Relationship between potential vorticity tendency and tropical cyclone motion

This paper proposes a consistent conceptual framework to explain tropical cyclone (TC) motion based on the concept of potential vorticity tendency (PVT) and to verify this framework based on analyses of different observational datasets. The framework suggests that a TC is likely to move toward an area of maximum wavenumber-1 (WN1) PVT, which is mainly contributed by the corresponding WN1 components of potential vorticity (PV) advection and diabatic heating (DH). The PV advection process consists of advection of symmetric PV by the asymmetric flow [AASPV, which includes, but is not limited to, the environmental "steering flow'' and the beta-induced circulation (the so-called ventilation flow)] and the advection of asymmetric PV by the symmetric flow (SAAPV). The asymmetric PV includes any asymmetry in the TC circulation, the beta gyres and contributions from asymmetric convective heating. The modification of PVT by the DH process depends on the vertical gradient of convective heating and the coupling between horizontal gradient of convective heating and vertical wind shear. In steady (i.e., without much change in direction or speed) TC motion, the PV advection processes are generally dominant while the contribution by DH is usually less significant. However, the latter process becomes important for irregular TC motion. Changes in TC motion are then not only caused by those in steering, but can also be induced by variations in the other processes. Composites of the Met Office operational analyses associated with TCs that had similar and relatively steady motion are first made to verify the contribution by the advection terms. In all motion categories examined, while the magnitude of the AASPV term is found to be generally dominant, its maximum is not downstream of the TC motion. The SAAPV term also contributes to the overall PV advection. The sum of these two terms gives a maximum at a location that generally aligns with the direction of TC motion. The contribution of the DH process to PVT, and hence TC motion, is then examined using satellite-derived temperatures from high-resolution geosynchronous satellite images for individual TCs. It is found that DH appears to be important especially for slow-moving TCs. Track oscillations as well as irregular track changes may be explained by changes in the convection pattern that lead to variations in the location of maximum WN1 DH. The entire PVT concept is further investigated using analyses from the Tropical Cyclone Motion Experiment TCM-90 for individual TCs with different track types. The results are consistent with those from the composites (for straight-moving cases) as well as from the satellite image analyses (for the irregular-moving case). Further, in the recurving case, the locations of the maximum in the advection terms rotate ahead of the turning motion of the TC, which is consistent with previous studies of TC motion based on the concept of absolute vorticity conservation. An integration of all these observational analyses generally verifies the validity of the proposed conceptual framework, which appears to explain most types of TC motion.