbeginning toencirclethewarmseclusion: thestrongest
surface winds were situated along this axis, at C (cf. detailed gust analysis in Fig. 9). The open cold-frontal
symbols in (a) represent the northern part of the primary cold front where it is mainly an upper-level feature.
Theothercoldfrontisthesecondary coldfrontwhichislinkedtothewarmfrontviaabent-back frontencircling
thewarmseclusion.
in 3 hours) after the passage of the cyclone centre at locations such as Netheravon
¤
(Fig. 7(a)). The later surface wind analyses indicate that the strongest gusts, in excess
of50ms 1,werecentredatCinFig.6.Thegradientwindalso reachedamaximumin
¡
thevicinity ofC.Thisisestimated tohavebeenabout43. 10/ms 1.
¡
§
Asimilarpressureriseof23mbin3hourswasrecordedfordamagingstormMartinasittravelledacrossFrance
¤
at23ms¡1inDecember1999.Anevenfasterpressure rise,29mbin3hours,wasrecorded forthestormLothar
whichtravelledacrossFranceat35ms 1ontheprevious day(Pearceetal.2001).
¡
384 K.A.BROWNING
Figure7. (a)Pressure(mb,unadjusted), (b)temperature (BC)and(c)relativehumidity(%)tracesfortheperiod
from1000UTC15Octoberto1200UTC16October1987,atNetheravon (51B140N,01B470W).Warmfront(WF),
warmseclusion (WS)anddryslot(DS)arelabelled.
Figures6(b)and(d)showtheanalysesofsurfacewet-bulbtemperature,T ,at0000
w
and 0300 UTC, respectively. For each station, hourly reports were used for 1, 0 and
C
1houraftertheanalysistime, thelocationofthemeasurementstakenbeforeandafter
¡analysistime beingdisplacedaccordingto thevelocityofthecyclonecentre(23m s 1
¡
from 217 ). T has been plotted rather than temperature, T, in order to identify the
B w
boundaries of air with different origins (some of the low-T air reached the surface
w
after descending from higher levels and was characterized by an anomalously high T
owing to adiabatic warming). The warm seclusion shows up as a 15 C bull’s-eye at
B
0000, diminishing to 14BC at 0300 UTC as it reaches the south coast of England. A
pronounced tongue of cold air can be seen wrapping around the warm seclusion. To
the west of the seclusion, T drops by 7 degC within a distance of 20 km or less.
w
Aradiosondereleasedat0000UTCfromCamborne,200kmbehindthewarmseclusion
(notshown),revealsthatthetongueofcoldairwasveryshallow,beingcon(cid:142)nedthereto
below900mbwheretherewasa25ms 1 northerlylow-leveljet (cold conveyorbelt).
¡
The axis of this tongue of cold air (dashed line) is seen to be within 50 to 80 km of
the bent-back front in Fig. 6(b) and it passes throughthe centre of the most damaging
winds (C). Comparisonwith the later Fig. 9 (allowing for time differences)shows that
the dashedline in Fig. 6(b)correspondsto the axis of maximum low-levelwind speed
and henceto the maximum in the cyclonic advectionof low-T air. The north-west to
w
south-east distance across the warm core of the encircling low-levelwindmaximum is
220 kmin Fig. 6(b), which is almost as small as the 150to 200km distance measured
intheERICAcyclonestudiedbyNeimanetal.(1993).
Horizontaladvectionof low-T air aroundthewarm core is onlypart ofthe story,
w
however, because the rate of advance of the leading edge of this low-T air at the
w
surface was exceptionally rapid. The 10 C contour, seen arriving at the French coast
B
at 0000 UTC, reached central England by 0300 UTC with a rate of advance of about
47ms 1. This exceeds the wind strength at the surface, even in gusts, at all places
¡
exceptnearC.Thisisattributed later(section3(c))tosomeoverrunningairwithlowµ
w
being broughtdownto the surface, in part by slantwise convectionand in part also by