What's Interesting and Diflerent About Australian Meteorology?*

What's interestingand diflerentabout Australianmeteorology?*

Peter G. Baines President,Australian Meteorological and OceanographicSociety CSIRO Division of AtmosphericResearch, Aspendale, Australia (Manuscript received February 1990)

Introduction This article is an attempt to summarisethe pro- south of the continent. A significant difference gressthat has beenmade in the past fifteenyears betweenthe two showsin the latitude of the mean or so in the understandingof Australia'sweather. high pressurebelt and the associatedsynoptic dis- It is necessarilya personalselection, but I have turbances,which are much further south in sum- tried to be comprehensiveand to covera long list mer. These synoptic disturbancesdominate the of topics briefly, but I hope not too superficially. flow patternon a day-to-daybasis, and the arrows Phenomenawhich occuron the hemisphericscale are(presumably) intended to representa resultant have been omitted in order to concentrateon of them. The summer pattern is, of course,also those confined to the Australian region. I have affectedby the northernmonsoon, and by greater alsoomitted recentwork in the tropics associated continentalheating which is discussedbelow. wirh the AMEX, EMEX and STEP etc. experiments,since most of the publishedresults are de- scriptiveand 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 dynamicalexplanation for the various phenomenadescribed. Most of the work already existsin publishedform, and I must apologisein Fig. I A schematicinterpretation of the characterof the atmospheric flow in the Australian region in advanceto anyonewhose work hasbeen omitted, July. From lrlas of Australian Resources, Third misrepresentedor inadequatelyreferenced. Some Series,Vol. 4, 1986,Division of National Map- outstandingunsolved problems are indicated. ping, Canberra, I have endeavouredto cater for a fairly broad (professional and non-professional)group of readers,but haveincluded technical details where I thought them interesting and important.

Large-scaleflow patterns 'cartoon' FiguresI and 2 showin form the nature of the atmospheric flows in July and January re- spectively.It is not possibleto provide precise meaningsto the arrows which dominatethe dia- grams,but the drawingsdo show,in a schematic manner,the differencebetween the summerand winter flow patterns. Both show easterlytrade winds in tropical latitudes and westediesto the

* This article is adapted from the President'sAddress to the Australian Meteorological and OceanographicSociety on 23 November1989. e 38:2June

a typical Fig.2 As for Fig. 1, but for January. Fig.3 Analysed synoptic charts showing - example of blocking in the Australian region (a) MSL pressureat 0000 UTC on 14 March f979. (b) 500 hPa height (dam)for 1200UTC on iJli'-:il; 13 March 1979. Note the relatively barotropic f 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 circulationcontains several unresolved mysteries 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 scaleto look at smaller phenomena. Much of the progressin recent years has been in the field of mesoscalephenomena, and I havediv- 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 world- wide becauseof its generalunpredictability, and the large length scalesand lifetimes of the phe- It generallyconsists of a synoptichigh- nomenon. 'blocks' low pair which becomes stationary and the flow, thereby requiring other synoptic disturb- The main problems concerningblocking are: place?; ancesto pass around it. In the northern hemi- what causesthe block to form in the first spherethe phenomenonoccurs on a much larger and, why does it persist once it has formed? scaleand may last for much longer(several weeks) Numerical WeatherPrediction (NWP) modelsin than is common in the southern,to the extentthat use around the world still have great difflculty in patterns. it is usually regardedas a hemisphere-wideevent forecasting the onset of blocking in the northern hemisphere.In the southern hemi- However; the proposal by Frederiksen (1982, 1989and referencestherein) that blocking is due to an instability processwhich is analogousto cyclogenesisseems to be gaining generalaccept- ance. The consequentproblem of recognising which {low states are likely to develop growing and Australia). There doesnot appearto be any disturbancesofthis type has,however, seen little correlation betweenthe occurrencesof blocking progress and is still difficult for both manual in thesethree regions.A representativeexample analystsand NWP models. for the Australian region,where the longitudeof The secondproblem of why blocking patterns most common and long-lastingevents is in the can be so long-lasting once they become estab- Tasman Sea,is shown in Fig. 3. lished has been the subjectofa number oftheo- Baines:What's interestingand differentabout Australianmeteorology? 125 retical studies in recent years,but again signifi- Fig. 4 A synopticexample of summerheat lows over the (a) MSL cant progresshas been elusive.A major factor Australian contin€ntin light winds. pressurefor 0000 UTC on 26 December 1973. to be that a dipole structure(with the low seems (b) 700 hPa height (dam)for the sametime. Note a westerlyzonal flow equatorwardofthe high) in the shallowdepth of the surfaceheat lowswhich tends to be a stable flow configuration in fluid havebeen replaced by heights at 700 hPa. From mechanicsin general,in a manner analogousto a Leslie (1980). vortex pair or a smokering. A surveyof the state ofthe subjectis given in Baines(1983). Hence,although the blockingphenomenon is a globalproblem, the Australianversion is regional in characterwith its own peculiarities,and it de- servesstudy in its own right. Studiesof specific casesare few and havemainly beencarried out via numericalmodelling (e.g. Noar 1983).

