Diagnosis of Upper-Level Processes Influencing an Unusually Intense Extratropical Cyclone over Southeast Australia

Abstract

On 1 December 1987, an unusual midlatitude cyclone affected much of southeastern Australia. The storm was characterized by unforced rapid deepening to a near record low (locally) mean sea-level pressure, high winds, anomalously cold surface temperatures, and near-record rainfall in some areas. The storm resulted in extensive damage, including a massive livestock kill. Comparison with storm tracks over southern Australia from the past 20 years shows that the path of this storm was quite unusual for this time of year. Utilizing a series of analyses prepared from an incremental limited area data assimilation system, it is shown that: 1) an amplifying upper-tropospheric wave influenced the initial development and path of the cyclone as it crossed the southern coast of Australia, 2) transverse circulations associated with two juxtaposed upper-level jet streaks embedded in the wave focussed upper-level divergence and midlevel ascent over the low during its rapid intensification phase, and 3) a distinct upper-tropospheric isentropic potential vorticity maximum was identified well upstream of the developing low, but with no evidence of an extrusion of this air penetrating and enhancing the low-level circulation as has been found in other cases of rapid cyclogenesis. Given that inadequate operational numerical weather prediction (NWP) guidance was partially to blame for the underforecast of this event, the operational limited area NWP forecasts are presented and compared with forecasts based on the research analyses from the assimilation system. 11 is shown that improved forecasts of cyclone intensification and of precipitation result when the model is initialized with the assimilation analyses. Further improvements are obtained when the grid resolution of the forecast model is increased. With the operational implementation of the assimilation system into the Australian Bureau of Meteorology (BOM) in 1989, the improved guidance resulting from the assimilated analyses is currently available to forecasters in Australia.