Geometric Characteristics Of Eyes And Eyewall Tilts In Sheared Tropical Cyclones

The geometric characteristics of tropical cyclone eyes before landfall in South China are examined using ground-based radar reflectivity.It is found that the median and mean eye area decrease with tropical cyclone intensity,except for the severe typhoon category,and the eye size increases with height.The increasing rate of eye size is relatively greater in upper layers.Moreover,3the ratio of eye size change in the vertical direction does not correlate with tropical cyclone intensity.No relationship is presented between the ratio of eye size change in the vertical direction and the vertical wind shear.No relationship between the vertical change in eye size and the eye size at a certain level is found.No relationship exists between the vertical change in eye size and the intensity tendency.The eye roundness values range mainly from 0.5 to 0.7,and more intense tropical cyclones generally have eyes that are more circular.Four numerical simulations are conducted with TCM4 to investigate the geometric features of eyes and eyewall contraction of sheared tropical cyclones,including experiments with(1)a stationary environment(SHOO),(2)5 m s-1(SH05),(3)10 m s-1(SH10),and(4)15 m s-1(SH15)shear.The results show that the roundness value becomes larger with increasing shear.The elliptical eye shape is associated with wavenumber-2 vortex Rossby waves.Eye size increases with height,which grows more slowly at lower levels than at upper levels.The vertical change of eye size in vertical wind shear with different magnitudes seems not significant.Eye contraction nearly concurs with tropical cyclone intensification,but such contraction ceases before the cyclone matures.The ratio of eye size change in the vertical direction is proportional to intensity change.The quasi-concentric eyewall structure plays an important role in intensity change.The change of eyewall slope depends on the diffenrence of changes of the radius of maximum wind at upper and lower levels.Distinct structures of the simulated tropical cyclones thus leads to different eyewall tilt characteristics.For instance,the upper-level RMW in SH10 first shrinks followed by an oscillation on the first stage while the lower-level RMW always contracts,making the eyewall tilt barely changed first and then increasing.The upeer-level RMW oscillates first and then expands followed by rapid contraction during the second stage in SH10 while the lower-level RMW rapidly shrinks first and then contracts slowly,resulting in an increase followed by an decrease in eyewall slope.During the third stage,the upper-level RMW shrinks first and then expands while the lower-level RMW changes oppositely.As a result,the eyewall tilt decreases first and then increases during that period.It is indicated that the change of RMW is governed by the radial gradient of the tangential wind tendency and the tangential wind curvature near the RMW,and the radial gradient of the tangential wind tendency plays a relatively important role.Absolute angular momentum budgets show that the verticaltransport of absolute vorticity differs on the inner edge and on the outer edge of the RMW,largely influencing the radial gradient of the tangential wind tendency in the vicinity of the eyewall.