Sepat台风（0709）登陆过程中眼放大现象研究

台风登陆过程中常发生结构变化,从而引起其强度、路径以及风雨分布等一系列变化,导致登陆台风灾害十分复杂.0709号台风Sepat在穿过台湾岛时结构变化明显,出现了台风眼放大现象.基于上海台风研究所台风资料、FY-Ⅱ卫星半小时一次的遥感资料、台湾雷达逐时合成回波图像以及NCEP每日4次1°×1°格距的再分析资料,研究了Sepat登陆过程中的眼放大现象.结果表明:(1)Sepat登陆台湾后眼墙塌陷、眼消失,但随后在从台湾海峡移向大陆过程中重新出现了台风眼并伴有眼放大现象,眼直径扩展至约600 km;(2)这种眼放大现象,实际上是台风内核区对流云团分裂扩散过程中与外围螺旋云带一起重新发展出的环状结构.台风眼的扩大与眼区下垫面温度降低、低层大气不稳定度减弱、径向外流加强、下沉运动区范围扩大等因素有关;(3)在台风外围,环境干空气侵入台风环流并在其西部形成了弧状湿度锋.锋区既促进对流运动发展,也阻碍了台风眼区云团进一步向外扩散,使对流云团在锋区附近排列成半圆弧状云带,并在台风气旋性环流组织下与台风东部的螺旋云带一起形成了环状眼墙;(4)台风的减弱消亡与其眼区放大现象密切相关.台风眼放大过程中,由于眼内干空气下沉范围加大、对流凝结潜热加热减弱,不利于暖心结构维持,台风强度亦随之衰减.同时,其增强的径向外流在一定程度上阻止水汽能量向台风内核区输入,促使台风内核对流运动的减弱和消亡.

A study on the eyewall expansion of Typhoon Sepat (2009) during its landfall process

Structure change would occur when typhoons approach or cross an island, and that will usually give rise to their intensity change, track turns, and variations in associated high winds and heavy rains. A phenomenon of eyewall expansion related to Typhoon Sepat (0709) was found when it was crossing the Taiwan Strait. A study was conducted to investigate the causes of the eyewall ex-pansion and its relationship with the typhoon intensity change using data from Shanghai Typhoon Institute, FY- Ⅱ satellite remotely sensed images, Taiwan hourly radar echoes, and NCEP 1°× 1° reanalysis fields. It is found that (1) Sepat' s eyewall actually broke down and dissipated after it made landfall at the island of Taiwan, but a new and expanding eyewall formed as soon as Sepat entered the Taiwan Strait and approached China mainland. It expanded into a big eye with about 600 km in diameter finally. (2) The eye-wall expansion actually was a formation process of cloud band ring, which was composed by the diffused convective cloud clusters from the core region of typhoon and its spiral bands, It was associated with many factors including the decrease of underlying temperature, the weakening of unstable atmospheric stratification, the enhancement of outgoing winds, and expanding descending motion in ty-phoon core region, etc. (3) An arc humidity frontal zone occurred in the western periphery of the typhoon in lower layer due to the intrusion of dry air from environmental circulation. The front was favorable for the development of convection, but disadvantageous for the further expansion of eyewall clouds because of the descending motion on the dry side of the frontal. In this connection, diffused convective cloud clusters were organized by the circular typhoon circulation into an arc form near the front and joined the spiral cloud clusters in the eastern part of the typhoon circulation, leading to the form up of a ring eyewall. (4) The typhoon decaying was closely related to the typhoon eyewall expansion. Because condensational heating was weakened and the heating source was dispersed over the typhoon center in this process, the typhoon warm core couldn't be sustained, resulting in the dereasing of typhoon intensity. On the other hand, the input of environmental water vapor and energy was held back by the enhancing radial outflow of the typhoon in lower to middle layers, convection in the typhoon core region was thus attenuated.