O the Physical Processes Responsible for Tropical Cyclone Motion.

This paper investigates the possible physical processes involved in tropical cyclone motion. The first part analyzes the relation between the synoptic-scale flow around tropical cyclones and cyclone movement. It was found that, in most cases, tropical cyclones in the Northern (Southern) Hemisphere move to the left () of and faster than their environmental flow. To understand these observations, an analytical study using a simplified form of the vorticity equation was performed. The results show the importance of the environmental flow and change in the Coriolis parameter across the cyclone in determining cyclone motion. The locations of maximum local changes of relative vorticity can be interpreted to correspond to the observational results. The interaction between the vortex and the environmental flows was further investigated by analyzing the radial equation of motion. Results from a simple model and observations both indicate the presence of radial accelerations which can be explained in terms of parcel trajectories. The differences of radial accelerations between the right and left sides of the cyclone (with respect to cyclone direction) was found to be proportional to the speed of the cyclone. Differences in radial accelerations between the front and the back of a cyclone appear to be related to a change in cyclone direction. Vorticity budgets for cyclones with different speeds and turning directions were then analyzed. The horizontal advection and the divergence terms were found to dominate over the tilting and vertical advection terms. These results are consistent with those from the analytical study of the vorticity equation described above. Combining the theoretical and observational results, a hypothesis is proposed to describe the possible physical processes involved in tropical cyclone motion. Interaction of the vortex and environmental flows brings about a vorticity increase to the front of the cyclone. This increase in relative vorticity forces a mass adjustment through subsidence, producing a net tropospheric warming and eventually a new center of vortex circulation. The movement of a tropical cyclone is therefore one of "propagation", with eye wall clouds continually reforming around the new "eye" and dissipating around the old center.