Study Of An Extratropical Cyclone Which Reintensified Explosively While Occluding Over The Northwestern Pacific

On 6-9 January 2007 an explosively intensifying extratropical cyclone occurred over the Northwestern Pacific. The cyclone experienced two explosive developing stages during its life time, one of which occurred when the cyclone occluded. Similar feature has rarely been discussed in the historical case studies of explosive cyclone. Diagnostic analysis and numerical simulations of this unique cyclone are performed using the Zwack-Okossi equation and WRF (Weather Research and Forecasting Model) model. The evolutionary processes of the explosive cyclogenesis are described in detail, the causes of the reintensification are analyzed, and the developing mechanisms of the cyclone are further discussed.It is found that the cyclone formed along the baroclinic zone in the lower troposphere, and favorable baroclinic environment is the basic condition to promote its development. During the process of moving toward northeast, the surface cyclone met the cyclonic-vortex systems in the mid- and upper-troposphere which propagated eastward. The cyclonic-vortex systems tilted westward vertically from the low-level to the upper-level. The cyclone intensified explosively when it was in front of the cyclonic-vortex systems aloft. At the onset of occlusion, the favorable forcing factors moved downstream from the cyclone center, resulting in the division of the cyclone and the formation of a new cyclone center downstream. The new center of the surface cyclone was again ahead of the cyclonic-vortex systems aloft. Such configuration led to the reintensification of the cyclone.Diagnostic analysis based on the Zwack-Okossi equation indicates that the forcing factors in the two explosive development stages are not exactly the same. In the first explosive development stage, the cooperation of cyclonic-vorticity advection, warm-air advection and diabatic heating results in the explosive development and diabatic heating is the most important factor to promote the development while in the second explosive development stage, diabatic heating has little effect on the development and baroclinic forces in the upper-level (i.e. cyclonic-vorticity advection and warm-air advection) are the main factors to promote the development.The sensitive experiment without latent heat release indicates that there exists a positive feedback process between latent heat release and baroclinic forces in the upper-level to promote the development of the cyclone. The positive feedback process can be described as follows:The cyclonic-vorticity advection in front of the low/trough and warm-air advection at the ridge area in the upper-level forced upper-level divergence over the surface cyclone while latent heating increased the intensity of the upper-level trough/ridge and thus enhanced the baroclinic forces in the upper-level; Upper-level divergence led to compensatory ascending motions below and pressure decreases in the low-level, which further led to low-level convergence and the generation of cyclonic-vorticity; The development of the low-level systems enhanced the southerly low-level jet at the eastern portion of the cyclone, which transported large amounts of warm moist air to the cyclone and led to further release of latent heat due to severe lifting and condensation of the water vapor. That is the positive feedback process which results in the extremely intensification of the cyclone.The sensitive experiment without surface fluxes indicates that heat and vapor fluxes at the ocean’s surface play different roles during different stages of the cyclone, which depend on the intensity of the cyclone, the position of the cyclone and the horizontal distribution pattern of the fluxes.