tropical–extratropical cloudbands and australian rainfall: i. climatology

INTERNATIONAL JOURNAL OF CLIMATOLOGY, VOL. 17, 807±829 (1997)

TROPICAL±EXTRATROPICAL CLOUDBANDS AND AUSTRALIANRAINFALL: I. CLIMATOLOGY

W. J. WRIGHT

National Climate Centre, Bureau of Meteorology, GPO Box 1289K Melbourne, Australiaemail: [email protected]

Received 31 October 1994Accepted 4 December 1996

ABSTRACT

This paper examines the contribution of tropical±extratropical cloudbands, and of interactions between tropical cloud andmid-latitude systems, to cool season (April±October) rainfall in agriculturally marginal areas of Australia. A following paperdescribes inter- and intra-annual variability of these features. A classi®cation scheme for these tropical in¯uences based onGMS satellite imagery is described, and used to compile a 15-year archive of events. It is shown that cloudbands extendingfrom the tropical oceans bordering Australia (`Oceanic' Cloudbands) are most frequent and in¯uential between April and July,but decrease sharply after August, at which time bands originating over the continental interior (`Continental' Cloudbands)increase.

The contribution of these systems to rainfall at stations representing agriculturally marginal areas is assessed. OceanicCloudbands originating west of 120�E contribute 70±90 per cent of cool-season rain in north-western Australia, with thecontribution decreasing to the south and east. North-eastern Australia receives a signi®cant portion of its rain from Cloudbandsoriginating east of 120�E. Tropical±mid-latitude interactions are more important over eastern than western Australia, andproduce some 30±40 per cent of rain over much of inland eastern Australia. The overall tropical in¯uence (Cloudbands plusinteractions) on rainfall is least in South Australia and western Victoria, but still amounts to some 35±40 per cent of cool-season rain in those areas. The proportion of events producing signi®cant rainfall (>10 mm) is also examined: almost two-thirds of the Oceanic Cloudbands to affect western Australia produce signi®cant rain, and about half of those affecting easternAustralia. # 1997 by the Royal Meteorological Society. Int. J. Climatol., 17: 807±829 (1997)

(No. of Figures: 10. No. of Tables: 4. No. of References: 33.)

KEY WORDS: cloudbands; tropical±extratropical cloudbands; tropical±extratropical interactions; Australian rainfall; synoptic climatology.

INTRODUCTION

Extensive cloudbands extending from tropical to mid-latitudes are prominent features over certain parts of the

globe (Flohn, 1971; Streten, 1973; Thepenier, 1981; Thepenier and Cruette, 1981; Kuhnel, 1989), and represent

important channels for transporting latent heat and moisture into higher latitudes. These poleward excursions of

tropical moisture often trigger widespread and heavy rain, an effect that may extend into quite high latitudes

when, as often happens, the cloudbands, or other tropical cloud-masses (associated with, e.g., thermal convection

or easterly waves) link, or `interact', with a mid-latitude system.

Such `tropical±extratropical' cloudbands and interactions are an important rainfall source in many subtropical

and mid-latitude regions. For instance, the South Paci®c Convergence Zone (e.g. Vincent, 1994), a recurrent

cloudband feature typically extending from near the Solomon Islands to the mid-latitude South Paci®c, strongly

in¯uences rainfall over many South Paci®c islands. Further east, cloudbands extending from the Amazon Basin to

the South Atlantic (the South Atlantic Convergence Zone) produce most of southern Brazil's spring/summer

rainfall (Cavalcanti, pers. comm, 1994). Over southern Africa, cloudbands associated with `tropical±temperate

troughs' contribute the bulk of summer rain in inland areas (Harrison, 1984a,b), but may generate ¯ooding (e.g.,

Lindesay and Jury, 1991). North of the Equator, cloudbands extending from the tropical Atlantic are important in

wet Mediterranean winters (de Felice and Viltard, 1976), and cloudbands extending from the tropical eastern

CCC 0899-8418/97/080807-23 $17.50

# 1997 by the Royal Meteorological Society

Paci®c have been linked to heavy winter rains in the southern USA (Douglas, 1980; Douglas and Englehart,

1981).

Similar systems produce heavy rains over the Tasman Sea±New Zealand region (e.g., Hill, 1964, 1969), and

over Australia (e.g., Woolcock, 1960; Gentilli, 1974; P. B. Wright, 1974; Hill, 1977; Lajoie, 1980). However, so

far the climatological signi®cance of these systems has been assessed only in south-eastern Australia. Lajoie

(1980) and Wright (1988a) demonstrated that rain from frontal systems affecting this area was increased

substantially when they `interacted' with cloudbands, or with other cloud-masses of tropical origin. Wright

(1988a,b) further showed that such features contribute about 50 per cent of winter to early spring rainfall in

northern Victoria, and dominate interannual variability.

The purpose of this paper is to describe several types of tropical±extratropical cloudbands and interactions that

affect Australia in the cooler months, and then quantify the contribution of these types to rainfall in a broad belt

of agriculturally and pastorally signi®cant but marginal country between the Australian coast and the arid inland.

These areas support extensive wheat, sheep and cattle enterprises, with more diverse activities in wetter areas

towards the coast. Annual rainfall in this area varies between 250±700 mm on 40±120 days a year, is often erratic

Ð but vital for water storages, pastures and plant growth when it does occur. The success or failure of grazing

and cropping activities may therefore hinge on the occurrence or otherwise of a few signi®cant rain events. Figure

1(a) is a location map, showing general locations mentioned in the text, and Figure 1(b) shows annual average

Figure 1. (a) Location map, showing general features mentioned in text. (b) The study area, showing rainfall stations used in the study, andisohyets of average annual rainfall. The two-letter code for each station is an identi®er for later diagrams and tables. (c) Proportion (per cent)

of annual rainfall falling during the April±October period.

