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A case of coastal interaction with a cool change

It has long been known that summertime fronts in southeastern Australia show pronounced diurnal changes in intensity, movement and times of arrival. As long ago as the 1940s this behaviour was linked to the coastal thermal and frictional gradient (Cassidy 1945; Loewe 1945). A series of field studies in the 1950s (Berson et al. 1957, 1959; Clarke 1961) demonstrated diurnal variations in speed and arrival time of southeastern Australia cool changes, and attributed some of these to the variations in land-sea heating contrast during the diurnal cycle. Following the inception of meteorological satellite imagery it was recognised that fronts in southeastern Australia, particularly dry fronts during late spring and summer, can appear to rapidly accelerate across the coastline during the day, or ‘surge’ along the southern coastline of Australia. Other fronts can appear to have multiple change lines (e.g. Jasper and Taylor 1977). The Cold Fronts Research Program (Ryan et al. 1985), hereafter CFRP, was designed to address some of these issues. The CFRP observational programs were based in the late springtime period, and so tended to sample ‘moist’ changes, with multiple ‘change lines’ associated with convection. In addition, a series of studies addressing the issue of accelerating dry fronts was stimulated by the CFRP (e.g. Hanstrum et al. 1990a,b; Physick 1988; Reeder 1986; Garratt 1986,1988; Garratt and Physick 1986,1987; Garratt et al. 1989). Hanstrum et al. (1990a,b) proposed the model of frontogenesis in the pre-frontal trough which develops over land ahead of the hyperbolic deformation zone between two anticyclones. In this model, they proposed an intensifying thermal gradient between pre-frontal oceanic west/northwesterly flow and hot continental northerly flow. Once the thermal gradient was established, quasi-geostrophic arguments were used to propose a positive feedback process of frontogenesis. The issue of what processes led to the formation of the trough was neglected, as were diabatic processes. Physick (1988) used an idealised three-dimensional model of a front approaching a coastline to demonstrate that the relative phase of the sea-breeze circulaAust. Met. Mag. 51 (2002) 203-221