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Annular Hurricanes 204 WEATHER AND FORECASTING VOLUME 18 Annular Hurricanes JOHN A. KNAFF AND JAMES P. K OSSIN Cooperative Institute for Research in the Atmosphere, Colorado State University, Fort Collins, Colorado MARK DEMARIA NOAA/NESDIS, Fort Collins, Colorado (Manuscript received 15 January 2002, in ®nal form 15 October 2002) ABSTRACT This study introduces and examines a symmetric category of tropical cyclone, which the authors call annular hurricanes. The structural characteristics and formation of this type of hurricane are examined and documented using satellite and aircraft reconnaissance data. The formation is shown to be systematic, resulting from what appears to be asymmetric mixing of eye and eyewall components of the storms involving either one or two possible mesovortices. Flight-level thermodynamic data support this contention, displaying uniform values of equivalent potential temperature in the eye, while the ¯ight-level wind observations within annular hurricanes show evidence that mixing inside the radius of maximum wind likely continues. Intensity tendencies of annular hurricanes indicate that these storms maintain their intensities longer than the average hurricane, resulting in larger-than-average intensity forecast errors and thus a signi®cant intensity forecasting challenge. In addition, these storms are found to exist in a speci®c set of environmental conditions, which are only found 3% and 0.8% of the time in the east Paci®c and Atlantic tropical cyclone basins during 1989±99, respectively. With forecasting issues in mind, two methods of objectively identifying these storms are also developed and discussed. 1. Introduction parison of the intensity evolution of annular hurricanes in the context of intensity forecasting will be examined The satellite appearance of tropical cyclones can vary in this paper. widely from case to case and from day to day. This The rather persistent structural appearance displayed study introduces a category of tropical cyclone, termed annular hurricanes. When compared with the greater in annular hurricanes suggests that there are special cir- population of tropical cyclones in an archive of infrared cumstances related to their formation and persistence. (IR) tropical cyclone imagery, these storms are distinct- Based on modeling and theoretical studies (Jones 1995; ly more axisymmetric with circular eyes surrounded by Bender 1997; Frank and Ritchie 1999, 2001) environ- a nearly uniform ring of deep convection and a curious mental wind shear should be intimately related to the lack of deep convective features outside this ring. Be- symmetry of these systems. Using environmental data, cause of this symmetry, these storms have also been this paper will document the vertical shear and other referred to as truck tires and doughnuts. This appearance environmental conditions that are associated with an- in satellite imagery can persist for days. Accompanying nular hurricane occurrences and how they differ from this structure is a nearly constant intensity1 with an av- a large sample of tropical cyclones. erage of 107.6 kt (1 kt 5 0.514 m s21). This charac- The lack of in situ wind and radar observations is an teristic represents a potential source for large intensity unfortunate reality that is common when studying trop- forecasting errors, which may be reduced by better iden- ical cyclones; however, animated IR satellite imagery ti®cation of annular hurricanes. For this reason, a com- combined with aircraft reconnaissance, when available, can be used to infer the dynamics and thermodynamics associated with annular hurricanes and their formation. 1 Because the maximum winds for tropical cyclones are forecast This methodology will be pursued in the context of a and archived (Davis et al. 1984; Jarvinen et al. 1984) in knots, knots are the units used for intensity throughout this paper. number of recent studies (Schubert et al. 1999; Kossin et al. 2000; Montgomery et al. 2000; Kossin and Schu- bert 2001) that have considered the role that barotropic Corresponding author address: John A. Knaff, NOAA Cooperative instability and asymmetric potential vorticity (PV) mix- Institute for Research in the Atmosphere, Colorado State University, Fort Collins, CO 80523-1375. ing processes might play in the region of the hurricane E-mail: [email protected] inner core. Using aircraft ¯ight-level data from hurri- q 2003 American Meteorological Society APRIL 2003 KNAFF ET AL. 205 canes, Kossin and Eastin (2001) showed that radial pro- lution after being remapped to a Mercator projection. ®les of vorticity and equivalent potential temperature This IR dataset includes all storms for the period 1995± (ue) often undergo rapid and dramatic changes from a 2001 in the Atlantic, and 1997±2001 in the eastern Pa- barotropically unstable regime (i.e., ]2y/]r2 . 0 and ci®c, for this reason in this study we concentrate on ]ue/]r . 