In Australia

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SevereLocal Wind Storms in Australia

R. H. Cr.enxn

DIVISION OF METEOROLOGICAL PTTYSICSTECHNICAL PAPER NO. 13 COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH ORGANIZATION, AUSTRALIA 1962 SevereLocal Wind Stormsin Australia

By R. H. Clarke

Division of MeteorologicalPhysics Technical Paper No. l3

CommonwealthScientific and Industrial ResearchOr ganization,Australia Melbourne 1962 SEVERELOCAL WIND STORMSIN AUSTRALIA

By R. H. CLanrE*

lManuscript received April 26, 1962|1

Sumntary A searchof files on the occurrenceof severelocal wind stormsin Australia has rnade possiblesome tentative conclusionsconcerning the frequency,intensity, and geographicaldistribution of thesetornadoJike stortns.They are on the whoie about as frequent as, but much lessintense than, North American tornadoes. Among the worst affiicted of the n-roresettled areas in Australia are coastal Queensland,an area around Bendigoin Victoria, and the south-westcoast of WesternAustralia. An investigationof synopticfactors operating prior to the occurrenceof storms has shedsome light on conditionsnecessary for their genesis,and has suggestedsome forecastingrules. Briefly, necessarypre-conditions are: the existenceof a large-scale Iifting mechanism,such as an intensefront with strong circulationsaround the leading edge; the presenceof adequatemoisture to a sufficientheight; and "parcel" and convectiveinstability in low levels. Case historiesof six individual storms are describedin considerabledetail. Field exan-rinationof storm damagehas establishedthat cyclonicrotation is a regular featureof such storms,and has indicatedthat wind speedsat tree-toplevel of about 100m.p.h. over a diameterof 50 to 200yd n-rayoccur in stormsdescribed in the press. The estintationof wind speedfrom tree damage,and the quantitativerelation between wind speedand danragemore generally,are discnssedin Appendix I.

I. InrnorucrloN 'osevere For the purpose of this study, a local wind storm" is a locally restricted stroltg wind on a lateral scale of less than one mile, sumcient to causenoteworthy damage to buildings. There is ample testimony that tornadoes occur in Australasia, and some of the storrls analysedhereunder undoubtedly merit this title, while others equally qlearly do not. Betweenthese extremes lies a large classof storms for which adequatedetails are not available tojudge whether they ought or ought not to be classedas tornadoes. As this classconstitutes the majority, we are unable to make any satisfactorydeduc- tions about the frequency of occurrenceof tornadoes. The term "tornado" has been freely used in pressreports of local storms in somecities and States,whereas in others what is evidently a similar phenomenon is apt to be described as a "wiily-willy", "cockeyed bob", "hurricane", "tornadic squall", or even "cycione". An analysis based on the use of the word "tornado" wouid not therefore be meaningful. Referencesto tornadoes and local wind storms in Australasia are listed in Appendix II(a). Storms presentedin the following analysis have been culled from thesepublications and from newspaperfiles held amo,ngtheil records by the various Bureaux of Meteorology. Some other sourceshave been consulted,aud it is con- R. H. CLARKE sidered unlikely that any large body of rnaterial has escapednotice, although direct searchingof old newspapershas not been attempted. In this paper a number of referencesare made to place names, the locations of which are given in Figure 1.

NEW SOUTH WALES

-.ShepDorlon 8endigo. . .Viotet Town Derrinol 'Mongolor€ 'Alexondro Essmdon'6u*1on

Fig. 1.-Map showingplace names mentioned in thetext.

II. FneeueNcv op OccURRENcE (a) Historical Distributiort of Reported Storms It is to be expectedthat, with increasingpopulation density and the development of a more thorough news coverage by the press, the number of reports of local storms would show, on the continental scale, a historical increase. This is borne out by the frequenciesin Table l.

Tnst-e I RECORDSOF SEVERELOCAL WIND STORMSIN AUSTRALIA BY DECADES

I 888-97 1918-21

45 SEVERE LOCAL WIND STORMS IN AUSTRALIA 5

Thus the bulk of our information is drawn from the last three decades,depleted during the war years 194V45 by an offcial ban on publication of current weather information. (b) Frequency of Local Storms in the Capital Cilies Since most of Australia is sparselypopulated, it is certain that many storm occurrencesare not recorded. If we wish to estimate even roughly their frequency, we must choose areas and times where the population and real estateconcentration are so great, and the news coverageso extensive,that we can be sure that no significant number of eventshas escapedmention. Only the capital citiesand their suburbs, during post-war years,occupying an area of about 2300 sq. miles, meet theserequire- ments. Because of the paucity of reports from Tasmania, Hobart has not been included. Table 2 sets out the frequency of reported local storms in the capital cities durins 1947-58 inclusive.

Trer-E 2 FREQUENCY OF SEVERE LOCAL WIND STORMS ]N AUSTRALIAN MATNLAND cAprrAL ctrtrs. 1947-58 TNcLUSIVE

Occurrences Area (1947) Occurrences per Sq. Mile (sq. miles) per Century

Perth 190 18 0.79 Adelaide 160(350*) 7 0.36(0.19) Melbourne 700 11 0.13 Sydney o/u 20 0.25 Brisbane 385 10 0.22 All cities 210s(229s) oo 0.26(0.24)

* Assumedeffective area (see text).

The "area" quoted above is that given by the Commonwealth Statistician, but is, of course, not necessarilyaccurate as an estimate of "effective area" in the present context. Only Adelaide appeared to require substantial modification, and the bracketed figures are based on al1 assumed "effective area" for Adelaide of 350 sq. miles. A 12 test was applied to thesedata (both bracketed and unbracketed) to deterrnine whether the differencesin "occurrence per sq. mile per century" were significant. A high degreeof significancesupported the view that, assumingthe post- war period to be representative,Perth reports considerably more, and Melbourne fewer, storms per sq. mile than the other cities.

(c) Frequencyof Local Storms over Country Areas Since buildings are, on the whole, sparsein country areas,in the application of our definition we must use indirect evidence where no buildings are involved. In fact, however, accountsare rarely published unlessbuildings are affected,and the difficulty in practicehardly arises. Thus, in effect,records of storms in a given areaare largely determined by the proportion of that area which is built up, which in turn 6 R, H. CLARKE

of a is closely dependenton population density. Table 3 shows local storms in each number of areas tepresetltedin Figure 2, together with statistics of population and area, and date of the first storm report available, for comparison with capital city occurrences. TAel-r 3 or 1957) occuRRENcEoF sEVERELocAL sroRMs rN AUSTRALIA(nLL nvntl-nsLE DATA To ENo

1947 Area Frequency Population Ref. Occur- (1000 per 101 Density Fig. 2 rences sq.miles) Sq. Miles per Sq. Mile _l--_- I WesternAustralia r918 Metropolitau 019 30 ttuo 1400 t.1 | .79 rB9? Southcoast 17 7 | 4.1. 5.2 0 t897 Westcoast, etc. B 205 5 | 024 017 14 t9l0 Agriculturalbelt c 100 t7 | 1.7 0.72 2'4 l9l0 Gold-fields D 172 il i 0'64 0.16 4.0 SouthAustralia t9t2 Metropolitan 0.16 18 ll20 2400 0.47 1912 Coastaiplain 30 6 2.0 4.0 0.50 1926 Central highlands B 30 5 1.6 3'4 0.47 1910 Interior 320 4 012 0.13 0.92 Victoria a1 I 890 MetroPolitau 0'70 300 1700 0.18 18 1861 Coastal plain' A 19 B 4.1 22.8 0' D 1904 Easternhighlands 22 5 z' -) 6'4 0.36 1911 Westernhighlands 12'5 t8 t4'5 14.6 0.99 North, north-west D 33.5 10 30 5.8 052 r920 New SouthWales 189'l Metropolitan 0.67 33 490 1900 026 .6 34.9 039 1894 CoastalPlain A 22 30 13 1894 Highlands b 60 l2 20 6.0 033 '16 3.4 0'85 1891 Slopes L 22 2.9 1902 Interior D 150 1l 0'73 0.6'l 1.1 Queer-rsland t9z4 Metropoiitan 0'38 15 390 1000 0.39 t920 East coast A 125 69 5.9 ,4.3 r'4 )'L 0.80 40 1910 Flighlands D 195 62 .46 1920 Interior (- 350 l6 0 0.l1 4.2 TerritorY Northern .021 Whole <14 5 0.095 0 4.5 I leri Tasmania 1.6 9.8 016 Ln,, Whole 26 4 I

While it would be unwise to geueralizefrom Table 3, it is worth noting that frequenciesrelated to population lead to the saureconclusion as we reachedbefore: that stormsare reportedmore frequentlyat Perth and lessfrequently at Melbourne than at other cities, and this is borne out also by independent frequenciesin the and coastal areas where the cities are located. This correspondencebetweeu city SEVERE LOCAL WIND STORMS IN AUSTRALIA coastal area does not obtain in Queensland, where the capital is at the southern extremity of a long coastline: it is probable that storms are more frequent in the northern portion of the coastal plain. While there is some reason to believe that "occurrences per person" is the best single index of relative frequency,it rnay well be that this index, which is higher in the scantily populated interior of Queensland,Westeru Australia, and Northern Territory, is misleadinghere. We conclude, however, that frequency of severelocal wind storms is higher in coastal Queensland,an area around Bendigo in Victoria, and the coastal area in south-westernWestern Australia, than in other of the more settled parts of the continent.

Fig. 2.-Map of Australia,showing the divisionsof each Statefor thepurpose of estimatingstorm frequency'

There is quite a hint of similarity between severelocal stornl occurrence,as deduced here, and thunclerstormfrequency (Ashton 1960) in Queensland,Western Australia, ancl western Victoria, but obvious dissimilarities in the highland areas of Victoria and New South Wales. Severelocal storrn data fror.r-rmollsoon regions are clearly inadequatefor comparison'

(d) Comparison v,ith North American Tontado Frequencies Bearing in mind that our storms are probably on ti'le whole less intense (as judgecl by the winds in Table 5) than those which are classifiedas tornadoes in the United States of America, we call neverthelessmake comparisons of frequency by nea11sof data published by Winston (1956) and the U.S. Weather Bureau (1952). Table 4 is based on the latter publication, which gives tornado occurrencesfor 1916-50 inclusive. In the United States the difficulty exists, as here also, that many tornadoes are not tecorded, as Tepper's (1951) analysis of the effect of an improved R, H. CLARKE reporting Systemshows. Hence there is Some ground for preferring "occurrences per person" as a guide to relative frequency there also' If we can assume that damage areas of United States tornadoes and of Aus- tralian local storms are comparable, that the orgar.rizationof buildings with respect to population is similar, and that news reporting habits with regard to the two phe- nomela are similar in each country, we carl make direct comparisortsbetween the population-based frequencies of Tables 3 and 4. This suggeststhat the storm frequenciesin parts of Queenslandand Western Australia are somewhat similar to

Tesre 4 TORNADO STATISTICSCOMPARABLE WITH TABLE 3 FOn SOptEOF THE UNITED STATES (The effectivenumber of yearsin Table 3 is roughly 35, but variesregionally)

1950 Area Occurrences Population Occurrences (r000 per 104 Densitv per IOa sq. miles) Sq, Miles per Persons Sq.Mile

Kansas 82 618 75 22 3.4 Iowa 56 498 89 43 /'I Oklahoma 70 399 57 29 2.0 S. Dakota 77 lr3 15 7.8 1.9 Arkansas 53 324 6l 34 1'8 Nebraska 77 195 25 l4 18 N. Dakota 70 57 8.1 83 0.98 Louisiana 45 178 40 47 0.85 Missouri 69 204 30 44 0'68 Texas 267 481 l8 27 0.67 Alabama 5l 188 JI 56 0.66 Flonda 54 I59 29 47 0.62 Minnesota 80 tt1 15 Jt 0.48 Tennessee 42 128 30 65 0.46 Illinois 56 164 29 9l 0.32 Delaware 2 5 25 104 0.24 Virginia 40 4l 10'2 77 0.13 .041 Massachusetts 8 16 20 486 0 California 157 16 1.0 63 0.017

those of tornadoes in the worst afllicted States of the United States. Frequerrcyof severestorms in coastalareas of New South Walesand soltthernAustralia may also be similar to that of tornadoes in the easternStates of the United States,while fre- quetlcy of severestorms in the south-westof Australia is almost certainly much higher than that of tornadoes in the western United States.

