What's interestingand diflerentabout
Australianmeteorology?*
Peter G. Baines
President,Australian Meteorologicaland OceanographicSociety
CSIRO Division of AtmosphericResearch,Aspendale,Australia
(Manuscript received February 1990)
Introduction
This article is an attempt to summarisethe progressthat has beenmade in the past fifteenyears
or so in the understandingof Australia'sweather.
It is necessarilya personalselection,but I have
tried to be comprehensiveand to cover a long list
of topics briefly, but I hope not too superficially.
Phenomenawhich occur on the hemisphericscale
have been omitted in order to concentrateon
those confined to the Australian region. I have
alsoomitted recentwork in the tropics associated
wirh the AMEX, EMEX and STEP etc. experiments,sincemost of the publishedresultsare descriptiveand interpretationof the observationsis
still in progress,and it would not be possibleto do
justice to this work at this stage.The main accent
hereis on a dynamicalexplanationfor the various
phenomenadescribed.Most of the work already
existsin publishedform, and I must apologisein
advanceto anyonewhosework hasbeenomitted,
misrepresentedor inadequatelyreferenced.Some
outstandingunsolvedproblemsare indicated.
I have endeavouredto cater for a fairly broad
(professional and non-professional)group of
readers,but have includedtechnicaldetailswhere
I thought them interesting and important.
Large-scaleflow patterns
FiguresI and 2 show in 'cartoon' form the nature
of the atmospheric flows in July and January respectively.It is not possible to provide precise
meaningsto the arrows which dominate the diagrams,but the drawingsdo show,in a schematic
manner, the differencebetweenthe summer and
winter flow patterns. Both show easterlytrade
winds in tropical latitudes and westediesto the
* This article is adapted from the President'sAddressto the
Australian Meteorological and OceanographicSocietyon 23
N o v e m b e r1 9 8 9 .
south of the continent. A significant difference
betweenthe two showsin the latitude of the mean
high pressurebelt and the associated
synopticdisturbances,which are much further south in summer. These synoptic disturbancesdominate the
flow pattern on a day-to-daybasis,and the arrows
are(presumably)intendedto representa resultant
of them. The summer pattern is, of course,also
affectedby the northernmonsoon,and by greater
continentalheatingwhich is discussedbelow.
Fig. I
A schematicinterpretationof the characterof the
atmospheric flow in the Australian region in
rces,Third
July. From lrlas of Australian Resou
Series,Vol. 4, 1986,Division of National Mapping, Canberra,
e 38:2June
Fig.2
As for Fig. 1, but for January.
f iJli'-:il;
Fig.3
Analysed synoptic charts showing - a typical
example of blocking in the Australian region
(a) MSL pressureat 0000 UTC on 14 March
f979. (b) 500 hPa height (dam)for 1200UTC on
13 March 1979. Note the relatively barotropic
nature ofthe flow in the blocking region.
This overall mean flow and synoptic pattern is
establishedby the factorswhich maintain the general circulation of the earth's atmosphere. This
circulation containsseveralunresolvedmysteries
in itself (e.g.Lorenz 1967),but we will passover
them here to look at the regional effectsthat exist
within this overall circulation. We will start at the
larger scales and then, in a general way, move
down the scale to look at smaller phenomena.
Much of the progressin recent years has been in
phenomena,and I havedivthe field of mesoscale
ided them up into three main areas;continental
heating efects, cold fronts, and topographic effects.Before addressingthese,however,we must
considerthe phenomenonof blocking.
Blocking
Synoptic-scateblocking is probably the most significant problem for weather forecasting worldwide becauseof its generalunpredictability,and
the large length scalesand lifetimes of the phehighnomenon.It generallyconsistsof a synoptic
'blocks'
low pair which becomes stationary and
the flow, thereby requiring other synoptic disturbancesto pass around it. In the northern hemispherethe phenomenonoccurson a much larger
scaleand may last for much longer(severalweeks)
than is common in the southern,to the extentthat
it is usually regardedas a hemisphere-wideevent
in the northern hemisphere.In the southernhemi-
and Australia). There does not appearto be any
correlation betweenthe occurrencesof blocking
in thesethree regions.A representativeexample
for the Australian region, where the longitudeof
most common and long-lastingevents is in the
Tasman Sea,is shown in Fig. 3.
The main problems concerningblocking are:
what causesthe block to form in the first place?;
and, why does it persist once it has formed?
Numerical WeatherPrediction (NWP) modelsin
use around the world still have great difflculty in
forecasting the onset of blocking patterns.
However; the proposal by Frederiksen (1982,
1989 and referencestherein)that blocking is due
to an instability processwhich is analogousto
cyclogenesisseemsto be gaining generalacceptance. The consequentproblem of recognising
which {low states are likely to develop growing
disturbancesofthis type has,however,seenlittle
progress and is still difficult for both manual
analystsand NWP models.
