Synoptic analyses of a subtropical cold front observed during the 1991 Central Australian Fronts Experiment

Using objective analyses, the synoptic environment and large-scale controlling mechanisms for two frontal events (Events 1 and 2) observed during the Central Australian Fronts Experiment (CAFE) are presented. The analyses are based on the Australian Bureau of Meteorology's Regional Assimilation and Prognosis system (RASP). While Event 1 is the main focus of the paper, we discuss briefly Event 2, mainly highlighting the similarities between the two systems. Although both events are associated with well-developed mid-latitude lows over southern Australia, the strongest gradients in temperature and velocity are analysed in the subtropics. Moreover, the low-level temperature gradients, relative vorticity, divergence and frontogenesis function all weaken during the daytime and rapidly strengthen at night. The nocturnal evolution is emphasised in the paper. In both events the analysed wind fields show that during the afternoon to the rear of the front, the boundary-layer minds are subgeostrophic because of the strong turbulent mixing there. After sunset the turbulent mixing subsides and the boundary layer stress is rapidly reduced. Consequently, the post-frontal ageostrophic winds rotate anticyclonically and a low-level nocturnal jet develops. Localised increases in the ageostrophic deformation and convergence accompany the formation of the nocturnal jet, which in turn rapidly strengthens the northern section of the front. This rapid frontogenesis may be the generation mechanism for the large-amplitude bore-waves observed during CAFE to propagate ahead of Events 1 and 2.