IIl. WrNp SpspptN AusrR,q,l-IA'xSpvens Loce.l Sronus Estimatesof wind speedin local storms are rather rarely available,and in uost cases their source is not stated. Hence we should accept published figures with caution, although in some casesthey can be traced to qualified engineers. The 62 SEVERE LOCAL WIND STORMS IN AUSTRALIA 9 recorded cases where estimates of wind speed were given have beenanalysed in Table 5. An estimateof 200 m.p.h. was made after a study (Hounam 1958) of the "Brighton tornado", Melbourne, February 2, 1918,which severelydamaged strongly constructed buildings in a bayside suburb.

Trst-r 5 FREeuENcyANALysrs oF PUBLISHEDwrND spEED lN 62 nusruuAN LocALsroRMs

Some very fierce storms have been reported from Western Australia, and that on April 6, 1960, (Southern 1960) would appear to have had wind velocities commensurate with those in many American tornadoes. For information on this, one should see recent analysesby Hoecker (1960) and Fujita (1960). The speedof the strongestwind recorded in local storms in a denselybuilt-up areain Australiais 95 m.p.h. at 1515hr E.A.S.T. on October 31,1940,at the Sydney Bureau of Meteorology, when a rather narrow belt of damagecould be traced through thousands of houses over a distance of about 12 miles. The centre was said to be

feeI-r 6 FREeuENcyoF REroRTEDwrDTH oF IATH oF 89 I-ocal sronus

width No. of Width No. of (yd) Storms (vd) Storms

0-20 t3 201-300 2140 4 301-400 5 4t-60 7 401-600 8 61-I00 20 601-800 1 I01-150 2 > 800 l3 I 51-200 il

near the Bureau at the time of maximum development. No estimate of damage is available,but there were two deaths. The three recording anemometersin the metropolitan area (Bureau, Rose Bay, Mascot) recordedmaximum gusts of 95,61, and 45 m.p.h. respectively. A well-developedwaterspout was observed to form as the stormmoved outto sea. Accordingto Hounam (1958)a gust of 95 m.p.h. in Sydney should have a return period of 50-100 yr, which may be compared with the figure of 400 yr for Sydney (Table 2) for the return period per sq. mile for severelocal wind storms. it

10 R. H. CLARKE

IV. DttvtEt'tstoNsoF Ssvene Locnr- Wnp Sronus 7 Dinrensions of storms are sometimesquoted in reports, and Tables 6 and than I sumrnarizethe information given. Latetal dimensionsare by definitio[ less

irrvestigatefully the length of the longer ones.

Trsrt 7 FREeuENcYoF REPoRTEDLENGTH oF PATH or 49 local sronrras

Length(miles)

No. of stortns

V. DlunNar ,qNp ANuual DIStRreuttoN oF SroRI\ls given in Distribution of local storms with tirne of day aud month of year are Tables 8 and 9, where storms have been dissectedinto "southern" and "ttortireru": The boundary between the two is shown in Figure 3'

TASLP 8 pnv DrsrRrBUrroN oF sroRMs wlrn TIME or (%)

Tirne Interval Northern Whole of (L S.T.) Section Australia

> 00-02 5.2 >0244 0 0'9 > 04-06 0 T.J > 06-08 t'J > 08-10 3'9 Jl > 10-12 2.6 8.9 > 12-14 6.5 8.5 > 14-16 19.5 14'3 .4 > 16-18 20'8 21 > 18-20 14'3 tr.z >20-22 5'2 4.0 >22-24 0 2."1

No. of storms SEVERE LOCAL WIND STORMS IN AUSTRALIA II lnaximltm are found in southernAustralia, suggestingthe effectsrespectively of large- scaleascent and of surfaceheating, while in the north the winter months are virtually free of local storms

Fig. 3.-Locatior-r of the 167 storrnsreported in Australia duringthe periodJanuary 1, 1950,to June30, 1961,and the boundarybetween "southern" and "northern" storms.

Tann 9 DrsrRrBUTIoNoF sroRMsWITH MoNTH or vs,q'n(96)

Southern Northern Whole of Section Section Australia

12.7 20'9 15'9 8.0 9'1 8.4 O'L 4.0 5.3 April 6.5 3.4 5'3 May 7.3 1.1 4.9 June 0 6.0 July 0.6 5.1 August 0.6 3.1 September 4.0 5.1 '7.9 't'7 October November L)' L l o'o December 25'4 16.6

No. of storms r77 452

VI. Tne PRrnrcrtoN oF SEVERELoca.l- WtNp Sronnas A good deal of effort has been directed towards evolving mealts of predicting tornadoes in the United States,and a number of empirical rules has proved at least partly successfulin practice (see Appendix II(b)). l2 R. H. CLARKE

A brief study has beenmade of I 67 Australian storms occurring sinceJanuary l, 1950,whose distribution is shown in Figure 3, with a view to elucidating some synoptic indicators. The choice of this decade was dictated by the existenceof daily synoptic maps at standard pressures,and twice-daily mean sea-levelmaps' on micro- film. The ready avallability of temperature soundiugs,ranging from 14 daily continental souldings i1 1950to 18 in 1961,also makespossible an analysisof temperature structure on days of local storms. The observatiolal inadequacieswill be readily apparent. It seemsclear that a few storms have been attributed to the wrorrg date; and the wide diversity of Aus- tralian climate inevitably leadsto blurring effectswheu all storms are lunped together. To counteract the grossesteffects of this inhomogeneity, we have distinguished,as before, "northern" and "southern" storms'

Trslr l0 wEATHERAccoMpANYlNc LocAL sroRMs (pnrQueNcv PER cENT.)

Australia 4l Southernsection 40 Northern section A<

One point requiring to be establishedis whethercondensation processes are a necessaryingredient of local storms, and for this purpose the records were searched to fi1d in what kind of weather they occur. Accompanyitrg weather was classified as "thunder", "hail", "rain", or "nothing", and each storm was assignedto one of these classifications,"thunder" taking precedenceover "hail" and so on. High accuracy is unattainable, since many press reports ntade no referenceto accompanyingweather and in suchcases synoptic reports at the uearestavailable point in spaceand time wereused. Table 10 summarizesthe results' The inclication is thus that most, if not all, local storms have their origin in moist processes,while deep convection plays an important part in at least the majority. Dust devils due to intense surface heating contribute few, if any, casesof severe local wind storms.

(a') Suface Dew Point as an Indicator The predominance of moist processesled to an investigation of the surface dew point as a factor in storm genesis,by noting the distribution of storms with respectto dew point anomaly at the (three-hourly)synoptic observation immediately precedingthe storrn, at the nearestobserving point. The resultsare showu in Figure 4' Means of theseheterogeneous dew points are as follows: I I8 "southern"storms: meandew point 55'l"F anomalY +4'1 49 "northern" storms: mean dew point 65'5"F anomalv +3'2. SEVERE LOCAL WIND STORMS IN AUSTRALIA IJ

Surface dew points are thus generally above averageprior to local storms, the proportion of exceptiousrising from 19Yofor "southern" storms to 33o/ofor "northern". This suggestsa decreasingdependence in low latitudes on surface dew point, which is at all eventshigh there,in the wet season.

(b) The Importanceof Static Stability Initially neglecting possible changes in static stability between sounding and storm we survey the dependenceof local stortns on stability by means of (l) the "showalter index", which assumes the pseudo-adiabaticlifting of a parcel from 850 mb to 500 rnb, and (B) the "surface index", which assumessuch lifting to occur from the surface. Under (A) athree-fold subclassificationwas rnadeinto: (i) storms for which the soundingnearest in time was lessthan 200 miles away (162 storms);

NORTHERN STORMS

Eo U frto f, 3 tr- tr

Fis.4.-Distritu,ion oiil*"ililH; l"ro..o..,..."n." of storms.

(ii) storms for which the nearestprecedent sounding occurred less than l2 hr before ald within 100 miles (69 storms); and (iii) storms for which the nearestprecedent sounding occurred less than 4 hr before and within 20 miles (5 storms). Under (B) only subclassifications(ii) and (iii) were used, with surfacetemperatures and pressure from the nearestpreceding synoptic observation. The results are given in Figure 5, where the 162 A(i) storms are subdivided as before into "northern" aud "southern". We seethat deep instability, as measuredby a negative"Showalter index", was rather rare some hours prior to storms, more so in the "southen" (9o/o)than in the "northern" (2911)storms. Confining our attention to subclassifications(ii) and (iii), however, does not alter this situation, which suggeststhat a local storm can, and frequently does, occur with positive "Showalter index" at least up to within a few hours of its occurrence. The distribution of storms with "surface index", shown in B(ii) and B(iii), however, shows that, on the assumptions of the "parcel method", instability for ascentfrom low levelsis nearly always presentshortly before a storm. The few exceptionsoccur in the presenceof a shallow nocturnal inversion. t4 R. H. CLARKE

days in l(ii) showed A x2 test applied to the Showalter indices on the storm that they differed significantly from those on days chosen at random (see below), the mean indices being respectively3 and 7" C'

; U z l o U tr E

-15 -1o u 5 10 rs atou,rl."('.) Fig. 5.-Distribution of "parcel" instability before occurrence of storms.These are in three groups: (i) 167 storms, divided between "northern" (49) and "southern" (118); (ii) 69 storms; (iii) 5 storms. Seetext for meaningof categories(i), (ii), (iii).. I refersto Showalterindex, i e' the lifting is assumedfrorn 850 mb to 500 mb, and 'B to "surface index", i.e. the lifting is from the surfaceto 500 mb' The "index" in eachcase is (500-mbterxperatuie-"lifted" tem- peraturer.

