Finescale Radar and Airmass Structure of the Comma Head of a Continental Winter Cyclone

25 Pages·2015·7.67 MB·English

University of Wyoming
Wyoming Scholars Repository
Atmospheric Science Faculty Publications Atmospheric Science
11-1-2014
Finescale Radar and Airmass Structure of the
Comma Head of a Continental Winter Cyclone:
The Role of Three Airstreams
Robert M. Rauber
University of Illinois at Urbana–Champaign
Matthew K. Macomber
University of Illinois at Urbana–Champaign
David M. Plummer
University of Illinois at Urbana–Champaign
Andrew A. Rosenow
University of Illinois at Urbana–Champaign
Greg M. McFarquhar
University of Illinois at Urbana–Champaign
See next page for additional authors
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Publication Information
Rauber, Robert M.; Macomber, Matthew K.; Plummer, David M.; Rosenow, Andrew A.; McFarquhar, Greg M.; Jewett, Brian F.; Leon,
Dave; and Keeler, Jason M. (2014). "Finescale Radar and Airmass Structure of the Comma Head of a Continental Winter Cyclone:
The Role of Three Airstreams."Monthly Weather Review142.11, 4207-4229.
This Article is brought to you for free and open access by the Atmospheric Science at Wyoming Scholars Repository. It has been accepted for inclusion
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Authors
Robert M. Rauber, Matthew K. Macomber, David M. Plummer, Andrew A. Rosenow, Greg M. McFarquhar,
Brian F. Jewett, Dave Leon, and Jason M. Keeler
This article is available at Wyoming Scholars Repository:http://repository.uwyo.edu/atmospheric_facpub/29
NOVEMBER2014 RAUBER ET AL. 4207
Finescale Radar and Airmass Structure of the Comma Head of a Continental
Winter Cyclone: The Role of Three Airstreams
ROBERTM.RAUBER,MATTHEWK.MACOMBER,DAVIDM.PLUMMER,ANDREWA.ROSENOW,
GREGM.MCFARQUHAR,ANDBRIANF.JEWETT
DepartmentofAtmosphericSciences,UniversityofIllinoisatUrbana–Champaign,Urbana,Illinois
DAVIDLEON
DepartmentofAtmosphericScience,UniversityofWyoming,Laramie,Wyoming
JASONM.KEELER
DepartmentofAtmosphericSciences,UniversityofIllinoisatUrbana–Champaign,Urbana,Illinois
(Manuscriptreceived10February2014,infinalform21July2014)
ABSTRACT
Datafrom airborneW-bandradar, thermodynamic fieldsfromthe WeatherResearchandForecasting
(WRF)Model,andairparcelbacktrajectoriesfromtheHybridSingle-ParticleLagrangianIntegratedTra-
jectory (HYSPLIT) model are used to investigate the finescale reflectivity, vertical motion, and airmass
structureofthecommaheadofawintercyclonethatproduced15–25cmofsnowacrosstheU.S.Midweston
29–30January2010.
Thecommaheadconsistedofthreeverticallystackedairmasses:frombottomtotop,anarcticairmassof
Canadianorigin,amoistcloud-bearingairmassofGulfofMexicoorigin,andadrierairmassoriginating
mostlyatlowaltitudesoverBajaCaliforniaandtheMexicanPlateau.Thedrierairmasscappedtheentire
commaheadandsignificantlyinfluencedprecipitationdistributionandtypeacrossthestorm,limitingcloud
depthonthewarmside,andcreatinginstabilitywithrespecttoice-saturatedascent,cloud-topgenerating
cells,andaseeder–feederprocessonthecoldside.Convectivegeneratingcellswithdepthsof1.5–3.0kmand
verticalairvelocitiesof1–3ms21wereubiquitousatopthecoldsideofthecommahead.
