Travelling waves and bores in the lower atmosphere: the ‘morning glory’ and related phenomena

Abstract

Observations of solitary-wave and travelling bore-like disturbances in the lower atmosphere are reviewed, together with relevant theoretical, laboratory and numerical studies concerning the structure, propagation and genesis of such disturbances. Primary emphasis is given to a discussion of the ‘morning glory’ phenomenon in northern Australia, the example par excellence of these types of disturbance, and the best documented observationally to date. Observations of related phenomena elsewhere are reviewed briefly. While such occurrences appear to be less frequent in any single location than in the southern Gulf of Carpentaria region of northern Australia, reported occurrences are widespread, and there is mounting evidence that such disturbances are more common than is generally supposed. The evidence suggests that whenever a suitable wave-guide exists for the propagation of disturbances, the latter can be generated as the natural response of quite general forcing mechanisms, examples being the collision of two sea breezes from opposite directions on a peninsula; the interaction between a thunderstorm outflow and a sea-breeze front; or simply the transformation of a thunderstorm or squall-line outflow as it moves into a low-level stable layer. Because of the large vertical displacements they produce, the larger amplitude disturbances (typically with surface pressure jumps of 1 mb or more) must be considered as potential triggers for deep convection if the atmosphere is sufficiently conditionally unstable, and the large low-altitude wind shears associated with them present a serious hazard to low-flying aircraft.