Non-Barotropic Processes in tropical cyclone motion. Part I: Composite results

During the last few years, much interest has been generated on the relationship between potential vorticity (PV) distribution and tropical cyclone (TC) motion. Most recently, Wu and Wang (1999) proposed a diagnostic framework to relate PV tendency with TC motion. This is the first of a series of three papers that attempt to apply this approach to study TC motion through observational analyses. Part I (this paper) will present results from a composite study of TCs moving in specific directions. In Parts II (Lei and Chan 2000) and III (Ko and Chan 2000), the composite results will be verified with individual case studies using different data sets.

Operational analyses from the UK Meteorological Office global model for the years 1991-96 form the dataset for this study. Composites are made for TCs that had steady directions and speeds of motion. Four categories are defined: westward, northwestward, northward and northeastward. For each category, various terms contributing to the PV tendency at the mid and upper troposphere are computed and composited. It was found that in the mid troposphere, the horizontal advection of PV has the most significant contribution. However, in the upper troposphere, other terms in the tendency equation can play an important role in modifying the PV tendency distribution.