HIGH-RESOLUTION NUMERICAL-SIMULATION OF TROPICAL CYCLONE DRENA UNDERGOING EXTRA-TROPICAL TRANSITION

Tropical Cyclone Drena, a relatively long lived cyclone lasting from J anuary 2, 1997 to January 10, 1997, crossed over three well separated island groups and affected a fourth in the south western Pacific Ocean during different stages of its life cycle. Midway through its transit ion into an extra-tropical low, it passed over the eastern edge of Nor folk Island, an isolated island located in the western Pacific Ocean m idway between New Zealand and New Caledonia, at 0615 UTC (1745 local) on January 9, 1997. The tropical cyclone exhibited markedly different characteristics during each phase of its life cycle, with thick fog be ing reported during the eye passage at Norfolk Island. Although routin e global and Australian region numerical models were able to provide t he operational forecasters with broad scale guidance as to the movemen t of the tropical cyclone, the level of detail available at these reso lutions is insufficient to identify the characteristics important to c ommunities in the tropical cyclone's path. The high resolution numeric al model (HIRES) developed by the University of New South Wales, was r un at a resolution of 25 kilometres to investigate the evolution of th e tropical cyclone from a warm cored, quasi-symmetrical vortex into a highly asymmetrical mid latitude low pressure system. The model captur es the observed wind and precipitation structure of the cyclone very w ell during this crucial transition phase. The comparison extends into the vertical with a model derived vertical cross section depicting the key features observed in a sequence of ten specially requested radios onde flights, released at six hourly intervals, from Norfolk Island wh ich transect the passage of the tropical cyclone over this remote isla nd.