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 surfacecharacteristics compared with the sur- rounding oceans (this may be contrasted with New Zealand,where topographic effects are relatively much more important).Heating results in a numberof atmosphericphenomena which we will examinein 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 combinedeffects of radiation and surfaceheating

Fig.5 Ten-yearmean MSL pressurefor 0900 in Jan- uary. The trough lines are shown dotted. From Fandry and l*slie (f984). two main centresfor heatlows are commonly seen - one centredin the northwestof WesternAus- tralia, and the other in westernQueensland. It is a combined radiation/dynamical phenomenon and, with its complexvertical structure,is not as well understoodas it shouldbe. When the externallyimposed zonal winds are not soweak, the'heat loweffect'manifestsitself in the form of surfacetroughs. The summer zonal flow consistsof easterliesat low levelsand wes- terlies at upper levels,and as for the heat lows, thereare frequently two troughsembedded within the easterlies.Figure 5 showsa 10-yearmean at 0900 for Januarywhich showshow common the phenomenonis - it showsup in mean flow patterns as well as synopticones. Fandry and Leslie 126 AustralianMeteorological Magazine 38:2 June 1990

(1984)have made a studyofthese effects using a Fig.7 (a) Iower layer flow pattern from the model of twoJayer quasi-geostrophicmodel with easterlies Fandryand lrslie (1984)for theforcing shown in in the lower layer and westerliesin the upper.The Fig. 6 and a spatial pattern approximatingAus- tralia. pattern (uniform) motion of the lower layerwas perturbed This shouldbe comparedwith Fig. 5. (b) As for (a) but for the upper layer. 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 crudeand hasyet to be properly justified, but it seemsto give the right answer,as shown in the correspondingflow pat- ternsin the lower and upperlayers in Fig. 7 where the trough lines are reproducedreasonably well. Heatingcauses the westerntrough (overwhelming the effect of the local topography),whereas the 'Cloncurry eastern low' is due to the combined effectsof the easternrange and inland heating.

Fig.6 Verticaleast-west section from the centrelinein the two-layerquasi-geostrophic model of Fandry and Leslie (1984)showing the forcingdue to topographyand heating(represented by topography).The horizontalpattern approximateb the Australiancontinent. l' upper tayer Wesl

+ lower layer F ,{(.. - \\ heating topography Daly Waters,NT, hasextended this dataset to the tropical environment.Here katabaticeffects due These studies indicate that we have a partial to nocturnalcooling and small surfaceslopes are understandingof the effectsof continentalheat- more significantthan at higherlatitudes (Garratt ing on the synoptic flow pattern,but that it is still I 985). somewhatsuperficial. A large number of articleshave been written about,or haveutilised the datafrom, theseexper- Deep convectivelayers iments. I would like to single out one of these 'zoom We now in' and look more closelyat the becauseit bringsout somesimple and fundamen- processesgoing on nearthe surface.Probably the tal propertiesofthe boundarylayer. Ifthe ambi- most significant atmospheric field experiment ent (geostrophic)wind is not too large,the day- carried out in Australia wasthe'Wangara'(Abor- time surface mixed layer may be depicted as 'west iginal for wind') experiment, which took shown in Fig. 8. The depth of the mixed layer place near Hay, NSW. This produced what is beginsgrowing shortly after sunrise and continues widely recognisedinternationally as the standard to grow duringthe day dueto surfaceheating. The data set for the midJatitude atmosphericboun- potentialtemperature, 0, and horizontalvelocity, dary layer, becauseofthe quality and quantity of u, areapproximately uniform with heightthrough the observationsmade. The principal resultswere the layer. At the top of the layer there may be a describedby Clarke (1970), and its significance relatively abrupt jump in potential temperature, and achievementshave been summarised by Hess A0, and a correspondingchange in velocity,Au. et al. ( I 98 I ). Theseinclude the accuratedetermi- The most important parameterfor the boundary nation of the various functions for the Monin- layer is the Obukhov length scaleL, definedby Obukhov and Rossbynumber similarity theories I : -ul(tgHs/pco0) for the atmosphericboundary layer. A similarly ...1 conceived (also principally by R.H. Clarke) but whereu* is the surfacefriction velocity,k : 0.4I , smaller scale experiment at lower latitudes, the g is the accelerationdue to gravity, H6 is the sur- 'Koorin' (eastwind) experiment,carried out near faceheat flux to the atmosphere,p is the density Baines:What's interesting and different about Australian meteorology?

Fig.8 Schematicof the surfacemixed layer of thick- Fig, 9 A three-dimensionalperspective of the interpret- ness,d, and swface heat flux, H6, modelledas a ation of the observationsduring the 1970 con- slabwith typicalincrements in potentialtemper- vectionexpedition by Webb (1977), showinga ature,0, and velocity,u, at the top. polygonalcellular pattern of convectionwithin the mixed layer.Heated fluid nearthe groundin a 'thermal Iayer of thickness lL,l flows towards walls', along which it then flows towardsjunc- tions of thesewalls. It rises up from the wall junctionsto nearthe top of the layer in 'thermal columns'.Within the cellsthe fluid sinks slowly towardsthe ground,completing the circulation.

