Morning Glories" ^S'as* Over Northern Australia And

Interacting "Morning Glories" ^S'aS* over Northern Australia and Abstract some of the place names mentioned in the text is displayed in Fig. 1a; the remaining place names are It is just over 15 years since the first major expedition was indicated on Fig. 1b, which focuses on the Gulf of organized to investigate the "morning glory" phenomenon of north- Carpentaria region.) A photograph of a morning glory eastern Australia. The authors review briefly what has been learned roll cloud is shown in Fig. 2. The clouds are parallel to about the generation and evolution of morning glory disturbances during this time and present data for a particularly interesting event one another, often stretching for several hundred that occurred on 3 October 1991, a day on which two morning glory kilometers. The leading roll is usually smooth, whereas wave formations, one from the northeast and one from the south, were successive rolls are more ragged in texture reflecting detected. The morning glories were seen to interact over the Gulf of a higher degree of turbulence. As the name suggests, Carpentaria. Spectacular National Oceanic and Atmospheric Ad- ministration/Advanced Very High Resolution Radiometer satellite imagery together with comparatively good surface data are pre- sented for this event. Aspects of the interaction between the northeasterly and southerly morning glories are shown to be consistent with theoretical predictions concerning solitary wave interactions. 1. Introduction Solitary waves are isolated finite-amplitude waves of permanent form. Although their mathematical prop- erties have been studied for the past century and a half, only recently have they been viewed as funda- mentally important in the evolution of a wide variety of dynamical systems in the physical and biological sciences. Solitary waves have been observed in the lower atmosphere, often at the leading edge of an evolving undular bore disturbance, which is the atmospheric analog of an undular bore on a tidal river. The "morning glory" phenomenon of northeastern Australia is by far the best documented and most widely known example of an atmospheric undular bore. Morning glory is the name given to a distinctive cloud formation, characterized by a spectacular low- level roll cloud or series of roll clouds that may be seen in the southern part of the Gulf of Carpentaria region of northern Queensland. (A map of Australia indicating *Centre For Dynamical Meteorology and Oceanography, Monash University, Clayton, Australia. +Research School of Earth Sciences, Australian National University, Canberra, Australia. #Meteorological Institute, University of Munich, Munich, Germany. Corresponding author address: Dr. M. J. Reeder, Centre for FIG. 1. (a) Map of Australia indicating places named in the text, Dynamical Meteorology and Oceanography, Monash University, and the Australian Bureau of Meteorology mean sea level isobaric Clayton, Victoria 3168, Australia. analysis for 0700 LST 3 October 1991. The trough axis is marked In final form 12 December 1994. with a dot-dash line, (b) Map of the Gulf of Carpentaria region of ©1995 American Meteorological Society northern Australia indicating places named in the text. Bulletin of the American Meteorological Society 1165 Unauthenticated | Downloaded 10/07/21 03:53 AM UTC atmosphere and is observed to propagate southwestward on the low-level stable layer produced by the onshore branch of the sea-breeze circulations along the west coast of the peninsula and the southern coast of the Gulf of Carpentaria. The disturbance evolves as energy is focused into stable solitary- wave components. The characteristic morning glory roll clouds are formed as moist low-level air is lifted and cooled adiabatically in the crest of the approach- ing wave. In contrast to northeasterly morning glories, the origin of southerly morning glories is far less certain, although we know that the passage of cold fronts FIG. 2. Morning glory roll cloud over Burketown, Queensland, at about 0700 LST taken during the Central Australian Fronts Experiment (photographed by R. K. across the center of the continent can Smith). lead to their generation. Moreover, it is known that the synoptic conditions con- ducive to the formation of northeasterly these disturbances form in the early hours of the morning glories are favorable also for the generation morning and normally decay within a few hours after of southerly morning glories. Much insight was ob- sunrise. Although they are generated sporadically at tained from the recent Central Australian Fronts Ex- all times of the year, they are most frequent during the periment (CAFE). The experiment was conducted late dry season (September-October). Morning glo- jointly by Monash University (Australia), the Australian ries are accompanied by pressure fluctuations of National University, and the University of Munich several millibars, wind gusts up to about 10 m s~1, and (Germany) and ran from 7 September to 4 October usually propagate with a speed of about 10-15 m s1 1991. Its aim was to