Cold Fronts Research: the Australian Summertime "Cool Change" A

Cold Fronts Research: The Australian Summertime "Cool Change" A Abstract Much of the significant weather of southeastern Australia is associated with the passage of cold fronts. In summer, such passages are often accompanied by rapid and extreme temperature falls, as hot continental northerly winds are replaced with much colder southwesterlies from the Southern Ocean; for this reason, they are popu- larly and aptly known as "cool changes." These summertime fronts, which normally form part of a front-trough complex sandwiched between two anticyclones, are ill-understood and lead to many forecasting problems. In early 1979, a Cold Fronts Research Programme was established as a coordinated long-term project to study front- trough systems affecting this region of Australia. The program, which involves all of the major Australian meteorological centers, has been designed to include three observational phases over five years, with emphasis being placed on summertime frontal systems. Each phase of intensive observations is of four weeks duration, and Phases I and II have now been completed. This article summarizes the philosophy behind the program, outlines its scientific objectives, and describes the observational networks employed. A brief review of the results of Phases I and II and an outline of future activities also is presented. South Wales, an extreme manifestation of which is the "southerly buster" (Colquhoun, 1980; Baines 1980). Weather forecasters in Australia have been aware for a long time that models of frontal structure, developed during 1. Introduction many decades on the basis of observational studies in the northern hemisphere, are of limited value in the Australian In summer, frontal passages in southeast Australia often region in late spring and summer. At this time, when the sub- bring extreme temperature falls, as strong hot and dry north- tropical ridge is displaced south of the continent, it is com- erly winds from the continent are replaced with much cooler mon for southeast Australia to be affected by cold fronts southwesterly winds from the Southern Ocean. Surface formed in the col or trough between two anticyclones (ana- temperatures in the northerlies occasionally exceed 40°C and lyzed by the Australian Bureau of Meteorology as the "pre- temperature falls of 10-15°C in a few tens of minutes are not frontal trough") as well as the tail ends of fronts extending uncommon. This explains the popular name "cool change" from the depressions of higher latitudes. Anticyclones affect- for such fronts. Only about half of summer changes are ac- ing southeast Australia tend to "bud off" from the semi- companied by precipitation, the remainder being completely permanent high in the Indian Ocean in this season, and this dry; this is a reflection of the relative dryness of the warm air. may be associated with a higher latitude cold front. Interac- Accordingly, severe thunderstorm activity is unusual over tion between this front and the pre-frontal trough presents a South Australia and Victoria, but the interaction of fronts very complex situation for forecasting. An example of this with the coastal dividing ranges between Melbourne and type of situation is shown in Fig. 1. There is also a large land- Sydney in the presence of more humid air to the east of the sea temperature contrast along a long and predominantly ranges can lead to severe squalls along the south coast of New east-west coastline, with an extensive ocean area poleward, a unique geographical situation which presumably has an im- portant bearing on frontal behavior (also the north-south coastlines of western Australia and southeast South Aus- 1 Geophysical Fluid Dynamics Laboratory, Monash University, tralia between Adelaide and Mount Gambier provide a suit- Clayton, Victoria 3168, Australia. ably oriented strong temperature gradient for frontogenesis 2 CSIRO Division of Cloud Physics, Epping, NSW 2121, Australia. 3CSIRO Division of Atmospheric Physics, Aspendale, Victoria to occur). Notwithstanding these special aspects, models de- 3195, Australia. veloped on the basis of observations of fronts in the northern 4 Research and Development Branch, Bureau of Meteorology, hemisphere currently provide the main guide to frontal ana- P.O. Box 1289K, Melbourne, Victoria 3001, Australia. lysis and forecasting in the Australian region. This is an un- 0003-0007/82/091028-07$05.75 fortunate situation, since most of the significant weather in © 1982 American Meteorological Society the densely populated southeast of Australia is associated 1028 Vol. 63, No. 9, September 1982 Unauthenticated | Downloaded 10/06/21 10:48 PM UTC Bulletin American Meteorological Society 1029 season (Berson, 1958; Berson etal., 1959). The data were col- lected from two experimental networks. The first was at Mount Gambier and the second in the Melbourne region. During the observing period both radiosonde temperature and humidity soundings as well as pilot balloon (pibal) ascents were made. The upper-air observations were supported by a network of self-recording surface stations. A light aircraft