Sloble Ioyer

and cDis the specificheat at constantpressure. Manins (1982)has shown that if L is small(and negative)the mixed layer growth is governedby Hs and the condition that ae is effectivelyzero. 'encroachment' This very simpleprocess is 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 sophisticatedturbulence models. For larger (negative)L, the layer growth is substan- layer.Within the cells,the fluid sinksslowly into a 'Froude tially governedby number dynamics'. regionof depth IL | , closeto the ground, whereit The Froudenumber for the mixed layeris defined is heated,and then moves laterally to 'thermal by walls' or fines,again of width lI- | , uto.rgwhich it risesto a heightof about 200 m, moving horizon- F : au/(^ecd/e)1/2 ...2 tally to junctions of thesewalls. At thesejunctions and Manins has shown,from a discussionof the it risesin thermal columnswhich maintain their energyequation for the layeras a whole,that the identity to the top ofthe convectinglayer, and on layershould grow in a mannerwhich keepsF < l, a hot day over central Australia d may equal which actsto givea closurecondition for the equa- severalkilometres. Figure 10 shows a re-drawn tionsgoverning layer growth. The resultingsimple versionof Fig. 9 by Hessand Spillane(1990), model againgives much better agreementwith the but including height scales,for the casewhere Wangara data than some much more sophisti- d/lll : 100. cated models.This model obviously cannot de- Another phenomenon which seems to occur scribedetailed turbulent or small-scaleproperties independentlyofthese convection patterns is that of the boundary layer, but it doesshow that the ofdust devils,which seemto occurin the centreof bulk propertiesmay be understoodin relatively the cells rather than at the rising columns.Dust simple terms. devils differ from tornadoesin that the convec- A closerlook at the flow within the daytime tion driving them is solelydue to heatingfrom the mixed layer revealsa complexstructure of con- groundand doesnot involve cumulusconvection vectioncells. Webb (1977)has describedobser- aloft. The mechanismscausing them are still very vations from an experiment near Hay, NSW muchthe subjectof debate.Hess et al. ( 1988)have (again)in 1970,and Fig. 9, takenfrom his paper, shownthat they seemto occur when d/ | L | : 50; shows the convection pattern observed when on the other hand, Webb (1964) has arguedthat Ollt I is large(i.e. winds not too strong).This con- the turbulent viscositymust be small compared sists of a polygonal cellular structure with cell with the surface inflow towards the base of a width comparablewith layerdepth d, which drifts buoyantupcurrent, and this requireslight winds, acrossthe countrysideat the meanvelocity of the smooth terrain and strongheating. 128 AustralianMeteorological Magazine 38:2 June 1990

Fig. l0 Figure9 redrawnby Hess and Spillane(1990) hasa convolutedshape, as in the SouthAustralian includingscales, for the casedllll : 166. gulfs (Physick 1976) and the Port Phillip Bay region(Abbs 1986). The collision of opposing sea-breezeson the westernside of CapeYork Peninsulagives rise to ir 'Morning +t the Glory' of the Gulf of Carpentaria region,which hasthe characterofan undularbore propagating on a stable layer (Clarke 1984; -:l\? Noonanand Smith 1986, 1987).Undular bores probablyoccur in places, more hjl ----.1 other but they are FE!o|.|+ spectacularin northernAustralia because the con- densationof the moisture in the uplifted stable layer makesits structurevisible. )--..\

Fig.11 Schematicof a gravitycurrent advancing into fluidat rest,If thehead of thecurrent trayels at speedc, the lowJevel fluid behind the head trav- elsat speedU1, with U1>c. This fluidmixes withthe environmental fluid behindthe head to form an upperJevellayer of mixedfluid with meanvelocity U2, with 0

Sea-breezes The diurnal variation ofcontinental heatingis the causeof sea-breezes,and they aremore proninent herethan in most otherplaces because of the magnitude of the heating.Clarke (1955) was the first to suggestthat they were not just coastaleffects, but propagatedinland for largedistances ofup to severalhundred kilometres.As a result of a num- Cold fronts ber of field, laboratory and numerical modelling studies,they arenow reasonablywell understood. Our primary concernhere is with the effectsof the In particular, it is now well-establishedthat they Australian land mass on the structure and be- havethe dynamicalcharacter ofa gravity current, haviour of the southernhemisphere cold fronts shown schematicallyin Fig. 11, a characterthey which approachthe continentfrom the southwest. share,in a generalway, with thunderstorm out- In order to discussthis it is appropriateto first flows, duststorms,and some cold fronts, as dis- considerthe structureof the southernhemisphere cussedbelow. The noseofa gravity currenttravels fronts in their unaffectedstate. Since there have at a speedc which is approximately beenvery few observationsofthe fronts far from land, most of our knowledgeof them is basedon s : k(6Qgd/Q)trz .. .3 cloud patternsand theory. We may presumethat (e.g.Simpson 1987) where k is a constantof order the structureof the cold fronts approachingthe unity, d is the height of the nose and a0 is the continent over the SouthernOcean is similar to difference in potential temperature across the that ofthe frontsobserved approaching Europe in leadingedge. The cold air far behindthe nosetrav- the northern hemisphere.An example of the elsat a fasterspeed to reachthe nose,where it rises theoreticaldescription of a representativefrontal and mixes with the surroundingfluid. The result- structure,chosen for its simplicity and southern ing mixed fluid then forms a layer abovethe cold hemisphereconfiguration, is shownin Figs 12, I 3 air below. Sea-breezesseem to be strongestin and 14 takenfrom Reederand Smith (1987).A Western Australia, as visitors to the Freemantle two-dimensionalflow state in geostrophicand dockson a summer'safternoon can attest.This is thermalwind balancewas integrated with a prim- as might be expectedfrom the magnitude of the itive equationmodel for a period of 24 hours.The continental-heatingin that region,but they are a initial state(at time t:0) is shown in the frames li common featureof the afternoonweather in most marked (a), and consistsof a localisednortherly il 1l coastalregions of Australia,and someinteresting jet embeddedwithin a zonalwind which increases interaction effectscan occur where the coastline linearly with height.The only variablewhich var- What's interest and differentabout Australian t29