There is a possibility, which we now investigate, that large-scale ascent is au importalt factor in producing instability, as is apparently the case for tornadoes ip the Ulited States. [t is known that a convectively unstable air mass, if lifted to its condellsation level, becomes absolutely unstable' SEVERE LOCAL WIND STORMS IN AUSTRALIA t5

Figure 6 showsthe frequency distribution of As-,0u,,Aa-gdu, and Az-sd,,for the 69 "precedent soundings" in subclassification(ii) above. The symbol Ae-sd.ullloons (d,, at 900 mb - 0,, at the surface), and so on, d,ubeing the wet-bulb potential temperature. Here d," at the surface was obtained from synoptic observations,as above. It is seenthat there is a strong tendencyfor convectiveinstability to exist in the lower layers prior to storms, probably largely as the result of surface heating and evaporation, but no marked preponderaltceof such instability above 900 mb. Layers above 700 mb are not presented,because of the uncertainty in humidity when radiosonde"rnotor boating" occurs; but the apparenttendency is for couvectiveinstability

^20s

o- tr E5

srABlLlrY (oc) Fig. 6.-Distributionof convectiveinstability, measured by Lst-"0,,, As,e0u, Lz -s0 u , beforeoccurrence of 69storms. prior to storms to decreasemarkedly above 700 rnb. The values of Aa-09,,and A;.-sd,,preceding storms were colrlpared by meaus of a yz test with values on 267 days,chosen at randon from capital city sourrdingSin mid-sulnmerand mid-winter. This corlparison revealed no signiflcant difference between the two samples of ,\6-ed,,,,while the value of Az-a0. prior to local storrus had a mean value of 0, aud differed only at the 5% significancelevel from the random soundings, whose meau was positive. We conclude,then, that the possible role of asceutprior to a storm is usually to produce instability in the lower layers, provided asceltt frorn llear the ground to the condensatioulevel occurs. Destabiiization by ascentof layers initially above 900 mb is not a regular feature of Austraiian iocal storms. It was not, therefore, surprising to find that the presenceof a moist layer beneath a dry layer was not a common feature preceding Australian local storms.

I'r: 16 R. H. CLARKE

In the SevereLocal Storm Center, Kansas City, Mo., (Winston 1956)this feature is not held to have any especialsignificance, except in so far as it affects convective ilstability, but it has often been cited as a prerequisite for tornado formation. In Figure 7 the frequency distribution in the "precedent soundings" of Aa-nR,Az-eR, a1d Ae rR, where R is relative humidity, is shown. Large decreasesin relattve humidity are seento be rather infrequent over each of the three layers,and on only 5 of the 69 soundines was Ae-eRor A;-eR lessthan -40o,L.

7. CHANGE IN RELATIVE HUMIDITY

Fig.7.-Distribution ol relativehurlidity changes with height,l8-eR, .\r-sR,lo iR, beforeoccurrence of 69storms.

T|e mean rrixing ratio and temperatureof these precedentsoundings from 900 to 500 mb, together with the d," derived from them, extendedto 1000 nib by rneansof surface observations,are shown in Figure 8. Acknowledging the inhomogeneity of the data, we note that the "surface index" is -0'5'C, the "lifted index" +l .5"C. "showalter index" +3'0'C, while the "lifted corrdensationlevel" is 850mb and no "level of free convection" on the Kansas City definition exists. The most striking feature is the marked convective instability to 800 mb. SEVERELOCAL WIND STORMS IN AUSTRALIA I1

A further possibility favoured as an explanationby sorne forecasters(e.g.' Southern 1960; Winston 1956; Veryard 1946)is that stability changespreceding a storn are effectedby differential horizontal advectiou, and tirat, in particular, oyer- runuing cold air aheadof a cold front producesthe necessarydestabilization. This also seemsto agreewith Wallington's(1961) idea of kata-cold-frontstructure.

.^o ^O co qo \o *o

Fig. 8.-Mean soundingbefore occurrence of 69storms.

Assuming horizontal motion and conservation of potential temperature, may write t0 n0 r0 (1) dl dx dy

lus gn0,ud0 (2) )z f0lx'0tz' lus -fra_oi'gt0,ut| E: (3)

a : f (a-ur), (4)

p : -f (u-u), where the symbolshave their conventional meaningsand the dot indicates total time differentiation. From theseequations it can be readilv shown that #: -il"#-,u#,-lpv)-*?Y)l (6) 18 R. H. CLARKE

the secotldtwo terms If we apply the reasoningof charney (1941)we can show that smalier thal on the iight of (6) in synoptic scalemotions aie an order or maglitude pre-frontal environ- the first two, although it is not certain that this will be so in the ment. We proceed on the assurnptiorrthat

)29 |fu tzu -N (1) )l )z g d;' where the x-axis ilas now beeu oriented in the direction of the wind at the level under storms for discussion. The right-hand side of (7) was evaluated for each of the 31 15,000ft which adequateprior wind soundingsto 25,000ft were available, for both the results' and 20,000 ft, and using height increments of 5,000 ft. Table 11 shows

Tanle I 1 RATEoFCHANGEOFSTABILITYPRIoRToLocALsToRMs'DUEToHoRIzoNTALADvEcTIoN

Mean of 15,000 and 20,000 Ft

Mean rate of stabilization(deg per hr per 5000ft)

No. of cases(n) (no. of southernstortns in brackets) 3t(2s) 30(2s) 0.84 Standarddeviation (o) (degC per hr per 5000ft) 0.i6 o/ln 5.5 Mean time beforestolr-u (hr)

Proportion of caseswhere destabilizationwas estimated (9i,) OJ

There is thus no definite indication that destabilizationby differential horizontal advection some hours before a storm is a regular feature of Australian storms' a Estimates derived from the few soundings closely preceding a storm do not show significant differencefrom the above sample.

(c) The SynopticSituation in v'hichLocal Stornts occur The dependenceof storms on "synoptic" processes,as revealedby the M.S.L. a synoptic chart, was assessedby attempting to iocate each storm with respect to fiont, a depression, a17area of cyclogenesis,or a northerly or a southerly stream. Each storm was allocated to one of these as the most satisfactory "explanation"' priority being given to "fi'ont" over "depressiott",and so on. A small residual of hve storms could not be thus allocated. If a storrn occurred "sirnultaneously" with a front on tl.reanalysed chart (with the precisionattainable), it was classedas "frontal"; "post- if within 5' of latitude of a rnovingfront, but not on it, it was classified"pre-" or frontai". The remainder were classifiedas "in a depression" if they were within 5o latitude of a low-pressurecentre; "in a region of cyclogenesis"if a closeddepression SEVERELOCAL WIND STORMS IN AUSTRALIA 19

later developed; or in the appropriate stream if the north or south component of geostrophic wind predominated. Table 12 gives the numbers in each categorirfor 48 "northern" and 119 "southern" storms. It is seen that the maiority of local storms in both areas are connectedwith fronts, depressions,or cyclogenesisin quite a close mantter. Their frequent association with fronts should be compared with the North American figure of 87o/oquoted by Fawbush, Miller, and Starrett (1951)for tornadoes. Cold fronts apparently pro- vide the lift necessaryfor the release of convective instability, and also the con- vergencenecessary for the concentration of angular momentum. Even a weak cold front or sea breezecan produce sudden ascentof a kilometre or more (Clarke 1960, 1961). The violent tornado-scale motion is superimposedott the less violent and larger-scalecyclonic ascendingmotion.

IasI-e 12 cLAssIFIcATIoNoF sroRMsAccoRDINc ro rHE M.s.L.cHART (%)

Category ! ,"*n*" No,tt'.,n Alt I l_ At a cold front oz 58 61 At a secondarycold front 8 0 o At a stationaryfront 2 6 3 Pre-frontal 3 10 5 Post-frontal 3 0 2 In a depression 12 6 10 In a region of cyclogenesis 7 6 7 In a northerly stream 1 6 2 In a southerly stream 2 0 I Unciassified 2 6 3

Many of the storms in the more Southern part of the "llorthern" division occurred at disturbancesin the westerlies,like those in the "southern". The difference betweenthe two categories,as revealedin Table 12, suggeststhat fronts and depressions of synoptic scale are less important in determining local storm genesisin low latitudes than elsewhere. The association between fronts and depressionsoll one hand and local storms on the other is probably quite slight iu very low Australian latitudes. The connection between upper flow and Iocal storm occurrence was studied by means of stream-linesand isotachs on the 300-mb surface, where well-defined troughs and jets are commonly distinguishabie. Figure 9 shows the distribution of 167 local storms with respectto the trough line at 300 mb in a westerlyflow. It suggeststhat storms are to a large extent conditioned by upper flow, tending to occur at or downstream from a trough axis (80o/oof "southern" and 57o1oof "uorthern" storms were thus located). The proportion of storms occurring with no trough differed significantly between the two regions, being 9o/s for southern and 30o/ofor northern storms. Evidently the larger-scale organrzationof tnotion lepresentedby a trough in westerliestends to play a less important role in lower latitudes in the Eenerationof local storms. 20 R. H. CLARKE

A similar conclusionis reachedby exanining Figure 10, which showsthe displacementof local storms with respectto the 300-mbwest wind maximurn. Of the 'Jet", "southern" ston.ns,279; occurred on the high-pressureside of the and for l0o/othere was no jet within 15' latitude on either side. Correspondingfrequencies for "northern" stormswere 380,6and 3l].iorespectively. From the forecaster'spoint of view, we cau assertthat over southern Australia, severelocal storms should be expectedmainly within about 6o latitude of the jet, and at about 2 to 14' longitude ahead of a trough. The statisticsare inadequateto enableus to venture any such generalizalion for rrorthern Australia.

I1g SOUTHERN STORMS

-5O5lO15NO TROUGH LONGITUDE {C)

9.-Location of 167storms with respectto the 300-mbtrough axls.

VII. Soue CaseSruotES oF LocAL Sronus IN AUSTRALIA Some published accountsof local wind storms have been listed. Photographic evidenceis availableto show that funnel cloudshave certainly been observed in Aus- tralia, for example,the Maroug tornado, near Bendigo, or.rSeptenrber 27, l9ll (ConrmonwealthBureau of Meteorology19ll). There seerrrsno reasonto doubt that a few of thesehave beeucommensurate in violencewith their more famous overseas collnterparts. Furthermore,quite a number of deathshave beencaused by tornado- like stormsalthough, on a per capita basis,far fewer than in the United Statds,where annual deaths ascribedto tornadoes are about 200. SEVERELOCAL WIND STORMS IN AUSTRALIA 2l

o'I Russell(1891) stated, inter alia'. fear we must adrnit that they (the storms) are just as terrible in their destructive powers as those known in America; when sound growirlg trees three, and even four, feet in diameter, are snapped off within four feet of the ground, it is evident that they wield a force before which ordinary town buildings would be destroyedas if by magic", and again, ". . . if we cannot do anything to prevent thern, we can at leastacquire such a knowledge of their frequency and force that it will be possiblein the future to insure against the damage caused by them." He went on to describea violent storm on February 23, 1891, "with unnristakableevidence of gyratory motion", which moved at about 50 rn.p.h. and

1o l u10 e

-10 NO

LATTTUDE(o) Fig. 10.-Location of I67 stormswith respectto the 300-mbjet axis. was traceableover 175 miles south from Goondiwindi, Qld. The path of damagewas said to have expanded from 300 yd to f mile during its later life. A description of the large-scalepressure field left little doubt that the situation was a frontal one, with cyclogenesisoccurring in southern Queensland. The same author in a later publication (1898) also listed many watersponts occurring in coastal areas of New South Wales, some of which "became tornadoes" when they moved inland.

(a) The Collie Storm of April 6, 1960 The most recent published case study is that of Southern (1960) describinga local storm near Collie, approximately 100 miles south of Perth. Darnage to the iarrah forest 20 miles lons and 12 chains wide in a direction from about WNW. 22 R. H. CLARKE

to ESE. was unusually severe,and evidence of rotary motiou is supplied by the remark that trees fell towards SSE. on the uorthern side and towards NNE. orr the southern. This feature is apparently unique among Australian storm descriptions' It accords well with some of Letzmann's types, as quoted by Miildner (1950), and with Miildner's own observationsnear Nuremberg. Both the latter and the Collie storm, as far as can be judged, agreemost closely with Letzmann's type III(c), which implies spiral inflow at 60" to the radius vector, combined with translatiou at one- sixth of the symmetrical spiralling motion. The translation rate of the Collie storm is not kr-rown,but a speedof 20 m.p.h. superimposedon a spirallinginflow of 120m'p'h' would appear to be roughly consistentwith the facts known'

Fig.1 1.-Isohyets for 24hr endedMarch 14,195'7 , and loca1 storm tracks on the afternoon of March13.