was used to make soundings over Bass Strait. The observations successfully established the existence of a low-level shear zone associated with the cool change. In 1959 a mesoscale network was set up to study the structure of dry cold fronts (Clarke, 1961). Serial pibal flights in the vicinity of frontal zones revealed flow patterns that indi- cated a marked deformation of the frontal surface. It was found that if the air in the vicinity of the fronts had been satu- rated at the 900 mb level, both the cold fronts and the sea breeze fronts would have developed significant prefrontal FIG. 1. MSL synoptic chart for the Australian Region showing a rain. Further, such flows would have triggered convective typical summertime situation with a front-trough system in the southeast of the region, sandwiched between two anticyclones. Sur- rain if the warm air was potentially unstable. Clarke (1961) face fronts, representing the boundary of significantly colder air, are found evidence to suggest that the strong circulations about often preceded by a 'pre-frontal trough' in which thunderstorms the leading edge of the cool change were favored by the pres- may develop. With the passage of the front, hot continental north- ence of the coastline. The observational studies at this time erly winds are replaced with much colder southwesterlies from the Southern Ocean. were complemented by the theoretical work of Ball (1960). It is noteworthy that the pioneering studies of Clarke and Berson et al. described above were made before the advent of satellites and suffer from an almost complete lack of data with the passage of cold fronts. In summer, the bush fire sea- over the ocean (except for the occasional ship report). In son, accurate forecasts of frontal wind changes and precipi- more recent years, synoptic research in Australia has concen- tation may be critical for successful fire control; such fires trated on satellite observations of cloud and its application to occur frequently during hot spells and are especially danger- the forecasting of cyclogenesis over the Southern Ocean. ous in the ambience of strong, dry northerly winds ahead of However, the cloud structure of frontal bands has not been fronts. investigated specifically. Indeed, Streten (1977) emphasized Research into the characteristics and behavior of cold the lack of observational studies of bands of organized cloud fronts in the Australian region is not without a history, al- of substantial horizontal dimensions which are associated though previous studies, in particular early observational with the "front" of the synoptic chart. programs, were limited in a number of ways. Intensive research into cold fronts in Australia has been In the immediate post-war period, attempts were made to revived in recent years, the initial stimuli resulting from a systematize southern hemisphere frontal analysis (Clarke, two-day workshop on fronts organized by the Australian 1954), but not until the period from 1953 to 1959 were special Branch of the Royal Meteorological Society in May 1977 and observations made describing in detail the surface features of from the report later that year of an ad hoc Bureau of Meteor- the summer cool change (Berson et al., 1957,1959). The term ology working group on the 'Progress and Prospects of "change" was used to describe a sequence of events which Weather Forecasting' which recommended, inter alia, that give the appearance of fronts and included both cold fronts there be a comprehensive study of cold fronts as they affect and sea breeze fronts. During the seasons 1953-54 and southeastern Australia. Partly as an outcome of the report, a 1954-55, Berson et al. (1957) analyzed the frequency, move- Cold Fronts Research Programme (CFRP) was established in ment, and certain structural features of fronts. The observa- 1979 with a particular view to increasing understanding of tional material used in the analysis was based on routine ob- the summertime cool change and to facilitate improved fore- servations made by the Bureau of Meteorology and casts of such events. Central to this is an observational sub- supplemented by other surface-based observations. It was program, with field studies closely coupled to theoretical in- shown that the summer differential heating between the land vestigations. This ambitious project has harnessed the and sea exerted a strong influence on the behavior of cool resources of the major meteorological institutions in Aus- changes. Two, and sometimes more, discontinuities in a se- tralia, including the Bureau of Meteorology, the CSIRO Di- quence frequently accompanied the main trough, the leading visions of Atmospheric Physics and Cloud Physics, the Aus- change being as a rule more pronounced. Over a wide coastal tralian Numerical Meteorology Research Centre and Monash, region, single and leading changes tended to arrive in the Melbourne, and Flinders universities. The CFRP is adminis- afternoon, consistent with a diurnal variation of the speed of tered by a Joint Planning Committee whose main function is the front deduced from synoptic analysis.

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