Multiscale interactions in the formation of a Tropical Cyclone simulated in a global cloud-system-resolving model

The simulation of global cloud-system-resolving model NICAM with a horizontal resolution of about 7 km successfully reproduced the lifecycle of Tropical Storm Isobel that formed over the Timor Sea in the austral summer 2006. Initialized with the atmospheric conditions 2 weeks before the Isobel's formation, the model captured reasonably not only the timing of the observed cyclone genesis but also its motion and mesoscale structure. In this study, both the evolutions of the large-scale circulation and the mesoscale processes in the formation of the simulated Isobel were analyzed. The westerly wind burst accompanying the onset of a Madden-Julian Oscillation (MJO) event over the Java Sea enhanced the cyclonic shear in the lower troposphere around the Maritime Continent with the high convective available potential energy (CAPE), providing the pre-conditioned large-scale environment for the genesis of Isobel. In the pre-conditioned favorable environment, mesoscale convective vortices (MCVs) developed in the mesoscale convective systems (MCS), and the cyclonic potential vorticity (PV) anomalies associated with deep convection (model VHTs) were generated in the MCVs. The PV budget analysis showed that convection strengthened cyclonic PV in the parent MCV (the primary vortex enhancement). The multiple VHTs evolved in the parent MCV and merged to form an isolated VHT with an upright monopole PV structure with a warm core in the upper troposphere. The intensification of the MCV was enhanced through the system scale intensification (SSI) process (the secondary vortex enhancement), leading to the formation of Isobel. We show that the large-scale environmental flow was critical to the genesis and structure change of Isobel, while the mesoscale and the storm-scale processes, such as the MCVs and merging VHTs were responsible for the genesis.