Four large‐amplitude wave families observed simultaneously over northern Queensland, Australia

Large-amplitude internal waves* are generated in the northern Queensland region of Australia with predictable regularity. For this reason the region has proved to be an ideal natural laboratory for studying their generation and propagation. (The locations of all places mentioned in the text are marked on Fig. 1.) Invariably these waves are generated in the evening or early hours of the morning and propagate on the strong surfaced-based nocturnal inversion that commonly forms in the region. The waves generally dissipate shortly after sunrise when convective mixing destroys the stable waveguide layer. At times the crests of these waves are marked by spectacular roll clouds known locally as 'morning glories'. Comprehensive reviews of the morning glory and related phenomena are given by Smith (1988) and Christie (1992). Morning glory waves in the region are known to propagate from three preferred directions and, by implication, are preferentially generated in three general areas. The most frequently observed waves propagate from the north-east and are the most well known and widely studied. They form out of the collision between the east-coast and west-coast seabreezes over the Cape York Peninsula; for further details see Smith (1988) and Christie (1992). A second class of waves propagate from the south and are thought to be generally, although not always, associated with the passage of troughs and cold fronts across the central parts of the continent. A recent field experiment, known as the Central Australian * Internal waves are those found in the interior of a fluid. Fronts Experiment (CAFE), was designed to examine the origin of southerly morning glories, the results from which are described in Smith et al. (1995). Christie (1992) reported that a t h r d class of waves, although much less frequently observed than their north-easterly and southerly cousins, are found to propagate from the south-east. The dynamical origin of waves in this class is less certain than that of those belonging to the first two classes, although moist convection over the ranges that run along the east coast of Queensland and katabatic drainage are likely candidates. (Figure 1 shows details of the regional topography.) On occasion, north-easterly and southerly morning glories have been observed simultaneously in the region. One particularly spectacular example, observed during the final stages of CAFE, has been reported by Reeder er (11. (1995). Only once, in the early morning of 10 October 1990, have large-amplitude waves been observed to propagate simultaneously from all three preferred directions. Christie (1992) has presented a preliminary analysis of this case based on microbarograph data. The purpose of the present article is to re-examine the case of 10 October 1990 in the light of a spectacular N O M (National Oceanic and Atmospheric Administration) A V H M (Advanced Very High Resolution Radiometer) satellite image which was not available at the time of the original analysis. The satellite image, which forms the cornerstone of the present analysis, shows at least four distinct morning glories present in the region in the early-morning hours, and suggests that the south-easterly waves were triggered by deep convection. Ex-