Tropical cyclone Katrina developed over the western south Pacific on 3 January 1998, and spent 3 weeks meandering over the tropical oceans. During this time, it underwent numerous changes of intensity. From 14 to 16 January, the storm moved westward toward the northeast coast of Australia, and rapidly intensified as its direction of motion changed to the south southwest under the influence of a midlatitude trough.
Observations clearly indicate that in this case, the traditional view of intensification, with enhanced outflow ahead of the upper trough did not occur. Indeed as the trough from the southwest approached the TC circulation at upper levels, convergence developed over local areas just west of the storm's center. This occurred as a secondary windstream branched from the trough and converged towards the storm's upper level (cyclonic) circulation.
The BMRC high resolution (15km) tropical cyclone prediction system - with sophisticated vortex specification and initialisation - has been used to run a series of forecasts during the intensification. The system shows considerable skill at foreshadowing both the motion change and intensification, and reproduces quite realistically the observed upper level flow changes.
Diagnostics from the high resolution forecasts suggest the following scenario. As the upper trough approaches from the southwest, the anticyclonic flow - between the trough and the TC circulation at 200 hPa - becomes inertially unstable. Possibly as a result of the release of the instability, a branch of anticyclonic flow exits from the trough, converges towards, and wraps around the cyclonic portion of the storm's upper level circulation. The wrap-around flow indicates capture of the TCcirculation by the midlatitude trough, and appears as a new amplifying trough developing over the top of the TC. Local subsidence develops just west of the TC center (west of the new trough axis), in regions of upper convergence, while much enhanced ascent develops east of the TC center (east of the new trough axis). The associated increase in convective heating appears sufficient to produce rapid surface pressure falls and intensification.
SSM/I data and water vapor winds (not used in the forecasts) validate the evolution
of convective asymmetries and structure change