Theairmassboundarieswithinthecommaheadlackedthethermalcontrastcommonlyobservedalong
frontsinothersectorsofextratropicalcyclones.TheboundarybetweentheGulfandCanadianairmasses,
althoughquitedistinctintermsofprecipitationdistribution,wind,andmoisture,wasmarkedbyalmostno
horizontalthermalcontrastatthetimeofobservation.Thehigher-altitudeairmassboundarybetweenthe
GulfofMexicoandBajaairmassesalsolackedthermalcontrast,withtheless-stableBajaairmassoverriding
thestableGulfofMexicoair.
1. Introduction 1993;Martin1998a,1999;Stoelingaetal.2002;Schultz
and Vaughan 2011). In the classical warm occlusion
Thecommaheadofwintertimeextratropicalcyclones
process (Bjerknes and Solberg 1922), air behind the
isacommonlocusofheavysnow,blizzards,icestorms,
cyclone’s cold front ascends the warm-frontal surface,
andotherhazardsasthesestormsmoveeastwardacross
isolating a wedge of warm air aloft. This wedge, first
theNorthAmericancontinent.Inmaturecyclones,the
investigated by Crocker et al. (1947), Godson (1951),
commaheadisoftencharacterizedbyawarmoccluded
and Penner (1955), is sometimes referred to as the
frontalstructure(e.g.,Kuoetal.1992;SchultzandMass
trough of warm air aloft, or trowal (e.g., Penner 1955;
Galloway 1958, 1960; Martin 1999) and is associated
withthecommaheadcloudsandprecipitationevidentin
Correspondingauthoraddress:RobertM.Rauber,Departmentof
satelliteandradarimagery.Airflowintoandthroughthe
AtmosphericSciences,UniversityofIllinoisatUrbana–Champaign,
comma head has also been described in storm-relative
105S.GregorySt.,Urbana,IL61801.
E-mail:[email protected] coordinates as the westward-flowing branch of a warm
DOI:10.1175/MWR-D-14-00057.1
(cid:1)2014AmericanMeteorologicalSociety
4208 MONTHLY WEATHER REVIEW VOLUME142
conveyor belt (Schultz 2001). Madonna et al. (2014) fromKansastoIndiana.Thedatafromthisflightprovide
provide a recent review of research related to warm a unique perspective on structural features of comma
conveyorbeltflows,SchultzandMass(1993),Stoelinga headclouds,precipitation,andtheirrelationshiptover-
et al. (2002), and Schultz and Vaughan (2011) provide ticalmotion,flowfields,andairmassstructurethatarenot
thorough reviews of the literature on occlusions, and easilydeducedwithconventionalandverticallypointing
Martin(1999),Grimetal.(2007),andHanetal.(2007) radars.
provide analyses of thermodynamic structure and dy- Inthispaper,ananalysisoftwohigh-resolutionradar
namicforcingofcommaheadcloudsystems. crosssections derivedfromWCRmeasurementsmade
When viewed with scanning radars, precipitation during the 30 January 2010 storm is presented. The
withinthecommaheadistypicallyorganizedinlinear, analysis uses data from the WCR in conjunction with
banded features of enhanced reflectivity (e.g., Nicosia thermodynamic fields from the Weather Research and
and Grumm 1999; Novak et al. 2004, 2009, 2010). The Forecasting(WRF)Modelandairparcelbacktrajectories
propensityofbandingtooccurwithinthecommahead calculated using the National Oceanic and Atmospheric
has been thoroughly documented (e.g., Martin 1998b; Administration (NOAA)/Air Resources Laboratory
Novak et al. 2004; Browning 2005; Moore et al. 2005; HybridSingle-ParticleLagrangianIntegratedTrajectory
Nicosia and Grumm 1999; Novak et al. 2008, 2009, (HYSPLIT)model(DraxlerandHess1998;Draxlerand
2010). These banded precipitation structures are nor- Rolph 2014), to elucidate the finescale features of the
mally identified from low-level radar scans typical of comma head clouds and precipitation and their rela-
operational S-band (10-cm wavelength) radars such as tionship to the air masses in which they are embedded.
