On the relative contribution of inertia-gravity wave radiation to asymmetric instabilities in tropical cyclone-like vortices

Intense atmospheric vortices such as tropical cyclones experience various asymmetric instabilities during their life cycles. This study investigates how vortex properties and ambient conditions determine the relative importance of different mechanisms that can simultaneously influence the growth of an asymmetric perturbation. The focus is on three-dimensional disturbances of barotropic vortices with nonmonotonic radial distributions of potential vorticity. The analysis is aided by a diagnostic formula for the growth rate of a perturbation that is derived from an equation expressing conservation of angular pseudomomentum. The primary modes of instability are examined for Rossby numbers between 10 and 100, and Froude numbers in the broad neighborhood of unity. This parameter regime is deemed appropriate for tropical cyclone perturbations with vertical lengthscales ranging from the depth of the vortex to moderately smaller scales. At relatively small Froude numbers, the main cause of instability inferred from analysis typically involves the interaction of vortex Rossby waves with each other and/or critical layer potential vorticity perturbations. As the Froude number increases from its lower bound, the main cause of instability transitions to inertia-gravity wave radiation. In some cases, the transition occurs abruptly at a critical point where a mode whose growth is driven almost entirely by radiation suddenly becomes dominant. In other cases, the transition is gradual and less direct as the fastest growing mode continuously changes its structure. The radiation driven instabilities examined herein are shown to be quite fast and potentially relevant to real-world tropical cyclones. Their sensitivities to parametrized moisture and outer vorticity skirts are briefly addressed. This work is supported by NSERC and NSF.