OCTOBER 2005 C ORBOSIERO ET AL. 2905 The Structure and Evolution of Hurricane Elena (1985). Part I: Symmetric Intensification KRISTEN L. CORBOSIERO AND JOHN MOLINARI Department of Earth and Atmospheric Sciences, The University at Albany, State University of New York, Albany, New York MICHAEL L. BLACK Hurricane Research Division, NOAA/AOML, Miami, Florida (Manuscript received 1 December 2004, in final form 17 March 2005) ABSTRACT One of the most complete aircraft reconnaissance and ground-based radar datasets of a single tropical cyclone was recorded in Hurricane Elena (1985) as it made a slow, 3-day anticyclonic loop in the Gulf of Mexico. Eighty-eight radial legs and 47 vertical incidence scans were collected aboard NOAA WP-3D aircraft, and 1142 ground-based radar scans were made of Elena’s eyewall and inner rainbands as the storm intensified from a disorganized category 2 to an intense category 3 hurricane. This large amount of con- tinuously collected data made it possible to examine changes that occurred in Elena’s inner-core symmetric structure as the storm intensified. On the first day of study, Elena was under the influence of vertical wind shear from an upper-tropospheric trough to the west. The storm was disorganized, with no discernable eyewall and nearly steady values of tangential wind and relative vorticity. Early on the second day of study, a near superposition and construc- tive interference occurred between the trough and Elena, coincident with upward vertical velocities and the radial gradient of reflectivity becoming concentrated around the 30-km radius. Once an inner wind maxi- mum and eyewall developed, the radius of maximum winds contracted and a sharp localized vorticity maximum emerged, with much lower values on either side. This potentially unstable vorticity profile was accompanied by a maximum in equivalent potential temperature in the eyewall, deeper and stronger inflow out to 24 km from the eyewall, and mean outflow toward the eyewall from the eye. Within 6–12 h, intensification came to an end and Elena began to slowly weaken. Vorticity and equivalent potential temperature at 850 hPa showed indications of prior mixing between the eye and eyewall. During the weakening stage, an outflow jet developed at the eyewall radius. A strong 850-hPa updraft accompanied the outflow jet, yet convection was less active aloft than before. This feature appeared to represent a shallow, outward-sloping updraft channel associated with the spindown of the storm. 1. Introduction of the time evolution of the inner-core structure of tropical cyclones has been inhibited by the shortage of The dynamics of the inner-core region of tropical continuous observational data. cyclones (defined as the eye, eyewall, and spiral rain- With the general lack of observations spanning a suf- bands within 100 km of the center) are crucial to our ficiently long period of time over which the tropical understanding of hurricane intensity change. It is within cyclone evolves, observational studies of hurricane this region that most of the energy generation and con- structure have commonly gone in one of two directions: version that drives the hurricane circulation takes place 1) case studies or “snapshots” of a hurricane’s structure (Riehl 1950; Ooyama 1969; Emanuel 1986). Despite spanning 1–3 h, or 2) construction of composites over considerable research over the last 50 yr, understanding longer periods, sometimes spanning a day or more. Landmark studies by Marks and Houze (1987), Marks et al. (1992), Franklin et al. (1993), and Reasor et al. Corresponding author address: Kristen L. Corbosiero, Dept. of Earth and Atmospheric Science, The University at Albany, State (2000) have gone the first route by using airborne dual- University of New York, 1400 Washington Ave., Albany, NY 12222. Doppler data to form detailed pictures of the radial and E-mail: [email protected] vertical structure of hurricanes that were strengthening © 2005 American Meteorological Society MWR3010 2906 MONTHLY WEATHER REVIEW VOLUME 133 (Alicia 1983), weakening (Norbert 1984; Olivia 1994), of Mexico while intensifying into a strong category 3 and at peak intensity (Gloria 1985). The second method hurricane (Case 1986). Previous studies on Elena’s was employed by Hawkins and Imbembo (1976), Jor- rainfall pattern (Burpee and Black 1989), interaction gensen (1984b), and Frank (1984) to construct longer- with an upper-level midlatitude trough (Molinari and term time composites of flight-level reconnaissance Vollaro 1989, 1990; Molinari et al. 1995), erratic track data of steady Hurricane Inez (1966), weakening Hur- (Velden 1987), and tangential wind spinup (Willoughby ricane Allen (1980), and rapidly intensifying Hurricane 1990) have provided some details of Elena’s evolution, Frederic (1979). Snapshots of hurricanes show struc- but none of these previous studies has examined the tural detail, but not time change. Composites over evolution of Elena’s inner-core structure with all avail- longer periods optimize data coverage, but might miss