Quart. J. R. Met. Soc. (1985),111, pp. 479-493 551.515.821:551.574.73:551,.576.71,
The frontal transition zoneand microphysicalproperties of associated clouds
By B. F. RYAN, W. D KING and S. C. MOSSOP Cloud PhysicsLaboratory, Diuision of AtmosphericResearch, CSIRO, PriuateBag No. 1, Mordialloc, Victoria 3195,Awtralia
(Received11 January 1984;revised 29 November 1984)
SUMMARY A study of the cloud structure and precipitation in the transition zone of cold fronts over westernVictoria showsthat ihe importantprecipitation parameters, such as cloud top temperatureand depth, are determined synoptically. Pre-frontal cioudsexperience slow_uplift over substantialdepths; this resultsin cold deep clouds wittriitetimesof the order of tensbf hours.In the post-frontalclouds the uplift is mesoscaleand shallowand causesshortlived cloud elements. These dynamic and thermodynamicfeatures determine the cloud microphysicsand largely the associated rainfall. Preiipitation in pre-frontal clouds forms mostly through lightly rimed ice crystalsproduced at low temperaturesithe procesican be very ef{icientiil glaciatingthe cloud but n-otnecessarily efficient in ptodlcing ice crystals large enough to survive the fall to the surface. In the post-frontal systemsthe_ice crystals are produced-,atmich higher temperaturesin the presenceof higher liquid water contentsand the precipitation dependsvery sfiongly on cloud depth and duration.
1. INrnooucrlou This paper presentsthe resultsof studiesof cold fronts in south-easternAustralia over the past eight years. During the years 1975-78and 1979-80,as part of a major cloud-seedingexperiment, an extensivestudy was made of the microphysicsof cloudsin the Wimmera region of westernVictoria (King 1982a).These observations provide an extensivedata set on the microphysicalproperties of cloudsin the region for the spring months of August to October. Typically the fronts occuring in spring are wet and produce-4mm of rain per event.An additionalexperiment, the Cold Fronts Research Progtum*e (CFRP), wasestablished in 1979(as a joint CSIRO/Bureauof Meteorology undertaking) to study the summertimecool changein the whole south-easternAustralian region. The summerfronts are generallyquite dry and may produceno rain at all. The emphasisduring the CFRP was on the dynamic and thermodynamic structure of the summertimefrontal systems(very few detailedmicrophysical measurements were made). Sincecold fronts havean importantinfluence upon the weatherof the southernhalf of Australia, and sincethe meteorologicalliterature in generalis lackingin descriptions of clouds associatedwith thesefronts and of the precipitationprocesses within them, thesestudies should be of interestto all workersin the field. The aim of section2 of this paper is to show that the important dynamic and thermodynamicfeatures that definedthe frontal transitionzone, observedin the CFRP to be a commonfeature of the summertimecool change,are alsofeatures of the systems studiedin the westernVictorian cloud-seedingexperiment during spring. In section3 relativeflow isentropicanalysis and aircraftcross-sections from a single casestudy taken from the cloud-seedingexperiment will be usedto showthat the structure of the upper-levelhumidity front is dynamicallyand thermodynamicallysimilar to that found in the CFRP, The particular case study was chosenfrom the cloud-seeding expedment becauseit provided the most complete data set of aircraft observations, rainfall measurements,satellite imagery, etc' In section 4 the microphysicsof the clouds studied during the spring cloud-seeding experiment is describedin terms of the properties of clouds associatedwith the frontal 4,79 B. F. RYAN, W. D. KING and S. C. MOSSOP
transition zone of the summerCFRP.
2. FRoNrar,rRANSrrroNZoNE (a) Conceptualmodel basedon CFRP obseruations Observationsof some seven events made during the CFRP identified a frontal transition zone (F lZ) as a common feature in'the summertimecool change. The important featuresfound in a typicalcross-section through theFTZ are shownin Fig. 1 and are discussedin detail by Ryan and Wilson (1984).