Southern's photographs rnake intelligible a referencein aboriginal rnythology. In Elkin (1956) we read that Kunapipi, the legendaryancestt'al mother, "gave birth to men and women. Her way was prepared by the rainbow serpent. This syrnbolises the storms, u,hichclear a road through the trees . . .".

(b) The Storms of March 13, 1957, in Victoria and the Riveritn On March 13, 1951, a seriesof tornado squailsof narrow dimensionand con- siderable intensity crossed Victoria and southeru New South Wales. Their paths are shown in Figure I 1. The detailsof this picture are uncertain,brtt there are grounds for believing that the interpolations on which the storm tracks are basedare approximately correct. Length in miles and width in yards are entered where known, but information is naturally incomplete. Other storm tracks may have occurred. and SEVERE LOCAL WIND STORMS IN AUSTRALIA ZJ those marlcedcould have beenlonger than shown. Track C was inadequatelyinvesti- gated and could have resulted from incorrect interpretation of information relating to two or n'rore tracks, differently oriented. These storm paths were traced partly by personai examination some weeks after the event, but largely by telephone and postal communicationwith eyewitnessesand property owners.

Fig. l2.-storm damage,March 13, 1957,near Swan Hill, Y)c. (a)An isolatedriverred gum tree (E. camaldulensrs)on the outskiftsof a woocledarea; (b) debrisfror-n a defoliatedyeliqw box (E. melliodora\;(c) danage to soundtimber in a standof riverred gum; (d)destruction of isolatedyellow box.

Damage consistedmainly of destruction of trees,some large yellow box (Euca- lyptus melliodora) and red gums (E. cantaldulensis)having been blown over or had branchesremoved. Some typical damageto the two types of tlee is shown in Figure 12. A woolshedwas unroofed near Moulamein, a houseur.rroofed and chimneyblown l

24 R. H. cLARKE

down near Swan Hill. Eyewitnessaccourlts from this area did not mention a funnel cloud or whirling motion, but one stated,". the most severestorm I call ever l remember.It grew dark as night (at 5 p.m.) and the wind wasvery high. I got fright-

.L ened and took shelterin the house." A garagewas blowu away by storm C at Derrinal, Vic., and D and Eblew roof tiles offhousesand dernolishedchimneys and fencesin narrow paths through Melbourne suburbs. Also shown in Figure 1l are isohyetsfor the 24 hr ended 0900 E.A.S.T., Marclr 14, 1957,which are believedto represent,at leastin tl,e plain areas,maiuly rain which fell about the time of the frontal passage. Over this featurelessterrain there is a nTarkedter.rdency for "streakiness"of the rainfall in a WNW. to ESE. direction,which suggeststhe presenceof travellinglreso-systems. Hail was reported from somepoints in the Riverina.

Fig.13.-Fronts and pressure field, 1500 E.A.S.T., March 13, 1957.

Details of wind speedestirnates in storm track -8,near Swau Hill, are given iu Appendix L The mean value of 143m.p.h. by method A, and 94 m.p.h. (corrected to 110 m.p.h. by means of the ratio ViVn to rnake tl-retwo estimatescomparable at approximatelytree-top height) by method .8, are in poor agreemeut. Estin-rates by nrethodI arrdtheir standarddeviation appear excessive, and somepossible reasons are advancedin Appendix I to accourrtfor this. It seemspossible, on the basisof method.B and other reasoningsuggested in AppendixI, to concludethat the strongest wind exceeded100 m.p.h., which comparesclosely with the figure of 100 kt derived by Lamb (1957)for an Englishtornado. Valuesof e"given in Table 16,measuring the deviationof the wind as indicated by the displacemelltof broken branchesto the right of the storm track, areallpositive. This is evidenceof cyclonicmotiorr superimposedon translation,although the evidenceof variation of z acrossthe track is not as conclusiveas could be wished. This SEVERE LOCAL WIND STORMS IN AUSTRALIA 25 is the pattern, mutatis mutandis,indicated by Letzmann's type II(d) (Mtildner 1950). A velocity of translation of about 40 m.p.h., coupled with rotation of 60-80 m.p.h. at a radius of about 100 yd, appearsto fit the facts as known. An attempt to reconstructthe detailedpressure field at i500 E.A.S.T.,roughly 2 hr before the locai storms began,is shown in Figure 13. The method used was that of Fujita (1955)and Fujita, Newstein,and Tepper (1956),usingindividual barograms to interpolate pressurevalues between observing points, and projecting in an easterly direction. The barograms available in Victoria are aimost all weekly, so that time resolution is poor, their spatial disposition was unsatisfactory, and the "average density" of I per 7300 sq. miles comparesunfavourably with that in Kansas of I per 1100sq. miles, so that one could not expectmore than mediocresuccess with this technique.

WEAK CALMAT ]IMEs ;sr*d"c cALFr TO MODERATE i, *-J tviv V ..4

t l

4 L lr us

,;,- -;,,.; ;".. ;$:n:;,,;";;;",""o","'o**** (adjusted for separation) superirnposed." The pressure trace has been ntagnified and slightly sin-rplified. The wind direction trace has been analysed into six periods; the limits and median direction have been entered for each period, togethel with a subjective estimate of the speed, based on tl-redirection record.

Wind data were unfortunately very inadequate in the neighbourhood of the meso-high and its "wake" low, but the wind direction sequenceat Alexandra shows cleariy the airflow from the meso-high,frorn the west during the plessurejump, and weakly from the east into the wake low during the period of falling pressure,before settling down to a steady sorith-westwind during the period of steady pressurerise. Figure 14 shows the pressule trace at Mangalore with Alexandra's wind direction (and qualitatively estimated speed), adjusted for spatial separation, superinposed or-rit. One may note the decayingleading cold front, which had for some hours been untraceable over Victoria. The vigorous cold front, near which the meso-features occurred, had developedovernight as a "secondary" frout on a line NNW. through Kangaroo Island, where cyclogenesisoccurred. Widespread thunderstonns accon- panied this development. The meso-systensalternate along the front, and very steep pressuregradients 'Jump". are to be found at the The two mesoJows appear to have been conttected in some way with the local storm systemsA, B oll one hand, and Con the other, as is frequently the case in North A-merica. R, H, CLARKE

Isochrouesof the main cold front at two-houriy intervals,showing accelera- tion after the initial forrnative stage,are given in Figure 15. A markedpressure jump occurred with the frontal passageat all the barograph stationsit passed,at leastas

Fig. 15.-Isochronesof the main cold fror-rtacloss Victoria, March 13, 1957.

140 150

N C. (7OO ME) I

(9s) \

o 5OO MB f, F35 F \t urr\/"' A5O MB

13o 15o ro,]llru'.,', Fig. 16.-Map showingthe 850-mbjet, the 500-rttb jet, and the 700-rnb "no-changeline" at 1400 E.A.S.T.,March 13, 1957. Maximunrmeasured winds in the two jets and their iocation are given in brackets. lbr east as Canberra. It reached an amplitude of 4 mb at Gabo Island, Sale, and Scoresby (altitude 300 f0, but only half that value at Mt. Dandenong (2080 ft) onl-y 5 miles away. This strongly supports the hypotiresis tliat the pressure jump is mainly due to ]ow-level cold air advectior.r. SEVERE LOCAL WIND STORMS IN AUSTRALIA 27

Beebe and Bates (1955), Winston (.1956),and Fawbush, Miller, arrd Starrett (195i) have used a connection betweenNorth American tornado ocaurrenceand the 500-mb flow pattern. For purposesof comparison with Beebeand Bates,the 500-mb and 850-mboJets"at 1400E.A.S.T., March 13. 1957,are reproducedin Figure 16, which is seen to be sirnilar to their Figure 8, which they claim attends many major outbreaks of tornadoes. The forecastingsystem proposed by Beebeand Bates would have resulted in an alert for possibletornadoes being issuedfor a strip roughly east- west through Mildura. It was in this strip that storms A and B occurted, some 3 hr afterwards.

1l l I I I I \: \t I

r3s Fig. 11.-Map showingM.S.L. fronts, isotherms,and dew point, 1500hr, March 13, 1957.

Whiting (1957)flnds, in a study of tornado occurrencein relation to the M.S.L. chart, that it is necessary,for tornado fonnation in North America, to have an isobar (these being drawn at intervals of 1 mb) crossing 4 isotherms (drawn at intervals of 5"F) nearly perpendiculady in less than 200 miles, that the dew point should be above 50"F, the temperature above 80"F in summet, and that there should be at least 2 isobars below 1016mb. It is seen(Fig. l7) that all theseconditions were satis- fied in the Riverina at 1500hr on March 13, 1957,in a strip where storms A andB occurred. The so-called "no-change line" at 700 mb, separating an area of v/alm advectionfrom one of cold advection,is held to have someprognostic value in tornado situations (van Thullenar, quoted by Winston 1956). Its position at 1400 E.A.S.T. is shown in Figure 16. Our analysis above of horizontal advection and destabiliza- tion tends to discredit the idea that the "no-change liue" is an important prognostic tool, which also agreeswith the comment by Winston (1956,p. 20). The pre-frontal "moist tongue" at ground level appears at 1500hr as a dew point maximum near Hay, N.S.W. (Fig. 17). It moved rapidly eastward (about 70 m.p.h.) and appearsto have been due to rainfall. It could not have been trans- ported atlhat speedby winds in the lowest 10,000ft. 28 R. H. CLARKE

Tepper's (1950) concept of the origin of tornadoes on pressurejump lines is not supportedby the SevereLocal Storms staffat KansasCity. In the caseof the storms of March 13, 1957,there certainly was a pressurejump line, most intenseson,e 200 miles south of the main storms A and B. The exact time relation betweenthe storms ar-rd 'Jump" the is not known, but the latter is well explained in the present caseas the static result of very rapid cold air advection in low levels,although dynamic effects, due to vertical accelerations, are doubtless also present. On March 13-14, 1957, four radiosonde flights were made in the Melbourne area: the first (I) at 0911 E.A.S.T. on March l3 at Aspendalewas abandonedowing to a receiverdefect at 883 mb, while II (1300at Laverton),Ill (1930at Aspendale), and IV (1300 on March 14 at Laverton) reachedinto the stratosphere. A flight (V)

ti i\l I tl

2422v rrvE (e n s,r )

MARCH ta, t957 MARCH 13, 1957 Fig. i8.-Wet-bulb potentialtemperature ('C) plottedon time-heightaxes, Melbourne, March 13-14,1957. Sounding V wasmade at Adelaideat 1300E.A.S.T. The front is suggestedby shading. at Adelaideat 1300E.A.S.T. on March l3, sotne4 hr after the frontal passage,has also been used in the construction of Figure 18, together with surface data from Melbourne. Figure l8 shows the distribution of wet-bulb potential temperaturewith time and height. If we assumeconservation of 0,,, and also that variations normal to the front far outweigh those parallel to it, we seethat the front usheredin air of lower total energy, the decreasebeing measurable,less than 2l hr after the frontal passage,up to the 300-mb level. The diagram suggestsparticle trajectories rising from a pre-frontal 700-800 mb to about 400 mb between 1300 and 1930 hr, and, with lessconfldence, a subsequentsinking to about 500 mb by 1300hr on March 14. The suggestedtrue frontal zone in Figure l8 is shown by hatching. It indicatesa mean slope of I in 20 over a distanceof 90 miles, about twice that of Newton's (1950)squall line, if one applies the samecriterior, to fix it. SEVERELOCAL WIND STORMS IN AUSTRALIA 29

A rather different picture emergesif one attempts to delineate the front by means of potential temperature (Fig. 19). An apparent frontal inversion occurs on flights III, IV, and V which can be compared with Newton's "post-squall subsidence inversion" and with Wallington's (1961) "temperature front". We cannot demon- strate by soundingsthat the front of Figure 18 did not "overhang", on the model of Wallington's "air mass front", but this seems improbable, since the weather was quiet before the passageat 1706 hr. Nevertheless,both of Wallington's "fronts" i can be found on flight III, in the form of inversions at 750 and 387 mb. An important difference between our case and Newton's is the post-squall cold front in the latter, certainly not presentin the former, if the main event at 1706hr is equatedto Newton's squall.

e tr

ooa06 022422V I 16 AIME (EA5T) MARCHt4 1957 Fig. I9.-Potentialtemperature on time-heightaxes, Melbourne, March 13-14,1957. Soundingv wasmade at Adelaideat 1300E.A.S.T. The front is suggestedby shading.

