NUMERICAL-SIMULATION OF CUTOFF LOWS ON THE AUSTRALIAN EAST-COAST - SENSITIVITY TO SEA-SURFACE TEMPERATURE

The occurrence and synoptic-scale structure of cut-off lows near the Australian east coast are documented. These systems are associated with severe weather in the south-east Australian states of New South Wales and Victoria. Two types of cut-off low formation are identified, described here as coastal lows and blocking lows. Twenty-four hour numerical simulations are performed for four systems in order to investigate the model-reproduced structure of the large-scale rainfall and gale-force wind. The numerical experiments demonstrated that the regional-scale model is capable of simulating the heavy rainfall and gale-force winds associated with these systems. All systems are characterized by a major rain band curling around the south and east of the surface low. To the south of the system the rain band is paralleled by a curved band of gale-strength winds. The sensitivity of these parameters and of mean sea-level pressure to underlying sea-surface temperature (SST) fields also is investigated through numerical experimentation. When the SSTs were increased by between 2-degrees and 3-degrees, the simulated cut-off lows had deeper central pressures, usually of 1-2 hPa. In one case the extra deepening was dramatic, being 7 hPa at 24 h. The impact on rainfall was considerable, especially on the peak values, which were increased by between 45 per cent and 80 per cent, and on the areal mean values in 'catchment size' areas surrounding the peak value. In contrast the peak surface winds were increased only by about 10 per cent, but the areal extent of gale-force winds increased greatly, by between 50 per cent and 70 per cent. The large sensitivity of these weather elements to SST is discussed in terms of possible implications for both short-term operational forecasting and for regional effects of enhanced greenhouse climate change.