808 W. J. WRIGHT

rainfall (with `marginal' country being de®ned approximately by the 300 and 700 mm isohyets), along

with the locations of stations used in this study. The stations used, represented by the two letter codes on

Figure 1(b), are:

(i) western Australia Ð La Grange (LG), Halls Creek (HC), Roebourne (RO), Marble Bar (MB), Learmonth

(LE), Newman (NW), Three Rivers (TR), Meekatharra (MK), Mount Magnet (MM), Geraldton (GE),

Corrigin (CO), Trayning (TG), Kalgoorlie (KG), Wiluna (WI), Giles (GI), and Forrest (FO);

(ii) eastern AustraliaÐWinton (WI), Windorah (WD), Barcaldine (BA), Charleville (CH), Cunnamulla (CU),

Thargomindah (TH), Miles (MI), Bourke (BO), Cobar (CO), Collarenbri (CL), Ivanhoe (IV), Mudgee (MG),

Deniliquin (DQ), Wagga (WG), Hay (HY), Wangaratta (WA), Kaniva (KV), Eudunda (EU), and Butler

(BU).

The study focuses on the cooler months (April±October), the time when cloudbands and interactions are most

frequent, and also a crucial time for agricultural activities outside the tropics. Figure 1(c) shows that the

proportion of annual rainfall occurring in these months varies over western Australia from 15±25 per cent in the

north of the study area to 70±80 per cent in the south, and over eastern Australia from 30±40 per cent in the north

to 60±70 per cent in the south. Even within the tropics, where summer is the wet season, water storages and soil

moisture levels still bene®t from cool-season rain.

Figure 1. (Continued )

CLOUDBAND SOURCE AND AUSTRALIAN RAINFALL 809

CLASSIFICATION OF CLOUDBANDS AND INTERACTIONS

The classi®cation scheme, described below and summarized in Table I, is based mainly on the location and nature

of the low-latitude source region for tropical±extratropical systems (in turn representing differences in the

systems' moisture source), and the subsequent evolution of these systems, as observed on a sequence of satellite

images. Figure 2 illustrates the various cloudband types, their source regions, and the area of origin of low-

latitude cloud involved in interactions.

Tropical±extratropical cloudbands (hereafter `Cloudbands')

All cloudbands described consist of predominantly layer cloud generated by steady ascent (upslide) of moist

air along sloping isentropic surfaces associated with a mid-tropospheric baroclinic zone. The dynamics of such


Table I. Summary of Cloudband and Interaction types.

Tropical±extratropical cloudbands Tropical±extratropical interactions

North-west oceanic Ð origin west of 120�E Cloudband-associatedÐinteraction with CloudbandNorthern oceanic Ð origin east of 120�E ContinentalÐinteraction with cloudmass over continentContinental Ð origin over land Eastern±interaction with Coral Sea Cloud/moisture

810 W. J. WRIGHT

bands have been described by, e.g., Hill (1977), Downey et al. (1981) and Bell (1982), and various climatological

aspects of the bands have been described by Tapp and Barrell (1984; hereafter TB), Wright (1988a,b) and Kuhnel

(1990).

Cloudbands are subdivided into two types.

(i) Òceanic' Cloudbands. These extend across Australia from the tropical oceans to the north and north-west

(which form an obvious moisture source) as long, coherent bands of middle-level cloud. This study

distinguishes between so-called `north-west' Cloudbands originating over the tropical Indian Ocean, i.e.,

west of 120�E, and those originating east of 120�E (hereafter `northern' Cloudbands). Figure 3(a) displays a

typical well-developed Oceanic Cloudband, which produced substantial rain over a broad belt of the interior,

and culminated in ¯ood rains over South Australia when it interacted with a mid-latitude depression.

(ii) `Continental' Cloudbands. In most respects similar to Oceanic Cloudbands, except that they extend from

cloud clusters over the northern continental interior instead of the oceans (see source region, Figure 2). The

cloud clusters are often convective, but sometimes consist of relatively low-level cloud, formed from

advection inland of moist air from adjacent tropical oceans. Figure 3(b) shows a mature band some 30 h

after its initial development from convective activity over the south-west Northern Territory. (In this time,

the low-latitude end of the band has shifted south-east and lost its convective nature.)

Tropical±extratropical Interactions (hereafter Ìnteractions')

In this study, Ìnteraction' refers to the capture of low latitude (i.e., north of 23�S) moisture, as represented by

cloud-masses, in the circulation of a mid-latitude front or depression, and the subsequent amalgamation of mid-

and low-latitude cloud systems. Three classes of Interactions are de®ned, depending on whether the low-latitude

cloudmass consists of:

(i) a Cloudband (`Cloudband-associated' interaction);

(ii) low-level or convective cloud over northern Australia (`Continental' interaction);

or

(iii) cloud advected west, then south, from the Coral Sea (Èastern' interaction).

Figure 2. Schematic of the cloudband types, their source regions, and the low-latitude source regions for continental and eastern interactions.


In each case, strong uplift of moist, tropical air by a mid-latitude barocline can cause signi®cant, often rapid,

cloudband development. Examples of such cases have been investigated by, e.g., Downey et al. (1979), Wright

(1988a), and Mills (1989). For rainfall attribution purposes, an interaction is not de®ned until amalgamation of

the mid- and low-latitude cloud systems has occurred, or, in the case of cloudband-associated interactions, until

cyclonic curvature is clearly evident near the higher latitude end of the band.