0), which they refer to as regime 1, to a bar- annular hurricanes that occurred after 1995. The dataset 2 2 otropically stable regime (i.e., ] y/]r # 0 and ]ue/]r is part of a tropical cyclone IR image archive maintained ù 0), or regime 2, and they hypothesized that these at the National Oceanic and Atmospheric Administra- changes may result from horizontal mixing processes. tion's (NOAA) Cooperative Institute for Research in the The current study concentrates upon six annular hur- Atmosphere (Zehr 2000). ricanes that have been observed in the Atlantic (1995± The track and intensity of each storm come from dig- 99) and eastern North Paci®c (1997±99) tropical cyclone ital databases of best-track information discussed in Da- basins. A factor in determining the number and location vis et al. (1984) and Jarvinen et al. (1984), which are of storms investigated in this study is the availability maintained at the National Hurricane Center (NHC). of a tropical cyclone IR satellite dataset, which exists The Atlantic best-track dataset starts in 1870 and the for 1995±2001 in the Atlantic and 1997±2001 in the east Paci®c best-track dataset starts in 1945. Another eastern Paci®c. Additionally, uniform measurements of dataset referred to as the extended Atlantic best-track environmental conditions calculated for the purpose of dataset exists for the years 1988±2001 and contains ad- intensity forecasting are available from 1989 to 1999. ditional storm-scale information including radii of sig- We know that annular hurricane have occurred in other ni®cant (maximum, and 34, 50, and 64 kt) winds, and tropical cyclone basins; however, neither of these da- eye size. The extended Atlantic best-track data (De- tasets is readably available in other tropical cyclone ba- Maria et al. 2001) are used to discuss structural differ- sins. It is the desire to perform quantitative analysis ences between annular hurricanes and the Atlantic mean, involving both the environmental conditions and the IR since reliable estimates of these structural quantities, brightness temperatures that excludes annular hurri- namely wind radii and eye size, often do not exist for canes in other basins and earlier times from this study. other basins, where routine reconnaissance is unavail- In addition to the IR imagery and the environmental able. For these comparison purposes, eye sizes of an- conditions, a variety of other datasets is also used to nular hurricanes were estimated using the IR imagery. investigate these axisymmetric storms. To compare the life cycle of tropical cyclones with The structural and intensity characteristics of annular intensities greater than 64 kt with the life cycle of an- hurricanes suggest methods to objectively identify them nular hurricanes, we rely upon the past work of Emanuel may be both possible and useful. The identi®cation of (2000) who showed that the life cycles of Atlantic hur- annular hurricanes, especially in a real-time forecast set- ricanes and west Paci®c typhoons are remarkably sim- ting, may possibly be utilized to improve intensity es- ilar. Unfortunately, Emanuel (2000) did not examine timations and forecasts. For this reason objective iden- east Paci®c hurricanes explicitly; however, the remark- ti®cation techniques, which use structural and environ- able similarity between the intensity evolutions in these mental characteristics found during the course of this two quite different basins suggest, that the intensity evo- study, will also be explored. lution in the eastern Paci®c is likely similar. To begin the examination of annular hurricanes, sec- The rate at which tropical cyclones weaken is also of tion 2 offers descriptions of the various datasets that are interest. To determine standard weakening rates, two used in this study. Following this brief discussion of methods are used: one from empirics and another from datasets, the features of annular hurricanes, including theory. The weakening of tropical cyclones is a fun- their axisymmetry, intensity characteristics and forecast damental aspect of the Dvorak method for tropical cy- errors, formation, and associated environmental con- clone intensity analysis (Dvorak 1984). The rate of ditions, will be shown in section 3. Using the evolu- weakening is given in terms of T numbers, which are tionary, structural, and environmental characteristics of related to current intensities. The rules associated with these storms detailed in section 3, section 4 discusses this method limit the number of T numbers that is al- the development of objective techniques to better iden- lowed to change in a 24-h period, which is a maximum tify these hurricanes in a real-time forecasting setting. ®lling rate. These same rules give guidelines for average The ®nal section will present a summary of this study and slower than average weakening rates. These em- along with a few concluding remarks and relevant ob- pirically derived rates are compared to those created servations. using the theory of Eliassen and Lystad (1977), which has been applied to tropical cyclones in Montgomery et al. (2001). In this theory vortex spindown is con- 2. Datasets trolled by the toroidal circulations (u, w) forced by the Several datasets are used to study annular hurricanes.
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