theNationalWeatherServiceWeatherSurveillanceRadar- The analyses are unique in that they 1) represent very
1988 Doppler (WSR-88D). Because of their scanning high-resolution(15m)cross-sectionalradardepictionsof
strategy,operationalradarsprovidelittleinsightintothe commaheadcloudandprecipitationstructurefromthe
verticalstructureofprecipitation,anditsrelationshipto elevatedcloudshieldonthecoldsideofthestormtothe
the airmass and frontal structures that characterize the dry slot on the equatorward side; 2) relate flow field,
comma head. Information about the finescale structure airmass structure, and stability to finescale vertical mo-
ofprecipitationcomesprimarilyfromverticallypointing tion fields; 3) show how air masses from diverse source
radars, which view precipitation as storms pass over a regions juxtapose to create the structural features char-
fixed location. Vertically pointing radar measurements acterizingcommaheadprecipitation;and4)demonstrate
within the comma head have consistently shown the importantrolesofthreeairstreamsintheformationand
presenceofgeneratingcellswithinextratropicalcyclones distributionofprecipitationacrossthecommahead.
(e.g., Marshall 1953; Gunn et al. 1954; Wexler 1955;
Douglasetal.1957;WexlerandAtlas1959;Carboneand
2. Datasourcesandanalysismethods
Bohne1975;HobbsandLocatelli1978;Syrettetal.1995;
Stark et al. 2013; Rosenow et al. 2014). The cells form The measurements reported here were made during
nearcloudtopwellabovefronts,are1–2kminhorizontal the2009–10ProfilingofWinterStorms(PLOWS)field
extent,haveupdraftswithmagnitudesof1–3ms21,and campaign(Rauberetal.2014;Rosenowetal.2014).This
produce streamers of precipitation that merge during paper focuses on data from the WCR collected on 30
descentintothestratiformradarecho.Microphysicalchar- January 2010. The WCR is described in Wang et al.
acteristics of generating cells are described in Plummer (2012) and its use in PLOWS is described in Rosenow
etal.(2014). et al. (2014). Summarizing key points from Rosenow
The advent of high-resolution, airborne Doppler etal.(2014),theWCRhadtwodownward-lookingbeams,
W-bandradar(Wangetal.2012)providesanewoppor- oneatnadir,andone34.38aftofnadir,andasingle,up-
tunity to investigate the finescale structure of precip- wardbeam,withthewidthofallbeams,18.TheWCR
itation across the comma head of winter storms and its dataareavailableatahorizontalscaleof4–7.5matC-130
relationshiptofrontsandotherairmassboundaries.The nominalairspeedsbetween100and150ms21.TheWCR
advantage of airborne radar measurements is that they typically transmitted a 250ns (37.5m) pulse, sampled
provide spatial information aboutstructure, ratherthan at 15-m resolution. The unambiguous range was 9km,
temporalevolutionoverasinglelocation.On30January andtheunambiguousvelocitywidthwas26.3ms21.The
2010, the NationalScience Foundation/NationalCenter orientation of the WCR beams in ground-relative co-
forAtmosphericResearch(NSF/NCAR)C-130aircraft ordinateswascomputedusingtheaircraftroll,heading,
carryingtheUniversityofWyomingCloudRadar(WCR) andpitchanglesandtheground-relativeaircraftvelocity.
flewtwotransectsacrossthecommaheadofawintercy- These were usedtoidentifyandcorrectforthecompo-
clonethatproduced15–25cm(6–10in.)ofsnowinaswath nentofaircraftmotionalongeachofthebeams,andto
NOVEMBER2014 RAUBER ET AL. 4209
determine the vertical component of the radial velocity diffusion. The model was initialized at 0000 UTC
fromtheupward-anddownward-pointedbeams.Thevar- 30 January. Forecast fields for 0300 UTC, the central
iance of the computed radial velocities was ,0.04m2s22 time of the flight legs across the comma head, were
atreflectivityfactors.223dBZ .Rosenowetal.(2014) interpolatedtotheWCRcrosssections.