Fig.l2 Isotachs of along-frontvelocity v in a two- Fig. 13 As for Fig. 12 but showingpotential tempera- dimensionalstudy by Reederand Smith (1987): ture0. The thick solid linesindicate the axis of (a) shows the initial state with uniformly maximumtemperature gradient. shearedzonal velocity u"(z) in thermal wind balance;(b) showsthe flow after 12 hours; and (c) l) after 24 hours.Dashed contours denote negative values,and the thick solidline indicatesthe axis of maximumcyclonic (negative) vorticity. € t5 J C)

l0 j

r500 3000 c500

l5 b

l0 €

) q500

r500 3000 q500

^10 E :" ) r500 300c q500 x (km)

this ageostrophiccirculation may be regarded q500 as 0 I 500 3000 beingforced by the divergenceofa vectorfield, Q x IkmJ (Hoskinset al. 1978),where Q is definedby the rate of changeof v0 following the geostrophic flow. ies in the y (northward)direction is the potential temperature, 0, which increaseslinearly north- - '- " ...4 o:&ve:+99rxgr*Dt 0o dso ward in thermal wind balance with the zonal dn" shear;there is no externalheating or forcing.With Here 0e is a referencevalue of 0, v" is the geos- x denoting eastwardsand z upwards, Fig. 12 trophic wind and k is a unit veitor directed showsthe north-southvelocity, Fig. 13 the east- upwards,s denotesa coordinatealong the linesof westpotential temperaturecross-section, and Fig. constant 0 in a horizontal plane, and n is the 14 the stream function for the ageostrophicx-z horizontal coordinate perpendicularto them in circulation which is a consequenceof the initial theright-handedsense. Q is non-zerointhe vicin- stateand is described(for example)by quasi-geo- ity ofthe northerlyjet becauseofthe variation in 0 strophic theory. ln the quasi-geostrophicmodel, in the north-southdirection. The verticalvelocitv. 130 AustralianMeteorological Magazine 38:2 June 1990

Fig. 14 As for Fig. 12 but showing the ageostrophic of a southernhemisphere front on the quasi-geo- streamfunction for the circulation in the vertical strophicscale. plane. Thick solid lines indicate the axis of For observations,by far the most completede- maximum convergence. scription of southern hemisphere cold fronts has beenobtained by the Cold Fronts ResearchPro- l) gramme (CFRP), carried out in November and Decemberof1981, 1982 and 1984andcentred on Mt Gambier,SA. Observationsfrom a widelydis- tn persednetwork including pilot balloons,aircraft, = additional wind stations and radiosondes,and J shipsgave data which coveredthe synopticand

N- mesoscalesdown to about 50 km. The resultson f the synopticscale are summarisedby the'conceptual model' (denoted CM here) put forward by Wilson and Stern(1985) and Ryan and Wilson (1985),which is basedon a distillationof the c500 observationsof ten fronts (or rather,events) observedin phasesone (1981)and two (1982).Fig- ure 15 shows the CM vertical time-sectionof l) potential temperature, 0, which may be inter- a pretedas a spatialcross-section, for a front passing through the site in the afternoon (the stable _10 layerahead ofthe front is dueto a nocturnalinver- E sion).Note the shallownessof the cold air behind + the front. Figure 16 showsa plan view of three N_ isentropicflow streamsin the CM relativeto the f, moving front; the correspondingpotential tem- peraturesin Fig. l5 are indicated.At the lowest level(denoted B), a largeproportion ofthe coldair q500 subsidingbehind the front is shownas originating r 500 3000 aheadof the front and moving south, before it againmoves north behind it. At the middle level (denotedA), air which has spentsome time over the continent to the north rises as it moves southwardjustahead ofthe frontal line. This fea- 'conveyor tn ture hasbeen termed a belt' in northern : hemispherefronts. At higher levels(denoted C), j the air ascendsand movesin a southeasterlydirec- N_ tion. This conceptualpicture of the air motion f, associatedwith the front may be comparedwith that obtainedfrom the relativelysimple model of Reederand Smith (1987,1988) described above, Figure 17 showsparticle trajectorieswithin this 0 I 500 3000 q500 modelviewed from threedirections. The trajecto- x Ikm) ries which correspondto the threepotential temperature levels of tire observationalconceptual model are indicated, and there is a remarkable w, of the ageostrophiccirculation is then given resemblancebetween the two. by The similarity betweenthese two setsof trajec- toriesimplies, firstly, that the'simple' two-dimen- N2viw+P 5 sional model provides a good descriptionof the #:r"O cold fronts observedby the CFRP on the quasi- where N is the buoyancy frequency and f the geostrophicor synopticscale, and capturesmuch Coriolisfrequency, and thehorizontal component ofthe essentialdynamics. Secondly, since the 2-D then followsfrom continuity. As the changein the model is presumedto give a reasonabledescrip- flow pattern after 12 and 24-hour periodsshows, tion of a Southern Ocean cold front, continental the ageostrophiccirculation causesthe front to heating effects have not changed the overall sharpenup and inducesa southerly flow in the synopticfrontal structurevery much. One signifi- cold air at low levelsbehind the front but, other cant continentaleffect is that the air enteringthe 'conveyor than this, the overall picture is not greatly belt'is significantlydryer than it would changed.We may take this picture to be a rep- be otherwise,resulting in highercloud or noneat resentative description of the local behaviour all in this middle levelregion ahead of the front. A Baines:What's interest and diferent about Australian

