Interaction of an Asymmetric Double Vortex and Trochoidal Motion of a Tropical Cyclone with the Concentric Eyewall Structure

Radar echo images demonstrate that mature tropical cyclones frequently have a concentric eyewall structure, which consists of the inner eyewall, echo-free moat, and outer eyewall regions. Near the inner and outer eyewalls, well-defined wind maxima are generally observed. This indicates that two large vertical vorticity regions exist just inside radii of the two wind maxima near the inner and outer eyewalls. Therefore, the concentric eyewall structure can be considered to be a double vortex composed of the inner vortex and the outer vortex ring. In this study, the contour dynamics model is used on the f plane to analyze the characteristics of flows with either a symmetric double vortex or an asymmetric one, and examined the relationship between the movement of the inner vortex in an asymmetric double vortex and a trochoidal motion of a tropical cyclone with an asymmetric concentric eyewall structure. Results show that, depending on the degree of an interaction of a double vortex, the evolution of the inner vortex is classified into three patterns: the first is that the center of the inner vortex is stationary, which is seen only for the symmetric double vortex; the second is that the track of the center of the inner vortex draws a circle; and the third is that it draws a spiral. A numerical experiment based on an observed flow around Typhoon Herb was also performed. The time evolution of the double vortex is very similar to that of radar echo intensity of Typhoon Herb. Also the rotation period and amplitude of the inner vortex in the numerical experiment were comparable with those of the trochoidal motion in the observation. These suggest that, in tropical cyclones with the concentric eyewall structure, the interaction of an asymmetric double vortex can become a cause of trochoidal motion.