Evolution of Hurricane-like Eyewall Structures using a Divergent, Normal-Mode Spectral Model

The evolution of both vortex Rossby waves (VRWs) and inertia-gravity waves (IGW) within hurricane-like eyewalls is examined using a normal-mode, shallow-water model on a doubly-periodic domain. The shallow-water model offers an advantage over non-divergent, barotropic models because it allows the propagation of IGWs. The normal-mode technique offers the advantage of allowing the contributions to the mass and momentum fields from both the gravity-inertia modes and the Rossby modes to be easily partitioned and examined independently. Another advantage of the normal-mode technique is the ability to control unwanted gravity wave reflection from the model domain boundaries without impacting the Rossby modes. To do this, a circular sponge layer is applied only to the gravity-inertia modes near the model boundary.

Eyewalls are simulated in the model through a variety of idealized initial annulus-shaped regions of high vorticity in which the both the size and magnitude of the core vorticity region and the annulus vorticity are varied. In addition, the evolutions of elliptical and other asymmetric eyewall structures are examined as well. Once the initial vorticity field is specified, the corresponding height and wind fields are determined through use of the non-linear balance equation.

Barotropic instability, resulting from counter-propagating VRWs near the eyewall radius of maximum winds causes the eyewalls to amplify into a variety of polygonal patterns depending on the shape of the initial eyewall. The different initial vorticity patterns have differing effects on the evolution of the eyewall as well as both the central pressure and tangentially averaged wind fields.