Dynamics of a Rapid Cloud Band Development over Southeastern Australia

Abstract

On the night of 27?28 August 1985, a cloud band associated with a cold front dramatically broadened over southeastern Australia, leading to unforecast rain and overforecast daytime temperatures. This paper presents a detailed synoptic description of this event, describes the vertical circulations leading to cloud band development, and, by means of back trajectory analysis, determines the origins of air parcels entering the cloud band. The analyses which were used in this diagnostic study were prepared using an intermittent insertion incremental limited-area data assimilation system. The consistency and coherence of the diagnostic quantities calculated from these analyses demonstrates the value of linking the analyses with a dynamic forecast model, even with the relatively coarse resolution of 250 km used in this study. It is shown that, following the reorganization of the jet patterns over Western Australia (WA), pressure falls lead to an amplification of the surface pressure trough and the acceleration of both the southerly flow west of the trough and the northerly flow east of the trough. The thermal gradient increased over WA in this period under the influence of the increased convergence and increased deformation associated with these flows, which themselves were strongly influenced by the direct vertical transverse circulation at the jet entrance region. Back trajectory analysis shows that air parcels which formed the developing part of the cloud band had their origins in the boundary layer over central Australia and moved into the ascending warm conveyer belt as a northeasterly isallobaric flow towards the rising branch of the vertical circulation at the jet entrance and thereafter ascended at a rate of up to 330 hPa day?1.