Recent observational and numerical modeling work has documented the importance of understanding the dynamics of the asymmetries within the core of tropical cyclones. These asymmetries have been found to influence not only storm structure, but storm intensity and intensity change through the interaction of the asymmetries with the parent vortex. This interaction has been well documented in numerical models, but the three-dimensional, high resolution observational data sets necessary to diagnose the evolution of asymmetric motions are rare, and thus only a handful of observational studies have documented their existence and speculated on their formation mechanisms and feedback to the parent vortex. In this study, one of the most complete aircraft reconnaissance and ground-based radar data sets of a single tropical cyclone (Hurricane Elena (1985)) will be used to analyze the evolution of both the symmetric structure and azimuthal asymmetries within the storm and the complex interaction between them that controls the intensity of the hurricane.
On the first day of study, Elena was under the influence of strong vertical wind shear from an upper tropospheric trough to the west. The storm was disorganized, with no discernable eyewall and nearly steady values of tangential wind and relative vorticity. Early on the second day, a near superposition and constructive interference occurred between the trough and Elena, initiating height falls and tangential wind spin-up in the core. During this period of intensification, Elena’s rainfall pattern exhibited a strong wavenumber one asymmetry with the highest reflectivity in the eyewall to the north of the center, or to the left of the vertical wind shear vector.
Once convection and a diabatic heating maximum were established in the eyewall, a sharp localized vorticity maximum emerged, with much lower values on either side. This barotropically unstable vorticity profile was noted to breakdown near the time of peak intensity, accompanied by the appearance of an elliptical eyewall, asymmetric mixing between the eye and eyewall, and propagating inner spiral rainbands with properties consistent with vortex Rossby wave theory. The asymmetric mixing between the eye and eyewall appeared to act as a brake on intensification from which Elena was unable to recover due the storm’s proximity to land and the ingestion of low equivalent potential temperature air.