A cloud-resolving simulation of Typhoon Rusa (2002) : Polygonal eyewall and mesovortices structure

Understanding the details of inner structure of tropical cyclones is one of the most important things because the inner-core structure near the eyewall is related not only to severe weather such as torrential rainfall and hazardous winds, but also to the change in the intensity of a tropical cyclone.

However, the numerical experiments of the core region in a real tropical cyclone with full-physics and high resolution have rarely been conducted due to the limited computer resources until recent years.

In this study, to elucidate the inner structure of Typhoon Rusa (2002), a numerical simulation was conducted using a two-way triple-nested cloud-resolving nonhydrostatic model with a 2 km horizontal grid size on the finest nested mesh. The model successfully reproduced the features of polygonal eyewall structure observed in Typhoon Rusa. The simulated asymmetric structure in the inner-core region was dominated by a number of mesovortices within or near the eyewall in the lower and middle troposphere. Meso-lows on the horizontal scale of 20-30 km were found at the kinks of polygonal eyewall, and between them meso-highs existed. The modification of radial flow by the mesovortices affected not only the location of eyewall, but also the convective activities in the eyewall by causing the interaction between the eye and the eyewall. The radial outflow that was modified by the mesovortices transported the high equivalent potential temperature air from the eye into the eyewall, and consequently the convective activities in the eyewall were enhanced there.

The horizontal distribution of the potential vorticity showed the wave-like pattern related to the mesovortices near the eyewall, which was quite similar to the ideal numerical experiments of Schubert et al. (1999) and Kossin and Schubert (2001).