The secondproblem of why blocking patterns
can be so long-lasting once they become established has been the subjectofa number oftheo-
Baines:What's interestingand differentabout Australian meteorology?
retical studies in recent years,but again significant progresshas been elusive. A major factor
seemsto be that a dipole structure(with the low
equatorwardofthe high) in a westerlyzonal flow
tends to be a stable flow configuration in fluid
mechanicsin general,in a manner analogousto a
vortex pair or a smokering. A surveyof the state
ofthe subjectis given in Baines(1983).
Hence,althoughthe blockingphenomenonis a
globalproblem, the Australian versionis regional
in characterwith its own peculiarities,and it deservesstudy in its own right. Studiesof specific
casesarefew and havemainly beencarriedout via
numericalmodelling(e.g.Noar 1983).
125
Fig. 4 A synopticexampleof summerheatlowsoverthe
Australian contin€nt in light winds. (a) MSL
pressurefor 0000 UTC on 26 December 1973.
(b) 700 hPa height (dam)for the sametime. Note
the shallowdepthof the surfaceheat lows which
havebeenreplacedby heights at 700 hPa. From
Leslie (1980).
Continental heating effects
The efect of the Australian continent on the
atmosphereis mainly felt in the way in which it
affectsthe heating, becauseofthe marked change
in surfacecharacteristicscomparedwith the surrounding oceans (this may be contrasted with
New Zealand,wheretopographiceffectsare relatively much more important).Heatingresultsin a
numberof atmosphericphenomenawhich we will
examine in a sequencewith decreasingscale.
Heat lows and troughs
When the externally imposed mean winds are
weak,the relativelyhigh albedoof the Australian
land surfaceand the common occurrenceof clear
skiesimply that the air column over the continent
losesmore heatby radiation than it gainsfrom the
combinedeffectsof radiation and surfaceheating
Fig.5
two main centresfor heatlowsarecommonlyseen
- one centredin the northwestof WesternAustralia, and the other in westernQueensland.It is
a combined radiation/dynamical phenomenon
and, with its complex vertical structure,is not as
well understoodas it should be.
When the externally imposed zonal winds are
not so weak,the'heat loweffect'manifestsitselfin
the form of surfacetroughs.The summer zonal
flow consistsof easterliesat low levels and westerlies at upper levels,and as for the heat lows,
therearefrequentlytwo troughsembeddedwithin
the easterlies.Figure 5 showsa 10-yearmean at
0900 for Januarywhich showshow common the
phenomenonis - it showsup in mean flow patterns as well as synoptic ones.Fandry and Leslie
Ten-yearmeanMSL pressurefor 0900 in January. The trough lines are shown dotted. From
Fandry and l*slie (f984).
126
AustralianMeteorologicalMagazine38:2 June 1990
(1984)have made a study oftheseeffectsusinga
twoJayer quasi-geostrophic
model with easterlies
in the lower layer and westerliesin the upper.The
(uniform) motion of the lower layerwasperturbed
by realistic topography,and the heatingwas also
representedby an'equivalent topography,'where
heating : holes, cooling : mountains. The resulting'topography'inthe modelis shownin Fig.
6. This parametrisationis crude and hasyet to be
properly justified, but it seemsto give the right
answer,as shown in the correspondingflow patterns in the lower and upperlayersin Fig. 7 where
the trough lines are reproducedreasonablywell.
Heating causesthe westerntrough(overwhelming
the effect of the local topography),whereasthe
eastern'Cloncurry low' is due to the combined
effectsof the easternrangeand inland heating.
Fig.7
(a) Iower layer flow pattern from the model of
Fandryandlrslie (1984)for the forcingshownin
Fig. 6 and a spatial pattern approximatingAustralia. This pattern shouldbe comparedwith Fig.
5. (b) As for (a) but for the upper layer.
Fig.6 Verticaleast-west
sectionfromthe centrelinein
the two-layerquasi-geostrophic
modelof Fandry
and Leslie (1984)showingthe forcingdue to
topographyand heating(represented
by topography).The horizontalpatternapproximateb
the Australiancontinent.
l'
uppertayer
Wesl
+
lower layer
F
,{(..
\
\
heating topography
These studies indicate that we have a partial
understandingof the effectsof continental heating on the synoptic flow pattern,but that it is still
somewhatsuperficial.
Deep convectivelayers
We now 'zoom in' and look more closely at the
processesgoing on near the surface.Probablythe
most significant atmospheric field experiment
carried out in Australia wasthe'Wangara'(Aboriginal for 'west wind') experiment, which took
place near Hay, NSW. This produced what is
widely recognisedinternationallyas the standard
data set for the midJatitude atmosphericboundary layer, becauseofthe quality and quantity of
the observationsmade.The principal resultswere
describedby Clarke (1970), and its significance
and achievementshavebeensummarisedby Hess
et al. ( I 98 I ). Theseinclude the accuratedetermination of the various functions for the MoninObukhov and Rossbynumber similarity theories
for the atmosphericboundary layer. A similarly
conceived (also principally by R.H. Clarke) but
smaller scale experiment at lower latitudes, the
'Koorin' (eastwind) experiment,carried
out near
Daly Waters,NT, hasextendedthis datasetto the
tropical environment.Here katabaticeffectsdue
to nocturnalcoolingand small surfaceslopesare
more significantthan at higher latitudes(Garratt
I 985).
