Southerly Nocturnal Wind Surges and Bores in Northeastern Australia

Abstract

Observational data are presented on southerly nocturnal wind surges, which sometimes give rise to spectacular ?morning glory?-type roll clouds in the southern part of the Gulf of Carpentaria region of northern Australia. Like their more frequent northeasterly counterparts in the region, southerly surges are shown to have the character of an undular bore propagating on the nocturnal low-level stable layer. Synoptic mean sea level pressure patterns conducive to the formation of southerly surges are identified and possible mechanisms for the genesis of surges are discussed. The possibility that some southerly surges are generated by the movement of a front across central Australia, as suggested by Smith et al., does not appear to be the most usual generation mechanism. In fact, an analysis of time-height cross sections of velocity components derived from six hourly rawinsoundings from Mount Isa point to a strong association between the occurrence of southerly surges and the formation of the nocturnal jet over the continent. We are led to the hypothesis that many southerly surges are spawned in the interaction between the inland nocturnal jet and the sea breeze front which originates from the southern coast of the gulf. Others may be the result of katabatic drainage from the Barkly Tablelands, but at present the data are inadequate to assess the importance of this mechanism. The paper concludes with a discussion of the extra difficulties in synoptic analysis and forecasting in the region south of the gulf presented by the occurrence of these southerly surges.