Figure 1. A cross-sectionnormal to the frontal transition zone (F'lZ) showingthe important featuresof the conceptualmodel. Encircled dots refer to a northerly wind, encircledcrosses to a southerly wind and dashed lines to precipitation from the convectivelines. The solid lines beiow 3 km show the typical I structure below the middlelevel cloud and the dashedline showsthe typical 9" structure in the middle-Ievelcloud layer.
For this paper the following featuresof the FTZ arc important. At the surfacethe leading edge of the FTZ is characterizedby either the pressureceasing to fall or the passageof a weak changehaving the characteristicsof a seabreeze. The most invariant surfacefeature is the rear of the F'IZ, wherc there is a steadypressure rise. Between thesetwo lines there may be one or more convectivecloud linesthat propagaterelative to the FTZ. Ahead of the final line there is middle-levelcloud with basenear 3 km. The moisture for this cloud is provided by isentropicupglide (seeWilson and Stern 1984). Behindthe final line the windsback and there is a substantialincrease in low-levelrelative humidity. This air has a maritime origin and generatesstratocumulus layers with tops in the vicinity of 2to 3km, abovewhich the air is very dry again.In most of the events studiedduring the CFRP there was a gap betweenthe middle-levelcloud and the low- level moist air (seeFig. 1), althoughin two eventsthe surfacefront was beneaththe middle-levelcloud and no gap was observed, An important feature of the model is the upper humidity front aheadof the surface front; this suggeststhat the model is consistentwith the split cold front model proposed by Browning and Monk (1982).The essentialdifference between this model and that proposedby Browning and Monk is that in the Australian situationthe pre-frontal air risesover the continentand is much drier in the lower levels.Typically the air needsto rise to 3 km before saturationis reachedwhereas in the United Kingdomthe lowest2 to 3km aheadof the surfacecold front is alreadynear saturation.In this paper the upper humidity front will be termed the upper front.
(b) Obseruationsfrom the westernVictoria experiment The focusof the westernVictoria experimentwas the automaticraingauge network THE FRONTAL TRANSITION ZONE 481.
Fig:ure2. Themean sea level pressure chart for 23crr.rr on 16September 1980 prepared by theBureau of Meteorology.The map shows the re8ion sampled andthe pluviograph network to thesouth of :ilffitt".tXl shownin Fig. 2. The microphysicalproperties of cloudsin the vicinity of the raingauge network were observedfrom an instrumentedCSIRO F-27 researchaircraft. During 1980altostratus cloud systemswith the characteristicsof the upper front were sampled on 10 occasions(see Table 1). Satelliteimagery and hourly surfaceobservations from Mildura (a Bureau of Meteorology upper air station some 240km from the centre of the
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Figure 3. GMS satellite imagery (infrared) at 0000cvr 17 September1980. 482 B. F. RYAN, W. D. KING and S. C. MOSSOP
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4p B I A I I o o s Ai .j> o .co€d6d6; d *iJP...; ?iltitib" d \ov)tt)(^dd::X o svrca*vvu .i : ;r--N-.-dr N n z6 R R \ THE FRONTAL TRANSITION ZONE 483 raingaugenetwork) were usedto establishwhether or not thesecloud systemsfitted the conceptualmodel of the upper front basedon the CFRP. The first criterionwas whether or not a F'tZ could be identified from the surfacedata and the secondwas that there was a cloud band severalhundred kilometres ahead of the surfacecold air as diagnosed from the cumulusactivity on the satellitepicture. Table l showsthat the systemsstudied in 1980all haduppel fronts definedby thesecriteria. However, with two of thesesystems, on 5 and 23 October, the final line wasdifficult to determinefrom the surfaceobservations at Mildura. In these casesit appearedthat the upper front overlappedthe low-level surfacefront. To be more specificwe will usedata from 17 September1980 as an exampleof the above (we will leave the isentropic analysisof this day until section3). The surface synoptic chart for 23crrrr on L7 September(0900 local time in western Victoria), as preparedby the Bureauof Meteorology,is shownin Fig. 2. This showsa cold front lying approximatelyno4h-west to south-eastin the area of interest centred approximately 1.43"8.between 35" and 35"30'3.The cloudsassociated with the front are shownin the GMS satelliteinfrared imageryfor 0000cvr (Fig. 3). The front analysedby the Bureau of Meteorology correspondedto the trailing edge of the middle-levelcloud band and is the upper front. The shallowcold air associated