Summertime Cold Fronts in Southeast Australia—Behavior and Low-Level Structure of Main Frontal Types

Abstract

The Cold Fronts Research Programme (CFRP) concentrated upon cold frontal systems (synoptic scale fronts) occurring in southeast Australia during late spring and early summer (November and December); many of these tend to the complex, with change lines associated with sea breezes, prefrontal squall lines and related cold outflows (Garratt et al.). The present paper extends the earlier work to include additional observations of frontal events from field phase 3 of the CFRP held in late 1984. In addition we distinguish between this frontal system (Type 1) and a frontal system (Type 2) occurring throughout summer, but predominantly late in the season, which is often dry with a single major change line identified as a mesoscale (coastal) front. 0bservations from the CFRP and other sources are described covering a wide range of frontal intensities, with frontal speeds varying between 5 and 25 m s?1. The nature of the low level flow behind, and in the vicinity of, the active surface cold front (SCF) is described. In the coastal region both systems show significant falls in ?e across the SCF in the lowest 500 m or so, but only Type 2 has a corrresponding fall in ?. Type 1 shows gradual falls in ? throughout the prefrontal zone related to subcloud evaporative cooling. Wind observations for both types reveal a feeder flow of cold air towards the front, below 500?1000 m height and within about 50?100 km of the SCF. Together the data suggest that the surface cold front has the local structure of an unsteady gravity current. Nevertheless the Type 1 SCF is identified with a synoptic scale (Southern Ocean) cold front having an associated large-scale, deep tropospheric three-dimensional flow configuration and so must be predominantly under large-scale control. This is consistent with the lack of any significant diurnal influence on the eastward movement of the SCF. In contrast such a diurnal influence is found for the Type 2 SCF (which form ahead of a synoptic scale cold front often only distinguishable as a cloud band on satellite imagery) with the implication of substantial mesoscale forcing. Both the Type 2 SCF and prefrontal squall lines of the Type 1 system tend to traverse the coastal region between late morning and early evening, with squall-line activity tending to be concentrated near the coast. This implies a strong influence of the diurnal cycle of boundary-layer heating over the land, as suggested in the case of the Type 1 leading change line by Berson et al.