Fig. t5 The vertical section of potential temperaturee Fig. 17 Trajectoriesofair parcelsrelative to a cold front from the 'Conceptual Model' of southeastern simulated by the model of Reeder and Smith coldfronts, expressedas a time-seriesand based (f987) with propertiesshown in Figs 12, 13 and on observationsfrom the Cold Fronts Research 14. (a) Three-dimensionalperspective, (b) pro- Programme(from Ryan and Wilson 1985). jection on horizontal plane, (c) project on vertical plane.Trajectories II and III correspondto 295K and (B) in Fig. 16,trajectory I 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 rela- tive to an observermoving with 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 I40.e r60'E 3l 20's 20's ^lI I E T I I t +10OOkm+

r (km) 40"s 40.s

--Q'

160'E

+ 1O0Okm+ x (km) particuladyinteresting feature ofthis flow pattern is that the cold air behind the front does not Modifications by the Australian land appearto comefrom latitudesfar to the south,as mass is implied by simpleair-mass boundary models of fronts.Instead it comesfrom a regionahead of the Frontogenesison the pre-frontal trough front which is only slightlyfurther south,if at all. This phenomenon is a common occurrence This implies that on this synopticscale most of the between September and March (Hanstrum et al. air may be regardedas flowing through thefront, 1990)and it constitutesa significantforecasting ratherthan moving alongwith it, or catchingit up problem in the southern States.As an example, from behind as in a gravity current. Fig. l8(a) showsa common situation(from phase t32 AustralianMeteorological Magazine 38:2 June 1990

Fig.18 An exarnpleoffrontogenesis on the pre-frontal (wherev60 is the horizontalgradient of0) so that trough. Mean sealevel charts for 22 November the'rate offrontogenesis'orincrease in the poten- 1981.(a) 0000UTC. (b) 1630UTC. The north- tial temperaturegradient is forcedby the com- ern Ocean front has 'faded Q end ofthe Southern ponent acrossthe o-contours.Figure l9 shows away'and beenreplaced by a new front in the locationof the hough. computedQ-vectors and 0-contours13 hoursbe- fore Fig. l8(a) and this indicatesan increasein lv60l in the Bight region, where the trough is locatedahead of the front. In spite of this diag- nostic consistency,the dynamicalprocess which causesthis phenomenonis still not clear,although the processis apparently due to the quasi-geostrophic circulation associatedwith the original front interactingwith the temperaturecontrast set up by the continentalheating and the relatively coolerBight water. Ofthe frontsobserved in the CFRP,from which 'conceptual the above-mentioned model' wasob- tained, more than half had formed on the pre- frontaltrough near the Bight,so that theywere not directlyderived from SouthernOcean fronts. The dynamicalpicture obtainedfrom the CM and the Reederand Smith modelis thereforeapplicable to fronts in the region irrespectiveof their origin.

l i0't Fig.19 Potentialtepperature contours (broken lines) andanalysed Q-vector field at 850hPa (arrows) at 1100UTC on 21 November1981. The Q- vectorfield is obviouslyconvergent, implying a forcingofvertical motion (Eqn 5), and the com- ponentsacross the 0-contours imply an increase in thepotential temperature gradient (Eqn 6), so that the frontogenesisappears to be a quasi- geostrophicprocess. From Wilson and Stern (198s).

I of the CFRP) wherea cold front is approaching from the southwestand a heat trough extends acrossthe continent from the northwest.As the front movesfurther east,frontogenesis occurs in the region of the heat trough, which becomesthe locationofa new front asshown in 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 phenomenonseems to be Iargelyan quasi- Mesoscalefrontal geostrophicone. From the definition of the Q- structure and classification vectors(Eqn 4) one may readily show that The abovedescription ofthe observedcold fronts from the CFRP wasconfined to the synopticscale. & lvnelz: 2Q.vno --.6 The'conceptual model' abstractedfrom the ob- Dt servationson the mesoscaleis shown in Fie. 20 Baines:What's interestingand differentabout Australianmeteorology? I JJ

