The morning glory of the Gulf of Carpentaria: a paradigm for non-linear waves in the lower atmosphere

Observations of morning glory waves and other related non-linear wave disturbances are reviewed along with laboratory and theoretical studies which describe the formation, evolution and decay of disturbances of this type. Emphasis is focussed primarily on the properties of morning glories which are at best ill understood. Special attention is given also to those areas which illustrate the wide diversity of dynamical processes associated with morning glory wave phenomena. Some new observations are described which help to clarify the interpretation of morning glories and their propagation characteristics. It is proposed that morning glory waves which occur in northeastern Australia can be divided into at least three independent groups, each of which is characterised by a particular source and a particular direction of propagation. A new type of commonly occurring morning glory cloud formation is identified in the form of thin cumulus capping clouds centred above each of the leading solitary waves at an altitude of about four kilometres. Extensive microbarometer array observations are used to determine the area over which morning glories propagate as coherent disturbances . Evidence is presented to show that some northeasterly morning glories, which originate in the early evening over the Cape York Peninsula, continue to propagate inland over north-western Queensland during the following day as coherent low-level disturbances for distances of at least 300 km. It is shown that some northeasterly morning glories excite waves, apparently on an upper-level waveguide, which are detected during the following night at Warramunga near Tennant Creek in the Northern Territory, some 1000 kilometres from their point of origin over the Cape York Peninsula. Array observations show also that morning glories of different types frequently interact in the area along the southern margin of the Gulf. It is proposed that isolated breaking waves and other similar localised instabilities which are seen from time to time near the crest of morning glory roll cloud formations are manifestations of strong non-linear interactions between intersecting solitary waves. Other observations of related disturbances both in Australia and elsewhere are critically reviewed. Finally, we present some numerical solutions based on fully non-linear wave theory which illustrate the morphology of typical large amplitude solitary waves in the lower atmosphere. Solitary waves play a significant role in the initiation and organisation of thunder-storm squall lines, the modulation of rainfall and the dispersal of atmospheric pollutants. Since solitary waves usually occur without warning as clear-air disturb-ances, waves of this type represent a particularly insidious wind shear hazard to low-flying aircraft, especially during landing or take-off.