1446 MONTHLY WEATHER REVIEW VOLUME 138 Eye Excess Energy and the Rapid Intensification of Hurricane Lili (2002) GARY M. BARNES AND PAUL FUENTES University of Hawaii at Manoa, Honolulu, Hawaii (Manuscript received 14 July 2009, in final form 28 October 2009) ABSTRACT Over 4.5 days, NOAA and U.S. Air Force personnel in reconnaissance aircraft deployed 44 global posi- tioning 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 ue of the eye with that in the eyewall revealed that like many other hurricanes, the eye was a reservoir for the warmest ue. The authors define a variable called eye excess energy that is a function of the difference in ue 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 ue 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. 1. Introduction a. Prior eye studies Occasionally a tropical cyclone (TC) in the Atlantic Earlier studies utilizing radiosonde and dropwindsonde Ocean basin was sampled over several days by the Na- observations have revealed the thermodynamic struc- tional Oceanic and Atmospheric Administration (NOAA) ture of the eye to be characterized by warm, dry air in WP-3Ds and U.S. Air Force (USAF) C-130 aircraft. These the mid- to upper levels of the troposphere, an inversion extended investigations may have included the regular layer located from 1- to 3-km altitude, and a cool and moist deployment of the global positioning system drop- air layer adjacent to the sea that may contain stratiform windsondes (GPS sondes) into the eye. As an example, cloud, sometimes referred to as a ‘‘hub cloud’’ (Jordan Hurricane Lili (2002) was repeatedly visited over 4.5 days, 1952; Malkus 1958; Stear 1965; Hawkins and Imbembo during which time Lili evolved from a tropical storm (TS) 1976). The air at 300 hPa was usually 108–158Cwarmer to a category-4 hurricane. The aircraft-borne in situ sensor than the air located far from the TC center; this warming observations of the eyewall and 44 GPS sondes deployed was attributed to a combination of subsidence-induced in the eye captured both the rapid intensification (RI) and adiabatic warming (Malkus 1958; Jordan 1961; Gray and rapid decay (RD) phases of Lili’s life. We use these ob- Shea 1973; Willoughby 1998) and mixing of warm air from servations to investigate the thermodynamic evolution of the eyewall into the eye (Rotunno and Emanuel 1987). the eye below 700 hPa to ascertain if there is evidence of Malkus (1958) argued that the subsidence was deep and eye to eyewall transfers that have the potential to impact nearly continuous as air detrained from the eyewall and hurricane intensity. refreshed the eye multiple times throughout the TC’s life. In contrast to this view, Willoughby (1998) believed that the air above the inversion was trapped from the Corresponding author address: Gary M. Barnes, Dept. of Meteorology, University of Hawaii at Manoa, 2525 Correa Rd., TC’s inception, resulting in a long residence time, and Honolulu, HI 96822. that the sinking in the eye extended only a few kilome- E-mail: [email protected] ters rather than the entire depth of the troposphere. The DOI: 10.1175/2009MWR3145.1 Ó 2010 American Meteorological Society Unauthenticated | Downloaded 09/28/21 11:41 PM UTC APRIL 2010 B A R N E S A N D F U E N T E S 1447 saturated or nearly saturated conditions observed below et al. 2005b; Montgomery et al. 2006, Sitkowski and the inversion level were attributed to frictional inflow Barnes 2009). This air, occupying the lowest 2–3 km, if underneath the eyewall, inward mixing across the eye– mixed into the eyewall updraft, could increase the buoy- eyewall boundary, and evaporation from the ocean by ancy enough to provide a convective boost (Holland Malkus (1958) and Willoughby (1998). 