Figure 4 shows a typical Continental Interaction situation. In Figure 4(a) an area of scattered, mainly

convective, cloud lies over Australia; a frontal trough crosses the Great Australian Bight. Twenty-one hours later

(Figure 4(b)), an extensive pre-frontal cloudmass has formed, with widespread rain. The amalgamation of two

initially separate cloud-systems (one low-latitude, one mid-latitude) distinguishes this case from a Continental

Cloudband (which extends into mid-latitudes from an initial cloud cluster over inland Australia), and illustrates

the need to track the evolution of cloud systems over a series of images.

Classi®cation criteria

The classi®cation was based on inspection of three-hourly GMS infrared and visible satellite imagery (12

hourly in part of 1978). The following criteria were used to identify Cloudbands; italicized criteria apply to both

Cloudbands and Interactions.

(a)

Figure 3. (a) Tropical±extratropical cloudband (Oceanic), 0000 UTC 29 August 1992. This Cloudband produced widespread rain over inlandAustralia. The cloudband is just beginning to `interact', as shown by the incipient development of cyclonic curvature at the poleward end of

the band. (b). A continental cloudband affecting south-eastern Australia, 12 00 UTC 5 August 1992.

(a)

812 W. J. WRIGHT

(i) Origin within area (0�±23�S, 80�±140�E), and extending east, south-east or south over part of the Australian

continent.

(ii) Dimension at least 20� longitude or 15� latitude; length substantially greater than width; width at least 4� of

latitude or longitude; persist as discrete entity for at least 24 h.

(iii) A coherent link with tropics (at any level) must be evident, at least in the development phase.

(iv) Band essentially stratiform at middle and upper levels.

The end of a cloudband's in¯uence is marked by:

(i) disintegration of coherent band into disorganized mass;

(ii) No part of band remains north of 25�S;

(iii) Entire cloudband cyclonically curved;

(iv) Band shrinks below minimum size of 4�615�.

The criteria for Cloudbands are similar to, but broader than, those of TB, whose strict criteria excluded many

major rain events identi®able as cloudband features (e.g. `Continental' bands).

To ensure that only rain associated with the primarily stratiform Cloudband or Interaction component was

included, certain types of cloud system were speci®cally excluded from the analysis (three hourly imagery was

(b)


(b)


generally adequate for making these distinctions). Figure 5 illustrates the classi®able (broken hatching) and non-

classi®able (shaded) components in a typical Interaction situation; note that only the baroclinic leaf portion of this

idealized frontal cyclone is classi®ed as Interacting. The non-classi®able components are:

(i) cloud accompanying any front, depression or trough not clearly linked to the tropics on satellite imagery. (It

is possible that low-level tropical moisture may sometimes `feed' mid-latitude systems without a visible

cloud-link; however, for the sake of objectivity, only systems where this cloud-link appears are regarded as

tropical±extratropical Interactions). Any portion of an Interacting mid-latitude system not linked directly to

the tropics (Figure 5);

(ii) any tropical cloud-cluster not extending as an organized band into higher latitudes. The low-latitude

cloudmass comprising the origin of a Cloudband or Interaction;

(iii) any predominantly convective cloud-mass, whether or not it formed part of a cloudband. Isolated

thunderstorms probably occur in many Cloudband and Interaction cases, but the intention here is to omit

predominantly convective systems, e.g., convective clusters, or organized convection along a low-level

convergence line, which show a strong diurnal tendency, and produce patchy, uneven rainfall (such cases

seldom met the size and/or duration criteria for cloudbands in any case);

(iv) disorganized or incoherent cloud-masses.

(a)

Figure 4. `Continental' interaction, September 1992. (a) 2100 UTC 22 September 1992: an area of well-scattered convective cloud lies overcentral Australia, with a frontal cloudband approaching from the Bight. (b) Approximately 21 h later (1800 UTC 23 September 1992) the two

systems have amalgamated, with the development of an extensive pre-frontal cloudmass and widespread rain.

814 W. J. WRIGHT

Figure 5. Distinguishing the tropical±extratropical Interaction component (broken hatching) from non-interacting components (shaded) in amid-latitude system. Rainfall corresponding to any shaded cloud, except that overlain by broken hatching, is excluded from the analysis.

(b)



The classi®cation scheme described above was used to form an archive of Cloudband and Interaction events for

the months April±October, 1978 through to 1992. This archive contains the start±®nish times of each event,

location of origin of cloudbands or low-latitude cloud-masses, and (where appropriate) the longitude and time of

interaction.

THE RAINFALL STATION NETWORK

Rain for each event was evaluated from 24 h totals at networks of stations representing marginal areas in western

Australia, and eastern Australia (Figure 1(b)). Where possible, stations were selected from the Bureau of

Meteorology's `high quality' set (Lavery et al., 1992), with reliable siting and recording practices. Unfortunately

a substantial area of central Western Australia strongly in¯uenced by Cloudbands contained no `high quality'

sites; for this area, several stations of lesser quality, but regular reporting practises, were used. Most stations had

complete records over the 15-year period; no attempt was made to estimate data in the (very few) missing

months.

