Interactions between tropical cyclones and the baroclinic flow

The poleward movement of a tropical cyclone usually initiates complex interactions with the middle latitude environment and then results in strong winds and large amounts of precipitation in the region of the tropical cyclone. To understand the impact of baroclinic flows on the evolution of tropical cyclones and the modifications of the tropical cyclones on the middle latitude environment, a series of ideal numerical experiments are performed. By simply representing the mid-latitude flow regime as a straight jet, two sets of numerical experiments are performed. One is designed to discuss the evolution of tropical cyclones that are in different developing stages when they impinge on the baroclinic flow. Another is carried out with the intention to study the interactions between tropical cyclones and baroclinic flows with different structures. In order to make the simulations realistic, the variation of the Coriolis parameter is considered. In this case, the tropical cyclone is carried northward by Beta-drift and the interaction between it and the baroclinic flow takes place.

It is found that a strong tropical cyclone tends to weaken at the edge of the baroclinic zone, while a weak tropical cyclone intensifies for a relative long time in the baroclinic environment. Stronger tropical cyclones tend to influence the upper troposphere at greater distances. Therefore, the interaction between the upper-level jet and a strong tropical cyclone can be significant enough to impact the evolution of the tropical cyclone even when the cyclonic vortex is only at the edge of a baroclinic area. In addition, when the beta effect is taken into account, the vertical shear is composed of the environmental westerly shear and the beta shear. In the case that a tropical cyclone is strong, the beta shear is usually conspicuous. The resultant shear over the tropical cyclone is large and unfavorable for further intensification of the cyclone.

As a tropical cyclone moves poleward, the baroclinic flow is greatly modified. One prominent feature is a jet streak forming in the region where the outflow of the tropical cyclone impinges on the jet. As the decaying tropical cyclone moves into the right entrance region of the jet streak, rapid re-development of the system usually takes place. The numerical results indicate that, under the condition that the straight jet is strong enough, a PV anomaly may form downstream of the poleward-moving tropical cyclone. If the PV anomaly happens to lie beneath the right entrance region of the jet streak, surface cyclogenesis may happen. In this case, the new cyclone rapidly develops and the original cyclone associated with the tropical cyclone cannot undergo the re-development process and finally dissipates.