The generation of north Australian cloud lines and the 'morning glory'

The generation of cloud line disturbances over northern Australia is investigated using the three-dimensional mesoscale numerical model developed by R. Pielke and collaborators. These disturbances include the so-called North Australian Cloud Line (NACL), a line of convective cloud that forms regularly along the west coast of Cape York Peninsula in the early evening, and the 'morning glory', a propagating wave cloud line that occurs in the late evening and early morning over the southeastern part of the Gulf of Carpentaria region. Recent two-dimensional model simulations supported by field observations show that the generation of both phenomena can be accounted for by the convergence produced by the collision of the east- and west-coast sea-breezes over the Peninsula. In the present paper we investigate the collision of the sea-breezes and its aftermath in a three-dimensional simulation to explore further the relationship between the NACL and the morning glory and the flow patterns accompanying their genesis. The simulation described is for a typical dry-season situation when there is also a moderate easterly component of geostrophic flow over the Peninsula. Attention is focussed on three-dimensional effects such as the northward tapering of the Peninsula, the marked bulge thereof near latitude 14°S, and the concave coastline shape in the southeastern part of the Gulf. In the calculation, collision of the sea-breezes occurs in the early evening north of the bulge and progressively later as the Peninsula widens to the south. North of the bulge, the times and locations of maximum convergence associated with the collision correspond well with those typically observed for the first appearance of the NACL in satellite imagery, but in the absence of moist-convective processes in the model, the convergence is not sustained and the resulting disturbance has largely decayed by midnight. In the south of the Peninsula, sea-breeze collision produces a bore-like disturbance in the late evening, the model analogue of the morning glory surge. This accords with the observation that morning glories are confined mostly to the southern Gulf region. A comparison of model generated data with those available for a particularly well-documented morning glory surge shows good agreement, suggesting that the model captures the essential dynamics of the diurnal low-level circulations over the Peninsula under typical dry-season conditions.