In this study we present observational evidence of the modulation of eyewall convection through the interaction of vortex Rossby waves with a shear-induced velocity-potential dipole in Hurricane Olivia of 1994. During a 3-h period vertical shear increased from near zero to 15 m/s between 1 and 9 km, and the upper circulation weakened substantially. The storm maintained its intensity near the surface, while tangential wind decreased by nearly 20 m/s at the 9-km level. Recent full-physics simulations may provide some insight into these observations.

The data for this study were collected on 25 September 1994, as two NOAA WP-3D aircraft probed Hurricane Olivia, approximately 1500 km SW of Puerto Vallarta, Mexico. Each aircraft carried two scientific radars, one Doppler radar scanning vertically to document the three-dimensional wind and precipitation structure, and another non-Doppler radar scanning horizontally to document the evolving precipitation structure in plan view.

Low-wavenumber analyses of the wind field show a substantial wavenumber-two component of vorticity, and the authors suggest that the wavenumber-two structures are azimuthally propagating vortex Rossby waves. Radar animations show precipitation cells and clusters propagating around the hurricane at .5 to .7 times the maximum tangential wind speed. In some cases the clusters can be tracked more than once around the storm, and they appear to pulse as they pass northward through the (low-level) convergent half of the velocity-potential dipole.