A large number of articles have been written
about,or haveutilised the datafrom, theseexperiments. I would like to single out one of these
becauseit bringsout somesimpleand fundamental propertiesofthe boundarylayer. Ifthe ambient (geostrophic)wind is not too large, the daytime surface mixed layer may be depicted as
shown in Fig. 8. The depth of the mixed layer
beginsgrowingshortlyaftersunriseand continues
to grow during the day dueto surfaceheating.The
potential temperature,0, and horizontalvelocity,
u, areapproximatelyuniform with heightthrough
the layer. At the top of the layer there may be a
relatively abrupt jump in potential temperature,
A0, and a correspondingchangein velocity,Au.
The most important parameterfor the boundary
layer is the Obukhov length scaleL, definedby
I : -ul(tgHs/pco0)
...1
whereu* is the surfacefriction velocity,k : 0.4I ,
g is the accelerationdue to gravity, H6 is the surfaceheat flux to the atmosphere,p is the density
Baines:What's interesting and different about Australian meteorology?
Fig.8 Schematicof the surfacemixed layer of thickness,d, and swface heat flux, H6, modelledas a
slab with typicalincrementsin potentialtemperature,0, and velocity,u, at the top.
perspective
Fig, 9 A three-dimensional
of the interpretation of the observationsduring the 1970 convectionexpeditionby Webb (1977), showing a
polygonalcellular pattern of convectionwithin
the mixed layer.Heatedfluid nearthe groundin a
Iayer of thickness lL,l flows towards 'thermal
walls', along which it then flows towardsjunctions of these walls. It rises up from the wall
junctionsto nearthe top of the layer in 'thermal
columns'.Within the cellsthe fluid sinks slowly
towardsthe ground,completingthe circulation.
Sloble
and cD is the specificheat at constant pressure.
Manins (1982)has shownthat if L is small (and
negative)the mixed layer growth is governedby
Hs and the condition that ae is effectivelyzero.
'encroachment'
This very simple processis called
- the layer encroacheson the fluid above by
entrainingit, and the rate is controlledby Hs and
the overlying density profile. The model fits the
(famous)day 33 of Wangara(for example)better
than some sophisticatedturbulencemodels.For
larger (negative)L, the layer growth is substan'Froude number dynamics'.
tially governed by
The Froudenumberfor the mixed layeris defined
by
F : au/(^ecd/e)1/2
...2
and Manins has shown,from a discussionof the
energyequationfor the layer as a whole,that the
layershouldgrow in a mannerwhich keepsF < l,
which actsto givea closureconditionfor the equationsgoverninglayergrowth.The resultingsimple
model againgivesmuch better agreementwith the
Wangara data than some much more sophisticated models.This model obviously cannot deproperties
scribedetailedturbulent or small-scale
of the boundary layer, but it doesshow that the
bulk propertiesmay be understoodin relatively
simple terms.
A closerlook at the flow within the daytime
mixed layer revealsa complex structure of convectioncells.Webb (1977)has describedobservations from an experiment near Hay, NSW
(again)in 1970,and Fig. 9, taken from his paper,
shows the convection pattern observed when
Ollt I is large(i.e.winds not too strong).This consists of a polygonal cellular structure with cell
width comparablewith layerdepthd, which drifts
acrossthe countrysideat the mean velocityof the
Ioyer
layer.Within the cells,the fluid sinksslowly into a
regionof depth I L | , closeto the ground, whereit
is heated,and then moves laterally to 'thermal
walls' or fines,againof width lI- | , uto.rgwhich it
risesto a heightof about 200 m, moving horizontally to junctions of thesewalls.At thesejunctions
it risesin thermal columnswhich maintain their
identity to the top ofthe convectinglayer, and on
a hot day over central Australia d may equal
severalkilometres.Figure 10 shows a re-drawn
versionof Fig. 9 by Hess and Spillane(1990),
but including height scales,for the case where
d/lll : 100.
Another phenomenon which seems to occur
independentlyoftheseconvectionpatternsis that
ofdust devils,which seemto occurin the centreof
the cells rather than at the rising columns. Dust
devils differ from tornadoesin that the convection driving them is solelydue to heatingfrom the
ground and doesnot involve cumulusconvection
aloft. The mechanismscausingthem are still very
muchthe subjectof debate.Hesset al. ( 1988)have
shownthat they seemto occur when d/ | L | : 50;
on the other hand, Webb (1964) has arguedthat
the turbulent viscosity must be small compared
with the surface inflow towards the base of a
buoyant upcurrent,and this requireslight winds,
smooth terrain and strong heating.
