The prediction of low-level mesoscale convergence lines over northeastern Australia

One of the most important, but difficult forecasting problems in northern Australia is to predict the occurrence of deep convection. On occasion, convective storms are triggered along convergence lines that originate as a result of organised mesoscale circulations such as sea-breezes. If one can forecast these lines, it might be possible also to forecast the storms that they initiate. The Gulf of Carpentaria and Cape York Peninsula region of northeasternAustralia is distinguished for the regular occurrence there of a range of convective and wave-cloud lines which appear to be associated with mesoscale convergence lines. These include the celebrated ‘morning glory’ phenomenon, a spectacular travelling wave-cloud system commonly observed over the southern part of the gulf and adjacent seaboard (see Smith 1988 and Christie 1992 for recent reviews), and the so-called north Australian cloud line (NACL), a line of convective cloud that is seen in satellite imagery to extend frequently across the entire gulf (Drosdowsky and Holland 1987; Drosdowsky et al. 1989). Both types of disturbance form on the western side of the Peninsula, the former typically in the late evening and the latter in the late afternoon, and they both move with a westward component across the gulf. Moreover, both are responsible for significant weather events in the region. The morning glory is accompanied by sudden wind squalls, intense low-altitude wind shear and a marked vertical displacement of air parcels, sometimes sufficient to initiate showers or thunderstorms in the wake of the disturbance. The NACL brings a wind change also and while the depth of convection is sometimes limited by an inversion at a height of about 3 km, embedded showers and thunderstorms may occur at times of the year when the air over the gulf is sufficiently moist. Sometimes the morning glory forms the southern extension of the NACL (Smith and Page 1985). Aust. Met. Mag. 51 (2001) 13-23