e
describetheproceduretoderivetheverticalcomponent The 3-h WRF forecast was used rather than the
oftheradialvelocityW. 0000 UTC NAM initialization because 1) the nearest
Theequivalentradarreflectivityfactor(Z ,hereafter NAM initialization was several hours prior to aircraft
e
reflectivity)andWmeasurementswereregriddedfrom data collectedbetween0225and0422 UTC, 2)thever-
aircraft-relative to ground-relative coordinates. The W tical resolutionoftheNAManalyseswasinsufficient in
valuesareshownpositiveupwardinallfiguresdepicting the layer of the generating cells and in the vicinity of
theWCRverticalradialvelocitymeasurements.Rosenow airmass boundaries to properly interpret the structure
et al. (2014) showed that in stratiform regions of the andstabilityofthesefeatures,3)the3-hforecastallowed
comma heads of winter storms observed during spinupanddevelopment of vertical motionswithinthe
PLOWS, the reflectivity-weighted terminal velocity of model,and4)the3-hWRFforecastcloselymatchedthe
icecrystalsandsnowflakesV ,afteraccountingforthe structuresrevealedbytheWCRairborneobservations.
T
decrease of atmospheric density with altitude, ranges As a test, we also examined WRF simulations with
from about 20.7 to 21.0ms21 over most of the cloud the same parameters but without cumulus parameteri-
depth.Therefore,aroughestimateofw,theverticalair zation, and a 9-h simulation initialized at 1800 UTC
motion,canbemadebyadding1ms21totheWvalues. 29 January and valid at 0300 UTC. There were minor
The 34.38 aft beam simultaneously sampled the ver- differences in the vertical velocity distribution at the
tical motion and horizontal motion of hydrometeors. generating cell level between these simulations. The
Thehorizontalmotionofhydrometeorssampledbythe differencesweresufficientlysmalltohavenoimpacton
radarisrelatedtothehorizontalwindcomponentinthe thefindingsofthispaper.Theutilized3-hforecastwas
directionofflight.Theverticalmotionofhydrometeors found to best match the WCR structures and was
is related to the reflectivity-weighted fall speed of the therefore selected for analyses overlaid on the WCR
precipitationplusanyverticalairmotion.Asaresultof figures to provide larger-scale context for the detailed
thegeometry,the34.38slantbeamradialvelocitymea- WCRobservations.
sures 56% of the horizontal and 83% of the vertical Airparcelbacktrajectorieswerecalculatedusingthe
component. HYSPLIT model, August 2013 revision (515) version
A 3-h forecast with the WRF Model (version 3.5.1) (DraxlerandHess1998;DraxlerandRolph2014).The
wasusedtoobtainfieldstooverlayuponandaidinin- meteorologicaldatasetusedtoinitializeeachHYSPLIT
terpretationoftheWCRdata.Forecastfieldsincluded trajectorywastheNCEPNAMDataAssimilationSys-
potential temperature (u), equivalent potential tem- tem(NDAS;Rogersetal.2009).Thearchiveddatahave
perature derived with respect to ice (u ), relative hu- a grid spacing of 12km and are available every 3h.
ei
midity with respect to water (RH ) and ice (RH), HYSPLIThasbeenusedinpaststudiesofwinterstorms
w i
convective available potential energy with respect to (Grim et al. 2007; Fuhrmann and Konrad 2013) to
ice-saturated ascent (CAPE), and large-scale vertical determine airmass source regions. In this paper, the
i
airvelocity(w),allinplanescorrespondingtotheWCR HYSPLIT model is used to calculate 48-h back trajec-
crosssections.TheWRFsimulationwasinitializedwith toriesforairparcelslocatedatevery1kminthevertical
NationalCentersforEnvironmentalPrediction(NCEP) and ;40kmin thehorizontal (5-minflight time)along
North American Mesoscale Model (NAM) analyses theWCRcrosssections.