Magazine38:2June 1990
AustralianMeteorological
128
Fig. l0
Figure 9 redrawnby Hess and Spillane(1990)
includingscales,for the casedllll : 166.
i
r
+ t
-:l\?
FE!o|.|+
hjl
----.1
hasa convolutedshape,asin the SouthAustralian
gulfs (Physick 1976) and the Port Phillip Bay
region(Abbs 1986).
The collision of opposing sea-breezes
on the
westernsideof CapeYork Peninsulagivesriseto
'Morning Glory'
the
of the Gulf of Carpentaria
region,which hasthe characterofan undularbore
propagating on a stable layer (Clarke 1984;
Noonan and Smith 1986, 1987).Undular bores
probablyoccur in other places,but they are more
in northernAustraliabecausethe conspectacular
densationof the moisture in the uplifted stable
layer makesits structurevisible.
)--..\
into
Fig.11 Schematic
of a gravitycurrentadvancing
fluidat rest,If theheadof thecurrenttrayelsat
speed
c,thelowJevelfluidbehindtheheadtravelsat speedU1,with U1>c. This fluid mixes
withtheenvironmental
fluid behindtheheadto
form an upperJevellayerof mixedfluid with
meanvelocity
U2,with 0<U2<c.
Sea-breezes
The diurnal variation ofcontinental heatingis the
causeof sea-breezes,
and they aremore proninent
herethan in most otherplacesbecauseof the magnitude of the heating.Clarke(1955)was the first
to suggestthat they were not just coastaleffects,
but propagatedinland for largedistancesofup to
severalhundred kilometres.As a result of a number of field, laboratory and numerical modelling
studies,they are now reasonablywell understood.
In particular, it is now well-establishedthat they
havethe dynamicalcharacterofa gravity current,
shown schematicallyin Fig. 11, a characterthey
share,in a generalway, with thunderstorm outflows, duststorms,and some cold fronts, as discussedbelow.The noseofa gravity current travels
at a speedc which is approximately
s : k(6Qgd/Q)trz
.. .3
lii l
1l
(e.g.Simpson1987)wherek is a constantof order
unity, d is the height of the nose and a0 is the
difference in potential temperature across the
leadingedge.The cold air far behind the nosetravelsat a fasterspeedto reachthe nose,whereit rises
and mixes with the surroundingfluid. The resulting mixed fluid then forms a layer abovethe cold
air below. Sea-breezes
seem to be strongestin
Western Australia, as visitors to the Freemantle
dockson a summer'safternooncan attest.This is
as might be expectedfrom the magnitude of the
continental-heatingin that region,but they are a
common featureof the afternoonweatherin most
coastalregionsof Australia,and some interesting
interaction effectscan occur where the coastline
Cold fronts
Our primary concernhereis with the effectsof the
Australian land mass on the structure and behaviour of the southernhemispherecold fronts
which approachthe continentfrom the southwest.
In order to discussthis it is appropriateto first
considerthe structureof the southernhemisphere
fronts in their unaffectedstate.Sincethere have
beenvery few observationsofthe fronts far from
land, most of our knowledgeof them is basedon
cloud patternsand theory. We may presumethat
the structureof the cold fronts approachingthe
continent over the SouthernOceanis similar to
that ofthe fronts observedapproachingEuropein
the northern hemisphere.An example of the
theoreticaldescriptionof a representativefrontal
structure,chosenfor its simplicity and southern
hemisphereconfiguration,is shownin Figs 12, I 3
and 14 taken from Reederand Smith (1987).A
two-dimensionalflow state in geostrophicand
thermalwind balancewasintegratedwith a primitive equationmodel for a period of 24 hours.The
initial state(at time t:0) is shown in the frames
marked (a), and consistsof a localisednortherly
jet embeddedwithin a zonalwind which increases
linearly with height.The only variablewhich var-
and differentabout Australian
What's interest
Fig.l2
Isotachs of along-frontvelocity v in a twodimensionalstudyby ReederandSmith (1987):
(a) shows the initial state with uniformly
shearedzonalvelocityu"(z)in thermal wind balance;(b) showsthe flow after 12 hours; and (c)
after 24 hours.Dashedcontoursdenotenegative
values,andthe thick solidline indicatesthe axis
of maximumcyclonic(negative)vorticity.
t29
Fig. 13 As for Fig. 12 but showingpotential temperature 0. The thick solid linesindicatethe axis of
maximumtemperaturegradient.
l )
€
t5
J
C)
l0
j
J
r500
l5
€
3000
c5 0 0
b
l0
q500
)
r500
3000
q500
^10
E
:"
r500
)
300c
x
0
I 500
3000
x
q500
IkmJ
ies in the y (northward)direction is the potential
temperature, 0, which increaseslinearly northward in thermal wind balance with the zonal
shear;thereis no externalheatingor forcing.With
x denoting eastwardsand z upwards, Fig. 12
shows the north-southvelocity, Fig. 13 the eastwestpotential temperaturecross-section,
and Fig.