able reconnaissance aircraft and ground-based radar some important structural characteristics of intensity data. change. The organization of the paper is as follows. Section 2 Notable exceptions to the above are papers that have gives details of the data used for this study. Section 3 sought to document structural changes in hurricane ki- describes the two compositing methods used to display nematic and thermodynamic fields as intensity changes. the data. Section 4 explores the time evolution of the A series of three papers by Hawkins and Rubsam azimuthally averaged flight-level and ground-based ra- (1968a,b,c) documented the life cycle of Hurricane dar data. Section 5 examines changes in Elena’s radial Hilda (1964) over four consecutive days. Willoughby et and vertical symmetric storm structure that occurred al. (1982) and Willoughby (1990) examined changes in during five composite time periods based on Elena’s the tangential wind profiles of intensifying tropical cy- intensification rate. And finally, section 6 concludes clones that occurred in association with convective with a summary and discussion of the results of this rings. More recently, Kossin and Eastin (2001) noted a study, with an eye toward future work. “regime change” in equivalent potential temperature and relative vorticity in hurricanes between peak inten- 2. Data sity and weakening. a. Flight-level data With the increased spatial resolution in numerical models, the inner-core structure of tropical cyclones is As part of a planned reconnaissance mission to study currently being resolved with greater and greater accu- eyewall replacement cycles in tropical cyclones, the Na- racy. Using the fifth-generation Pennsylvania State tional Oceanic and Atmospheric Administration University–National Center for Atmospheric Research (NOAA) WP-3D N42RF and N43RF aircraft flew six Mesoscale Model (MM5), Braun (2002), Rogers et al. separate reconnaissance missions into Elena, with a to- (2003), and Yau et al. (2004) have simulated realistic tal of 88 radial passes, in the 55 h from 0100 UTC 31 intensification rates and inner-core structures with August to 0800 UTC 2 September 1985. Isobaric height, model grid lengths of 1.3–2 km. However, with large temperature, dewpoint, liquid water content, wind variability in model outcome based on initialization of speed and direction were recorded at 1 Hz by the in- the vortex (Pu et al. 2002), boundary layer scheme strumentation aboard the aircraft (Jorgensen 1984a). (Braun and Tao 2000), and input data quality (Davis The data were transformed by personnel at the Hurri- and Bosart 2002), numerical models alone are not yet cane Research Division (HRD) of NOAA into storm- sufficient to address all of the details of intensity relative coordinates through construction of the cy- change. clone track by the method of Willoughby and Chelmow In the current study, a hurricane will be analyzed (1982) and interpolation of the data to a 0.5-km radial using an unusually large number of observations col- grid using a 2-km overlapping filter window. The result lected over a 55-h period during which significant was a 100–150-km radial profile of the vortex structure changes in storm structure and intensity occurred. The at flight level (850 hPa in Elena) for each radial leg. In goal is to provide a more complete picture of the way addition to the atmospheric quantities collected aboard the inner core of a tropical cyclone evolves in time. Part the aircraft, radial profiles of equivalent potential tem- ⌰ I of this study focuses on the intensification process perature ( e) and the symmetric vertical component of through the examination of axisymmetric fields. A fu- relative vorticity (␨) were calculated, the former using ture paper will concentrate on asymmetric structures the method of Bolton (1980) and the latter from the ץ ץ␨ ϭ ϩ with respect to environmental vertical wind shear and formula Vt /r Vt / r, with a centered differencing on observational evidence for vortex Rossby waves. scheme applied to the local tangential wind (Vt). The storm chosen for this study is Hurricane Elena One possible significant source of error in the flight- (1985), which made a slow anticyclonic loop in the Gulf level thermodynamic data is instrument wetting as de- OCTOBER 2005 C ORBOSIERO ET AL. 2907 scribed by Eastin et al. (2002a,b). Wetting of the tem- perature sensor by cloud droplets or rain can lead to evaporational cooling, spuriously low temperature readings, and the possibility of supersaturated thermo- dynamic profiles. In an attempt to reduce these errors and eliminate supersaturated temperature values, Zipser et al. (1981) proposed the following method: if the dewpoint exceeded the temperature, saturation was assumed and the dewpoint and temperature were set equal to the average of the two values. The thermody- namic data for Hurricane Elena, obtained from HRD, have been adjusted with the Zipser et al. (1981) method (hereafter referred to as the ZML correction). Examining 666 radial legs on which concurrent mea- FIG.