An evaluation of the use of the mean vortex wind as a proxy for the environmental wind

The results of this work apply to all atmospheric vortices, however, owing to the abundance of previous mean wind studies within tropical cyclones, this paper addresses issues that specifically concern these studies. Previous composite and individual studies of tropical cyclones have utilized the “steering flow” concept in which the tropical cyclone is assumed to be advected by the environmental wind like a cork in a stream. The proxy used for the environmental wind in these studies is the mean wind calculated as symmetrically as possible around the tropical cyclone. This work derives an explicit expression for the mean wind in terms of the individual contributions from the both the environmental wind and the intrinsic tropical cyclone winds relative to the environmental wind. It is demonstrated that the interpretation of the mean wind as representing the environmental wind is generally only a good approximation in special situations. In addition, the derived expression for the mean wind demonstrates that the terminology concerning the mean wind used in both aircraft and ground-based “Velocity Track Display” radar analysis methods should be re-defined.