(218 grid, 12-km spacing) for a domain centered near
348N, 898W with 9-km grid spacing and 220 vertical
3. Stormoverview
layers. The simulation employed positive-definite ad-
vection,agravitywavedampinglayernearthedomain The 29–30 January 2010 cyclone was a significant
top, Thompson microphysics (2013 version, 300cm23 winterstormthatproducedbetween15–25cm(6–10in.)
clouddropletconcentrationusedforcontinentalsettings), ofsnowacrossIowa,Missouri,Illinois,andIndiana.The
Rapid Radiative Transfer Model for Global Climate surfacelowpressurecenterassociatedwiththecyclone
Models (RRTMG) shortwave/longwave radiation, the formed east of the Sierra Madre Occidental in Mexico
Noahlandsurfacemodel,theYonseiUniversitybound- near 0000 UTC 29 January, moved over the Gulf of
ary layer treatment, the Kain–Fritsch cumulus parame- MexicojusteastofthesoutherntipofTexas,progressed
terization with the default (Kain 2004) trigger function, northeastwardoverwaterintosouthernLouisiana,and
and 2D Smagorinsky (diagnosed from deformation) was located in east-central Mississippi at the time that
4210 MONTHLY WEATHER REVIEW VOLUME142
FIG. 1. (a) 300-hPaheights(m) and isotachs (ms21), (b) 500-hPa heightsandtemperatures(8C),(c) 700-hPa
heightsandtemperatures,and(d)925-hPaheightsandtemperaturesfor0000UTC30Jan2010.Heightcontoursare
blackandisotachsandisothermsareyellow.Thewindbarbsareflags(25ms21),longbarbs(10ms21),andshort
barbs(5ms21).AlldataaresuperimposedoncompositeWSR-88Dimagesat0000UTC30Jan2010.
the C-130 aircraft was crossing the cyclone’s comma lowpressurecenter,the500-hParelativehumiditywith
head in Missouri and Illinois on 0300 UTC 30 January respecttowater,andsurfaceisobars.Surfaceisotherms
(Fig.1d).Duringthe27-htransitacrossMexicoandthe areshowninthevicinityofthecommahead.Theheavy
Gulf of Mexico, the central sea level pressure of the whitelineshowsthetrackanddirectionofflightofthe
cyclonedecreased3hPa,from1010to1007hPa.During C-130.Thewesternflightleg(WFL)wasflownbetween
the event, subfreezing surface temperatures extended 0204 and 0324 UTC, and the eastern flight leg (EFL)
southwardintoTexas,Louisiana,andAlabama. between0324and0422UTC.Threefeaturesofimpor-
The cyclone was associated with a wide upper- tanceinFig.2arerelevanttothesubsequentdiscussion.
tropospheric trough that formed within the southern First, from the water vapor imagery and the 500-hPa
branchofthejetstream.By0000UTC30January,just relative humidity distribution, note that the drier air
before the C-130 flight, the trough was located east of associatedwiththecyclone’sdryslotappearstooverride
the southern Rocky Mountains and over the southern moistairalongthesouthernsideoftheEFLinsoutheast
Great Plains(Fig.1a). A westward-tilted, closed circu- Missouri.Farthertothewest,thedryairstreamappears
lation was present between the surface and 500hPa to split over Oklahoma, flow along and override moist
within the trough (Figs. 1b–d). The comma-shaped airatlowerlevelsoneithersideofthecommahead,and
precipitation shield of the cyclone was located north crosstheWFLonbothitsnorthernandsouthernends.
andeastofthecenterofcirculationat500hPa(Fig.1b). Second,notethemoisturelocatedalongthecentralaxis
Figure 2 shows a Geostationary Operational Envi- ofthecommahead.Aswillbeshownlater,thisregion
ronmentalSatellite(GOES)enhancedwatervaporsat- correspondedtothelocationofthedeepestcloudsand
elliteimageofthecycloneduringtheflightat0300UTC. wascoincidentwiththeheaviestsnowfallaccumulation
Superimposed on the image are the surface fronts and at the ground. Finally, note the surface temperatures.
