Spiral Bands in a Simulated Hurricane. Part II: Wave Activity Diagnostics

The theory of empirical normal modes (ENMs) was applied in a diagnostic study of the inner spiral bands formed in a simulated hurricane using the high-resolution Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) nonhydrostatic mesoscale model version 5 (MM5). The ENM method has the capability to decompose simultaneously wind and thermal fields into dynamically consistent and orthogonal modes with respect to wave activities. For wavenumber 1 and 2 anomalies, it was found that the leading modes are vortex Rossby waves. These modes explain 70%–80% of the statistical variances in a 24-h period. Gravity waves have small contribution in terms of wave activities. Analysis of the Eliassen–Palm (EP) flux and its time-mean divergence shows that vortex Rossby waves are generated in the eyewall 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 as indicated by the divergence of the EP flux, an acceleration of the mean tangential wind in the lower and middle troposphere inside and outside the eyewall and a deceleration aloft in the eyewall region.