On occasions where a system affecting a station, or group of stations, changed classi®cation such that the

stations were affected by (for instance) both a cloudband and its subsequent interaction within the same 24 h

period, it was necessary to estimate the relative contributions of Cloudbands, Interactions and other sources to

24 h totals at the stations affected. This was done by noting the time of `change of category' of rainfall (e.g. time

of interaction) from satellite imagery, and allocating rain between the categories on a pro-rata basis, unless there

was evidence (at, e.g., nearby stations) of a marked change in intensity of the rainfall associated with the

transition. At any one station, the number of such cases, and the amount of rain involved, comprised a relatively

small proportion of the total classi®ed, and any errors introduced would be insuf®cient to affect the results of the

study.

Over the 15-year study period, April±October rainfall was close to the long-term normal in south-eastern and

north-western Australia, above normal by 10±20 per cent in Queensland and northern New South Wales (NSW),

and below normal by a similar amount in south-western Australia.

RESULTS: CLOUDBAND FREQUENCY

Figure 6 shows the average number (frequency) of Cloudband events per month over the study period. Clearly,

Oceanic bands are most frequent between April and July (about ®ve events per month, peaking at six in May).

Most of these originate west of 120�E (dark shading), with fewer than one `northern' Cloudband per month (grey

shading). This frequency peak in the austral autumn to early winter period con®rms TB's results, although overall

frequencies in this study are higher because of the broader Cloudband de®nition used.

A marked decrease in frequency occurs from about August onwards, with no evidence for the secondary peak

in September found by TB (over the shorter period September 1978 to August 1982). This seasonal decline in

Oceanic Cloudband frequency is compensated for partly by an increase in Continental Cloudbands in September

and October. Continental Interactions (not shown) also increase at this time. The increased activity of the

Continental types probably re¯ects increased convection over the northern interior as this area heats up in spring,

and also a sharpening of the meridional temperature gradient over the continent.

CONTRIBUTIONS TO RAINFALL

Cloudbands

Figure 7 shows the contribution of Oceanic Cloudbands to April±October rainfall. Italicized numbers over

eastern Australia represent contributions from `northern' Cloudbands alone. Monthly values for selected stations

are shown in Table II. The following points are noteworthy:

(i) `North-west' Cloudbands account for some 70±90 per cent of cool-season rainfall over north-western and

central Australia. In fact, these systems contribute almost all north-western Australia's rain between May

and August (Table II).

816 W. J. WRIGHT

Figure 6. Average monthly frequency of Oceanic and Continental Cloudbands over Australia. The Oceanic bands are subdivided into`northern' and `north-western' Cloudbands, according to whether they originated east or west of 120�E.

Figure 7. Percentage contribution of Oceanic cloudbands to total April±October rainfall over Australia (the contribution from `northern'cloudbands is shown in italics).


Table II. Percentage contribution of each Cloudband/Interaction type to total rainfall, by month and for the entire cool season (April±October), at selected stations.

Eastern Australia Western Australia

Cloudband/Interaction Barcaldine Charleville Bourke Wagga Eudunda La Grange Newman Mount Magnet Trayning

Oceanic Cloudband (North-west) April 9�2 12�7 5�4 8�8 12�9 49�4 50�4 33�0 20�2May 13�1 17�8 15�5 12�9 14�5 91�7 89�3 62�7 46�0June 28�0 32�7 35�0 9�6 10�9 100�0 98�6 65�7 31�0July 16�2 31�7 29�6 13�4 11�6 100�0 98�8 56�6 32�9August 7�2 17�4 14�9 5�3 7�5 98�6 97�3 59�7 17�9September 27�9 16�5 6�9 0�6 3�2 94�4 40�0 20�1 3�1October 0�8 1�0 1�0 1�7 3�4 0�0 28�1 41�7 11�5Season 12�6 17�9 15�7 7�8 8�9 79�9 82�7 53�7 26�7

Oceanic Cloudband (Northern) April 45�9 26�4 13�6 0�7 0�3May 34�5 18�1 4�2 0�5 0�5June 33�7 9�5 5�0 0�0 0�2July 56�4 34�1 5�6 0�0 0�0August 30�7 9�8 0�0 0�0 0�0September 7�9 1�4 0�0 0�0 0�0October 18�4 2�5 2�0 0�8 0�0Season 37�8 17�2 5�4 0�3 0�1

Continental Cloudband April 0�0 6�7 2�0 3�5 9�9May 0�4 0�0 0�0 0�0 0�0June 8�2 18�8 0�0 0�0 0�0July 1�4 3�0 5�8 0�0 0�0August 24�2 17�4 10�0 9�8 2�0September 57�9 48�6 20�9 3�1 4�1October 32�9 34�8 21�4 12�4 11�9Season 9�9 13�4 6�2 4�0 3�4

818

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Cloudband-associated Interaction April 0�9 4�7 13�1 20�1 12�5 0�0 0�0 0�0 19�2May 9�5 25�9 31�3 41�6 23�3 0�0 0�0 15�3 14�4June 6�6 13�6 24�5 25�6 14�1 0�0 0�6 23�5 15�1July 0�4 9�4 24�9 24�5 19�8 0�0 0�0 16�2 9�9August 11�0 13�0 15�1 21�8 13�8 0�0 1�6 4�4 15�2September 0�0 3�3 18�8 15�0 8�0 0�0 0�0 18�8 5�2October 0�3 6�2 5�4 14�3 7�3 0�0 0�0 4�7 1�8Season 4�1 12�5 20�6 24�0 14�1 0�0 0�3 13�1 12�5