14 the stream function for the ageostrophicx-z
circulation which is a consequenceof the initial
stateand is described(for example)by quasi-geostrophic theory. ln the quasi-geostrophic
model,
q500
(km)
this ageostrophiccirculation may be regardedas
beingforcedby the divergenceofa vector field, Q
(Hoskinset al. 1978),whereQ is definedby the
rate of changeof v0 following the geostrophic
flow.
o- : &Dvt e' - : + 09o 9
r x" gd rs o*
dn"
...4
Here 0e is a referencevalue of 0, v" is the geostrophic wind and k is a unit veitor directed
upwards,s denotesa coordinatealongthe lines of
constant 0 in a horizontal plane, and n is the
horizontal coordinate perpendicularto them in
theright-handedsense.Q is non-zerointhe vicinity ofthe northerlyjet becauseofthe variation in 0
in the north-southdirection.The vertical velocitv.
AustralianMeteorologicalMagazine38:2 June 1990
130
Fig. 14 As for Fig. 12 but showing the ageostrophic
streamfunctionfor the circulation in the vertical
plane. Thick solid lines indicate the axis of
maximum convergence.
l )
tn
=
J
N
f
c500
l )
a
_10
E
+
N
_
f,
r500
:
3000
q500
3000
q500
t n
j
N
_
f,
0
I 500
x
Ikm)
w, of the ageostrophiccirculation is then given
by
N 2 v i w +# P
:r"O
5
where N is the buoyancy frequency and f the
Coriolisfrequency,and the horizontalcomponent
then follows from continuity. As the changein the
flow pattern after 12 and 24-hour periodsshows,
the ageostrophiccirculation causesthe front to
sharpenup and inducesa southerly flow in the
cold air at low levelsbehind the front but, other
than this, the overall picture is not greatly
changed.We may take this picture to be a representative description of the local behaviour
of a southernhemispherefront on the quasi-geostrophic scale.
For observations,by far the most completedescription of southern hemisphere cold fronts has
beenobtainedby the Cold Fronts ResearchProgramme (CFRP), carried out in November and
on
D e c e m b e r o1f 9 8 1 ,1 9 8 2a n d 1 9 8 4 a n dc e n t r e d
Mt Gambier,SA. Observationsfrom a widely dispersednetwork including pilot balloons,aircraft,
additional wind stations and radiosondes,and
shipsgave data which coveredthe synoptic and
mesoscales
down to about 50 km. The resultson
the synopticscaleare summarisedby the'conceptual model' (denoted CM here) put forward by
Wilson and Stern (1985) and Ryan and Wilson
(1985),which is basedon a distillation of the
observationsof ten fronts (or rather, events)observedin phasesone (1981)and two (1982).Figure 15 shows the CM vertical time-sectionof
potential temperature, 0, which may be interfor a front passpretedas a spatial cross-section,
ing through the site in the afternoon (the stable
layeraheadofthe front is dueto a nocturnalinversion).Note the shallownessof the cold air behind
the front. Figure 16 shows a plan view of three
isentropicflow streamsin the CM relativeto the
moving front; the correspondingpotential temperaturesin Fig. l5 are indicated. At the lowest
level(denotedB), a largeproportion ofthe coldair
subsidingbehind the front is shownasoriginating
aheadof the front and moving south, before it
againmoves north behind it. At the middle level
(denotedA), air which has spentsometime over
the continent to the north rises as it moves
southwardjustaheadofthe frontal line. This fea'conveyorbelt' in northern
ture hasbeentermeda
fronts.
At
higher levels(denotedC),
hemisphere
directhe air ascendsand movesin a southeasterly
tion. This conceptualpicture of the air motion
associatedwith the front may be comparedwith
that obtainedfrom the relativelysimplemodelof
Reederand Smith (1987,1988)describedabove,
Figure 17 showsparticle trajectorieswithin this
modelviewedfrom threedirections.The trajectories which correspondto the threepotentialtemperature levels of tire observationalconceptual
model are indicated, and there is a remarkable
resemblancebetweenthe two.
The similarity betweenthesetwo setsof trajectoriesimplies,firstly,that the'simple' two-dimensional model provides a good descriptionof the
cold fronts observedby the CFRP on the quasigeostrophicor synopticscale,and capturesmuch
ofthe essentialdynamics.Secondly,sincethe 2-D
model is presumedto give a reasonabledescription of a Southern Ocean cold front, continental
heating effects have not changed the overall
synopticfrontal structurevery much. One significant continentaleffectis that the air enteringthe
'conveyorbelt'is significantlydryer than it would
be otherwise,resultingin highercloud or noneat
all in this middle levelregionaheadof the front. A
Baines:What's interest
Fig. t5
:
and diferent about Australian
The vertical section of potential temperaturee
from the 'Conceptual Model' of southeastern
coldfronts, expressedas a time-seriesand based
on observationsfrom the Cold Fronts Research
Programme(from Ryan and Wilson 1985).