Temperature changes near the tropopause are often largely due to vertical movements. Neglecting temperature advection on constant pressure surfaces,and non-adiabatic processes,we can write, with conventional symbolism, dp ne co: lbq (8) dt nlw and )cr : -w (e) divoV * (#lv*) Tabte 13 shows mean values of co,w, and divrV derived from flights II and III on these assumptions,between 1300 and 1930 hr. These figures, although not conclusive, indicate the probability of strong ascentin the upper troposphereduring the period embracing the frontal passage. 30 R. H. CLARKE

An attempt has been made to representthe two-dimensional wind field relative to the front in Figure 20, in the lnanner usedby Clarke (1961). This picture helps to explain some of the observed facts. such as the subsidenceinversions, the steeply

Tenre 13 MEANvALUEs oF DIVERGENcEAND AscENT ovrn 6$ HR oN MARcH13' 1957

Level t0itt n0/np o @b) ("C/hr) ("C/mb) 0nb/hr)

200 1.6 -0'42 3.8 0'9x 10-5 250 0'68 -0.30 2.2 6'9x 10-5 300 -0' 51 -0 .05 - t0'2 0 350 -0. 51 -0.05 -10.2

)

09 Oa 05 ot 23 2l t9 7tat3l197 rrve(ensr,)

13, r957 MARCH 14,1957 MARCH 13-14, 1957. Fig. 20.-streamlines relativeto the front, on time-height axes,Melbourne, March

(lsoplethsof Vo : f' fr-,) dpinknotmbx 10-3') Js

front sloping front, and the sharp increasein wind with the frontal passage. The *u, uppur.ntly not in any sensea kata-front, at least up to the 400-mb level. The soundings of flights II and III are shown in Figure 21. The pre-frontal sounding exhibits convectiveinstability up to 700 mb' Comparing surfaceconditions SEVERE LOCAL WIND STORMS IN AUSTRALIA 31 at Swan Hill, we seethat intense instability could well have occurred in the Riverina, in the lower levels. To use the routine recommendedby Winston (1956)for tornado forecasting,we summarize the analysisof flight II in Table 14.

OO OrO

U )E o

t I

Fig. 2l.-soundings II and IlI, Melbourne,March 13' 1957 (respectivelypre-frontal and post-frontal)'

TAer-e 14 ANALystsoF PREcEDENT souNDING, i300 e.e.s,r.rtaarcH 13, 1957

Lifted I Level Lifted Showalter Condensation of Free Itidex Index i Moist Layer Level Convection (mb) (mb) ("C) ('c)

Meibourne 850 3'0 , Wholetroposphere -2.5 Riverina 870 840 I +1 0 , WholetroPosPhere (possiblevalues)

In view of the lifting deduced from Figure 7'7,we night expect deep, intense instability with the frontal passagein the Riverina. JL R. H. CLARKE

To summarize, the storms in south-eastern Australia on March 13, 1957, occurredon, or very near, anintense,steep cold front, which causedrapid ascentofthe warm air and release of convective instability. The Riverina storms probably pro- ducedwinds exceeding100 m.p.h., with lelatively weak gyratory motion superimposed on translation. They occurred in circumstances sirnilar to those occurring with North American tornadoes, and could have been forecast by the use of the techniques describeciby Winston (1956)and Whiting (1957). The structure of the front resembled in many respectsthat of a squall line studied by Newton.

(c) The Warriewood " Willy-wil[y" On July 9, 1951, in the outer Sydney suburb of Warriewood, at about 0930 E.A.S.T., a severelocal "willy-willy" was reported,and on-siteinvestigations were made two days after the event. A path of damage about 100 yd wide was tracked westward from a point about half a mile inland from the shore, near Turrimetta

,oot4l'

Fig.22.-B.roadfeatures of the pathroflhe Warriewood"wjlly-willy", July 9,

Head, for a distanceof approximatelythree miles,after which no further damagecouid be traced. Eyewitnessesdid not seea funnel cloud, but commented upon the roaring of the wind, which was by two witnessesobserved suddenly to reversedirection. A heavy shower accompaniedthe storm, and some hail was reported near the coast. Trees were blown down at a nnmber of points near the path of the storm, some of them clearly indicating the rotary wind structure. Two houseswere blown off their brick piers and largely destroyed, one of them a six-roomed fibro-cement cottage structurally complete but lacking roof tiles. This was moved bodily through 10 ft. The bulk of the damage, however, was inflicted on glasshouses,of which 66 were damaged; the losswas estimatedat f40,000and27,000 panes of glasswere replaced. During the storm glass was observedflying through the air at a distance of 400 ft to the right of the centre of the storrn track, although none could be found there later. Glass fragments were found embedded in a number of trees. Many minor sheds and buildings were unroofed, and the distribution of the wreckage clearly SEVERELOCAL WIND STORMS IN AUSTRALIA 3Z showsthe rotary nature of the storm. There wasa lso evidenceof "skipping", especiaily on lee slopes,and of intensification of danage oll windward slopes. Figures 22 and 23 represeutthe path of the storm, the flrst showing the broad features over the ascertainablelength of the path, together with physical features, and the secoud that part which was examined in most detail. Arrows indicate the direction of displacernentof trees,sheets of iron, and other debris. An unexpected feature is the apparent "skipping" aud swerviug into the centre of the clump of trees (Fig. 23) and out again, as indicated by considerableisolated damage in the copse. The observation of flying glass 400 ft to the north of the track centre is an indication of the scale of the whorl. Figure 24 illustrates the damage which occurred.

AIR.BORNE GLASS OBSERVED DURING fl sroRM To EAsr F Ll oF THls Housa t I

=+i>-=--- G -)a F ..- Til .- ..-- rtutt oF GLASS z flf \' \ FOUND ON GRoUND U I_" o ,J ;rE- E t:____- mF a- =zz?2.," o -ti1l1i.; *ra-a- tli,liij BARE PADDOCK

il GLASS EMBEDDED n tN FENCE PO5T5 II g^A t\:'l c€ ii ,ry 4\ '\ /) r\ .-L v-! :" -; -:-::f uNDAMAGET q' Q -\+\ .-. +'--- ".^.u-"orr.ilfll\illl 2.^\ - A .i tj; 7-- DAMAGEoGLAss-HousE IIUUUU w--L. =;- DAMAGED WALL (:; o e .r) c r DAMAGED RoOF

\ DlREcTloN OF THROW D FROM LOCATION ENCIRCLED

O ISOLATED OAMAGE I@ FT 3 {) woooeo neea {l Fig. 23-Some details of the damage causedby the "willy-willy" in the enclosedarea of Figvre22.

Wind speedestimates were made by the broken glassrnethod, and by the two methods, A and B, based or-rtree damage (seeAppendix I). Agreement betweenthe glassmethod and method B is satisfactory,showittg wind speedsof 83 and 82 m.p.h. respectivelyat low levels,while the estimateof 143m.p.h. by method I is thought to be unreliable,for reasonsgiven in Appendix I. The synoptic maps for this period (Fig. 25) show a deepeningeast aoast depression, at 0900 hr on July 9, 1957,r1ear Newcastle, while the 850-mbdepression was centred near Coonabarabran. A moderateESE. wind was blowillg in the Sydney area, with bursts of rain and occasionalthunder, Twenty-four hour isohyetsshow a maximum (3'03 in.) at Wyong for the day ended 0900 on July 9, and at Nowla (6.11in.) for that ended0900 on July 10,a translationrate of about 5 m.p.h. towards SSW. These locations roughly coincided with the axis of a trough which lay east- west through the storm area approximatelyat the time of the "willy-willy", and which appears to mark the position of maximum low-level convergence. For this reason,the 0900 sounding at Williamtown about 70 miles to the north on July 9 (Fig. 26) may not be representativefor Warriewood at 0930. J+ R. H. CLARKE

Fig. 24.-stortr damage,July 9, 1957,near Warriewood,N.S.W. (a) A cottagemoved from its piers, location 1, Figure 22; (b) sound timber' felled, location 2; (c) a partially destroyed glasshouse,location 4. SEVERE LOCAL WIND STORMS IN AUSTRALIA Jf

We summarizethis sounding, adjusting for the higher 0900 surfacetemperatures at Sydney,thus: L.C.L. (lifted condensationlevel) 930 mb; L.F.C. (level of free convection) 900 mb; L.I. (lifted index) 0; S.I. (Showalter index) *2'0"C, while the decreasein 0. betweenthe surface and 600 mb means that large-scaleascent in the trough could have enhancedthe conditional instability in these ]evels. As there was no front involved and the only jet streams were far (10" lat.) to the north, sotne of the tornado forecasting techniques advocated by Winston cannot be applied here. It is. however, an observed fact that with low-level cou- vergence,deep convection, a very moist, conditionally unstable stream, and suffici- ently light winds, waterspouts are observed to develop beneath cumulo-nimbus at

Fig. 25.-The M.S.L. isobars0900 E'A'S'T', July 9, 1957' The location of the storm at 0930is shownby a dot'

was investigated in detail, but is not described here' 'i1

I 36 R. H. cLARKE I I I I It is concluded that coastal thunderstorms occasionally evolve waterspouts, and that thesemay drift inland and produce effectssuch as those describedat Warrie- wood. This may happen with a frequency approaching one per year over an east coast area of the size of Sydney and suburbs, and sonewhat more rarely about the south coast of the continent, while observerson a single ship operatingin eastcoastal waters might expect to seean averageof roughly one waterspout per year, according : to Gordon's (1951)statistics. Gordon's (1951)map of waterspoutfrequency suggests

oo oo I ,ao ,' ,uo

Fig. 26.-Temperatureand humiditysounding, Williamtown,July 9, 1957,at 0900hr.

a maximurn in the vicinity of the Australian east coast and in the Soutli Tasman Sea, possibly linked with the warm ocearlcurrent found there. It may be noted that the mean ocean temperature off Sydney in July is 15.7"C and incleasesrapidly away from the coast (Konink. Ned. Met. Inst. 1947)so that the offshore stream on July 9 north of the trough should have been rapidly destabilizedin low levels.

