An Observational Study of the Physical Processes Responsible for Tropical Cyclone Motion

This paper presents an observational study of the physics of tropical cyclone motion. Analyses of the vorticity budget using both aircraft and rawinsonde composite data were performed. As expected, the results show a definite link between the local change in relative vorticity and tropical cyclone movement. The main contributor to this local change, at least in the middle troposphere, is the horizontal advection of absolute vorticity with the divergence term usually playing a secondary but not necessarily negligible role. The vertical advection and tilting terms are generally much smaller. The contribution of the divergence term as an extra component in determining the movement of tropical cyclone is discussed. The mass to wind adjustment as a result of the increase in vorticity is viewed as a combination of the advection of temperature (or mass) and subsidence. Substantiating evidence of this viewpoint is presented for cyclones undergoing turning motion.