Fig.20 Conceptualmodel ofthe mesoscalestructure of into two types,based on their mesoscalestructure, the frontal transition zonealong the sectionY-X as follows.Type I fronts contain multiple squall in Fig. 16.L; and L1denote the initial and final linesand have the structureobserved in the CFRP surfacelines (see text). A, B and C denote the and describedabove. They are common in spring isentropicsurfaces mentioned earlier. Encircled dots refer to northerly winds, encircledcrosses and earlysummer. Type 2 fronts havea singledry to southerlywinds. A single squall line (repre- changeline and arecommon in latesummer. They sentedby cloud,rain etc.)is shownin the frontal tend to havea gravity currentlike structure,with transition zone where there may be severalin cold air behind the front moving towards it (in any particular case. Modified from Ryan and someregions). The differencebetween these two Wilson (1985). typesis dueto the increasedcontinental heating in late summerand the consequentlydrier air being fed into the frontal system.This prevents moist convection and the consequent formation of squall lines.This mesoscalestructure also seems to belargely independent ofthe origin ofthe front, that is, whetherit is a true SouthernOcean front, or whetherit formed south of the continent on a pre-frontaltrough, as describedabove. Mesoscalecoastal effects Continental heating can affect frontal propaga- is I Frontoltrorsition zonc tion on the mesoscale.If a front offshore,the " developmentof the sea-breezeforcing iri the early 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 which representsschematically the flow on the also tend to propagatefaster over the seato the sectionY-X in Fig. 16. On this scalethe front is south of the continent than over land. Figure 2l identifiedwith a'frontal transition zone' marked showsfour examplesof type 2 cold fronts crossing by an initial liro, Li, &lrda final line, Lf. The initial southernVictoria and BassStrait. The generally line or leading edge of the frontal zone, Li, is largerspeed in the coastalregion is obvious.This marked by either the surfacepressure ceasing to is because:(a) the densitycontrast is greaterthere fall or by the arrival of a weak changehaving the (warm air offthe continent, cold air offthe sea), characteristicsof a sea-breeze.The final line, Ln 'significant giving greaterspeed according to Eqn 3; (b) fric- marks the end of the weather'in th-e tional dragis lessover the seathan over land; and frontal zone,after which the surfacepressure pro- (c) increasedelevation over land tends to retard gressivelyincreases, In the frontal zone between progress.These effects have been simulated in thesetwo lines there may be one or more squall mesoscale numerical models (Garratt et al. lines(of which only oneis shownin Fig. 20) which l 989). propagaterelative to the frontal zone itself. It is theseconvective systems within the frontal zone which bring the rain and gusts associatedwith Topographic effects frontal passage,and cooler, calmer conditions Australia does not have very high mountain prevail passed. after the final line has Details are ranges,but rnany of the complex and interesting describedin Garratt et al. (1985). mountain effects such as rotors and upstream Ryan et al. (1989)have consideredthe budgets blockingcan occurwith topographyof quite mod- of sensibleand latent heatwithin the frontal zone estheight, in light winds. For wind speedU, topo- and have shown that significant cooling may oc- graphy of height h and buoyancy frequency N cur in the lowest3 km due to evaporationofrain (supposeduniform for simplicity), the relevant falling from the rising'conveyor belt' air stream. dimensionlessparameter is Nh/U dnd nonlinear This coolingmay havethe effectof advancingthe effectsoccur for Nh/U greaterthan about 2 (e.g. cold air boundary,and hencethe leadingedge of Baines 1987a),which is easily realised in Aus- the cool change,by a distanceof order 50 km. tralian conditions. These features add to the complexity of the dynamicsof the frontal transition zone. The southerlybuster The abovemesoscale description applies to the As cold fronts passacross southeastern Australia, fronts observedin the CFRP, which was held in if they are oriented approximatelyperpendicular spring and early summer. Garratt (1988) has to the southeasternranges and maintain their examinedthe structureof fronts in late summer identity, they tend to be deformed into an S- and hasfound them to be generallysomewhat dif- shapedpattern. Figure 22 showsa typical exam- ferent.He hasproposed a classification.offronts ple. The speedof the front increasesto the eastof AustralianMeteorological Magazine 38:2 June 1990 ,rO

.type Victoria and BassStrait' (a) 8 February1983' Fig. 2l Isochronesoffour observed 2, cold^frontscrossing southern From Garratt (1988). (b) 16 February l9;X i;fiii;;"u"v is8s. (d) 7 Ma"rch 1986.

forcehas little effectand the flow alongthe coastis O.iu"n by local buoyancyforces, giving a gravity Baines:What's interestingand differentabout Australian 135

Fig, 22 MSL analysis of an example of a cold front Fig. 23 Numerical simulation of a southerly buster on passing across southeastern Australia and I Decemberf982 by Howells and Kuo (1988). developinginto a southerly buster in the NSW (a) Surface wind field and isotherms at 1800 coastal region, showing the S-shaped defor- Eastern Daylight Saving Time. (b) MSL pres- mation. Analysis by I. Mason, from Baines sure at the same time. (1e80).

40's

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experiments,that this eddy was primarily forced by flow separationfrom the Baw Baw plateau regionwhen the upperJevelgeostrophic flow was from the northeast. tn light winds and stably stratifiedconditions such as those which prevail at night, lowJevel air tends to flow around topo- graphicfeatures and separates,causiqg eddies in the wake regionin a similar manner to high Rey- nolds number flow past obstacles.Figure 26 showsan examplefrom a laboratoryexperiment 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 mesoscalemodel study by McGregorand Kimura (1989), although the sea-breezeand katabatic wind effects from the northern ranges can also contributeto eddYformation. Adelaide hills effects Nocturnal flows near Adelaide are also strongly L6:qOs affectedby topography.Figure 27 showsthe evol- ution 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 clearlyblocks the lowlevel flow which is deflected to the south towards the nearest gap, whilst the upperJevelgradient flow over the ridge remains unihanged.the flow patternis 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 sectiondisplaying isentropes (streamlines)when the upstreamwind is-fromthe 'gully southeast,on a night when winds' (a common local term for significantdownslope winds) wereobserved in Adelaide.Although the dataare

thereis evidence(W. Grace'private communica- tion) 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 differencebetween the Adelaideand Perthsituations is that the Adelaide hills are relatively narrow, while the eastedy winds in Perth come off a very broad plateau;the hydraulic phenomenain each of thesecases are 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 observationsmade in 1986and 1987.(The city ofAdelaide occupiesthe regionahead of the 'gradient'arrow, west of the ranges.)Blocking of the stablystratified nocturnal flow by the ranges is evident. From Tepper and Watson (1990). r38 AustralianMeteorological Magazine 38:2 June 1990