investigate the structure and relative to the ground. The most common direction of dynamics of subtropical cold fronts together with the movement is from the northeast, but cloud lines ori- inland heat trough that affects central and northeast- ented east-west and moving from the south are ob- ern Australia (Smith et al. 1995). It sought also to served also. Recently, Christie (1992) reported that investigate the generation of southerly morning glo- morning glories are occasionally observed to propa- ries. The observations made during CAFE highlighted gate from the southeast as well. Comprehensive re- the large diurnal variation of frontal structure associ- views and bibliographies on the morning glory, includ- ated with diabatic processes; the fronts were often ing summaries of the pioneering studies by R. H. very difficult to locate during the late morning and Clarke, are given by Smith (1988) and Christie (1992). afternoon when convective mixing was at its peak but quickly developed strong surface signatures in the evening as the boundary layer convection subsided 2. Generation mechanisms and a surface-based inversion developed. Moreover, there appeared to be a common tendency in the early Northeasterly morning glories are generated dur- morning for the formation of a borelike structure in the ing the late evening over the western side of the Cape frontal zone as the inversion strengthened. In each York Peninsula as the circulation associated with the case documented, as the bore evolved, it propagated east coast sea breeze develops a marked line of on the inversion, moving ahead of the airmass change convergence (see, e.g., Noonan and Smith 1987). and developing a series of solitary waves at its leading The formation of this convergence line so far inland is edge. aided by an easterly geostrophic wind across the The CAFE data are unique in providing the first peninsula, which, at the same time, restricts the inland clear evidence of the formation of a southerly morning penetration of the west coast sea breeze. Late in the glory in the Gulf of Carpentaria region linked to a cold evening, the east coast sea-breeze surge "collides" front in the south—the event of 16-17 October 1991 with the decaying west coast sea-breeze circulation described in detail by Smith et al. (1995). Neverthe- on the western side of the peninsula. As a result, a less, the precise generation mechanism for such pressure jump, or bore, is generated in the lower nonlinear waves remains uncertain and is the subject 1166 Vol. 76, No. 7, July 1995 Unauthenticated | Downloaded 10/07/21 03:53 AM UTC of continued research. One possibility is that the heat trough had merged to form a single trough that overall disturbances are generated by the enhanced curved northwestward through northwestern nocturnal convergence associated with the cold front/ Queensland and the Northern Territory. It is the most trough system, including possibly the inland heat equatorward part of this feature that was responsible trough, itself. Alternatively, they may be generated in for the generation of the southerly morning glory. the absence of a cold front through the development Northeasterly and southerly morning glories were of a nocturnal jet convergence within the heat trough observed 3 October; the possible generation mecha- itself. Whatever the case, there is evidence that the nisms for each of these waves have been discussed mechanism of formation of the solitary waves them- briefly in section 2. As the disturbances approached selves may not depend crucially on the precise details the moister air near the gulf coast, they each devel- of the forcing (Christie 1989). oped spectacular morning glory roll clouds. The sur- The origin of southeasterly morning glories is even face-data network across north-central Queensland less well understood than that of their southerly coun- was greatly enhanced during the CAFE project (Smith terparts. Indeed, their existence has only recently et al. 1995). Data from this surface network combined been reported (Christie 1992). It is likely that their with Japanese Geostationary Meteorological Satellite generation is linked to the mountain range that runs (GMS) imagery allowed us to construct an isochrone along the east coast of the continent, the Great Divid- map for the leading solitary waves of both distur- ing Range. Two candidate mechanisms are katabatic bances in unprecedented detail (Fig. 3). The isoch- drainage and the low-level outflow from regions of rones reveal a number of interesting features. In the deep convection. first place, they show that the disturbance with which the northeasterly morning glory was associated was over 1000 km long and extended much further north over the Gulf of Carpentaria than has previously been 3. Interacting morning glories on 3 recognized. The observed average propagation speed October 1991 of the northeasterly wave was about 15 m s~1 over inland northern Queensland and about 10 ms-1 over We present now a case study of two interacting the central Gulf region.

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