Tropical Cyclone Movement - Bill Gray's Contributions

In the 1970s, Bill Gray and his student, John George, noticed that the motion vector of a tropical cyclone (TC) generally deviates from that of its surrounding flow, which they defined as an azimuthal average around the tropical cyclone, from 1-7 degrees latitude from the TC center. In other words, there must be some other mechanism that causes the movement of a TC that is not contributed by the surrounding (steering) flow. This was the problem I began to work on at CSU. At the end of my PhD, we have identified the beta effect as the reason for such a deviation through vorticity budget analyses. Subsequently, this mechanism has been further confirmed through both data analyses by Lester Carr and Russ Elsberry, and numerical simulations using a barotropic model by myself and Terry Williams. This presentation will give an overview of the development of our understanding of the mechanisms of TC movement from Bill Gray's time to the early 2000s.