Fig. 17 Trajectoriesofair parcelsrelativeto a cold front
simulated by the model of Reeder and Smith
(f987) with propertiesshownin Figs 12, 13 and
14. (a) Three-dimensionalperspective,(b) projection on horizontal plane, (c) project on vertical plane.Trajectories II and III correspondto
295K and (B) in Fig. 16,trajectoryI to 305K and
(A), and hajectory IV to 3l5K and (C). From
Reederand Smith (1988).
500
7 ooo
a
/00
850
I 000
I+12
Fig. 16 Schematicplan view of the isentropic flow relative to an observermovingwith the cold front in
the 'ConceptualModel'. Flow (denotedby the
broadarrows)is shownat three levels,on the (B)
295K, (A) 305K, and (C) 315K surfaces.(From
Wilson and Stern 1985.)
120"E
20 ' s
I40.e
r6 0 ' E
20 ' s
?
3
^
l
I
l
I
E
T
I
I
t
+10OOkm+
r (km)
40"s
40.s
--Q'
160'E
II
+
1 O 0 Ok m +
x (km)
particulady interestingfeatureofthis flow pattern
is that the cold air behind the front does not
appearto come from latitudesfar to the south,as
is implied by simpleair-massboundarymodelsof
fronts. Insteadit comesfrom a regionaheadof the
front which is only slightly further south,if at all.
This implies that on this synopticscalemost of the
air may be regardedasflowing through thefront,
ratherthan moving alongwith it, or catchingit up
from behind as in a gravity current.
Modifications by the Australian land
mass
Frontogenesison the pre-frontal trough
This phenomenon is a common occurrence
between September and March (Hanstrum et al.
1990)and it constitutesa significantforecasting
problem in the southern States.As an example,
Fig. l8(a) showsa common situation(from phase
t32
Fig.18
AustralianMeteorologicalMagazine38:2 June 1990
An exarnpleoffrontogenesis on the pre-frontal
trough. Mean sealevel charts for 22 November
1981.(a) 0000UTC. (b) 1630UTC. The northern end ofthe Southern Ocean front has 'faded
away'and beenreplacedby a new front in the
locationof the hough.
l i0't
Fig.19 Potentialtepperaturecontours(brokenlines)
andanalysed
Q-vectorfieldat 850hPa(arrows)
1981.The Qat 1100UTC on 21 November
vectorfield is obviously
implyinga
convergent,
forcingofverticalmotion(Eqn5),andthecomponents
acrossthe0-contours
implyanincrease
gradient(Eqn6),so
in thepotentialtemperature
that the frontogenesis
appearsto be a quasigeostrophic
process.From Wilson and Stern
(198s).
I of the CFRP) where a cold front is approaching
from the southwestand a heat trough extends
acrossthe continent from the northwest.As the
front moves further east,frontogenesisoccursin
the region of the heat trough, which becomesthe
location ofa new front asshownin Fig. I 8(b),and
the original Southern Ocean front weakensand
disappears.This effectiveand substantialchange
in frontal location can be quite abrupt and take
placein about 24 hours.
The phenomenonseemsto be Iargelyan quasigeostrophicone. From the definition of the Qvectors(Eqn 4) one may readily show that
&
Dt
l v n e l z: 2 Q . v n o
(wherev60 is the horizontalgradientof0) so that
the'rate offrontogenesis'orincreasein the potential temperaturegradientis forcedby the Q component acrossthe o-contours.Figure l9 shows
computedQ-vectorsand 0-contours13 hours before Fig. l8(a) and this indicatesan increasein
lv60l in the Bight region, where the trough is
locatedaheadof the front. In spite of this diagnostic consistency,the dynamicalprocesswhich
causesthis phenomenonis still not clear,although
the processis apparently due to the quasi-geostrophic circulation associatedwith the original
front interactingwith the temperaturecontrastset
up by the continental heating and the relatively
coolerBight water.
Ofthe fronts observedin the CFRP,from which
the above-mentioned'conceptualmodel' was obtained, more than half had formed on the prefrontal troughnearthe Bight,sothat theywerenot
directlyderivedfrom SouthernOceanfronts.The
dynamicalpicture obtainedfrom the CM and the
Reederand Smith model is thereforeapplicableto
fronts in the region irrespectiveof their origin.
--.6
Mesoscalefrontal structure and classification
The abovedescriptionofthe observedcold fronts
from the CFRP wasconfinedto the synopticscale.
The'conceptual model' abstractedfrom the observationson the mesoscaleis shown in Fie. 20
Baines:What's interestingand different about Australianmeteorology?
structureof
Fig.20 Conceptualmodelofthe mesoscale
the frontal transition zonealong the sectionY-X
in Fig. 16.L; and L1denotethe initial and final
surfacelines (see text). A, B and C denote the
isentropicsurfacesmentionedearlier. Encircled
dots refer to northerly winds, encircledcrosses
to southerlywinds. A single squall line (representedby cloud,rain etc.)is shownin the frontal
transition zone where there may be severalin
any particular case. Modified from Ryan and
Wilson (1985).
