A Numerical Study of the Mesoscale Environment Of a Southerly Buster Event

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Abstract
A numerical case study of a Southerly Buster event that occurred on 1 December 1982 during the Southerly Buster Observational Program (SUBOP) is presented. Southerly busier refers to the leading edge of a coastally trapped gravity currentlike phenomenon that occurs over southeastern Australia during the passage of summertime cold fronts. The Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) mesoscale model has been used in a series of 36-hour experiments to determine the impact of orography, surface heating, boundary-layer physics and moist processes on the generation and maintenance of the broad features of the event. In addition, the effects of horizontal and vertical resolutions are examined. With 40 km grid separation, 15 vertical levels and a high-resolution boundary-layer formulation, the model captures the observed mesoscale flow pattern associated with the southerly busier event despite using coarse, although enhanced, U.S. National Meteorological Center global analyses for initialization. The simulated southerly flow is a relatively shallow and narrow cold-air current. Adequate horizontal and vertical resolutions are very important for a simulation of this phenomenon. With 80 km grid spacing and 10 vertical levels, the model is unable to resolve the broad features of the event. This has strong implications for the operational prediction of these types of mesoscale disturbances. Model sensitivity experiments indicate that strong gradients in physical properties associated with the land-sea contrast over southeastern Australia are responsible for the coastally enhanced flow. In particular, the synergy of orographic forcing and both differential surface friction and heating determines the structure of the current. It is shown that although the presence of the Great Dividing Range has a large influence on the events it does not seem to be essential for the development of the coastal jet in this particular study. This result contradicts most of the previous theories and hypotheses related to the dynamics of southerly busters.

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