A rare East Indian Ocean autumn season tropical cut-off low: impacts and a high-resolution modelling study

摘要

There are two aims of this study. The first is to provide a synoptic description of the lifecycle and impacts of a seasonally rare but intense cut-off low of tropical origin that developed off the northwest coast of Australia, during the Southern Hemisphere autumn months of late April/early May, 2005. The second, and more important, part of this study is an assessment of the capacity of a state-of-the-science numerical model, assimilating all available satellite-derived observational data, to predictthe track, intensity, duration, and severe weather that occurred during the life cycle of the cut-off low. The system was unusual in that its initial development occurred over tropical waters and was associated with a strong and highly involuted subtropical Jetstream, generating a cloud band that produced record 24 h rainfall totals in northwest Western Australia for this time of the year. The cut-off low then underwent further deepening which produced more unseasonable heavy rainfall. Confirmed tornado reports occurred over the populous southwest corner of Western Australia. The synoptic discussion focuses initially on the intensification phase of the system, next on the period of heavy rainfall and gale to storm force winds that were generated by the cutoff during its most intense phase, and finally on the severe thunderstorm outbreak over southwest Western Australia. The study reveals a possible additional reason for the intensification of the tropical low, namely, the presence of positive SST anomalies of up to 3 °C over which the storm track passed early in its life. The modelling study used a high-resolution version (10km horizontal grid spacing) ofa coupled atmosphere-ocean model developed at the University of Oklahoma. The 96 hour forecast covered the period 00 UTC April 28 to 00 UTC May 2, 2005. The initial and boundary conditions were obtained from the archived analyses and forecasts from the Australian Bureau of Meteorology. The numerical forecasts focussed on the salient features of the tropical cut-off low, including the development, track and the rainfall patterns and totals.