(d) The Adelaide"Tornedo" of July 17, 1958 At about 1430 E.A.S.T. on this day, what was describedin the pressas a tornado passedthrough Adelaide, and was timed by power breakdowns,which showed that it pursued a coursethrough southern suburbs,in a direction slightly north ofeast, SEVERE LOCAL WIND STORMS IN AUSTRALIA )l at a speedof 40 m.p.l-r.,and resulteclin damageto many houses,which wereunroofed, in a 50-ird-widestrip. Loss of life and severeinjury were feported' It was accom- panied by severethunder and hail. A cotton-palm tree in hospital grounds was broken lear the base,and applicationof method I for determiningwind velocity yielded 126 m.p.h. Evidence of cyclonic rotary rnotion in the storm was provided ty the throw of debris. Photographs of storrn damageare presentedin Figure 27' A similar stol.m, of such ferocity as to inspire the beholder to lie flat on the ground to avoid injury, was observed itl ar.ropen field at Mulgundawah, uear the

Photos:IaJ Atlvertiser, (b), (c), (cl) Ctuonicle' Adelaide.

Fig.27.-stornrdar.nage, luly 17,1958,at Adelaide.(a) A leaflessstreet tree falls on a carin thecity; (biroofingiron ripped offat Parkside(c)agarage at Beaumontlost its roofand a wallcollapsed on peopleand cars, causing a death;(r1) roofing iron in a frontgarden at Fullarton. mouth of the Murray, later in the afternoon, travellingrapidly towards the east, devastatingscrub it passedover, emitting a loud noise,aud whirling debrishigh in the air. It was rlot possibledefinitely to link this storm with that which earlierhad passed over Adelaide. The "toruado" appearsto have occurred ol1 or very near, aud to have been ilitiated by, a rapidly moving cold fror.rt,which resultedin ascent of pre-frontal air. alreadyconvectively and conditionallyunstable (Fig. 28). An analysisof the sounding yieldsthe following:L.C.L.910 mb; L.F.C.880mb; L'1. +0'5"C; S'I' +2'5"C' ---- - ril til iii iri iij ii 38 R, H. CLARKE i:1 evidencedby the decrease ,j, Strong convectiveinstability occurs in the lower levels,as of over 3'C in the 0*between the surfaceand 800 mb. ii; It is noteworthy that the 850-mb and 500-mb wind maxima at 0900 hr on this day intersectedat a point near Adelaide. No "moisture tongue" in the pre-frc'ntal air is to be found, since the front was a "secondary", i.e. the air on both sides of it had markedly polar characteristics,with only siight temperaturecontrasts. It arose as the result of cyclogenesissouth of Adelaide.

ocr bo

) @

28.-Temperature and humidity sound- Adelaide, July 17, 1958, at 0900 hr E.A.S.T.

Other American tornado criteria, such as those of Whiting (195'7),do not assist in the prediction of this storm.

(e) The Alice Springs "Huriicane" of October9, '/958 On this date at 1345 hr E.A.S.T. a "hurticane" was reported by the press to have causedconsiderable darnage in Alice Springs. It unroofed eight houses,whirled corrugated iron sheets300 ft high, blew down power lines and trees, and injured a local resident, and as a result of accompanyingrain, some houseswere "inches deep in water". SEVERE LOCAL WIND STORMS IN AUSTRALIA 39

The damaged arca was reported to be a strip 50 yd wide and 200 yd long' Cyclonic rotatiou was noted, and wind speedof more than 100 m.p.h. was estimated by a Bureau of Meteorology observer who witnessedit. Vigorous thunder activity was observed,but no hail. Autographic records from the aerodrome meteorological oftce, seven miles distant to the south, showed that very rapid cooling was simul- taneouswith a wind changefrom north to west and a squall to 53 m.p.h. The M.S.L. synoptic chart sequencesindicate a double cold front associated with a depressionnear the head of the Bight. The first member,which was untraceable south of Marree, in its northern section consistedof a line of thunderstormswith a

Fig. 29.-Temperatureand hurniditysounding at Alice Springs,October 9, 1958,at 0900E.A.S'T' squally wiud change,aud apparentlydissolved during the eveningof October 9, while the second passedAlice Springs some time before 0300 C.S.T. on October 10. The "hurricane" occurred on, or very near,the leadingfront, which was apparentlya major disturbance and not an isolated thunderstorm. Marked backing of the wind up to 15,000ft occurredin the 12 hr from 0930to 2130C.S.T., and up to 8000ft (the limit of the 1530 sounding) in the 6 hr from 0930 to 1530. It is likely that sufficient data would yield a picture for the strearnlinesrelative to this front qualitatively sirnilar to Figure 20, but showing a secondcirculation, correspondingto the secondfront. The sounding at Alice Springs at 0900 E.A.S.T. on October 9 is shown in Figure 29. This is remarkable-at this station-for the depth of the moist layer, and 40 R. H. CLARKE

to the for the strong convective instability (especiallyif temperatgresare adjusted yields: L'C'L' screenvalues at1230 hr) up to 550 rnb. The routine stability analysis 715 r.nb; L.F.C. 115 mb; L.I. -1'0'C; S.I. 0'C. Overalltemperature and humidity Figute 29, changefrom 0900 on October 9 to 0900 on October I 0 are also depictedin part of it at to show how radical a modificatior-rof the air mass was involved, at least the time of the storm. this Attempts to apply sofre North American tornado forecastingtechniques to 'Jets" of storm fail cofnpletely. The are far to the south,.and the surface criterja frontal Whiting (1957) are not met. It is concludedthat moisture,instability, and ascent are the only relevant factors:

(f) The Numurlcah " Wind Terror", September24, 1960 "tore path of A storm which occurred at 1300 E.A.S.T. at Nutnurkah, Vic., a blowitlg destruction" through the towu, "ripping the roofs off about 60 houses", hall to down trees,and blocking roads. An emergencydepot was set up at the town assistdistressed families. An on-site examinatiotr was made two days later, and the path of the stortn the plotted from information available. This revealedthat through the town itself path was nearly straight, on a heading of 170', and 50-75 yd wide. There was definite 400- evidenceof cyclonic rotation; for exaurple,sheets of iron were seen"whirling of the 500ft high". The speedof translation of the storm, as tirned over 10 miles south wind town, was estirnatedat 40 m.p.h., and the Shire Engineerestimated maximum photo- speed,from an inspection of the damage,at about 90 m'p'h' Typical damage graphs are showtl in Figure 30. other reports of damage on this day fronl Bearii, Bunbartha' Sheppartorr, Violet Town, and Buxton districts suggestthat this same storm may have retained a width its identity ovef about 100miles, moving towards 170". It was statedto have in excessof 200 Yd at Violet Town. miles Another storm on the same day devastated forest country about 10 a local north-west of Albury, before moving south into Victoria. A report from quarter grazierstated: "A heavily timbered red gum and yellow box forest about a of a mjle wide was almost completely wrecked. Huge trees that were uot uprooted the were almost completely stripped of their limbs and branches. . . . I would say wind must have been in the vicinity of 100 miles per hour'" The synoptic charts show a resemblanceto those for March 13, 1951. A leading cold front had almost completely dissolvedearly on September24, and the erstwhile secondaryfront increasedrapidly in vigour during the day, its deepeningdepression pressure moving ESE. acrossthe Bight at about 28 m.p.h. to be centred (with central reached 980 mL; near King Island at 1500 E.A.S.T, on September 24. This front 'Jets" were Numurkah at, or very near, the time of the storm. The 500- and 850-mb direction, almost coincident over Numurkah at 0900, and both lay in a north-south (cf. Fig. 16). as contrasted with the mainly east-westorientation on March 13, 1957 in the In both cases(March 73,7957, and septembet 24, 1960)the storms moved direction of the 500-mb flow. SEVERE LOCAL WIND STORMS IN AUSTRALIA 4l

:ri:.ii1i:# -;1,;','.11:

Photos f6, and (c): Herald and lileekly Times, Melbourne.

Fig. 30.-Storm damageat Numurkah, Vic., on September24, 1960. (a) A three- roomeddwelling scattered and treesstripped; (b) a store-roomcompletely demolished; (c) a church hall unroofed. T I

42 R. H. cLARKE

The 0900 E.A.S.T. soundingat Melbourneon septembet24 (Fig. 3l) was -2 analysedas follows: L.C.L. 860mb; L.F'C' 735mb; L.l, 5"C; S'I' *l'0"C; dept-hof moist layer:. 700 mb. Convectiveinstability at noon was evidentlyvery markedup to 675 mb. Other Americantornado forecasting criteria proved inappli- cablein this case.

E 3 0

E o

Fig. 3l.-Temperatureand hunriditysounding at Mel- bourne,Septerrber 24, 1960,at 0900E.A.S.T.

VIII. CoNct-usIoNS Severelocal wind storms in Australia have been shown to have a frequency sornewhat similar to that of toruadoes in some parts of North America. Few Aus- tralian storms approach American tornadoesin intensity, most being much lesssevere. Wind speedsin Australiall stolms appear rarely to exceed 120 m.p'h' and speeds in excessof 200 m.p.h., such as those deduced,for example,by van Tassel(1955)' Hoecker(1960), and Fujita (1960)in the U.S.A., are,in the light of availableevidence, of negligiblefrequency. There is sorre reasonto believethat the Perth area of Western Australia is worse affiicted, with regard to both frequency and severity, than other capital city areas, but data fronr tropical regions are too llteagre for gerteralization' Many points of similarity will be noted between Australian local storms and New Zealand "tornadoes" as describeclby Seelye(1945). SEVERE LOCAL WIND STORMS IN AUSTRALIA +J

Local wind storms in temperate latitudes may be divided for convenience into two broad,classes: (1) Those which form at cold fronts, or in intense depressions,on the eastern side of an upper level trough in the westerlies. These are probably, in the main, the more severe. (2) Those which form in thunderstorms in the absenceof a front or intense depression. In either case,a waterspout is observedwhen the storm is over water. Type 2 appears to be relatively common in the tropics and over the walm coastal waters of easternAustralia, and with general onshore winds may drift inland and causeconsiderable damage, Severalwaterspouts have been observed beneath a singlecloud mass (Russell1898). No satisfactory statement can be made about the mechanism of generation of local wind storms, in many of which evidenceof rotary motion has been found. Time-lapsephotography of roll clouds accompanyingfrontal squallstends to discount that proposed by Wegener (1928), for the rate of rotation is not spectacular. The conditions necessaryfor generationof local storms, as far as this survey can establish, include (l) the presenceof a powerful lifting rnechanism,such as a front, usually accompaniedby thunder and a "pressurejump". This condition probably becotnes less important in the tropics. (2) The presellceof sufficient moisture for deep convection in tire air to be lifted. Here pre-frontal rain and thunder, and perhaps the existenceof a preliminary decayingfront, may be a predisposing factor. We might summarize this condition by requiring the L.F.C. to be below the 700-mb level. (3) The presenceof moist instability, indicated by a negative "surface index", and a o'showaiter smail value of "lifted index" and index"; and convectiveinstability in at least the lowest kilometre. In addition, there is some evidencethat an intersection or coincidence of the M.S.L. projection of the 850-mb and 500-mbjets may be a factor of importance nnder some circumstancesin determining the location of storm gellesis,as is found to be the casein the U.S.A.