Continental Interaction April 0�0 0�0 0�2 1�8 2�8 0�0 4�3 13�4 2�6May 0�4 1�7 0�4 3�4 1�0 0�0 0�0 0�0 0�0June 0�0 0�9 5�0 1�9 0�9 0�0 0�0 0�0 0�0July 2�7 2�8 1�6 2�5 0�5 0�0 0�0 0�0 0�0August 11�6 6�2 11�6 6�3 3�4 0�0 0�0 0�0 0�0September 0�0 5�7 19�9 17�3 12�1 0�0 18�3 1�3 3�4October 27�5 33�3 24�1 21�9 26�3 0�0 0�0 7�9 2�7Season 4�9 6�7 6�0 7�6 6�2 0�0 1�6 2�5 0�7

Eastern Interaction April 5�3 9�3 12�7 14�1 2�8May 33�6 31�6 21�8 6�7 0�0June 9�0 11�8 4�0 5�6 4�4July 15�6 13�6 12�1 5�0 1�1August 6�0 27�5 25�7 6�2 0�0September 0�7 10�4 1�8 0�0 0�0October 4�0 4�3 15�0 5�8 0�0Season 13�3 16�7 14�6 6�1 1�1

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(ii) The contribution from Oceanic Cloudbands decreases towards the east. Over most of Queensland, `northern'

Cloudbands contribute at least as much rain as `north-west' Cloudbands, especially in the north, despite

making up only 24 per cent of the total Oceanic bands affecting eastern Australia. This probably is because

the area affected by `northern' cloudbands lies closer to the moisture source, compared with `north-west'

cloudbands. The contribution by both Oceanic types is greatest in late autumn and winter (Table II), when

ocean surface temperatures in the source region are warmest and the subtropical jet strong over northern

Australia (Gentilli, 1974).

(iii) Oceanic Cloudband contributions decrease in importance southward, accounting for about 25±40 per cent of

cool-season rainfall south of 30�S over Western Australia (which includes the agriculturally signi®cant

wheatbelt area), and 10 per cent or less in the agricultural areas of southern and south-eastern Australia. The

decrease in these areas is in part because Cloudbands tend to interact or weaken before getting there, but also

due to the increasing in¯uence of other synoptic types.

(iv) Continental Cloudbands (Table II) contribute only 10±14 per cent of cool-season rain in Queensland, and 7

per cent or less elsewhere. However they become important late in the season (September±October),

accounting for some 30±50 per cent of rain in these months over Queensland, and 20±25 per cent over

northern NSW. This probably re¯ects a tendency for convective cloud Ð the source of many of these bands

Ð to develop over the rapidly warming continent, although rainfall at this time of year is relatively low in

these areas.

Interactions

The mean contribution from Cloudband-associated Interactions is shown in Figure 8(a) and Table II, and that

by all types of Interactions in Figure 8(b). The following are points to note:

(i) Interactions are not an important rain source over western Australia, contributing only 10±18 per cent of rain

in the south, and almost none elsewhere. Their importance increases eastward Ð they contribute some 20

per cent of cool-season rain over agricultural areas of South Australia, and 30±40 per cent over much of

inland eastern Australia (over half this from Cloudband-associated Interactions). Their signi®cance

decreases again near the south coast, probably due to orographic `rain shadow' effects (Wright, 1989), and

also the increasing importance of other rain sources.

(ii) Of the remaining types, Eastern Interactions contribute most rain (10±15 per cent of total) in the northern

half of eastern Australia (Table II). This is again most likely due to the proximity of this area to the moisture

source over the Coral Sea. Although moisture originating from this area may be an important contributing

factor in many signi®cant rainfall events over south-eastern Australia (e.g., Hill, 1977), synoptic situations in

which moisture ¯uxes from this area across the south-east are the dominant feature of satellite imagery are

uncommon. Hence this type directly contributes less than 10 per cent of south-eastern Australia's rainfall.

Continental Interactions tend to produce less than 10 per cent of total rainfall, with no systematic variation

with latitude.

(iii) Within-season variability of rainfall from Interactions is generally less than for Cloudbands. However,

Cloudband-associated Interactions (Table II) and Eastern Interactions are most in¯uential in autumn and

early winter, and Continental Interactions from August. These changes probably re¯ect seasonal variations

in sea, and/or land, surface temperatures, as discussed more fully below.

(iv) The total contribution by Cloudbands and Interactions together (Figure 9) is about 80±85 per cent over most

of subtropical Australia. This contribution decreases rapidly to the south (further from the source region,

while non-interacting synoptic systems assume increasing importance), and north (further from the normal

location of upslide-producing baroclinic zones), and less rapidly eastward, but still amounts to 35±40 per

cent in the far southeast and southwest.

The variation in signi®cance of the various types with latitude is summarized in Figure 10, showing the

percentage contribution at stations near 120�E and 145�E. This highlights the importance of Cloudbands in the

820 W. J. WRIGHT

north, but not in the south, and shows that Interactions and other types assume greater signi®cance at the higher

latitudes, especially over eastern Australia.

CLOUDBANDS, INTERACTIONS, AND SIGNIFICANT RAIN EVENTS

A relevant feature of Cloudband/Interaction events is their effectiveness in producing signi®cant rain events: the

hydrological and agricultural value of a few events producing good rain is obviously different to that of many

events producing light falls. Here, a `signi®cant' event is de®ned as one producing at least 10 mm at one or more

stations, in either the eastern or the western Australian station networks. A `heavy' event is de®ned as one

producing relatively widespread signi®cant rainfall, and/or large totals, and is de®ned as: (i) at least 10 mm at

three or more stations; and/or (ii) at least 30 mm somewhere in the network.

