Elliptical and Polygonal Eyes of Typhoons Induced by Island Topography

It is well know that the eye of a mature tropical cyclone occasionally takes an elliptical or polygonal form, which rotates cyclonically with somewhat slower angular velocity than that of the swirling wind of the cyclone itself. In a special case, the eye keeps its specific form more than a half day and rotates several times with a constant period of a few hours. Such an elliptical or polygonal eye is supposed to be a manifestation of the multiple vortex structure of a tropical cyclone, which is known to be the most destructive form of a vortex in case of a tornado. Thus, it is important for the mitigation of damages on tropical (or subtropical) islands and coastal regions due to tropical cyclones to know under what condition, their eyes form such a distinctive and destructive structure.

Here, we investigated the eyes of typhoons hitting the Southwestern islands of Japan, located fundamentally in subtropical latitudes between 24 - 31 degrees N, with the ground-based operational radar data obtained by the Japan Meteorological Agency during 1994 - 2004. As the result of investigation, several examples of typhoons, which had typical elliptical or polygonal eyes, were found. There, it is noticeable that they were, in most of cases, formed when their eyewalls enclosing them came near to the island, not only mountainous islands such as Okinawa or Amami islands but also "flat" raised coral islands whose maximum size are as high as 120 m. In some typhoons, the elliptical or polygonal eyes were seen only when the cyclones have crossed the islands so that their eyewalls touched them. But in the remnant typhoons, their distinctive eye-shapes were kept intact for many hours even after they moved away from the islands. These facts, more or less, suggest the elliptical or polygonal eyes were formed through the interaction of typhoons with island topography, at least in the present cases.

Recently, the present author performed a linear singular value analysis investigating the optimal excitation of asymmetric perturbations on an idealized barotropic vortex, and revealed the case in which the pattern of perturbation leading to the maximum growth during a specified evaluation time, i.e. the most unstable singular mode, is far different from that of the most unstable eigenmode (Itano, 2015). We intend to discuss the possibility of such a singular mode, rather than eigenmode, to be excited by island topography.