Relationship between potential vorticity and tropical cyclone motion

Abstract

This study investigates the physical processes involved in tropical cyclone (TC) motion from the potential vorticity (PV) perspective. The relationship between PV distribution and the direction of TC motion is first investigated. It is found that, before recurvature, the TCs are generally moving towards an area of maximum PV that is about 10" latitude in front of them. In addition, the flow at the upper level is highly asymmetric. Cyclonic flow is dominant at the centre of the TC while anticyclonic flow is generally found on the south to southeast side of the TC at the upper level. However, the PV distribution can only indicate the large-scale interaction between the TC and its environment but may not be related to the dynamics of TC motion directly. To understand further the physical processes involved in TC motion, an analysis based on the PV tendency (PVT) concept is performed. It is hypothesized that a TC would tend to move towards the area of maximum PVT, which is mainly contributed by the PV advection and diabatic heating (DH). PV advection can further be separated into two terms: asymmetric advection of symmetric PV (AASPV) and symmetric advection of asymmetric PV (SAAPV). When the TC motion is steady, PV advection dominates the PVT while DH becomes important when the TC motion is erratic. The objective of this part of the study is to verify the hypothesis using the Final Analysis dataset from the Tropical Cyclone Motion Experiment (TCM-90) conducted over the western North Pacific in 1990. To determine the relative contributions from different physical processes on TC motion, the wavenumber-1 (WN-1) PVT components from AASPV, SAAPV and DH are examined for cases with different motion types (straight-moving, recurving and irregular motion). The results show that in general a TC will move towards the total WN-1 PVT maximum irrespective to the type of movement. Among the three main terms, AASPV usually dominates for all types but the relative contributions among the terms differ in different types of motion. For straight-moving TCs, the WN-1 PVT results mainly from the AASPV term and the SAAPV and DH terms are generally small. However, when a TC recurves, the magnitude of the SAAPV term increases significantly so that its contribution becomes important. Further, the maximum AASPV rotates to the direction of turning before the TC turns. When the motion of a TC is erratic, the motion is usually slow and the contribution from DH to the total PVT becomes more significant. The results from this study generally validate the hypothesis. That is, the sum of the advection and generation of PV gives a PVT distribution such that the maximum is a good indicator of TC motions.

Date of Award	31 Jul 2001
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chung Leung Johnny CHAN (Supervisor)