Structure and intensity changes during hurricane eye formation

This study characterizes the kinematic and thermodynamic changes that occurred before, during, and after the initial eye formations of a broad set of Atlantic tropical cyclones. To obtain the requisite structure and intensity parameters, a new data set has been synthesized from the Vortex Data Messages transmitted by routine aircraft reconnaissance from 1989-2008. Intensity ranges are determined for the times when the eye/eyewall structure first appears in aircraft radar and infrared satellite imagery. The mean intensity at which an eye is first observed in both aircraft or satellite imagery is found to be 58 kt, somewhat lower than reported in previous studies. Changes about the time of eye formation are examined for intensity, the radius of maximum winds, the minimum Rossby radius of deformation, and the maximum temperature and dew point temperature depression in the eye. Storms are found to intensify most rapidly near the time of eye formation, especially when a persistent eye is observed in infrared satellite imagery. Many storms which are forming eyes are found to undergo a substantial and rapid contraction in the radius of maximum winds during the 24-h period before the eye is observed; once the eye is present, this contraction slows or ceases. Strong warming at lower levels (850 or 700 hPa) of the eye is not observed to correlate well with the time in which the eye is first observed. Finally, observations suggest that the dynamical heating efficiency of the resulting eyewall increases even as the physical scale of the efficient heating region decreases. This allows a storm to continue intensifying even though its total inner core diabatic heating may be decreasing. Most eyes formed when the environmental vertical wind shear (850 to 200 hPa) was between 10 to 20 kt; very few eyes formed when the shear exceeded 25 kt. These results suggest that, to some degree, the formation of an eye is a manifold attractor of the system sometimes stymied by an unfavorable environment.