1997; Schubert et al. 1999; Braun 2002; Persing and The thermodynamic structure of the lower eye can Montgomery 2003; Eastin et al. 2005b; Montgomery et al. change dramatically during TC intensity changes (Jordan 2006; Cram et al. 2007). Persing and Montgomery (2003) 1961; Franklin et al. 1988; Willoughby 1998; Kossin and and Montgomery et al. (2006) have argued that a TC may Eastin 2001). Intensifying TCs frequently have enhanced achieve superintensity, a condition where the sustained warming and drying above a descending inversion while wind speeds in the eyewall exceed that estimated from weakening TCs have a rising inversion with cooling and maximum potential intensity theory (MPI; Emanuel moistening occurring from the sea surface to a less prom- 1986, 1988; Holland 1997), if the mass flux of air from the inent inversion layer. Jordan (1961) showed that as eye to the eyewall becomes substantial. Hurricane Grace (1958) filled 15 hPa over 6 h, warm and Warming of the lower portion of the eye usually dry air with an inversion near the sea surface was replaced makes an inconsequential contribution to the hydro- by a moist-adiabatic and saturated layer with no inver- statically induced surface pressure field, but the transfer sion. Franklin et al. (1988) reported remarkably strong of warm ue air from the lower eye into the eyewall could warming and drying far below the typical maximum conceivably reinforce convective elements in the eye- temperature perturbation level of 300 hPa during an wall that could subsequently deepen the TC. We will intensification period for Gloria (1995). Hawkins and explore the thermodynamic changes observed in the eye Imbembo (1976) also noted a large positive temperature of Lili to see if there are variations of ue that are corre- perturbation below 500 hPa in Inez (1966) when it had a lated with notable intensity variations. mean sea level pressure (MSLP) of 927 hPa. b. Goals Liu et al. (1999) implemented a high-resolution sim- ulation of the inner-core structure of Hurricane Andrew We will use the aircraft in situ sensors and the GPS (1992) that reproduced many of the observed thermo- sondes to address the following specific questions: dynamic eye structures. Equivalent potential tempera- 1) How do characteristics in the lower eye such as inver- ture (u ) in the lower eye was found to steadily increase e sion height, lifted condensation level (LCL), mixed as the TC intensified and to decrease as the TC decayed. layer depth, and hub cloud presence vary during the Some of the warm u air in the eye mixed into the eye- e intensifying, steady, and weakening phases? wall updraft, which reinforced convection. 2) How does u in the lower eye evolve in Lili? The transition from warm and dry to cool and moist e 3) Is there evidence of eye to eyewall mixing, when does conditions in the midlevels of the eye also has been it occur, and is there ensuing intensification? witnessed in Hurricanes Diana (1984) and Olivia (1994), 4) Can surface fluxes within the eye explain why u although these observed changes were explained through e observed there is usually higher than the eyewall? contrasting mechanisms (Kossin and Eastin 2001). The changes in Hurricane Diana were explained through as- 2. Data, methodology, and Lili (2002) cension of a well-mixed air mass below the inversion level. In Olivia, episodic horizontal mixing between the eye a. GPS sonde and eyewall via mesovortices was believed to be respon- 1) SAMPLING sible for the thermodynamic transition. Kossin and Eastin (2001) identified two regimes to describe the radial NOAA WP-3D and USAF C-130 aircraft deployed thermodynamic gradients between the eye and eyewall 44 GPS sondes in Hurricane Lili (2002) from altitudes above the inversion. During the first regime the eye was between 850 and 700 hPa. Figure 1 shows the temporal typically warm and dry with elevated ue in the eyewall and distribution of the sondes, from 0000 UTC 29 September lower values in the eye. The second regime occurred to 1200 UTC 3 October, along with the NOAA/Tropical after maximum intensity had been established and was Prediction Center/National Hurricane Center (TPC/NHC) characterized by a ue maximum in the eye with a mono- best-track MSLP. Successive sondes were deployed ev- tonic decrease radially outward. ery 2 h during individual flights with larger gaps of 5–7 h The lowest few kilometers of the eye often has been between flights. Forty-two of the 44 sondes released in shown to harbor some of the highest values of ue found the eye were within 4 km of the circulation center (Fig. 2), in a TC (e.g., Hawkins and Imbembo 1976; Jorgensen which was estimated by the aircraft using the center- 1984; Willoughby 1998; Schneider and Barnes 2005; Eastin finding techniques of Willoughby and Chelmow (1982).