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

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. Anumber 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-levelmicroscale 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.