Fig.28 Isentropic analysisof easterly flow over the whilst the centreremainsjust seaward of the coast Adelaideranges at 2300 local time (approx.)on and very strong local winds result. Leslie et al. 22 February1988, during a'gully winds'event. (1987) have made a study of a particular case The downslopeflow appearsto be supercritical, using a mesoscalenumerical model and have with a hydraulicjump overthe Adelaideregion. found that moist convection,topographyand sea- Dashed lines denote balloon sondes. dotted lines denotelocation of aircraft data. Figure surface fluxes are all important in the develop- courtesyof Warwick Grace, ment process,but the detailedmechanics of 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 800 oftropical eventshas been virtually excluded,and thereis also an interestingclass of phenomenain the upper tropospherethat has not been men- , 500 tioned at all and has a significant effect on

I weather.The field should be interestingand ac- 100 tive for a long time to come.

Acknowledgments The author is gratefulfor discussionswith numer- ous colleagueson the material in this review,and particularlyto Eric Webb for his carefulreading of the manuscript.

Fig, 29 An example of uniformly stratified hydraulic References flow over a ridge, with a transition from uniform subcritical flow upstream and super- Abbs,D.J. I 986.Sea-breeze interactions along a concavecoast- critical flow downstream.From Smith (1985). line in southern Australia: observationsand numerical modeflingstvdy. Mon. lleath. Rev.,114,831-48. Baines,P.G. 1980.The dynamicsofthe southerlybuster.,4rsr. lUz (Km) MeL Mag., 28, 175-200. Baines,P.G. 1983. A survey of blocking mechanisms,q'ith applicationto the Australian region.Aust. Met. Mag., 31, z t-Jo. Baines,P.G. 1987a.Upstream blocking and air flowovermoun- tains.Ann. Rev.Fluid Mech., i,9,75-97. Baines,P.G. 1987b. Upstream blocking and stratifiedflow hydraulics. Third International Symposium of Stratifed -Flows,Pasadena, IAHR. Baines,P.G. and Manins, P.C. 1989.The principlesof laboratory modellingof atmosphericflows over complexterrain. Jnl appl. Met., 28, l2l3-25. Charney,J.G. 1975.Dynamics of desertsand droughtin the Sahel.Q. Jl R. met Soc.,101,193-202. Clarke,R.H. 1955. Someobservations and comments on thesea 20 (m/s) (m/s) breeze.Aust. Met Mag., II,47-68. Clarke,R.H. 1970. Observationalstudies in the atmospheric boundarylayer. Q. JI R. met. Soc, 96,91-114. Clarke,R.H. I 984.Colliding sea breezes and atmospheric bores: two-dimensional numerical studies. ,4rsl. Met. Mag., 32, 207-26. Colquhoun,J.R., Shepherd,D.J., Coulman,C.E., Smith, R.K. East coastlows andMclnnes, K, I 985.The southerlybuster ofsoutheastern probably Australia: an orographicallyforced cold front, Mon. Llealh. Theseare the most significantsystems Rev..1 1 3. 2090-107. for causingfloods on the east Australian coast. Coughlan,M.J. 1983. Comparativeclimatology of blocking They occurat a frequencyofabout one or two per action in the two hemispheres.Aust. Met. Mag., 31,3-13. year(Holland et al. 1987),are small and intense, Fandry,C.B. and Leslie,L.M. 1984.A twoJayerquasi-geostro- andproduce massive downpours. Figure 31 shows phic model of summertrough formationin the Australian an exampleof the rapid development sub-tropicaleasterlies. J. Atmos. Sci.,41,807-18. of these Frederiksen,J.S. 1982.A unifiedthree-dimensional instability cyclones,where the pressureis observedto fall theoryof the onsetof blockingand cyclogenesis.J. Atmos. from l00l hPa to 986 hPa in about 18 hours. Sci.,39,969-82. Baines:What's interestingand differentabout Australianmeteorology? 139

Fig. 30 (a) East-westvertical sectionover Perth with easterlyflow (similar to Fig. 28 for Adelaide),showing isentropes inferred from balloon sonde(largq dots) and aircraft observations(dashed line). (b) Wind profile obiained from the radiosonde.From Pitts and Lyons (1989).

20 FEB 19A7 0630-074A IYST I 2320

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Analysisand satellite pictures showing the development ofan eastcoast low on 27July 1984.From Holland et al. (1987).