NOVEMBER2014 RAUBER ET AL. 4211
FIG.2.Watervaporsatelliteimageofthe0300UTC30Jan2010cyclone.Overlaidonthe
image are the C-130 flight track (straight whitelines), the surface low pressure center and
fronts,thesealevelpressurefield(blacklines,hPa),1000-hPatemperatures(8C,yellowlines,
onlyinregionofflighttrack),andthe500-hPa70%and90%RH contours(green).
w
Duringthis cyclone,arcticairhad intrudedfarenough Superimposed in Fig. 4a are analyses of u and RH,
ei i
south so that temperatures were well below freezing andinFig.4b,analysesofuandRH ,allat0300UTC30
w
acrossthecommaheadregioninthevicinityofboththe January.SuperimposedinFig.5aarepointsfromwhich
WFL and EFL. Consequently, most precipitation be- 48-h back trajectories were calculated using HYSPLIT.
neath both flight legs was in the form of snow. There Although these are backward trajectories, the points in
were some reports of freezing drizzle where the WFL Fig.5aarereferredtoastheendpointsofthetrajectories,
andEFLjoin. since,intime,airparcelsmovedfromsomeotherlocation
Figure 3 shows a composite WSR-88D image at at 0400 UTC 28 January to their position on the cross
0300 UTC with the WFL and EFL and surface frontal sectionat0400UTC30January.Thestartingpointsofthe
analyses superimposed. This image confirms that the trajectoriesat0400UTC28JanuaryareshowninFig.6.
locallystrongprecipitationradarechoextendedacross Bestestimatesofairmassboundaries,denotedbyfrontal
the central part of both flight legs. Based on the radar symbols,weredeterminedbyconsideringtheu field,air
ei
echointensity,onlylighttotraceprecipitationfellonthe parcelbacktrajectoriesinitiatedfrompointsonthecross
southern side of the comma head beneath the regions section (Fig. 5a), and the WCR 348 slant beam radial
wheretheWFLandEFLlegsjoininnorthernArkansas, velocity(Fig.5b).
andnoprecipitationfellatallalongthenorthernthirdof The comma head of the cyclone comprised three
the EFL. The reason for this snowfall distribution will vertically stacked air masses. For convenience, we will
becomeapparentwhenweexaminethefinescaleWCR refer to these air masses (from bottom to top) as the
andWRFanalysesinthefollowingsection. Canadian, Gulf, and Baja air masses, although some
trajectories did not originate directlyover Canada, the
Gulf of Mexico, and the Baja Peninsula. Air parcels
4. Finescalestructureofthecommahead
located within the Canadian air mass originated 48h
a. Easternflightleg earlier at locations across central Canada and the
northern United States (cf. square symbols in Figs. 5a
1) AIRMASSSTRUCTUREANDAIRPARCEL
and 6). The Canadian air was 1–2km deep along the
TRAJECTORIES
southernhalfoftheEFL.ExampletrajectoriesinFig.7a
The radar reflectivity measured by the WCR along show that air parcels arriving at the 1-km level (all
the EFL of the C-130 appears in Figs. 4a,b and 5a. heightsabovemeansealevel)underwentsubsidenceon
4212 MONTHLY WEATHER REVIEW VOLUME142
FIG.3.WSR-88Dcompositeshowingthe0300UTC30Jan2010cyclone.Overlaidonthe
image are the C-130 flight track (straight white lines),the surfacelow pressurecenter and
fronts,thesealevelpressurefield(blacklines,1000hPa),andthesurfacetemperatures(yellow
lines,8C).
their path to the cross section, typically descending 2– potential temperature (e.g., Bluestein 1993, 245–248;
4km during the transit from Canada to the EFL (see Martin2006,189–193).Theairmassboundaryisevident
trajectoriesk,«,andbinFig.7a,andtheinsetinFig.6). intheslant-beamradialvelocityinFig.5b,theboundary
Thetrajectoriesdidnotexhibitmuchcycloniccurvature, marked by the transition between air with a northerly
asexhibitedincoldconveyorbelttypeflowsinstronger flow component and air with a southerly component.
