The Structure of Polygonal Eye of a Typhoon

Abstract

A distinct polygonal eye was observed for 15 hrs in the PPI image of typhoon 8019 WYNNE. The shape of radar eye of approximately 30km diameter was variably square, pentagonal and hexagonal, and rotated counterclockwise around its center. The period of rotation was 41-43 minutes for the pentagon and hexagon compared with 47-50 minutes for the square. The period decreases (tangential velocity increases) as number of corner of polygon becomes larger. The pentagonal shape occurs most frequently and persists for 111 minutes, contrary to 12 minutes for square shape. There are no observations of a triangular or heptagonal shapes.
The deformation of the polygon is restricted within the narrow, limited portion with several kilometers width inside of inner eyewall of concentric double eye. The phenomenon coincides with the largest shear of tangential velocity. As the result of tracking small convective cells in the eye, the cells rotate in a Rankine vortex field; moreover, polygonal features have a higher rotational velocity than the cells inside the eye. The straight-sided portion of polygon becomes wavy as if instability exists at the inside boundary of eyewall. The phenomenon took place in the concentric double eye of well-developed tropical cyclones with approximately 920-950mb central pressure and 30-50km of eye diameter.