Thermodynamic structure and evolution of the low level eye of Hurricane Lili (2002)

Recently the concept of super-intensity has been argued (Persing and Montgomery 2003, Montgomery et al. 2006), where high energy air residing in the eye can be entrained into the updrafts of the eyewall. This results in a more buoyant updraft and ultimately a more intense hurricane than what one would expect based on maximum potential intensity theory. Satellites reveal the presence of eddies in the eye that would promote the mixing of air from the eye to the eyewall (Kossin and Schubert 2004), and aircraft measurements show a relationship between vorticity and equivalent potential temperature (theta_e) radial profiles through the eye-eyewall region (Kossin and Eastin 2001). Such results stimulate us to explore the thermodynamic structure of the lower-eye of more hurricanes to identify repeatable features and determine if the mixing between eye and eyewall is frequent.

Forty-four Global Positioning System dropwindsondes (GPS sondes) were deployed in the lower eye of Lili (2002) over a period of 4.5 days by NOAA WP-3D and USAF C-130 aircraft. Forty-two of the forty-four sondes were deployed within 4 km of the circulation center and provide vertical profiles of temperature and moisture while flight level data, mostly at 700 hPa, provided radial profiles across the eye and eyewall. The sampling distribution encompassed tropical storm to category 4 intensities which captured periods of rapid intensification (18 m s-1 over 24 h), extreme rapid decay (23 m s-1 over13 h) and steady state conditions while entirely over the Gulf of Mexico. These data provides us with a unique opportunity to observe the thermodynamic structure evolution of the lower eye of a hurricane with a temporal resolution of approximately a few hours.

The observations reveal that theta_e increased in the lower eye by 22 K as Lili matured into a category four hurricane and continued to increase even as Lili underwent extremely rapid decay. Estimates of oceanic heat fluxes, derived using the GPS sonde observations, are found to be insufficient in explaining the observed rapid energy increases occasionally witnessed in the lower eye. A plausible explanation is that the increase in theta_e in the lower eye is a result of episodic exchanges between the lower eye and eyewall, supporting the arguments of the previously mentioned authors. Increases in theta_e in the eyewall precede increases in the eye and show a cyclical structure change from elevated theta_e values in the eyewall and depressed values in the eye during intensification periods to a flat gradient across the eye and eyewall during steady or weakening periods. During periods when the convection in the eyewall has weakened some of the inflow does not ascend in the eyewall but instead penetrates into the eye where it remains to collect additional oceanic heat fluxes and builds a reservoir of high theta_e air. This reservoir goes almost completely unutilized in Hurricane Lili as it imparts little apparent influence on its rapid intensification. We will also discuss the Lili results in light of other eye studies conducted by Jordan (1958), Malkus (1961) and Willoughby (1998).