IX. AcTNowLEDGMENTS The author is glad to thauk Dr. F. A. Bersotl, who began this inquiry and has been closely associatedwith it; Mr. F. K. Ball for useful suggestions;Mr' R. Dowling, of "Glenesk", via Swan Hill, for continuedassistance; Mr. R. G. Pearson, of the Division of Forest Products,C.S.I.R.O.; Messrs.D. G' Reid, L. G' Veitch, N. G. Richards, aud others who contributed to the field work; the Director, Bureau of Meteorology, and his staff, for courteously providing accessto files and other matter; and others who assistedin the collection of material.

X. RsF'rnehtcrs AsuroN,H. T. (1960).-Aust. Met. Mag.No. 30: 44. Brrss,R. G., andBarrs, F. G. (1955).-Mon.Weatlt. Rev., Wash,83: 1. CHnnuev,J. (194'7).-Geofys.Publ. 11: 2. 44 R. H. CLARKE clAnxr, R. H. (1960).-"Seminaron Rain." vol. 5. Pap.1ll1. (Mimeo.) (commonw.Bur. Met.: Melbourne.) Cunre, R. H. (1961).-J.Met.18:'115. Corurvowwent-rnBunmu or MpreoRoI-ocv(191l).-lust. Mort. Weath'Rep' 2(9). DEAcoN,E. L. (1955).-Quart.J. R. Met. Soc.81:562. DuNsran, R. R. (1956).-Aust.Met. Mag. No. i3: 47. ElruN, A. P. (I956).-"The AustralianAborigines." 3rd Ed. p. 216.(Angus and Robertson:Sydney.) Fawnusu, E. J., Mrrr-pn,R. C., and St,rnnrrt, L. G. (1951).-Bull. Amer' Met. Soc' 32: l. FurIr,r, T. (1955).-Tellus 7: 405. FuJrrA,T. (1960).-U.S.Weath. Bur. Res.Pap. No. 42. Furru, T., NewsretN,H., and Tewen, M. (1956).-U.S. Weath. Bur. Res.Pap. No. 39' GolosrptN, S, (Ed.) (1938).-"Modern Developmentsin Fiuid Dynamics."Vol. I. p. 15. (Clarendon Press: Oxford.) Gonnox, A. H. (1951).-\il'eather6: 364. Hoecrrn, W. H., Jr. (1960).-Mon. Ileath. Rev., Wash.88: 167. HouNAM,C. E.'(1958).-lust. Met. Mag. No. 20: 1. KoNrNx. Nep. MBr. Insr. (1947).-'"SeaAreas around Australia, Oceanographicaland Meteorolo- gical Data." Lnrvrn,H. H. (1957).-Geophys.Ment., Lond. No. 99. Mijr-oNrn, W. (1950).-Ber. dtsch. lletterdienstesU.S. Zone No. 19: l. NEwroN,C. W. (1950).-J. Met.7:210. RossvnnN,F. O. (1958).-Weather13:259. Russrrr, H. C. (1891).-"r. Roy.Soc. N.S.lZ.25: 58. Russrr-L,H. C. (1898).-J. Roy. Soc. N.S.L/. 32: 132. SEELyE,C. J. (1945).-N.2. J. Sci.Tech.21 (2): 166. SHrurzu,I., and KtuuRA, S. (1957).-J. Met. Soc.Japan 35 (2): 73. Sou-rnrnN,R. L. (1960).-Aust. Met. Mag. No. 31:.1. vrN Trssrr, E. L. (1955).-Mon. LVeath.Rev., Lltash.83: 255. Trrrnn, M. (1950).-Bull.Amer. Met. Soc.31: 311. TEppER,M. (1957).-Mon. Weath.Rev., Wash.85: 159. U.S. Wenrnpn Buneeu (1952).-Tech. Pap. U.S. Weath' Bur. No. 20. Vrnvrno, R. G. (1946).-lleatherl:89. wALLrNcroN,C. E. (1961).-"Meteorology for Glider Pilots." (John Murray: London') Wncenrn, A. (1928).-Met. Z. 45:201. WHIrrNc, R. M. (1957).-Bull. Amer. Met, Soc.38: 353. WNsroN, J. S. (Ed.) (1956).-Forecastingtornadoes and severethunderstorms. Forecasting Guide No. 1. SEYERELOCAL WIND STORMSIN AUSTRALIA 45

. APPENDIX I

THE ESTIMATIONOF WIND SPEEDIN LOCAL STORMS (a) Tree Damage Since much of the destruction causedby local storms is inflicted on eucalypt trees of various species,it was considereddesirable to try to find a criterion for wind speedtherefrom. A first requirement is a knowledge of drag coefficientsof trees at high Reynolds numbers,and an experimentto determinethis was executedas follows. A common type of eucalypt (E. camaldulensrs)was chosen and a branch cut, of apparently avetageleaf density, about 4 m high, and mounted upright in a Land Rover, pivoted at the bottom, where the trunk diameter was l0 cm, and supported by a steelcable at l'25 m from the bottom. The cable,containing a butchers' spring balance,was lashedto the cabin of the vehicleand guide rails provided to limit lateral sway. An observerrecorded balance and speedometerreadings while the vehicle was

I = CROWN 1 O : CROWN 2 +: CROWN 3

n I \vn U \-xoo Q o.so -t\O+ u )-:+ -\+ + E + + + o x I-F=< x \)oL + x xx -+-+ Xx+

o '?o o102030405060 SFEED (M.P H) Fig. 32.-Measured drag coefficientson threecrowns of red gum treeattached to a moving vehicle. driven at constant speed. The area of the section presentedto the wind, and location of its centroid, were measured by means of a theodolite at known distance. The wind drag, assumedto act at the centroid, was thus measured for vehicle speeds up to 56 m.p.h. Variety was obtained by removing some of the crown and repeating the experiment. A light wind in the direction of the straight road was observedand allowed for, as was also the gravitational torque of the tree when displaced from the vertical. The drag coefficient was calculated from P : lpcAVz, (10) where -Fis measured drag, p air density, c the coefficient,I the area of cross section of the crown wheu at rest, and V the air velocity in the direction of the heading relative to the tree. According to Goldstein (1938),for a solid sphere,c should be 0'25 at the appro- Reynolds number. The plot of c against V (Fig. 32) shows that the value 46 R' H' cLARKE

(0.3) appropriate to a cylinder fits the points best for small V, although the scatter value, ir farg. frer.; but with increasing V, c apparcntlytends towards the expected of c o. u iittl" less,probably showing the effects of leaf alignment. The behaviour grossly at higher velocitiesremains unknown, but a value of 0'25 appears to be not in error under natural strong wind conditions' to One method, which we shall call A, of estimating the wind speed required the break a tree trunk or branch has beeu used by Miildner (1950). This equates maximum torque exertedby the wind, plus that of gravity, at the breaking point to the resisting torque of the tree, which has been determined by laboratory experiments for vartus types of timber, expressedas the "modulus of ruptute". Gravitational asym- torque at the moment of breaking is, of course,unknown, silce the beuding' breaking ^"lry,and weight are involved. Miildner arbitrarily ascribesa quarter of the torque to this factor. Neglectingthis, we write

gpcALVz:r/3(fr3), (11)

r the where Z is the distar,cefrom the breaking point to the cerrtroid of the crown, rupture' This effectiveradius of the trunk at the point of break, / the rnodulus of yields trz v : |.s (;lf 1+, (12) \nu/

where Zis in m.p.h., r in inches,/in lb wt',/in', and'A,L,inft2 and ft respectively; Here and the c has been taken as 0.25, and p as density at 80'F and 1000 rnb. .f of rotteu- neglectedgravitational torque are the least known factors. A srnall degree especially stressed ,-rrJ, strongly affect.f', and growing trees afe known to be "un as speed against wind. Trees with obvious sigus of rotten[ess have been rejected indicators. speed Another method, called here ,8, which has been evolved for estimating of a branch depends on measuring the displacement vertically and horizontally rolled after from the tree whencelt broke, provided it can be jLrdgednot to have vertical impact, and that its effective resistanceis the sarre for both horizontal and displaceraents. accurate . Using a wind-squared law for drag. which is known to be reasonably (Fig. 32) fpr lower velocities,we can write

du : ,# V - u),rr, - u)z*,,2 t' (13) dt I

dw iuIo (r4) dt I

velocity where Z is assumedcolrstant fron the crown to the grouud, a is the horizontal of the broken branch, u' its dowtrward velocity, and M its mass' A1 SEVERE LOCAL WIND STORMS IN AUSTRALIA

- Writing B: tpAC/M and neglectir^tgt4t2 by comparison with (V u)' in (13), we obtain du at,:B(v-u)" whence vr -:h@vr 1):x, (15) B + where X is the horizontal displacementof th,ecentre of gravity of the broken branch, and 7 the time taken to fall.

ooa

I G x- o06 o

o2 04 7l

Fig. 33.-Diagramsfor thesolution of equations(17) and (18). Seetext of AppendixI for rnethod of use.

In order to use tire measureddistance of fall (/1) of the centre of gravity, we observe that for the casesiuvestigated, the first term on the R.H.S. of (14) is small compared with g, and we can thus neglect the contributions of u and w under the radical sign, as being only a small fraction of Z. Then dw x -BVrv*g dt

I - exp(-BVT) 6 1 l-"- BViz- BY | "' 48 R. H. cLARKE

which can be We have thus two simultaneous transcendental equations fot V and T, solved graphically. Equations (15) and (16) can be rearranged: BX :'tt - 1n(1-| 1), (17)

BzHVz :'r)- r lexp(-1), (18) o'

B2HV2/g on the ordinate. : z : 0, Since I/ calculated in this way is a kind of time mean from z H to it is desirablebefore comparing results from methods A and -B to make a correction for the decreasein V near the ground. For neutral stabilitY, ln zfz6 t/ : t/a (19) lng/zo,

z where zo is roughnessparameter, Va the wind at height H, and V:0 for {zo' For the purpose only of estimating v/vu, we assumefree fall so that H-z:+9t2, whenceit can be shown that v:+ li ,a,,

: Vn (2t/Hln H/zo)-1 h rlro (H - z)-t i7, (21) t--1"" whence VlVo:(ogH/zo)-r + \/H- zo)l {--11- 7o) - '*'v'11- ^f 'va}' Q2) \^rrro ?'- whereN:loge.

Table 15 gives V/Vu according to this formula for some possible values of zo and H. The use of equation (19) in this context is, of course, open to criticism. Deacon

Vu. SEVERE LOCAL WIND STORMS IN AUSTRALIA 49

In Tables 16 and 17 a measuresthe deviation of the wind, as indicated by the throw of debris, to the right of the storm track; and "position" relatesthe location of the tree concernedto the estimatedcentre of the path of damage. Measurements of l, A, r, H, X, and M were made under field conditions and lack laboratory refine- ment. Only limbs appearingsound at the break and those showing no sign of rolling after impact were used for estimatesby methods A and B respectively. The considerablediscrepancy of estimatesby the two methods may be due to severalfactors. Mijldner's method was adapted by the omission of certain of his more arbitrary procedures. We havereduced r by f in. from the valuesmeasured in the field, in an attempt to allow for bark and sapwood. For the Riverina storm we have used. tabulated values off the modulus of rupture, for the type of timber concerned,but in other caseswe have relied on laboratory testson timber taken from the storm site. Adopting Miildner's arbitrary procedure to allow for gravitational torque, we seethat the estimateof 143 m.p.h. in the two storms by method I reducesto 124 m.p.h.,

TlsI-e 15 vALUES oF V/VH FRoM rqunrroN (22)

H(m) 5 l0 t5 20 30 (f0 16 zo(cm) 33 49 66 98

I 10 0'86 0.87 0.88 i 0'88 0.89 25 0.81 0'84 0.85 0-86 0.87 50 0'76 0.80 0.82 0. 84 085 100 0'69 0'76 0"79 0.80 0'82 200 0.56 0.69 0'73 0.76 0.79

which is probably nearerthe correct value. It is Iikely that the chief sourceof error in this method lies in the evaluation of effective r, and that the allowance made was inadequate. Our experiencesuggests that this method, becauseof the difficulties involved, is not to be recommended.

