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The Frontal Transition Zone and Microphysical Properties Of 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 7..t '' "i i. q4 Figure 3. GMS satellite imagery (infrared) at 0000cvr 17 September1980. 482 B. F. RYAN, W. D. KING and S. C. MOSSOP o c Niioi z o E..E E Yf ad A.= E N:ZZZN v\o Hdv d oo 'F 9^ B d ;,t4 o'= v OE v) bo g< q) lf d o Hj: g= R P P P P 9 P QA 0 J.E } E T 5 E E = E 5 E= o F C' r t F o J o z 'd & o !.1 o U) € s1 @ or O 't z C) tr] da H .G F riy T R \ P S E S R R3 - 'rtttttttl d c-l U .^ Y t.- I 9 I i 9 + f-o N \o\ovrhGh-oeh F <5 bo o (d .i g cr <) o o o o ooo r sf : bI) o A d @ 6 "+ Oc{m o ad .o e- I o a! o O o\ d d o d d H i rho\ d 4.U N N o 6 a.l ct c.l NOaa d 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 E z B--z'tiH,T -'-' 29oK Oistonce from {ront (km} Figure 4.
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