Southerly nocturnal bores over northeastern Australia

Abstract

Analyses of 1‐min data from automatic weather stations (AWSs) in the Gulf of Carpentaria region are used to study southerly nocturnal bores over northeastern Australia. The data cover the time period 1 August to 31 December in the three years 2012–2014. A total of 12 days on which a bore passed over the region of interest were found. Ten of the disturbances occurred between August and mid‐October in the presence of synoptic‐scale ridging across the continent behind a frontal trough. The other two occurred in early November in the presence of localized ridging behind an inland trough separate from a frontal trough. Six of the bores were followed some hours later by a clear air mass change. Four of the 12 cases are examined in detail wherein interpretations of the AWS data are aided by analysis fields from the European Centre for Medium‐range Weather Forecasts (ECMWF). The results indicate that the longevity of the bore is related to the time of day that the cold front interacts with the stable maritime layer to the north of it. The ECMWF analyses are able to capture the frontal troughs as well as the timing of their passage through the AWS network. In the analyses, the troughs are seen as narrow strips of elevated cyclonic vorticity at 950 mb. Vertical cross‐sections intersecting the troughs showed structural characteristics of a shallow cold front, but the horizontal resolution of the analyses is inadequate to capture the generation of bores ahead of the fronts.