(b) Damage to Glasshouses Another check on wind speedscan be derived from damage to glasshouses used for raising early vegetables. In the type examined at Warriewood (Section VII), rectangularpanes are held firmly at two edgesonly. At the moment of fracture, the bending moment isBds/6, where dis the thick- ness. The equation of a beam held firmly at two ends and uniformly loaded is

./ \ n'x2(s-x)2 /(.r) : _z.qet where w is the load per unit length, ,r the length, .r the horizontal and y the vertical 50 R. H. CLARKE

l\o; I tnro I I I o.\O.O\ | OH

lnoo lc\ lr-n t<_d I t-i- oll r o\ '<= : tfi I I IS€g I NN O I \e ls t8 | | 1333| I I z rF| | l$N$| ls

I! lfr-*ll+rJd# l-o$ lc.l l.+No F \oo

x=: x xxx rQf loon lc) lnqn \i 'ooa '- '0\0\6 F-z

6 R3R8ls€493 z 6 N rV-=-

H z lxxr F ls:Kr: @ I

F !!a! .e55-C E(IEE F U)bObI) r o o o o,q.- - - q c.+,JJJJQ4c4c( F

b0 onnoonnnoo r-\O$ --S@V

'6 o a 4 a 4 4 q.4 4 ,ss: r rss3=rssss 3333 *'X:Y**'X*r!** v -: x x{-: x-:{-i^i kl :-:=:::t:: tsts

siiuEiEESso kikikikidtlitjl{itikj SEVERE LOCAL WIND STORMS IN AUSTRALIA 51

O\\OCIO'NO-t+ -oo\s €rr@O€N! O€-O\

@dnO=fo..\Oe r o\ lees=BganH+Bn oi <= ;;ol\o6 -N r_l-^-

F] +t ..44r @€-\O\OO€hf-m@ NNN6n*o]o--.Sa.l

F @ r

F oi+ -r 4r qt + -- (\oN-N-n\i-m-Olo o r F-^ o 140 d3 xxxxxxxx-;iEi- rA \< hhhnhhbhhnna

q d-rrror*Or- z -6hhhh-Of

a z

a qqq qc,|qqqoo ,;J **JFl F

a a o a

a (d a z 52 R. H. CLARKE co-ordinate,Eis Young's modulus, and lthe moment of inertia. The bendingmoment : El(dzy/dxz), which has a maximum value of wsz/24 at the centre. Hence

Btg s'Lp (23) 6 24', where Ap is the pressureexcess on one side. The kinetic pressurdexerted by wind on sheetsof glassin a glasshouseis by no meanssimply a function of speed,and depends strongly on shapeand orientation. The experimentsreported by Shimizu and Kimura (1957)show that kinetic pressureexcess or deficit may reach but not exceedin magni- tude the value lpYz, which occurs when a flat surface is normal to the wind. If many,panes are broken, and if only the kinetic pressureis involved, minimum Z is given by SBtg ,r,| - (24) N2' provided there was no large hail. However, we may have to deal with pressurereduction in a vortex, which can produce effects commensurate with those expressedin (23), and damage to lee roof slopes,where there is adequateconstraint on the roof components,strongly suggests that this can be an important factor, for here kinetic pressure deficit combines with vortex pressure reduction to produce maximum deficit, and lee roof or wall components are displaced outwards after breakage. In a "straight blow", to which (23)is applicable,wall componentsare as likely, or more likely, to be displacedinwards after breaking. We now apply (23) to estimate pressuredifferences Ap required to break the glass in the Warriewood storm of July 9, 1957. For this purpose, the following measured valueswere adopted: B:5300 lb wt./in., l:0'113 in.; s = 16 in.; whence Ap : 8'5 mb and, on application of (24), V : 83 m.p.h. Sincein most locations in the storm path, destruction was complete, Lp may well have considerablyexceeded this value, and this value of V may be an underestimate.

(c) Damage causedby a Large-scale Storm Some comparisonswith the damagecaused by a cyclonic storm which ravaged south-east Australia on August 5, 1959, may help to answer the question, what wind speedsare required to cause the damage observed in storms such as those describedin Section VII? On this occasion, maximum gusts as follows were recorded on Dines anemo- graphs (except at Point Lonsdale, where a speedindicator was observed): Point Henry lighthouse 91 m.p.h. Aspendale(C.S.I.R.O.) 75m.p.h. Mt. Gambier aerodrome 88 Moorabbin airoort 14 Point Lonsdale 82 Carlton (Bureau of Meteorology) 64 Laverton airport 15 Essendon(Melbourne airport) 62 In the Melbourne area, to which belong all the above stations except Mt. Gambier, which is about 300 miles to the west, the decreasewith distancefrom the exposedcoast is typical of gusty onshore winds. SEVERELOCAL WIND STORMS IN AUSTRALIA 53

As a resultof this storm, widespreaddamage occurred; hundredsof thousands of pounds was quoted by the press as an estimate. At Mt. Gambier, for exarnple, it was reported that 90o/" of tiled roofs were damaged,most of them losing only a few tiles and the worst losing 35-40%. Iron roofs were damaged in about the same proportion, but severallost the whole roof. Many verandahswere demolished. Numerous wiudows were "blown in" on upper storeys. About 30o/oof windmills in the district were reported destroyed. Forty Pinus radiqta trees on the nearby hill (where the wind may have been stronger)were either uprooted or broken off near the base, the diameter there being about 4 ft. Eucalypts suffered numerous casualties. On the exposedparts of Port Phillip Bay some treeswere torn down, and at Franlcstonshop windowswere blown in and tiled roofs damaged.In the Aspendale- Moorabbin area,serious damage did not occur. A qualitativejudgment basedon this experienceis that guststo 90 m.p,h. are sufficientto causemost of the damagemen- tioned in SectionVIL 54 R. H. CLARKE

AppENox lI (a) Publicationls containing Referencesto Severe Local Wind Storm'g in Australia ANoN.(1958).-',Australian Encyciopaedia." vol. 8. p. 309.(Angus and Robertson: Sydney') BRooKs,E.M.(1951).-..CornpendiumofMeteorology'''(Ed.T.A'Ma1one.)p.673.(Waverley Press:Baltimore.) (9). CoMMoNWEALTHBUREAU or Mrrronor-ocy (1911).-AusI. Mon. Weath.Rep' 2 madein N.S W' C9MM6NWEALTHBgREAU or Mtrponolocv (1948).-Resultsof rainfall observations DuNsraN,R. R. (1956).-lust. Met. Mag.No.73:47' Bull.No- 12:43' Dwypn, w. N., and MclNrvne,D. G. (1949).-Bur.Met. Aust. Weath.Dev. Res. Houtuu, C. E. (1958).-Aust. Met. Mag. No. 20: l. (Oxford Press') KENDREW,w. G. (1953).-"The climates of the continents." p' 552. University No. 252' RoyAL AusTRAr-rrNAtn Foncr (1942).-Weather on the Australia station' Publ. Russer-r-,H. C. (1891),-J. Roy' Soc.N.S.W' 25: 58. Russnrr-.H. C. (1898).-"/. Rov.Soc. N'S.W.32: 132 SEei-ve,C. J. (1945).- N.2. I. Sci. Tech 2!7(2): 166. SournEnN,R. L. (I960a).-Aust' Met. Mag. No. 29: 49' SournenN,R.L. (1960b).-Ausr.Met' Mag' No. 3i: I' TAyLoR,G. (1920).:"Australian Meteorology." p. 205' (Clarendon Press: Oxford ) Vtsuen,S. S., and Hoocr, D. (1925).-Bur. Met' Aust. Bull No l6' VoLrrnrcHr, R. (1955).-lrtst' Met' Mag. No. 9: 61'

(b) some RecentPublications onTornado Forecastingin North America BEEBE,R. G. (1958).-Weatherwisell:' 13. BEEBE,R. G., and Bares,F. C. (1955).-Mon. Weath.Rev', Wash'83: 1' Soc'32:7' FAwBUSH,E. J., Mrt-len,R. C., and SrnnRlrr, L. G. (1951)'-Bull' Amer' Met' LEE,J. T., and G*wrv, J. Q. (1958).-Bull' Anter' Met. Soc' 39:217' Snowerrun, A. K. (1953).-rull. Amer. Met. Soc' 34: 250' S.rrrsrr,v, A. H. (1957).-Motl ll'eath. Rev', Ll/ash.85:141' WHrrrNc, R. M. (1957).-8ul!. Amer. Mer. Soc.38: 353' WnrrtNc, R. M., and BAILEY,R. E. (1957).- Mon. V/eath'Rev', Wash'85: 141' Guide WrNsroN,J. S. (Ed.) (1956).-Forecastingtornadoes and severethunderstorms' Forecasting No. 1. SEVERE LOCAL WIND STORMS IN AUSTRALIA 55

Norn apopn w Pnoor The following is a summary of information receivedtoo late for inclusion in the body of this report from Dr. H. M. Treloar, who, in company with Mr. J. Hogan, both of the Bureau of Meteorology, investigated the Geelong tornado of l:uly 22, 1926,the day afterit occurred. This storm is ofinterest becauseofits unusual ferocity, and becauseit was traced over 60 miles.

:;;:;a!rn"::

Fig.34.-Damage to churchesat Hightonon July22, 1926.

The tornado left a narrow path of destruction, about 300 yd wide, traceable from Lismore,Vic., through Cressy,Highton, Belmont(southern suburbs of Geelong), and near Queenscliffe,where it passedout to sea. Jts coursewas approximately from 290" and speed nearly 60 m.p.h. It moved along a rapidly advancirrgcold front, which was precededby the highest July ternperature(68'F) in Melbourne then on 56 R. H. CLARKE record, and arrived in Geelong about 5 p.m. Pronounced clockwise rotation was observed, and it displayed rapid variations in intensity and pursued a zigzag path ("two turns to the mile" according to one witness)confined betweenparallel straight lines 900 yd apart. Eyewitnessesdeclared that there was a gap betweenthe swirling debris and the cloud, suggestingthat the funnel was not well developed. The tornado was said by one observer to have given birth to another minor whirl which wandered off to the left of its parent's path, but after traversing several hundred yards, the two rejoined, acquiring maximum intensity at this time. Bedding and furniture from some houses were scattered over a radius of a quarter of a mile. A child was killed, and a number of people injured by flying debris. A man was hurled to the ground and rolled acrossa road by the wind. A piece of heavy iron lacework was stripped from a verandah and hurled 20 yd to smash a car, Two massive stone churches at Highton, with walls about 18 in' thick, were unroofed and partly demolished, as illustrated in Figure 34. The investigatorsdeduced that the heavy hail (up to pigeon-eggsize) and rain, which in some places precededand in others followed the tornado, were related to the accompanying thunderstorm, and not directly to the tornado.