with the surfacecold front was some 260km behindthis line. The CSIRO F-27 aircraftsampled the cloudsin the generallocation shownin Fig. 2. Pre-frontal altostratuswas sampledbetween about 2200 and 2300cur (080G-0900 localtime) andshallow Cu between0000 and 0200crvru (1000-1200local). Figure 4 shows the position of the cloudsand the samplingtrack of the aircraft relative to the position of the upper front. The positionof the upperfront at an aircraftsampling time is defined by assumingthat the front progressedat a constantspeed between the successivethree- hourly surfaceanalyses. The distancefrom the aircraft to the front along a line normal to the front is then readily calculated. Applying the conceptualmodel discussedabove to the surfirceobservations made at Mildura we find that the final line signifyingthe arrival of the surfacecold air reached Mildura at 0600crrar(1600local). Satellite imagery has been used to estimatea distance of 260km betweenthe upper front and the cold air. Observationsat Mildura showthat
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Figure 4. A cross-sectionof thdfrontal transitionzone sampledby the aircraft. Hatched areasshow the regions of cloud and the solid line showsthe flight path of the aircraft. Local time is shown at various stages oflhe aircraft flight path (local time is cMr + 10). The dashedlines show the 300, 295 and 290 K isotherms.A and B refer to the air massidentified in the relativeflow isentropicanalysis. 484 B. F. RYAN, W.D.KING andS. C. MOSSOP this is consistentwith the onsetof cumulusactivity and suggeststhat the FTZ hasa speed of about 12ms-1. The characteristicfeature of the final line is the start of a steady pressurerise (Fig. 5). This alsocoincides with a substantialtemperature drop and a rise in dew-pointtemperature. Consequently all the aircraft observationswere made in the frontal transition zone (FTZ). The altocumulusband (definedas the pre-frontalband) coincidedwith an increase in relativehumidity at the surfaceand a decreasein temperature-presumablyas a result of evaporativecooling. Behind the altocumulusband there were bandsof cumulus,a featurenot observedin the CFRP observation.Following the passageof thesebands the surfacetemperature rose, aswould be expectedfrom surfaceheating, and the dew-point fell steadilyuntil the arrival of the final line and the cold air. The rise in dew-pointis the reverseof the situationin the United Kingdom, and it occursbecause the dry pre-frontal air is replacedby cooler air with a maritime origin. The uncorrectedsurface pressures showa pressurerise below the rain band at 2300crr,lr which is consistentwith evaporative cooling in the lower troposphere. A recordingraingauge network developedfor the westernVictoria cloud-seeding experiment (King er al. 1979)was sited to the south of the area investigatedby the researchaircraft (Fig. 2). The network comprisessome 90 pluviographsusing a tipping- bucket systemto recordrain in incrementsof 0.2mm. The recordingraingauge network has been analysedfor L5-minintervals. The network analysisshowed that rain from the pre-frontalsystem fell between20 and22crvlr(0600 and 0800local time) at arate of 2mm h-1. The bandwas approximately 60km wide, spending-1h over any one stationand movingwith a speedof 17ms-1. This was fasterthan the movementof the FTZ andis consistentwith observationsfrom the CFRP. Aircraft observationsshow that the speedof the bandwas approximately that
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E 0000 GMT 2200 1000 r1m r000 0600 0200 2200 L@l tim I7 SEPTEMBER 16SEPIEMBER Figure 5. The half-hourly surfaceobseryations of dry-bulb temperature,dew-point temperatureand pressure as reported by the Bureau of Meteorology upper air station at Mildura. The figure also shows the times of significantcloud reports and rain. Filled triangles show the time of passageof the surfacecold front. THE FRONTAL TRANSITION ZONE 485
of the normal componentof the wind at the top of the cloud layer. The daily rainfall figuresarchived by the Bureau of Meteorologyalso show total rainfall of the order of 2 mm in the vicinity of the pluviographnetwork, indicatingthat the band studiedby the aircraftwas the only cloud to give substantialrain between09 on 16 Septemberand 09 on 17 September(local time). In the cloud-samplingarea the daily rainfall totals suggest that the cloudband produced about 1.5mm. The network analysisshows a rainfall pattem that is consistentwith the convective rain showersreported at Mildura between04 and 07crvrr(14 and 17 local time) on 17 September.The rainfall recordsshow showers entering the network at 04cvtt, with rates ' varyingfrom 10 to 25mmh-i.
