Southerly Nocturnal Wind Surges and Bores in Northeastern Australia

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 nocturna...