" I
Frontoltrorsitionzonc
which representsschematicallythe flow on the
sectionY-X in Fig. 16. On this scalethe front is
identifiedwith a'frontal transition zone' marked
by an initial liro, Li, &lrda final line, Lf. The initial
line or leading edge of the frontal zone, Li, is
marked by either the surfacepressureceasingto
fall or by the arrival of a weak changehaving the
The final line, Ln
characteristicsof a sea-breeze.
marks the end of the 'significantweather' in th-e
frontal zone,after which the surfacepressureprogressivelyincreases,In the frontal zone between
thesetwo lines there may be one or more squall
lines(of which only oneis shownin Fig. 20) which
propagaterelative to the frontal zone itself. It is
theseconvectivesystemswithin the frontal zone
which bring the rain and gusts associatedwith
frontal passage,and cooler, calmer conditions
prevail after the final line has passed.Details are
describedin Garratt et al. (1985).
Ryan et al. (1989)have consideredthe budgets
of sensibleand latent heatwithin the frontal zone
and have shown that significant cooling may occur in the lowest3 km due to evaporationofrain
falling from the rising'conveyor belt' air stream.
This coolingmay havethe effectof advancingthe
cold air boundary,and hencethe leadingedgeof
the cool change,by a distanceof order 50 km.
These features add to the complexity of the
dynamicsof the frontal transition zone.
The abovemesoscale
descriptionappliesto the
fronts observedin the CFRP, which was held in
spring and early summer. Garratt (1988) has
examinedthe structureof fronts in late summer
and hasfound them to be generallysomewhatdifferent. He has proposeda classification.offronts
I JJ
into two types,basedon their mesoscalestructure,
as follows.Type I fronts contain multiple squall
linesand havethe structureobservedin the CFRP
and describedabove.They are common in spring
and earlysummer.Type 2 fronts havea singledry
changeline and arecommon in late summer.They
tend to havea gravity currentlike structure,with
cold air behind the front moving towards it (in
someregions).The differencebetweenthesetwo
typesis dueto the increasedcontinentalheatingin
late summerand the consequentlydrier air being
fed into the frontal system.This prevents moist
convection and the consequent formation of
squall lines. This mesoscalestructurealso seems
to be largelyindependentofthe origin ofthe front,
that is, whetherit is a true SouthernOceanfront,
or whether it formed south of the continent on a
pre-frontaltrough, as describedabove.
Mesoscalecoastal effects
Continental heating can affect frontal propagation on the mesoscale.If a front is offshore,the
forcing iri the early
developmentof the sea-breeze
afternooncan causethe front to advanceto join
with the sea-breeze,
and to align itselfto be nearly
parallelwith the coastline(Physick 1988).
Fronts which have a gravity current character
also tend to propagatefaster over the seato the
south of the continent than over land. Figure 2l
showsfour examplesof type 2 cold fronts crossing
southernVictoria and BassStrait. The generally
largerspeedin the coastalregion is obvious.This
is because:(a) the densitycontrastis greaterthere
(warm air offthe continent, cold air offthe sea),
giving greaterspeedaccordingto Eqn 3; (b) frictional drag is lessover the seathan over land; and
(c) increasedelevation over land tends to retard
progress.These effects have been simulated in
mesoscale numerical models (Garratt et al.
l 989).
Topographic effects
Australia does not have very high mountain
ranges,but rnany of the complex and interesting
mountain effects such as rotors and upstream
blockingcan occur with topographyof quite modestheight,in light winds. For wind speedU, topography of height h and buoyancy frequency N
(supposeduniform for simplicity), the relevant
dimensionlessparameteris Nh/U dnd nonlinear
effectsoccur for Nh/U greaterthan about 2 (e.g.
Baines 1987a),which is easily realised in Australian conditions.
The southerlybuster
As cold fronts passacrosssoutheasternAustralia,
if they are oriented approximatelyperpendicular
to the southeasternranges and maintain their
identity, they tend to be deformed into an Sshapedpattern. Figure 22 showsa typical example. The speedof the front increasesto the eastof
,rO
Fig. 2l
AustralianMeteorologicalMagazine38:2 June 1990
.type 2, cold^frontscrossingsouthernVictoria and BassStrait' (a) 8 February1983'
Isochronesoffour observed
is8s. (d) 7 Ma"rch 1986. From Garratt (1988).
(b) 16 February l9;X i;fiii;;"u"v
forcehaslittle effectand the flow alongthe coastis
O.iu"n by local buoyancyforces,giving a gravity
135
Baines:What's interestingand different about Australian
Fig, 22 MSL analysis of an example of a cold front
passing across southeastern Australia and
developinginto a southerly buster in the NSW
coastal region, showing the S-shaped deformation. Analysis by I. Mason, from Baines
(1e80).
Fig. 23 Numerical simulation of a southerly buster on
I Decemberf982 by Howells and Kuo (1988).
