BAMOS 11 friction, showing that friction played an important role in determining the front’ s structure and speed.
— John Garratt.
Berson et al.( 1957,1959) noted that there was a marked preference for these cool changes to cross the coast in the afternoon or evening, a pattern that did not fit the fairly uniform distribution of fronts crossing the Southern Ocean. They attributed this to the“ variation of the horizontal pressure field caused by the daily cycle of differential heating between land and sea”. They also found that these cool changes often comprised more than one discontinuity and the majority of initial changes were non-frontal in origin, or rather developed in situ as a“ coastal front” that moves inland ahead of an expected cold front. Such initial changes were quite shallow and the deeper layer of cold air associated with the upper trough and front followed later. Due to this complexity the authors tried to avoid the term“ front” and so leave open the physical nature of these surface discontinuities.
Berson and his colleagues also explored the complexities of the passage of cool changes through mountain areas using automated observations from the Snowy Mountains Hydro- Electric Authority. They found that most changes were strongly distorted and delayed, with more rapid movement along the coast, particularly in the shallower leading changes. Some changes did not penetrate into the high parts of the mountains.
Similar evidence of the interaction among differential heating over land and sea, sea-breezes and“ fronts” had been garnered in the enterprising field studies by Reg Clarke( 1955). He explored the penetration of sea-breezes inland in the Canberra region and in the Kalgoorlie-Esperance region of Western Australia and observed that the sea-breeze could be identified up to 300 km inland. In observations of the passage of sea-breezes as far as Kalgoorlie he recorded changes in moisture associated with a wind change, although well inland the temperature change was small. However, Clarke describes these wind changes as“ frontal” in character. He commented:“ The similarity between frontal passages and sea-breezes around the Australian continent in summer is remarkable, and is thought to be a significant reflection of the effect of diurnal heating on frontal behaviour.” Clarke followed up this study with one focussed on the coastal region of south-eastern South Australia( Clarke, 1961). Like Berson et al., Clarke found that the leading edge of the cool change often originated as the sea breeze and subsequently developed into the main change identified as the front. He found closed circulations also and wave-like undulations at the leading edge.
Satellite era
A technological break-through in settling the debates was the advent of weather satellites in the 1960s. Gibbs( 1961) discusses the potential of weather satellite imagery and applies an early image from TIROS-1 to the synoptic situation of a tropical cyclone over the Tasman Sea on 10 April 1960. The first TIROS satellite lasted less than three months but TIROS-II was launched in November 1960. While it was not yet feasible to receive images in Australia, coded cloud analyses( nephanalyses) derived from the images were sent from the US Weather Bureau to the Bureau’ s Central Office during December 1960 and January 1961( Rutherford, 1961). The fact that these coded messages sometimes arrived within four hours of photograph time was regarded as remarkable at the time. Rutherford utilized these coded cloud reports to re-analyse a variety of synoptic situations to assess the utility of these observations. It is not surprising that fronts, tropical cyclones and extratropical vortices comprised the majority of the situations considered 2.
There were some basic issues to address such as where in an extensive cloud mass did the actual front lie. David Martin from the University of Wisconsin was attached to the International Antarctic Meteorological Research Centre( IAMRC) for a year and was instrumental in making great progress in satellite interpretation for the Southern Ocean. The summary of a seminar( Martin, 1967) reports on how he classified soundings from high latitude island stations( Macquarie and Campbell) according to the cloud signature in satellite images. From the concurrent surface analysis he was able to associate cloud signatures with synoptic features and hence the appropriate vertical temperature profile. Based on a winter case study for Macquarie Island he concluded that the structure of two systems passing the island showed“ a striking similarity to the Norwegian frontal model.” However, he felt that while the model was useful for explaining transient phenomena, his findings did not support the concept of a continuous polar front. His techniques were documented in Martin( 1968) and a detailed cold frontal case study was presented in Zillman and Martin( 1968). The Martin technique was further developed by Leon Guymer( 1978) who devised a methodology for using satellite imagery for specification the 1000-500 hPa thickness patterns. His technique became an integral part of the operational routine of the World Meteorological Centre Melbourne through the 1970s and early 1980s.
Mr W. J. Gibbs said that he had noticed that, following the introduction of satellite photographs on an operational basis, both CAO( Central Analysis Office) and Southern Hemisphere Analysis Centre( SHAC) were using more“ trailing fronts” than previously. He was unsure whether to attribute this to the influence of the satellite photographs, or to the influence of Mr. J. C. Langford( SHAC).
— quoted in Martin( 1967).
In preparation for the Global Atmospheric Research Program( GARP) which was aimed at progressing numerical weather prediction( NWP), the Bureau took on the task of preparing twice daily Basic Data Set analyses for the southern hemisphere to 20o S for November 1969 and June 1970. The charts were not required in real-time and so could test the application of new data types including a range of satellite images, satellite temperature soundings from the Satellite Infrared Spectrometer( SIRS), and, at least for the Atlantic and east Pacific Ocean, cloud drift winds derived from the US geostationary satellites, as well as traditional observations received after operational cut-off
2 A brief overview of the experiments( including aircraft reconnaissance, balloon and rocket flights) conducted to validate the new satellite imagery is given in Hart( 2021).