A Numerical Study of Spiral Rainbands in Typhoon Chanchu (2006)

Spiral rainbands are well-known features of tropical cyclones, and they play important roles in tropical cyclone structural evolution and intensity changes. In this study, spiral rainbands of Typhoon Chanchu (2006) are examined using a Weather Research and Forecast model simulation with the finest grid size of 2 km. The spiral rainbands can be classified into four types: principal, secondary, inner and distant rainbands, following Moon,Y. and D. S. Nolan (2015). All four types of rainbands are identified in the WRF simulation, and structural and thermodynamic differences among the different types are examined in this study. Principal rainbands are nearly static relative to Chanchu's circulation center and exhibit a wavenumber-1 feature with its outer boundary facing dry inflows. Secondary rainbands revolve around Chanchu's center. Both rainbands tend to lean radially outward with altitude, whereas distant rainbands are tilted radially inward with a cold pool in the lower troposphere. Inner rainbands are relatively shallow showing little vertical tilt. An analysis of the vorticity equation indicates that new convective cells at the tails of the spiral rainbands account for the generation of vorticity couplets due to the tilting of horizontal vorticity associated with vertical wind shear. As moving toward the eyewall, stretching of positive absolute vorticity by boundary-layer convergence plays a major role in increasing and maintaining cyclonic vorticity.