A Numerical Study of a Southeast Australian Coastal Ridging Event

Abstract

A numerical study of the 9?11 November 1982 southeast Australian coastal ridging event is presented. The mesoscale coastal features of this event have been previously described as a coastally trapped disturbance (CTD). However, the analysis presented in this paper shows that the model coastal trapping is of secondary importance in generating the coastal ridging. Given the potential controversy, particular emphasis in this paper is placed on identifying the causes of the ridging. The paper follows a previous study by the authors in which a Colorado State University Regional Atmospheric Modeling System simulation of this event was validated. In this event, contributions to ridging along the southeastern Australian coast came from both synoptic and mesoscale phenomena in the model. A Southern Ocean anticyclone that tracked east from the Great Australian Bight toward the southern Tasman Sea led to a large-scale steady pressure increase over the entire southeast Australian region. Ridging at the Victorian coastal stations developed with the passage of a synoptic-scale wave of cooler denser air at midlevels, and the arrival of low-level cooler marine air with a wind shift from the southwest to a more southerly direction. On the east coast, the pressure change was most abrupt in the south as warm continental air was replaced with cooler marine air after strong winds passed through Bass Strait and turned left around the southeast continental corner. The flow closest to the coast continued turning left, enhanced by the daytime lowering of pressure over the continent, and pushed inland to the mountain barrier. The most intense part of the wind surge crossed the coast during the evening near the Hunter Valley (a significant gap in the east coast mountain barrier). This led to flow splitting inland up the Hunter Valley and northward along the east coast. The coastal component developed into a CTD in the form of a wind surge that accelerated ahead of the region of onshore forcing until it encountered a reduced mountain barrier farther north, and the flow spilled inland signaling the end of the model event.