3. Rplattve FLow ISENTRopIC ANALysrs AND AIRCRAFT cRoss-SECTIoNS FoR 17 SnpreNasnn1980 (a) Method of analysis Wilson and Stern(1984) have applied relative flow isentropictechniques to the front studiedin the CFRP. Followingthe techniqueused by Harrold (1.973),they showedthat 'conveyor one of the main featureswas a warm belt' that originatedas a subsiding, relative easterly flow in low latitudes which on reaching the thermal ridge turned anticyclonicallyto flow parallel to and ascendahead of the surfacecold front. This conveyorbelt is a confluentstream in subtropicallatitudes, and it was found that the degree of ascent was sufficient to generate the characteristicpre-frontal middle-level cloud band with a baseat about 3km (700mb). Further it was found that the western edgeof the middle-levelcloud bandwas approximately coincident with the westernedge of the conveyorbelt. The air adjacentto the westernedge of the conveyorbelt had a lower equivalentpotential temperatureand had either recirculatedfrom low latitude troposphericflows or originatedin higherlatitudes. The upperfront correspondedto the boundaryof theseflows. (b) Casestudy A similaranalysis has been made of the air flow for 17September 1980 and the same featuresare evident.These analyses for 0:290 and 300K are shownin Fig. 6. The air massesare also identified along the potential temperature lines in the aircraft cross- sectionshown in Fig. 4. 0 : 290K (Figs.6(a), (b)):The 0 :290Ksurface depictsthe low-levelair beneathcloud base during the period of the researchflight. The arrows in Fig. 6(b) representthe objectivelyanalysed winds on the routine Bureau of Meteorologyanalysis. The region enclosedby the broad arrow B in Fig. 6(a) is pre-frontalair at this level.The post-frontal air is to the west of the observationalarea and is consistentwith the earlier analysis defining the FTZ. 0: 300K (Figs.6(c), (d)): The 0: 300K surfaceshows the objectivelyanalysed winds (Fig. 6(d)) while Fig. 6(c) showsthat the air massB enclosedby the broad arrow is pre- frontal in nature and is the conveyorbelt forcing the upslopeconvection. The western edgeof the arrow correspondswith the westernedge of the middlelevel cloud band in the observationalcross-section (Fig. 3). The air massmarked A and enclosedby the secondbroad arrow correspondsto the dry air that overliesthe stratocumuluslayer studiedby the researchaircraft. This air is colder and drier than the pre-frontalair and canbe tracedto higherlatitudes. The stratocumuluslayer observedby the aircraftis less than one grid spacingin width (250km) and thereforeunlikely to be predictedby the analysis. 486 B. F. RYAN, W.D.KING andS. C. MOSSOF
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Figure 7. A cross-sectionof the frontal transition zonesampled by the aircraft showingcontours of equivalent potential temperature 0". Hatching showsthe regionsof cloud. THE FRONTAL TRANSITION ZONE 487
(c) Aircraftcross-sections The potential temperaturecross-sections are entirely consistentwith the synoptic relative flow analysisin that they show no low-level barocliniczone acrossthe upper front. Below 700mbthe spatialgradient in I acrossthe upperfront andbelow the middle- level cloud bandis -1'per 100km.However, at 600mbthe gradientacross the upper front is -2.5" per 100km. The cross-sectionof equivalentpotential temperature (Fig. 7) relativeto the position of the upperfront showsthat the pre-frontalcloud corresponds with a regionof maximum 0" and is consistentwith the observationsleading to the conceptualmodel. A feature not found in any of the earlier CFRP studiesis the region of low-levelpotential instability that correspondsto the regionof Cu and Sc behindthe upper front. In the CFRP it was found that the low-levelCu and Sc formed to the west of the FTZ. The relative flow isentropicanalyses and the aircraft cross-sectionssuggest that in the south-easternAustralia region care needsto be taken in applyingthe terms post- frontal and pre-frontal to clouds.The presentstudies suggest that cloudsahead of the upper front should be calledpre-frontal while those behind the upper front should be called post-frontal. In the next section it will be shown that with this definition it is possibleto develop a consistentclassification of the microphysicalstructure of the clouds.
