BAMOS
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Theoretical advances in the origin of mid-latitude cyclones and anticyclones were made in the late 1940s through the work of Charney and Eady who introduced the concept of baroclinic instability, with the formation of fronts an integral part of the development of low pressure systems. Sutcliffe in the UK was concerned with the practical issue of forecasting cyclone development. His 1947 paper, among others, emphasised the role of upper forcing in cyclone development; it was not just a lower tropospheric process, although Pettersen’ s kinematic approach, considering deformation of temperature fields and defining a frontogenesis function, had aided understanding of the formation of fronts 1. In Australia, Bill Gibbs became an enthusiastic supporter and advocate for the Sutcliffe theory.
Clarke( 1954) writes that“ new fronts may also be created in a field of upper divergence. This produces low level convergence which results in frontogenesis in a suitable temperature field.” Such developments could occur in the cold air of a deep southerly outbreak, or at relatively low latitudes in warm air at the point of inflection of the 500 hPa contours between a nearly stationary trough and downstream ridge. In the latter there is“ the simultaneous creation of a cold front and low where neither had been before“. He noted that this process is common along the west coast of Western Australia in summer.
Langford( 1957) had assimilated these new findings and applied them to the task of prognosis:
“ The unique position fronts hold in this type of analysis is confirmed by the classical association of fronts and depressions in the form of waves, and the more general recent conclusion of the sufficiency of advection of large amounts of positive vorticity above a surface front for cyclogenesis. This intimate association between fronts and low pressure development permits delineation on any particular chart of areas of possible cyclogenesis. Because of this, meteorologists in southern Australia are alerted toward specific areas of the daily chart rather than being left suspicious of the whole of the adjacent Southern Ocean as the likely breeding ground of a cyclone.”
However, Tim Gibson( 1964), from the University of Melbourne, reported positively on the“ analysis philosophy” in the Canadian Meteorological Service Canada which was still very much based on air mass and fronts and had adopted a detailed“ three front model”. Gibson advocated a greater use of this approach for the southern hemisphere.
I was at the Bureau training school during the first half of the sixties and we were taught about the classical Norwegian model of fronts but we were also told that things were often not quite like that in the Australian region and, in particular, warm fronts were not often seen. I think the‘ frontal debates’ period was pretty well over by then.
— Jenny Hopwood.
As debate continued over the nature of fronts over the Southern Ocean, other researchers from the CSIRO Division of
1 A biography of Reginald Sutcliffe has recently been published. Martin( 2021).
Meteorological Physics were investigating the passage of fronts over the Australian continent and finding new complexities. One of those who developed a particular fascination with the Southern Australian summer-time cool change was Andrzej Berson who, in his earlier career, had served in the D-Day forecasting team( Operation Overlord) in World War II. In two pioneering papers with Derek Reid and Sandy Troup( Berson, Reid and Troup, 1957; 1959) he used available surface observations, augmented with private sources, upper air measurements, questionnaires to observers and a few flights by instrumented aircraft to analyse the behaviour of summer cool changes over south-east Australia.
Summertime cool changes in southern Australia from a CSIRO perspective.
The phrase“ cool change” in southern Australia, at least up until recent times, describes a summer phenomenon that often brings reductions in near-surface air temperature of 10 to 15oC in less than an hour along the southern coastline, providing sustained relief from high temperatures. The dynamical structure of the cool change lies somewhere between that of a classic cold front and of a sea breeze. In 1953 Bill Priestley and Andrzej Berson started work on understanding the dynamical structure of the cool change. Until 1955, Berson and co-workers analysed the frequency, movement, and structure of cold fronts using routine Bureau of Meteorology( BoM) surface and upper-air observations, supplemented by a network of surface stations operated by volunteer observers.
CSIRO’ s first true field experiments, involving considerable staff, took place in December 1954 and January 1956, when Berson and others, together with Clarke( still at BoM) went to Mt Gambier in South Austalia to study the cool change. Surface measurements across a substantial region of south-east South Australia together with observations from pilot balloon ascents were the result. Berson extended the programme into the 1956-57 season using instrumentation concentrated around Mt Gambier and Melbourne, both surface and upper air. In January 1959 Clarke( now at CSIRO) organised a month-long expedition to Renmark, with instruments distributed between Renmark and the coast and utilising light aircraft observations, to monitor arrival times of sea breezes. Finally, in January 1960 he undertook a larger expedition based at Lameroo in South Australia. Overall, the observations from these series of experiments revealed the mesostructure of both sea breezes and shallow, dry, cold fronts over featureless terrain. Evidence was found to connect the strong circulations at the leading edge of such fronts to frontal squalls and pressure jumps. Overall, a frontal transition zone existed over land consisting of a series of discontinuities, or change lines, extending over several hundred kilometres, rather than a single wind-shift line between the hot northerlies and cooler south-westerlies. The theoretical work Keith Ball( 1960) complemented these observational studies. He developed a simple model of a front that for the first time incorporated the effect of surface