In this study, a few events produced upwards of 100 mm (even up to 200 mm), mostly over north-western

Australia and Queensland. Such an event is likely to produce ¯ooding, especially as heavy rain from Cloudbands

and Interactions tends to occur over a substantial area.

Table III summarizes the frequency of Cloudbands and Interactions, and the proportion producing signi®cant

or heavy rain. In the average season 23 Oceanic Cloudbands affect western Australia, of which 63 per cent

produce signi®cant rain, and one in three, heavy rain. A small number of Continental Cloudbands affect inland

areas, but are rarely signi®cant. Signi®cant Interaction events are also uncommon. Month by month statistics (not

Figure 8. As for Figure 7, but for (a) Cloudband-associated Interactions; (b) all interactions.


shown) indicate that not only are cloudbands most frequent between May and July, but most likely then to

produce signi®cant rain (70±75 per cent of events signi®cant). Signi®cant events are least likely in October (31

per cent).

About 13±14 Oceanic Cloudbands normally affect eastern Australia. One in two of these produce signi®cant

rain, and one in three, heavy rain. Year to year variations range from 14 signi®cant events in 1978 and 12 in 1984,

to 1 in the ENSO year 1982. `Northern' Cloudbands are especially productive, with over half producing heavy

rain. Interactions outweigh Cloudbands as signi®cant rain-producers over eastern Australia Ð on average 13 of

20 cases per season produce signi®cant falls; about two-thirds of these involve Cloudbands. Signi®cant falls from

Interactions are more likely in the austral spring months, re¯ecting the greater incidence of Continental

Interactions Ð frequently a potent rain source (79 per cent of events signi®cant) Ð at this time.

The spatial distribution of signi®cant events is examined more closely in Table IV, showing frequencies over

the following subregions: north-east (eastern Australia from Bourke northwards); south-east (rest of stations in

eastern half, except Butler); north-west (western half from Meekatharra northwards); south-west (remaining

stations in western Australia). To assess the number of events at individual locations, frequencies are also

presented for Charleville (representative of the north-east region), Wagga (south-east), Newman (north-west),

and Trayning (south-west).

Table IV shows that in eastern Australia, Cloudbands are most likely to produce signi®cant falls in the north,

with about seven or eight events per season in Queensland and northern New South Wales. Here, the `northern'

and Continental types both contribute about the same number of signi®cant falls as the more frequent `northwest'


822 W. J. WRIGHT

Cloudbands, re¯ecting the importance of moisture sources north and north-east of Australia for rain in eastern

Australia. Individual stations in this sector can normally expect three to four signi®cant events per season, but

frequencies range from zero (in El NinÄo years 1982 and 1991) to 13 in 1978.

Over south-eastern Australia, nearly all signi®cant falls from Cloudbands arise from the `north-west' or

Continental types. However, signi®cant falls in this area are more likely to be produced by Cloudband-associated

Interactions (about eight cases per season), than by the originating band (®ve to six events). This is probably

because, as noted earlier, Cloudbands reaching south-eastern Australia are likely to interact, or have already

interacted. Signi®cant falls from Interactions are more likely in the south-east than the north-east: on average,

about 11±12 signi®cant events occur in the cooler months, again with considerable interannual variation (1982,

one event; 1978, 17 events). The Continental and Eastern types between them contribute about two signi®cant

events per season at individual stations.

The ®nal two entries for eastern Australia in Table IV summarize the overall in¯uence of Cloudbands on

rainfall, combining the frequency of signi®cant falls from Cloudbands and Cloudband-associated Interactions.

(Note that this number is not simply the sum of the frequencies earlier in Table IV, because some bands produce

signi®cant rain before and after interaction). On average, Cloudbands and their Interactions produce between one

and two signi®cant events per month in both eastern sectors (last entry), with four to ®ve events per season at

individual stations (even Ivanhoe, on the arid margins, averaged at least three events per season).

Over western Australia (Table IV), signi®cant falls from Cloudbands are equally frequent over the north-west

Figure 9. As for Figure 7, but for total contribution by Cloudbands and Interactions.


and south-western sectors, and far outweigh the contribution from Interactions. Interannual variations are greater

over the north-west than the south-west, but less than over eastern Australia.

DISCUSSION AND SUMMARY

This study has shown that tropical±extratropical Cloudbands and Interactions together account for most cool-

season rainfall over the Australian subtropics, and up to half that in marginal agricultural and pastoral areas of the

southern States. The importance of the different types may be summarized thus: `north-west' Cloudbands,

originating over the tropical Indian Ocean, are most in¯uential over the state of Western Australia north of about

27�S. This is an area of low rainfall, with a summer maximum; however, Cloudband activity is often suf®cient to

extend relatively moist conditions through autumn into early winter (many stations in north-west Australia

actually show a secondary rainfall maximum around May/June). Hence Cloudbands are vital to the hydrology of

this area, a site of much beef cattle grazing.

Figure 10. Percentage contribution of Cloudbands and Interactions to April±October rainfall over (a) eastern Australia around longitude145�E, and (b) western Australia (120�E (b)), as a function of latitude.

824 W. J. WRIGHT

The in¯uence of `north-west' Cloudbands decreases southward and (more slowly) eastward, as distance from

the primary moisture source increases. They produce between 20 and 40 per cent of cool-season rainfall in the

grain-growing areas of south-western Australia (their interactions with mid-latitude systems produce another 10±

15 per cent), and are recognized as an important source of early season moisture in the outlying wheat areas

(Arbrecht, pers. comm, 1994), i.e. areas bordering a north-west±south-east line through Trayning (Figure 1(b)).