4. MrcRopHysrcsoFclouDsrNrHeFTZ (a) Pre-frontalaltostratus Altostratus was sampledon 10 occasionsduring the cloud-seedingexperiment in 1980,including the casestudy of 17 September.The characteristicsof theseclouds as revealedby aircraft samplingare summarizedin Table 1. As mentionedearlier, the 'separationdistance' of the low-levelcold air is that deducedfrom the position of the cold air diagnosedfrom the typicalconvective pattern in the satellitepicture nearestto the time of flight. A preferableprocedure would have beento usethe surfaceanalysis, but as shown in Table 1 the cold air wasusually still over the oceanand thereforenot diagnosable. On 1.7September the pre-frontal cloud had a base altitude of 2'1 to 2.5 km and consistedof two major layers(see Fig, 8), the topmost extendingto 6.2km (-26'C). Above the 0'C level very little liquid water wasdetected and the cloudconsisted almost entirely of ice particles,The variationof particleconcentration with altitudeis shownin Fig. 8. Each point representsa 5s average(approximately 0.5km). Figure 9 showsthe crystal shapesand sizesat six different sampledcloud temperaturesand heights.The most remarkablefeature about the ice crystalstructure is the preponderanceof rimed Iumps-1 mm in diameterand the absenceof clearlydefined crystal shapes. King (1982b) has pointed out the general nondescriptappearance of crystalsfound in these deep systems-they bear little resemblanceto the classicalsnowflake shapes characteristic of diffusionalgrowth. This is partly due to the compactnature of ice crystalsgrowing at temperatures* -25 oC, and partly due to the limitationsof the measuringsystem. The PMS-2D-Cimaging probe usedfor this work had a pixel resolutionof 25 pm, and this is too coarsefor detailedexamination of crystalswhose median diameter near cloud top is in the range 10G-150,um.Lower down in the cloud, the crystalsare somewhatlarger, and the hexagonalshapes are more recognizable. Below the 0'C level, drops up to 0.8 mm diameterwere sampledand fall-streaks were observed.As mentionedearlier, precipitation measured in a network of recording raingaugessouth of the sampledclouds indicated an averagefall of 2 mm from this pre- frontal cloud. 488 B. F. RYAN. W. D. KING and S. C. MOSSOP
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Figure 8. The concentrationof ice crystalsper litre as a function height for the pre-frontal altostratusclouds on 1.7September 1980. Hatching showsthe two main cloud layers. tbft-tf LnLttf fruf thtrtrt-lllLl lfh*FttlFll'tt l'll'trtLftltIh+l't (.) l{f'FI[rt|tFr|*L rFilthItftFtrttilt (f)
scatc:bd (uottraxes) Figuree. Iceparticre images atf^l s (c)3.1 km, -6 "c;(d) 2.7 km, -2'c; f$\:t #, 3i8)6-rlfil]1:3f THE FRONTAL TRANSITION ZONE 489
All thesecases shown in Table t havecertain common features. The cloud is often layeredand usuallyreaches well abovethe -15'C level. In thesealtostratus-type clouds the total fraction of the in-cloudtime for which the liquid water contentwas >0.1 g m-r was only just over IVo. Further, becausethe crystalsare small and compactthere are few signsof aggregation. Figure10 shows the iceparticle concentrations as a functionof cloudtop temperature for all days sampledin 1980.They representan averageof the measurementsof ice particle concentrationwithin 200m of measuredcloud top. The dashedline is the 'average' concentration