The Australian Boxing Day Storm of 1998—Synoptic Description and Numerical Simulations

Abstract

An intense low pressure system formed in the Bass Strait region of southeastern Australia during the Boxing Day long weekend of 25‐28 December 1998. The explosive development of this low commenced on 26 December (Boxing Day) and reached peak intensity on 27 December 1998 with mean wind speeds reported in the 50‐60kt range. The event is referred to, for simplicity, as the Boxing Day storm. There are three objectives of this study. The first is to highlight the rarity of this type of storm during the Australian summer period and to investigate the potential of high-resolution numerical models to distinguish such storm events [winds 48 kt or greater (25 m s21)] from the less destructive and more common gale force (34‐47 kt or 17.5‐24 m s21) wind speed producing systems. A brief climatology of intense low pressure systems in the region underlines how unusual this system was. The second objective is to investigate the ability of available operational and research-mode global and regional numerical models to provide accurate and reliable advance warning of this major development. With the exception of a locally run high-resolution (10 km) limited area model, run in real-time research mode, all failed to depict accurately the storm force winds associated with this system in the 48-h time frame critical for operational warnings. However, most did predict gale force winds. The success of the locally run higher-resolution model over the lower-resolution models (50 km or greater) provided the motivation to investigate further how limited area models of increased resolution and complexity can assist in the operational prediction of extreme cyclogenesis events in a 48-h time frame. A 13-hPa improvement over the other models in the prediction of the central pressure of the system, accompanied by a significant area of surface mean wind speeds in excess of 50 kt (26 m s21), were achieved. The importance of horizontal model resolution in accurately depicting key wind speed thresholds with sufficient lead time for operational forecast and warning purposes was illustrated. Another critical aspect of the forecast problem, consistency, was then demonstrated through the ability of the model to predict successfully these key meteorological parameters in successive model runs. The third objective of this study is to understand the role of various dynamical and thermodynamical parameters in the development of the low. This is achieved by a series of model sensitivity studies, investigating the contributions of sea surface temperature structure, surface fluxes, jet streak resolution, cumulus convection, and topography, that were then conducted. Impacts on the finescale structure of the midlatitude cyclone were detected for the first three impact studies, with cumulus convection having a lower impact. Topography was found to have little effect on the critical features of the low for this maritime development.