Over eastern Australia, `north-west' Cloudbands occasionally produce substantial rain, but their overall

in¯uence is quite small in South Australia (at least in the agricultural areas), and far south-eastern Australia.

Inland Queensland receives about half its cool-season rain from Cloudbands, but here the importance of the

`northwest' type is at least matched by `northern' Cloudbands, and in the spring months, by Continental

Cloudbands. The latter types are less frequent than `northwest' Cloudbands, but tend to be potent rain-producers

when they do occur.

The importance of Interactions generally increases eastward, ranging from less than 15 per cent of south-

western Australia's rain, to about 20 per cent in South Australia, and 40 per cent over inland eastern Australia.

This trend in part re¯ects the proximity of moisture from oceans adjoining northern and north-eastern Australia. It

also follows from the general north-west±south-east orientation of Cloudbands, which are more likely to be

captured by mid-latitude systems as one goes east; indeed Cloudband-associated Interactions are easily the

dominant type.

It was shown that almost two-thirds of Oceanic Cloudbands affecting western Australia produce at least

10 mm; over eastern Australia the proportion is one in two, a ®gure elevated considerably if Interactions are also

included. Signi®cant falls over north-eastern Australia are mostly associated with Cloudbands, whereas in south-

eastern Australia their Interactions are more important. These results do not support Kuhnel's (1990) contention

that Cloudbands are `unreliable rain-bringing systems' over south-eastern Australia, and important on only a

local scale; in fact, this study clearly shows that Cloudbands and their Interactions have widespread, signi®cant

effects. Several factors may have contributed to Kuhnel's impression, among them the disproportionate in¯uence

of the extreme 1982±1983 El NinÄo in his 5-year data set; his use of several coastal stations subject to signi®cant

orographic attenuation in Cloudband/Interaction situations (see Wright, 1989); and a tendency to understate the

signi®cance of rainfall from Interactions in lower rainfall areas.

Table III. Mean frequency (April±October) of Cloudband and Interaction events, 1978±1992, and proportion of casesproducing `signi®cant' and `heavy' rainfall (see text)

Region Classi®cation Events Signi®cant Heavy Percentagesigni®cant

Percentageheavy

Eastern Australia `North-west' Cloudband 10�3 4�4 2�9 42�9 28�2`Northern' Cloudband 3�2 2�4 1�7 75�0 53�1All Oceanic Cloudbands 13�5 6�8 4�6 50�5 34�1Continental Cloudband 5�0 3�2 1�7 64�0 34�0All cloudband types 18�5 10�0 6�3 54�2 34�1Cloudband-associatedInteraction

14�3 8�4 4�8 58�6 33�6

Continental Interaction 3�5 2�8 1�8 79�2 51�4Eastern Interaction 2�2 1�9 1�6 84�8 72�7All Interaction types 19�8 13�1 8�2 66�2 41�6

Western Australia `North-west' Cloudband 23�2 14�7 8�4 63�2 36�2All Oceanic Cloudbands 23�2 14�7 8�4 63�2 36�2Continental Cloudband 1�5 0�4 0�1 31�8 4�4All cloudband types 24�7 15�1 8�5 61�3 34�4Cloudband-associatedInteraction

7�7 3�4 1�1 44�3 14�3

Continental Interaction 0�8 0�3 0�2 33�3 16�7All Interaction types 8�5 3�7 1�3 43�3 15�3


Table IV. Mean frequency, standard deviation, and high/low frequency extremes, of signi®cant rainfall events fromCloudbands and Interactions, by sector (see text) over eastern Australia and western Australia, and for representative stationswithin each sector. The last two entries show the combined total of signi®cant falls over eastern Australia

from Cloudbands plus Cloudband-associated Interactions

Cloudband/InteractionEastern Australia

North-east South-east Charleville Wagga

`North-west' Mean 2�5 3�5 1�4 1�1Cloudbands S.D. 2�2 2�0 2�0 1�2

High 9�0 7�0 8�0 3�0Low 0�0 0�0 0�0 0�0

`Northern' Mean 2�3 0�4 1�3 0�0Cloudband S.D. 1�8 0�8 1�3 0�0

High 5�0 3�0 3�0 0�0Low 0�0 0�0 0�0 0�0

All oceanic Mean 4�8 3�9 2�7 1�1Cloudband S.D. 3�5 2�1 2�6 1�2

High 13�0 8�0 10�0 3�0Low 0�0 0�0 0�0 0�0

Continental Mean 2�7 1�7 0�9 0�5cloudbands S.D. 1�8 1�5 1�1 0�5

High 6�0 5�0 3�0 3�0Low 0�0 0�0 0�0 0�0

All cloudbands Mean 7�5 5�6 3�6 1�5S.D. 4�2 3�1 3�2 1�3High 17�0 12�0 13�0 3�0Low 1�0 1�0 0�0 0�0

Cloudband- Mean 2�8 7�7 1�1 3�1associated S.D. 1�3 3�2 1�0 1�5Interaction High 6�0 14�0 4�0 7�0

Low 1�0 1�0 0�0 1�0Continental Mean 1�1 2�5 0�5 1�3Interaction S.D. 1�4 1�8 0�8 1�1

High 3�0 6�0 2�0 4�0Low 0�0 0�0 0�0 0�0

Eastern Interaction Mean 1�9 1�4 1�1 0�9S.D. 1�8 1�5 1�3 1�2High 6�0 4�0 4�0 4�0Low 0�0 0�0 0�0 0�0