OsGMT JULY27 Baines:What's interestingand differentabout Australianmeteorology? t4l

Frederiksen,J.S. | 989.The roleof instabilityduringthe onset of Noonan,J.A. and Smith, R.K. 1986.Sea breeze circulations blockingand cyclogenesis in NorthernHemisphere synoptic over Cape York Peninsulaand the generationof Gulf of flows.J. Atmos Sci., 46, 1076-92. Carpentariacloud line disturbances.J. Atmos Sci, 43, Carratt,J.R. I 985.The inlandboundary layer at low latitudes.I I 679-93. The nocturnaljet. Bound.Lay. Met , 32,301-27. Noonan,J.A. and Smith, R.K. 1987.The generationof north Garratt,J.R. I 988.Surnmer cold fronts in South-eastAustralia Australiancloud lines and the morning glory.Aust Met - behaviourand low-levelstructufe of main frontal types. Mag., 35,31-45. Mon. Weath Rev.,116,636-49. Physick,W.L. 1976.A numericalmodel of the seabreeze phe- Garratt, J.R., Howells,P.A.C. and Kowalezyk,E. 1989.The nomenonover a lake or gulf. J. Atmos Sci, 23,2107-35. behaviourof dry cold fronts travellingalong a coastline. Physick,W.L. I988. Mesoscalemodelling of a cold front and its Mon. l4teath.Rev, I 17, 1208-20. interactionwith a diurnallyheated land mass.J. Atmos.Sct , Garratf,J.R., Physick, W.L., Smith, R.K. and Troup, A.J. 1985. 45,3169-87. The Australiansummenime cool change. Part II: mesoscale Pitts, R.O. and Lyons, T.J. 1989.Airflow over a two-dimen- aspects.Mon. ll'eath. Rev. I I 3, 202-23. sionalescarpment. I: observations.Q. Jl R. met. Soc.,I i,5, Hanstrum,B.N., Wilson, K.J. and Barrell,S.L. 1990,Prefrontal 965-8r. troughsover southernAustralia. l: a climatology.lAeath. Reeder,M.J. and Smirh,R.K. 1987.A studyof frontaldynamics forecasting,5,22-31. with applicationto the summertime'coolchange'. J. Atmos Hess,D.D., Hicks, B.B. and Yamada, T. | 981.The impact of the Sci..44.687-705. Wangaraexperiment. Bound. Lay. Met., 20,135-74. Reeder,M.J. and Smith, R.K. 1988.On air motion trajectories Hess,G.D and Spillane,K.T. 1990. Characteristicsof dust in cold fronts.J. Atmos Sci.,45, 4005-7. devilsin Australia Jnl appl.Met, 29, (in press). Ryan,B.F. and Wilson, K.J. 1985.The Australiansummerrime Hess.G.D., Spillane,K.T. and Lourensz,R.S. 1988.Atmos- cool change.Part III: subsynopticand mesoscalemodel. pheric vorticesin shallowconvection. Jnl appl. Met., 27, Mon. ll'eath. Rev.,I 13,224-40. 30s-3t7. Ryan,B.F., Wilson, K.J. andZipser,E.J. 1989.Modification of Holland,G.J., Lynch, A.H. and Leslie,L.M. 1987.Australian the thermodynamicstructure of the lower troposphereby easl-coastcyclones. Part I: synoptic overview and case the evaporationofprecipitation ahead ofa cold front.Mon study.Mon lVeath Rev.,115,3024-36. ll'eath. Rev.,I 17, 138-53. Hoskins,B.J., Draghici, I. andDavis, H.C. 1978.A newlook at Simpson,J.E. 1987.Gravity Currents in TheEnvironment and the o-equation.Q. Jl R meL Soc.,104,31-8. the Laboratory.E[[is Horwood, Chichester,244 pp. Howells,P.A.C. and Kuo, Y.H. 1988.A numericalstudy of the Smith, R.B. 1985.On severedownslope winds. -/. Atmos. Sci, mesoscaleenvironment of a southerlybuster event. Mon. 42,2s97-603. Weath.Rev.. 116, l77l-88 Tepper,G. and Watson, A. I990. The winter-time nocturnal Leslie,L. M. 1980.Numerical modelling of the summerheat lo* northeasterlywind of Adelaide,S.A.: an exampleof topor over Australia.Jnl appl.Met., i,9,381-7. graphicblocking in stably stratifiedairmasses. Aust Met Leslie,L.M., Holland,G.J. and Lynch,A.H. 1987.Australian Mag., 38,(in press). east-coastcyclones Part II: numerical modelling study. Webb,E.K. 1964.Sink voflicesand whirlwinds.ln: Hydraulics Mon lleath.Rev.. 115.3037-53. and Fluid Mechanics,R. Silvester(ed.). Proc. lst Aust. Lorenz, E.N. 1967. The Nature and Theory ofthe General Cir- Conf., Pergamon,Oxford, 473-83. culdtionof theAlmosphere. WMO, Geneva,l6l pp. Webb,E.K. 1977.Convection mechanisms of atmosphericheat McGregor,J.L. and Kimura, F. 1989.Numerical simulation of transferfrom surfaceto global scales 2nd AusL ConI on mesoscaleeddies over Melbourne.Mon Weath Rev, I17, Heat and Mass Transfer, Univ. of Sydney, 523-39. 50-66. Wilson, K.J. and Stern,H. 1985.The Australiansummerrime Manins,P.C. 1982.The daytimeplanetary boundary layer: A cool change.Part I: synopticand sub-synopticscale aspects. newinterpretation ofWangara data. Q. Jl R met.Soc , 108, Mon. Weath.Rev, I13, 177-201 689-705. Noar,P.F. t 983. Modellingof blockingwith referenceto June 1982.Aust. Met. Mag., 31, 37-49.