cyclones [e.g., Mass and Schultz (1993, their Fig. 16), The boundary also coincides with sharp vertical and
Martin (1998b, his Fig. 16)]. The slope of the airmass horizontalgradientsofu (Figs.4aand5b)andclosely
ei
boundarychangednearthecenterofthecrosssection, matchestheresultsofthetrajectoryanalyses.However,
withthecoldairmassdeepeningto5kmatthenorthern theboundaryisnotobviousinthepotentialtemperature
endofthecrosssection.Unlikethenear-surfaceair,air field and thus cannot be characterized as a true front.
withintheCanadianairmassarrivingat3–4-kmaltitude NotethatthedescendingCanadianairwasunsaturated,
maintainedanear-constantaltitudealongitstrajectory whiletherisingGulfairwassupersaturatedwithrespect
(seetrajectoriesCandr).Airatintermediatealtitudes to ice (Fig. 4a). The trajectories suggest that over the
withintheCanadianairsubsided;1km(e.g.,trajectory 48-h period before the aircraft sampled the storm, dry
u).AsindicatedbytheRH andRH fields,theCana- adiabatic warming within the descending Canadian air
w i
dian air was sufficiently dry that precipitation falling and moist adiabatic cooling within the rising Gulf air
from aloft sublimated after entering the Canadian air, largely homogenized the temperature field across the
particularly on the northern side of the cross section commaheadattheleveloftheairmassboundaryatthe
wheretheCanadianairwasdeeper. timetheaircraftsampledthestorm.
The boundary between air originating over Canada The central midtropospheric air mass depicted in
andairoriginatingovertheGulfofMexico(depictedas Figs.4and5originatedoverorneartheGulfofMexico
acoldfrontinFigs.4and5)didnothavecharacteristics (see Fig. 7b, cf. the circle symbols in Figs. 5a and 6).
of a true front. From a dynamic perspective, fronts AlongtheEFL,airwithintheGulfairmasswassuper-
are associated with density gradients and marked by saturatedwithrespecttoice,providinganenvironment
steep isentropes and a discontinuity in the gradient of foriceparticlegrowth(Fig.4a).RH valueswithinthe
w
NOVEMBER2014 RAUBER ET AL. 4213
FIG.4.CrosssectionsoftheWCRequivalentradarreflectivityfactor(dBZe)fortheeastern
flightleg.(a)SuperimposedaretheWRFu (thinblacklines,K)andRH (thickbluelines)
ei i
fields.RH isshownonlyforregionswithRH $100%.(b)TheWRFu(thinblacklines,K)and
i i
RH (thickbluelines)fields.RH isshownforregionswithRH $70%.TheC-130flighttrack
w w w
isshowninred.Airmassboundariesaredenotedwithfrontalsymbols.
air mass approached or exceeded 90%, and even horizontal(Fig. 4b). However,theboundaryisevident
reached100%intheshallowcloudsonthesouthernend by a transition to much drier air aloft and a marked
of the EFL where freezing drizzle was observed at the changeinstability.Aswillbeshowninthenextsection,
surface (Fig. 4b). Example trajectories of the Gulf air cloud-top generating cells with vertical air velocity of
are shown in Fig. 7b. The trajectory analyses indicate 1–4ms21wereubiquitousnearthecloudtopalongthe
that Gulf air arriving at the cross section ascended 2– centralpartoftheEFLcrosssectionwithintheBajaair
4kmtoitspositionalongtheEFL(e.g.,trajectories1,I, mass.TheexactpositionoftheGulf–Bajaairboundary
L,R,MandEinFig.7b,alsocomparecirclesymbolsin was not as distinct as the Canadian–Gulf boundary. In
Figs.5aand6).Innearlyalltrajectories,Gulfairinitially Fig. 4 and subsequent figures, the boundary (marked
flowed northwestward, curving back northeastward to withupper-levelfrontalsymbols)wasplacedatthebase
arriveattheEFLcrosssection.Thesetrajectoriesdiffer ofthegeneratingcelllayer,whichcorrespondswiththe
fromthosedescribedinMartinetal.(1998a,hisFig.19) change in stability and most trajectories. A few trajec-