of ice nuclei suggestedby Fletcher (1962). The ice particle concentrationsare betweenone and three ordersof magnitudegreater than the average ice nucleusconcentrations, suggesting that an ice crystalmultiplication process is operat- ing. A more detailed discussionof the propertiesof the ice crystalsin theseclouds is givenby King (1982b).The naturalice crystalformation processes provide sufficientice particles to remove virtually all liquid water down to the 0 "C level. (This lack of supercooledwater renders these clouds (at least in western Victoria in the spring) unsuitablefor artificial seeding(King 1982b).)
(b) Post-frontalcumuli In Table 2 detailsare given of post-frontalcumuli sampledon five occasionsother than 17 September1980. In this table the distancebehind the front is definedrelative to the front analysedby the Bureauof Meteorology.These clouds form behindthe upper- level front in slightlyunstable air approachingfrom a rangeof directionsbetween west- north-westand south-west.The air is clean and the low concentrationof cloud nuclei and henceof cloud drops leadsto the production of large drops in comparativelyshallow clouds.Once the tops of suchclouds are supercooledto -5oC or -10'C they produce l
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Figure 10. Ice particle concentrationsas a function of cloud top temperature.Filled circlesrefer to the mean ice crystal concenttations in pre-frontal altostratus clouds (Table 1). Filled triangles indicate ice crystal concentrationsin L km runs in the post-frontal clouds(Table 2). The dashedline is the 'average'concentration of ice nuclei suggestedby Fletcher (1962). 490 B. F. RYAN, W. D. KING andS, C. MOSSOP
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(r) Post-frontalstratocumuli The inversion-limitedstratocumuli which often follow the post-frontalcumuli are generallylimited in depthto about 1km, as shownin the sampledclouds listed in Table 3. They seldomreach temperatures much below 0 oCand any rain theyproduce (generally <1 mm per day) forms by the drop-coalescenceprocess as drizzlein showersof typically 20 min duration.
5. CoNcrusroNs It hasbeen shownthat in classifyingthe air massesand microphysicalproperties of clouds it is essentialto distinguishbetween the upper humidity front as characterizedby 492 B. F. RYAN, W. D. KING and S. C. MOSSOP
TABLE3 Posr-FRoNTALSTRAToCUMULUS
Cloud base Cloud top Time Drop srnce concen- frontal Surface Alt. Temp, Alr Temp tratron Precip passage wind (km) fc) (km) fc) (cm -., (m-) (day$ direction -1 1 Aug. 75 t'2 +1 1.6 94 002 L2 w 3 Aug. 75 0.5 +5 11 +2 86 1.5 1 N 7 Atg.75 0.8 +3 1.8 01360 2 NE - L2 Aug.75 0.6 L.7 1 65 0'08 L2 S 13Aug. 75 0.5 +5 1.4 -1 43 0.07 2i S 14Aug. 75 0.9 +1 r'4 0980 3t S 16 Aug. 75 t'J -2 l.o 0870 )i SW 3 Sept.76 1'0 +5 2'3 0 150 005 2 wsw 1 7 Aug 77 1'1 +3 -2880 t SW 13 Aug.77 1.1 +1 z'0 -2800 1 SW the westernedge of the conveyorbelt cloud and the surfacecold front. It is suggested that relative isentropicflow analysisin conjunction with satellite imageryprovides a reliable techniquefor positioningthe westernedge humidity front. The most important differencebetween the cold fronts in the south-easternregion of Australia and thosenear latitude 50"Nin the northernhemisphere is the lack of low- level moisture aheadof the front in the former case.The data for the presentstudies were