(a) Surface wind field and isotherms at 1800
Eastern Daylight Saving Time. (b) MSL pressure at the same time.
40's
_\J._L- - - r - - r *
1:
\
-
\''\
\,
\
\
\ /
\. \
' r t -\, \ \ \
'\-r0'.-lr
\
/
T
)
r\
'\ '\ \
--x
I
/.0'S
Fig. 24 NOAA-6 satellite picture at 0730 Eastern
Daylight Saving Time on 2l February 1985,
showing re-circulating lowJying cloud in the
Melbourne region.
experiments,that this eddy was primarily forced
by flow separationfrom the Baw Baw plateau
region when the upperJevelgeostrophicflow was
from the northeast. tn light winds and stably
stratified conditions such as those which prevail
at night, lowJevel air tends to flow around topographicfeaturesand separates,
causiqgeddiesin
the wake regionin a similar manner to high Reynolds number flow past obstacles.Figure 26
showsan examplefrom a laboratory experiment
of low-levelstably stratifiedflow around a short,
angledridge.The flow is predominantlyhorizon-
AustralianM
Fig.25
-
Surfacewinds at 0500 Eastern StandardTime'
5 February 1976. I*tters denote locations,
figures wind speedsin ms-r. Figure by K.T'
Spillane,from Bainesand Manins (1989).
picture has beenlargely confirmed by a nu-m.erical
model studyby McGregorand Kimura
mesoscale
(1989), although the sea-breezeand katabatic
wind effects from the northern ranges can also
contribute to eddYformation.
L6:qOs
Adelaide hills effects
Nocturnal flows near Adelaide are also strongly
affectedby topography.Figure 27 showsthe evolution with time of the pattern of surface wind
observationsin the Adelaide region on a night
when the upper-level winds were from the
northwest, perpendicular to the range.The,latter
clearlyblocksthe lowlevel flow which is deflected
to the south towards the nearest gap, whilst the
upperJevelgradient flow over the ridge remains
unihanged.the flow pattern is similar to the flow
upstreamof the obstaclein Fig. 26'
Observationson the downstreamside in the
above situation are not available,but they are
when the upperJevelwind is from the opposite
direction so ltrat Adelaideis downstream.Figure
28 showsa vertical sectiondisplayingisentropes
(streamlines)whenthe upstreamwind is-fromthe
'gully winds' (a comsoutheast,on a night when
mon local term for significantdownslopewinds)
wereobservedin Adelaide.Althoughthe dataare
there is evidence(W. Grace'private communication) from surface wind observations that the
locaiion of the jump changesduring the night.
There is evidencethat the flow in the region
the observed undulations, with lighter winds
above. One interesting difference between the
Adelaideand Perth situationsis that the Adelaide
hills are relatively narrow, while the eastedy
winds in Perth come off a very broad plateau;the
hydraulic phenomenain each of thesecasesare
much the same.
Baines:What's interestingand different about Australianmeteorology?
t37
Fig. 27 Mean diurnal surfacewind variation in the Adelaideregion when the upperJevelgradient wind (denotedby the
broadarrow) is from the west-northwestsector,from observationsmadein 1986and 1987.(The city ofAdelaide
occupiesthe regionaheadof the 'gradient'arrow,westof the ranges.)Blockingof the stablystratifiednocturnal
flow by the ranges is evident. From Tepper and Watson (1990).
r38
Australian MeteorologicalMagazine38:2 June 1990
Fig.28 Isentropic analysis of easterly flow over the
Adelaiderangesat 2300 local time (approx.)on
22 February1988,during a'gully winds'event.
The downslopeflow appearsto be supercritical,
with a hydraulicjump overthe Adelaideregion.
Dashed lines denote balloon sondes. dotted
lines denotelocation of aircraft data. Figure
courtesyof Warwick Grace,
800
,
500
I
100
whilst the centreremainsjustseawardof the coast
and very strong local winds result. Leslie et al.
(1987) have made a study of a particular case
using a mesoscalenumerical model and have
found that moist convection,topographyandseasurface fluxes are all important in the development process,but the detailedmechanicsof these
systemsare still obscure.
To sum up, there is quite a long list of meteorological phenomena in the .A,ustralianregion
which are interestingin their own right, someof
which do not seem to be common elsewhere.
Many of theseare as yet poorly understood,and
this list is by no meanscomplete.The whole area
oftropical eventshasbeenvirtually excluded,and
there is also an interestingclassof phenomenain
the upper tropospherethat has not been mentioned at all and has a significant effect on
weather.The field should be interestingand active for a long time to come.
Acknowledgments
with numerThe author is gratefulfor discussions
ous colleagueson the material in this review,and
particularlyto Eric Webb for his carefulreadingof
the manuscript.
Fig, 29 An example of uniformly stratified hydraulic
flow over a ridge, with a transition from
uniform subcritical flow upstream and supercritical flow downstream.From Smith (1985).
l
U
z
(Km)
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AustralianMet
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