Three-Dimensional Airflow Through Fronts and MidlaXtude Cyclones

Three-DimensionalAirflowThrough
FrontsandMidla8tudeCyclones
ImportanceofAirFlows
•  Greatinsightsintocyclonestructuresand
evolu8oncanbederivedfromunderstanding
theairflowsinmidla8tudesystems.
•  Greatadvanceshavebeenpossibleduringthe
pastseveraldecadesusingmodeloutput.
•  Airflowsandtrajectoriesprovideamore
fundamentalunderstandingthantradi8onal
(frontal)approaches.(Notallkeystructures
areassociatedwithfronts!)
SomeHistory
1800’s
•  ThermalTheory
conceptualmodel
wasdominantin
the1830sandfor
severalsubsequent
decades.
•  Warmcorewith
hurricane-like
circula8on
Low
Espy 1831
MajorDebatesonCycloneAirflows
DuringtheMid-1800s
Espy
Redfield
Loomis
Loomis(1841):FirstAirFlow
Schema8cOverColdFront
By1860stheideaoftwomainairflows
(warmandcold)wasbecomingaccepted
cold
Fitz-Roy
1863
warm
Bythebeginningofthe20thcenturytheidea
ofthreemainairflowswasbeingsuggested.
TheNorwegianCycloneModel
(Bjerknes1918andlater)wasthe
FirsttoConnecttheConceptof
Three-DimensionAirflowswiththe
CloudsandTemperature
StructuresofMidla8tudeFronts
andCyclones
•  Ahugeadvance,butaswewillseeithadits
deficiencies
NorwegianCycloneModelConcept
ofAirFlowsinCyclones
MissingKeyIngredients
•  Drydescendingairstreamsinthemidtoupper
troposphere.
•  Forward-8l8ngfrontalstructures
•  Rela8onshipsofupperlevelshortwave
troughsandridgeswithlowertropospheric
structures.
•  Andmore…
1930s-1950s
•  Theavailabilityof
radiosondedata
paintedarevised
picturesofthreedimensional
airflowsand
structures.
PalmenandNewton(1969)
1950s-1980s
•  Manyofthesestudiesusedrela8veflow
isentropicanalysis---assumingsystemisinsteady
stateanddisplayedflowrela8vetothesystemto
giveapictureoftrajectoriesandver8calmo8ons.
•  Airtrajectoriesfollowthetaorthetaesurfaces
dependingwhetherairparcelsareunsaturated
orsaturated.
•  EliassenandKleinschmidt57,Browningand
Harrold69,Harold73,Carlson80,Browning86,
Youngetal.,87,Browning90
ConveyorBelts
•  Manyofthesestudiesdescribedthemajor
airflowsincyclonesasoccurringinalimited
numberofdiscreteairstreamsorconveyor
belts.
The
Conveyor
BeltModel
ofCyclone
Airflows
(Carlson,
1980)
ClearerVersion!
ThreeMainAirstreamsor
“ConveyorBelts”
•  Warmconveyorbelt(WCB)
–  associatedwithmostofcloudsandprecipita8on
incyclones.
–  beginsatlowlevelswithinthesouthernpartof
thewarmsectorandclimbsan8cyclonicallyabove
thewarmfront.
•  Coldconveyorbelt(CCB)
–  Originatesincold,low-levelan8cyclonicflowtothe
northeastofthecycloneandmoveswestward
(rela8vetotheeastward-movingcyclone)northof
thewarmfront.
–  Undercutsthewarmconveyorbelt(WCBmovesover
theCCB)
–  Twoideaswhathappensnext:
•  Carlson(1980):Coldconveyorbeltthenrisesandemerges
beneaththewesternedgeoftheWCB(producingthe
westernextensionofthecommahead)andthenascends
an8cyclonicallytomergewiththeWCB.
•  Browning(1990):partoftheCCBdescendscyclonically
aroundthelowcentertoaposi8onbehindthecoldfront.
•  DryAirstreamordryintrusion
–  Descendscyclonicallyfromtheuppertroposphere
orlowerstratosphereintothelowertroposphere
andthenascendoverthecyclone
–  Ofenadvancesoverthewarmsectorofthe
cyclone
–  ThewarmsectorisofenNOTaregionofuniform
warm,moistair!
AirflowandConveyor-BeltStudiesHaveSuggestedStructures
NotDescribedintheNorwegianCycloneModel
Splitand
Upper
“Cold”
Front
(Browning
andMonk
1982)
• 
• 
• 
Forward-8l8ng
Upperfrontis
moreofamoisture
thantemperature
front
Leadstopoten8al
instability
Split“Cold”Front
•  Ofenseethisonsatellitepictures,witha
separa8onbetweensurfacefrontandmiddle/
upperclouds.
Terminology:Anafrontversus
Katafront
•  Anafront:backwardleaning.Sinkingoncold
sideandrisingmo8ononwarmside.
warm
cold
•  Katafront:
descenton
bothsidesof
coldfront
(generally
stronger
descenton
warmside).
Notmuch
precipita8on
withfront
warm
cold
StrengthsandWeaknessesofthe
ConveyorBeltModel
•  Strengths
–  Ifyouignorethedetails,onecanofeniden8fy
threemainbroadairstreamsincyclonesandfronts
–  Getsusawayfromthinkingthatalltheweather
ac8onisrelatedtofrontalboundaries.Notonly
ver8calmo8onisdirectlyrelatedtofronts.
•  Weaknessess
•  Itiscanbeagreatsimplifica8ontoconsideronlythree
airstreams
•  Thereareallkindsofintermediarytrajectories
1980s-now:TheModelRevolu8on
•  Realis8cmodelsimula8onathighresolu8on
allowsthecrea8onofthree-dimensional
trajectories.
•  Moderngraphicspromotesvisualiza8on—a
majorchallenge.
•  Anearlyexample:ThePresident’sDayStorm
of1993:
hjp://www.atmos.washington.edu/
academic/videos/PresidentsDayStorm.html
TrajectoriesforaRela8vely
“Classical”CaseoverNorth
America:December14-16,1987
(MassandShultz,1993)
Realis8cMM5Simula8on
Model-Based
Trajectories
Dry
Moist
CanWeUseTrajectoriestoUnderstand
WhyPrecipita8onLeadstheColdFront?
TheOceanRangerStorm(1982)