南半球冷空气入侵与热带气旋的形成

THE EFFECTS OF COLD SURGES FROM SOUTHERN HEMISPHERE ON TROPICAL CYCLONE FORMATION

文中采用NCAR/PSU研制的非静力中尺度模式MM 5 ,研究了南半球冷空气入侵在热带气旋形成中的作用。初始场为纬向平均场 ,不含任何扰动 ,但为热带气旋的发生提供了基本条件 ;通过改变设在赤道的南边界条件 ,设计系列数值试验反映南半球不同强度冷空气的入侵。数值试验结果表明 :南半球冷空气侵袭后 ,在菲律宾以东洋面上形成热带气旋 ;没有冷空气入侵时 ,只有扰动产生 ,没有热带气旋形成。在对流不稳定的背景场中 ,即使没有冷空气入侵 ,低层小尺度辐合引起强上升运动 ,产生的非绝热加热 ,在热带洋面上也能形成扰动。但是非绝热加热使得稳定度增加 ,没有低层强辐合的支持 ,对流不能持续 ,扰动不能发展成为热带气旋。南半球冷空气的入侵 ,一方面气温降低 ,使得中低层层结稳定度降低 ;另一方面 ,冷空气形成向北的气压梯度 ,在低纬度产生南风 ,导致低层强辐合。稳定度因子和低层辐合的共同作用 ,驱动深厚的垂直环流 ,产生十分显著的非绝热加热 ,形成了暖心的热带气旋。

The effects of southern hemisphere cold surges on tropical cyclone formation, especially on initial vortex genesis, are studied by use of the fifth generation PSU/NCAR nonhydrostatic, mesoscale model (MM5). The initial data are zonal averaged fields with no disturbance but satisfy the basic requested conditions of tropical cyclone formation. A number of numerical experiments were designed by changing lateral boundary conditions to denote different intensity of cold surges from southern hemisphere. Numerical results show that tropical cyclones occur on the ocean to the east of Philippine with southern hemisphere cold surges and only disturbances form without cold surges. The experiment without cold surge shows that disturbances form on the tropical ocean. As the environment condition is convective instability, strong vertical motion and diabatic heating are caused by low level microscale mass convergence. But the disturbances can't develop into tropical cyclone for diabatic heating increases vertical stability and the convection can't maintain on the condition that there is no low level strong convergence. When there are cold surges from southern hemisphere, on the one hand, the temperature drops and vertical stability of low middle level decreases, on the other hand, the cold surges take place pressure gradient, which causes south wind in low latitude and strong mass convergence in low level. Deep vertical circulation is driven by both the convergence and the low stability. With appropriate basic flows, there is significant diabatic heating and a tropical cyclone with warm core is caused. The study confirms 'the theory of cold surge', which has been argued for long time.