A Numerical Study Of Spiral Rainbands In Typhoon Chanchu (2006):Fiue Structures And Propagation Mechanism

In this study, fine structures and propagation mechanism of spiral rainbands in super Typhoon Chanchu (2006) are examined using a Advanced Research Weather Research and Forecast (WRF-ARW) model simulation with the finest grid size of 2 km. Results show that the WRF model reproduces well the track, intensity and basic structures. Spiral rainbands are classified into principal, secondary,distant and inner rainbands. The principal rainbands remain quasi-stationary with respect to the Chanchu's center and exhibit a wavenumber-1 feature. The secondary rainbands that occur on the radial inward side of the principal rainbands rotate around Chanchu's center. Both the principal and secondary rainbands lean radially outward with height.The distant rainbands lie outside the principal rainbands and have deep convection Distant rainbands are inward-sloping with height with a cold pool in the low layers.The inner rainbands just populate outside the eyewall. They propagate rapidly both radially outward and azimuthally, whereas they do not show slope with height along the radial direction.The spiral rainbands in Typhoon Chanchu are made up of convective cells which have different shapes and structures in different spiral rainbands. In the upwind of the principal and secondary rainbands, the convective cells show positive and negative vertical vorticites along tangential direction. The convective cells in the upwind of the distant rainbands are banded positive and negative vertical vorticites.In the inner rainbands, convective cells are all positive vertical vorticites. The convective cells in the upwind of all spiral rainbands move cyclonic and merge with the eyewall. Vorticity budget calculations indicate that convective cells acquire their vertical vorticity through tilting of horizontal vorticity and stretching of vertical vorticity.In terms of the propagation mechanism of inner rainbands, the inner rainbands are not related with the gravity waves, the vortex Rossby waves or the mixed Rossby-Gravity Waves. It may be only associated with convection derived from the eyewall.