Tropical Cyclone Debbie (2017) caused 14 fatalities and an estimated US$2.67B and was ranked as the most dangerous cyclone to hit Australia since TC Tasi in 2011. In addition to the extreme flooding as TC Debbie moved onshore and down the east coast of Australia, Debbie also experienced rapid intensity changes just offshore prior to making landfall. As Debbie initially approached Australia, it intensified to Category-2 on 26 March, and then stalled. However, on 27 March, Debbie underwent a rapid intensification cycle intensifying from Category-2 to Category-4 in approximately 12 hours while just offshore. Debbie made landfall as a Category-4 TC near Airlie Beach on 28 March 2017, causing widespread and disastrous damage.

In this presentation, we focus on the inner-core structure and evolution during the offshore intensification period including both the slow intensification (SI) stage and the rapid intensification (RI) stage. To study the case, a high-resolution WRF simulation is used with 1-km horizontal, and 10-min temporal resolution. The SI stage is characterised by an asymmetric eyewall contraction. where the northern portion of the eyewall spirals inward and partly replaces the previous eyewall. In addition, a portion of the eyewall breaks down into several mesovortices, which mix vorticity inward toward the eye, as the radius of maximum winds rapidly contracts. By the end of this process, a smaller, but complete eyewall has formed. The following RI stage is characterised by a series of eyewall breakdown and re-development events. Initially the eyewall breaks down into mesovortices that mix high theta_e air and vorticity inward. Then followed by the axisymmetrisation, eyewall is re-developed and a more enhanced vorticity ring forms, which prepares for the next round of eyewall breakdown. Each round of breakdown and re-establishment brings high theta-e air back into the eyewall, re-invigorating thunderstorm activity and driving intensification.