The structure and evolution of the northern Australian dryline

Abstract

The diurnal cycle of the northern Australian dryline is examined through three 24-hour series of cross-sections taken from the Australian Bureau of Meteorology’s Limited Area Prediction System (LAPS) forecasts during the period 13–14 October 2002. The development of the dryline is analysed within the framework of the frontogenesis function applied to the mixing ratio. Close agreement is found between the LAPS forecasts of the low-level wind field and the network of automatic weather station (AWS) and manual observations, as well as with the available Atmospheric InfraRed Sounder (AIRS) satellite images. During the afternoon, a heat trough extending across northern Australia drives a sea-breeze circulation from the coastal side of the trough and a weak solenoidal circulation in the dry air inland of the trough. As boundary-layer turbulence subsides overnight and surface friction reduces, the low-level flow is accelerated toward low pressure, sharpening the mixing-ratio gradient in the region from just above the nocturnal boundary layer to 900 hPa. The contribution from convergence to the mixing-ratio frontogenesis reaches a maximum around midnight, while the contribution from deformation to the mixing-ratio frontogenesis reaches a maximum before sunrise as Coriolis deflection rotates the nocturnal winds and the axis of dilatation becomes aligned with the heat trough. The diurnal cycle of the northern Australian dryline is compared with that of the dryline which develops over the Great Plains of the United States. In contrast to the Australian dryline, the Great Plains dryline is strongest during the afternoon. While boundary-layer turbulence disrupts the dryline over northern Australia, it strengthens the Great Plains dryline by drying the moist boundary layer at higher terrain elevations and transporting westerly momentum to the surface. The resulting low-level westerlies oppose low-level easterlies in the moist air leading to strong convergent mixing-ratio frontogenesis which strengthens the Great Plains dryline in the afternoon