Eye Excess Energy and the Rapid Intensification of Hurricane Lili (2002)

Over 4.5 days, NOAA and U.S. Air Force personnel in reconnaissance aircraft deployed 44 global positioning system dropwindsondes (GPS sondes) in the eye of Hurricane Lili (2002). The vertical profiles derived from these GPS sondes were used to determine the evolution of the height of the inversion, presence, and height of the hub cloud, the height of the lifted condensation layer, and the depth of the mixed layer. As Lili deepened, underwent rapid intensification (RI), and eventually rapid decay, the lower portion of the eye moistened and the lapse rate became moist adiabatic. The inversion layer rose as Lili intensified and then quickly fell over 1500 m at the beginning of RI. Comparison of the equivalent potential temperature θe of the eye with that in the eyewall revealed that like many other hurricanes, the eye was a reservoir for the warmest θe. The authors define a variable called eye excess energy that is a function of the difference in θe between the eye and the eyewall and the depth over which this difference occurs and present evidence that this quantity became small during RI. The authors hypothesize that the warm θe in the eye served as a boost for convection in the eyewall that may, in turn, initiate RI. However, the small volume of eye excess energy available and the rapidity at which it was transferred to the eyewall demonstrate that eye excess energy cannot sustain RI, which typically continues for many hours. The results are discussed in light of eye–eyewall mixing arguments.