taken during spring;nevertheless they are consistentwith the summertimesituation investigatedduring the CFRP (Ryan and Wilson 1984).The lack of low-levelmoisture ensuresthat there are no feeder cloudsbeneath the upper front, as is frequently the situation in the northern hemisphere(Browning and Monk 1982).ln both the United Kingdom and the United Statesit hasbeen shown that feederclouds significantly enhance precipitationthrough the riming process(see e.g. Hobbs 1978).Consequently in south- easternAustralia the upper fronts generateonly a few millimetres of rain and this contrastswith, for example,rainfalls of the orderof 25 mm overthe Welshhills (Browning et al. 1974\. Post-frontalinstability typically producessmall cumulusand stratocumulusclouds whichwhen supercooled have a very efficientnatural ice-producing mechanism. However, when the post-frontalair has a polar origin deep convectionwill take place. The ice crystal production mechanismin such clouds has not been studiedin the Australian situation. r In the Australiancontext there are other seasonsand locations where further studies of cold fronts are desirable: (i) wintertime cold fronts; t (ii) cold fronts in regionswhere they are modifiedby strongorographic uplift, such as the Australian Alps, wherefeeder clouds may be generatedbeneath the upper front; (iii) cold fronts where the pre-frontal air has a maritime trajectory, as sometimes happenson the west coastof Australia.
ACKNOWLEDGMENTS The authors acknowledgethe assistanceof the Bureau of Meteorology in providing surfaceanalysis and satellitepictures; of Mrs Enid Turton with computing;and of Mr J. Meadows,who compiledsome of the backgroundmaterial for this paper. THE FRONTAL TR.A.NSITIONZONE 493
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Browning,K. A. and Monk, G. A. 1982 A simple model for synopricanalysis of cold fronts. Quart. J. R. l4et. Soc., 108, 435-452 Browning,K. A., Hil1, F. F. and L974 Structurd and mechanismof precipitation and the effect of Pardoe,C. W. orolraphy in a wintertimewatm sector.ibid., 96,369- 389 Fletcher,N. H. 1.962 The Physicsof Rain-clouds.Cambridge University Press Hallett, J. and Mossop,S. C. 7974 Productionofsecondaryiceparticlesduringthe rimingprocess. Nature.249.2G28 Harrold, T. W. 7973 Mechanismsinfluencing the distributionof precipitation within the baroclinicdisturbances, Quart. J. R. Met.\oc.,99, 232-257 Hobbs,P. V. L978 Organization and structure of clouds and precipitation on the mesoscaleand microscalein cyclonicstorms, Reu. Geophys.Space Phys., 16,741,-755 'Cloud King, W. D. 1982a ,u seedingscsufrg inrrr Australia:Auslra[a: Experimentsr,xpelrments 1948-81ly+d-6-t andano pros- pectsfor future develooment'.development', CSIRO Divisions of Cloudi PhysicsInternal Report CP 326 1982b Location and extent of supercooledwater regions in deep stratiform cloud in western Victoria. Awt. Met. Mag., 30.81-88 'Prospectus King, W. D., Manton,M. J., Shaw, 1,979 for a cloud-seedingexperiment in westernVictoria, D. E., Smith,E. J. and CSIRO Division of Cloud PhysicsInternal Report Cp Warner, J. 222 Ryan,B. F. and Wilson,F. J, 1,984 The Australiansummertime cool change: III. Subsynopticand mesoscalemodel. (To appearin Mon. WeatherReu.) Wilson,K. J. and Stern,H. 1984 The Australian summertimecool change:I. Synoptic and subsynopticaspects. (To appearin Mon. WeatherReu.)