All Interactions Mean 5�8 11�5 2�7 5�3S.D. 2�5 3�9 1�5 2�2High 10�0 17�0 5�0 9�0Low 2�0 1�0 0�0 1�0

Cloudband plus Mean 5�9 8�1 3�3 3�3Cloudband- S.D. 4�0 3�4 3�0 2�0associated Interaction High 14�0 12�0 12�0 7�0(oceanic only) Low 1�0 1�0 0�0 0�0Cloudband plus Mean 9�3 11�3 4�2 4�4Cloudband-associated S.D. 4�7 5�1 3�6 2�3Interaction (continental� High 20�0 20�0 15�0 8�0oceanic) Low 3�0 2�0 0�0 1�0

continued

826 W. J. WRIGHT

At Wagga, for instance, Cloudbands and their Interactions together produce between four and ®ve signi®cant

rain events per season, and twice as many in good seasons. Moreover, the frequency of signi®cant falls decreases

little as one goes further inland. It must be stressed that, given the low rainfall of the study area, the occurrence or

non-occurrence of these events could be critical to agricultural prospects. Indeed, Wright (1988b) has shown that

it is year to year variations in these tropical±extratropical in¯uences that dominate winter±spring rainfall in

northern Victoria, explaining much of the strong SOI±rainfall relationship in this area.

This study has concentrated on the cooler months, partially because of the agricultural signi®cance of this

period over much of the country, but also because conditions then are most favourable for Cloudbands and

Interactions to form. In particular, the strong mid-tropospheric barocline necessary for Cloudband development

requires incursions of cold air (associated with an upper level trough or depression) into subtropical latitudes.

Such incursions are quite common in the cooler months, but infrequent in the summer half-year. Nevertheless,

Cloudbands and Interactions do occur in other seasons (in 1992, they were frequent until the end of the calendar

year), with the Continental types probably pre-eminent. These Continental systems have much in common with

the `tropical±temperate troughs' of southern Africa in spring and summer (e.g., Harrison, 1984a).

Although upslide associated with a mid-tropospheric barocline represents a common denominator for the

various Cloudband/Interaction types, this analysis shows that the separate types exhibit quite different patterns of

seasonality, origins of low-latitude moisture, and conditions for formation. For instance, oceanic cloudbands are

most prominent in the autumn/early winter period when the oceans bordering Australia are relatively warm and

the subtropical jetstream frequently strong (Gentilli, 1974). They decline in the late winter/spring months,

probably due to the cooling of the oceans bordering Australia at this time. By contrast, the Continental types

predominate in the spring months, when the rapidly heating northern interior spawns convective activity at the

same time that frontal troughs frequently extend well into low latitudes. The point is that any attempt to

conceptualize or model these important rain-producing types will need to distinguish the differing seasonal

patterns and low-latitude origins of the various types, and a priori one might anticipate different relationships

with large-scale forcing parameters such as sea-surface temperatures and the SOI.

It might be expected from the above that Oceanic cloudbands and their interactions would be more active in

years where the oceans bordering Australia remained relatively warm. Similarly, the in¯uence of all types of

cloudbands and interactions might be expected to be greater when conditions favour increased atmospheric

baroclinicity across Australian longitudes. Evidence for these suppositions is provided by Wright (1987, chapter

7), who demonstrated a strong positive correlation between sea-surface temperature anomalies (SSTAs) over the

oceans bordering northern Australia and interaction events affecting south-eastern Australia, and also an

increased tendency for enhanced tropical±extratropical in¯uences when SSTAs over the east Indian Ocean are

relatively warm in the tropics and relatively cold over extratropical waters to the south-west. The latter result

implies that a north-west±south-east oriented gradient in SSTAs over the east Indian Ocean is re¯ected in the

overlying and downstream atmosphere as an enhanced baroclinic gradient (similar correspondences between

anomalous gradients in atmosphere and ocean have been demonstrated elsewhere by, e.g., Namias (1974) and

Table IV. (continued)

Cloudband/InteractionWestern Australia

North-west South-west Newman Trayning

Cloudbands Mean 9�6 9�6 2�3 2�6S.D. 3�3 2�8 1�7 1�0High 16 14 6 4Low 5 5 0 1

Interactions Mean 0�3 3�4 0 1�2S.D. 0�5 1�6 0�0 0�8High 1 7 0 3Low 0 1 0 0


Lanzante (1983)). Nicholls (1989) has also demonstrated a speci®c link between the strength of essentially the

same Indian Ocean SSTA gradient and winter rainfall over a broad belt of Australia between the north-west and

south-east of the continent.

Finally, apart from the rain they bring, other properties of Cloudbands and Interactions appear favourable for

agricultural activities, and in combination might enhance the overall bene®ts considerably beyond those of the

rain alone. Firstly, because of their method of formation, rain from these systems generally falls steadily over

large areas. Secondly, in the cooler months evaporation is generally low. These factors would tend to maximize

rainfall effectiveness. Finally, there is evidence that months with frequent Cloudbands (even where little rain

occurs), have a reduced incidence of frost, itself an important factor in crop growth.

ACKNOWLEDGEMENTS

The author wishes to thank Roger Tapp, Rob Allan, Trevor Casey and Mary Voice and two anonymous reviewers

for comments and suggestions that have considerably improved the manuscript. I also wish to thank Paul Leigh

and the Bureau of Meteorology Drafting Section for drafting the diagrams.

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tropical–extratropical cloudbands and australian rainfall: i. climatology

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