Non-Barotropic Processes in Tropical Cyclone motion. Part III: Potential Vorticity Advection

This is the third of a series of three papers to study tropical cyclone (TC) motion through observational analyses by applying the diagnostic framework of Wu and Wang (1999) that relate potential vorticity (PV) tendency with TC motion. Part I (Chan 2000) presents results from a composite study of TCs moving in specific directions while Part II (Lei and Chan 2000) focuses on the individual case results at the mid troposphere, with particular reference to the contribution of the diabatic heating term. In this paper, results from individual cases for the conditions in the upper troposphere are described.

While the vertical advection (VA) is relatively small and the friction is insignificant in the mid troposphere, the contribution of the VA term to the overall PV tendency (PVT) may need to be considered in the upper troposphere because of the large vertical gradient of PV. On the other hand, the diabatic heating term should be small. Therefore, the PVT is mainly contributed by the VA and the horizontal advection (HA) term. Preliminary results from individual cases from the Tropical Cyclone Motion Experiment TCM-90 suggest that this statement is true in a number of cases. It also appears that the Lagrangian PVT may need to be considered in some cases when studying the relationship between TC motion and PVT.