The theory of empirical normal modes (ENMs) was applied to diagnose inner spiral bands formed in an explicitly simulated hurricane using the high-resolution PSU-NCAR nonhydrostatic mesoscale model (MM5). The ENM method has the capability to decompose simultaneously wind and thermal fields into dynamical consistent and orthogonal modes with respect to wave-activities.

For wavenumber one and two anomalies on a time and azimuthal mean state which satifies the Rayleigh barotropic instability criterion, it was found that the leading modes are vortex Rossby waves. Some of these waves have characteristics typical of neutral modes and some typical of unstable modes described by the theory of linear waves. These modes explain 40% to 50% of the statistical variances in a period of 24 hours.

The Eliassen-Palm (EP) flux and its divergence show that the vortex Rossby waves are concentrated in the inner-core region where the radial gradient of the basic state potential vorticity is large. In general, these waves propagate outward in the lower troposphere and inward in the upper troposphere. Consequently, they transport eddy momentum radially inward and outward, respectively. The wave activities also propagate slowly upward inside the eyewall and downward outside. The associated eddy heat transport tends to warm the air in the eye region. The vortex Rossby waves lead to, through wave-mean-flow interaction indicated by the divergence of the EP flux, the acceleration of the mean tangential wind in the lower and middle troposphere inside and outside the eyewall and the deceleration aloft in the eyewall region.