Dynamical Interactions in a Field of Persistent Tropicalcyclones: Eddy Angular Momentum and Vorticity Exchanges

Although the steady-state construct has long served as an underlying basis for hurricane intensity theory, steady-state solutions without unrealistic sources of cyclonic angular momentum have proven difficult to find in three-dimensional, cloud-permitting, numerical simulations of hurricanes. Of particular physical interest is to identify the dominant sources of cyclonic angular momentum and vorticity required to offset the loss of cyclonic angular momentum to the ocean surface in association with surface friction.

In this project, we examine results from doubly-periodic simulations of tropical cyclones. The simulations generate either a single prolonged tropical cyclone or multiple prolonged tropical cyclones. In all simulations, an inherently unsteady eddy vorticity process in the upper-troposphere is identified that carries cyclonic vorticity toward the tropical cyclone center. This eddy process is found to be the principal mode of temporal variability in the mass-integrated angular momentum budget of the vortex. Moreover, this eddy process offsets the loss of cyclonic angular momentum associated with surface friction. In doubly-periodic simulations, the source of the cyclonic vorticity in the upper-troposphere cannot be separated from the zero-circulation constraint that applies at every vertical level.

To address a previous hypothesis of zero net surface drag torque to support prolonged hurricanes, we consider the distribution of surface drag torque around the vortex. The doubly-periodic boundaries naturally motivate an analysis of the image domains. It is found that the radius where zero net surface drag torque occurs is a radius that interacts with the four cardinal image vortices through radial outflow (relative to the vortex in question). Because the supplied torque necessary to offset the loss of frictional torque beneath the cyclonic circulation is processed through flow structures at even larger radius, it becomes untenable to define a closed flow structure within which a steady torque balance can be achieved.