in that they do not turn cyclonically westward into the toriesnearthenorthernedgeofthecloudshieldbelow
commahead. the marked boundary originated in the Baja region,
Theupper-troposphericairmassdepictedinFigs.4–5 while some above the marked boundary in the gener-
originated near Baja California and the Mexican Pla- ating cell region tracked back to the Gulf. These dif-
teau. The boundary between the Gulf and Baja air ferences may be related to differences between the
masses appears on satellite water vapor images (e.g., WRF simulation and the NAM/NDAS data (on which
Fig. 2) as an extension of the leading edge of the cy- the HYSPLIT trajectories were based). Trajectory
clone’sdryslotboundarywestwardalong(andover)the analysesoftheBajaair(seeFig.7candstarsymbolsin
comma head. Again, from a dynamic perspective, the Figs.5aand6)indicatethatmostoftheairarrivingatthe
boundary between the Gulf and Baja air did not have EFLcrosssectionoriginatedatloweraltitudesoverthe
characteristicsofatruefront—theisentropesarenearly Mexican Plateau and Baja California 48h earlier and
4214 MONTHLY WEATHER REVIEW VOLUME142
FIG.5.(a)WCRequivalentradarreflectivityfactor(dBZe)fortheeasternflightleg.Each
symbolin(a)denotesthe0400UTC30JanlocationofaHYSPLITtrajectory.Theoppositeend
ofeachtrajectory48hearlierat040028Jan2010appearsinFig.6.Selecttrajectoriesareshown
inFigs.7,8,and9.Thesquares,circles,andstarshadtrajectoriesthatoriginated48hearlier
overCanada,theGulfofMexico,andtheBajaCaliforniaregion,respectively.Thehexagons
andrectanglesveryclosetotheairmassboundarieshadtransitionaltrajectoriesthatended
betweenthethreeprimarylocations.(b)Radialvelocityfromthe348slantbeamoftheWCR.
Theboundarybetweenredandbluehasbeenadjustedby1ms21toaccountforthecontri-
butionofterminalvelocityofsnowtotheradialvelocity,sothatredrepresentsahorizontal
windfromsouthtonorth(lefttoright)andbluefromnorthtosouth.Theu fieldissuper-
ei
imposed(thinblacklines,K).TheC-130flighttrackisshowninred.Airmassboundariesare
denotedwithfrontalsymbols.
ascended to its position on the EFL cross section. For airmass(Fig.4b),aresultofitshistoryofascent,aswell
example,trajectories IandbinFig.7cascended6and as the cold temperatures in the upper troposphere. As
4.7km,respectively,fromjustwestofBajaCaliforniato will bediscussed later, this wasimportant to both gen-
theEFLcrosssection.SomeBajatrajectoriesloopnear eratingcellformationandtheformationofprecipitation
the New Mexico–Texas border. The reason for the particlesinthecloud-topregionofthestorm.
looping behavior is discussed below. Some trajectories Therewereseveraltrajectories(denotedbyhexagons
arriving in the upper troposphere on the EFL cross andrectanglesinFigs.5aand6)thatdidnotconformto
section originated at higher altitudes and underwent the three primary groups. The endpoints of these tra-
onlyasmallamountofascent.Alloftheseweresimilar jectories in all cases were very close to an airmass
to trajectory o, progressing southward from California boundary (Fig. 5a). The starting points of these trajec-
and Nevada and then northeastward to the EFL cross torieswereingeographiclocationslocatedbetweenthe
section.TheBajaairapproachedtheEFLcrosssection starting points of air arriving on the EFL cross section
from the southwest, effectively capping and secluding fromairmassesoneithersideoftheboundary(Fig.6).
themoistGulfaironitsnorthandsouthsides(seealso The evolution of trajectories in Fig. 7 can be better
Fig.2).AlthoughtheBajaairwasdrierthantheGulfair understoodbyconsideringtheirpositionsinthecontext
mass(Fig.4a),therelativehumiditywithrespecttoice of evolving tropospheric flow patterns. Figure 8 shows
still exceeded saturation within large parts of the Baja aseriesoffourpanelsat1200UTC28January(Fig.8a),