151FEBRUARY 2004AMERICAN METEOROLOGICAL SOCIETY |

OBSERVATIONS

n 12 September 2003,satellite images of At-lantic Hurricane Isabel

astounded the tropical cy-clone science community asa myriad of elaborate pat-terns revealed themselveswithin the clouds in Isabel’seye. Patterns of convolutedclouds in hurricane eyes havebeen previously documented(Fletcher et al. 1961; Kossinet al. 2002; Montgomery etal. 2002), but Isabel showedus something new. As thefirst visible satellite imagesbecame available after localsunrise (~1100 UTC), a re-markable pentagonal patternresembling a starfish wasclearly evident in Isabel’s eye.The pattern was due to thepresence of six distinct meso-vortices—one in the centerand five others arrangedfairly symmetrically aroundthe center—and remainedfairly steady for a few hourswhile rotating cyclonicallywithin the eye (Fig. 1).

One of the exciting as-pects of the appearance of thestarfish pattern in Isabel’s eye is that this pattern waspreviously found (Kossin and Schubert 2001) within

a theoretical framework of maximum simplicity—unforced two-dimensional barotropic flow. Using air-craft flight-level data, the authors found that the tan-gential flow in the eyewalls of intense hurricanes canresemble a vortex sheet (their Fig. 1), and that theseflows can support barotropic instability at high azi-muthal wavenumbers and fast growth rates. To studyhow such instability might evolve within the hurri-cane inner core, a fully nonlinear, two-dimensionalbarotropic model was initialized with flow analogousto the observed flows. One outcome of such numeri-cal integrations is shown in Fig. 2 (their Fig. 9). Themaximum growth of the instabilities of the initial flow

MESOVORTICES IN HURRICANE ISABELBY JAMES P. KOSSIN AND WAYNE H. SCHUBERT

AFFILIATIONS: KOSSIN—Cooperative Institute for MeteorologicalSatellite Studies, University of Wisconsin—Madison, Madison,Wisconsin; SCHUBERT—Department of Atmospheric Science,Colorado State University, Fort Collins, ColoradoCORRESPONDING AUTHOR: Dr. James P. Kossin, CooperativeInstitute for Meteorological Satellite Studies, University of Wiscon-sin—Madison, Madison, WI 53706E-mail: kossin@ssec.wisc.eduDOI: 10.1175/BAMS-85-2-151

©2004 American Meteorological Society

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FIG. 1. Defense Meteorological Satellite Program (DMSP) image of HurricaneIsabel at 1315 UTC 12 Sep 2003. The starfish pattern is caused by the presenceof six mesovortices in the eye—one at the eye center and five surrounding it.

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occurred at wavenumber 13, and 13 mesovortices were “rolledup” from the vorticity in the eyewall. Rapid merger events fol-lowed as the mesovortices migrated into and around the eyewhile rotating cyclonically. After 4.5 h, the six remainingmesovortices formed pentagonal (starfish) patterns in the lo-cal streamline field that were remarkably similar to the fea-tures observed in Isabel. Further integration of the modelshowed the mesovortices alternating between pentagonal andhexagonal configurations.

During 11–14 September, while Hurricane Isabel was atpeak intensity, super-rapid-scan operations (SRSO) were ac-tive on the eastern Geostationary Operational Environmen-tal Satellite (GOES-12). Animations of the SRSO visible imag-ery from this period are available at http://cimss.ssec.wisc.edu/tropic/isabel 2003.html. As shown in these animations, anotherremarkable period occurred at around 1400–1500 UTC on 13September. At least eight small mesovortices appeared to bearranged in a circle near the interface between the eye andeyewall. These mesovortices underwent multiple mergerevents in the following few hours. At 1745 UTC, only fourmesovortices—larger than the original eight and arranged ina square pattern—remained. A very similar evolution wasfound in another of the experiments of Kossin and Schubert(2001; their Fig. 4). In that experiment, the model eyewallvorticity rolled up into eight small mesovortices that mergedover the following 6 h into four larger mesovortices.

At some time after local sunset on 13 September, the four-mesovortex pattern in Isabel’s eye evolved to a less organizedmass of convoluted clouds, as seen in the early near-sunriseimages on 14 September. Contrary to this evolution, the four-vortex pattern in the unforced two-dimensional barotropicmodel is in fact stable and can rotate in that configuration foran indefinite period of time. Thus, while the mesovortices inIsabel’s eye on 12–13 September aligned themselves in vari-ous patterns that were predicted using a model of maximumsimplicity, the evolution and associated time scales of the ac-tual mesovortices measurably differed from the idealized cases.The local divergent flow is a likely culprit for the disparity.According to preliminary analyses performed at the NOAAHurricane Research Division, some of the clouds associatedwith the mesovortices in Isabel’s eye appeared to have a sig-nificant vertical depth and contained substantial vertical ve-locities, as determined from various aircraft-based measure-ments taken on numerous missions into Isabel’s eye (M. Blackand S. Aberson 2003, personal communication).

FIG. 2. Evolution of vorticity (shaded) and streamfunction con-tours (bold) for the numerical experiment of Kossin andSchubert (2001). Values along the label bar are in units of10-----4 s-----1. The shape of the streamlines transitions from a penta-gon to a hexagon and back to a pentagon over 6 h.

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The mesovortices that form in hurricanes play anessential role in hurricane intensification. Previousstudies have shown that axisymmetric models—whichfilter the barotropic instability mixing mechanism de-scribed above—cannot produce a strong hurricanevortex without introducing an artificial mixing pa-rameterization (Emanuel 1997). The parameteriza-tion is usually in the form of eddy diffusivity. How-ever, Kossin and Schubert (2003) showed thatdiffusion is a particularly poor parameterization whenmixing is accomplished via mesovortices. This is be-cause the mesovortices can protect their inner coresfrom further mixing and are thus able to transport veryhigh angular momentum from the eyewall directly intothe low angular momentum environment of the eye.

Hurricane Isabel gave us an unprecedentedglimpse into the turbulent dynamics of the hurricaneinner core and helped to validate previous predictionsbased on the equations of motion. The appearanceand behavior of the mesovortices in Isabel’s eyestrongly suggest that barotropic dynamics play a keyrole in hurricane evolution and that diffusive param-eterization in numerical models may be a poor choicewhen considering the structure and intensification ofthe inner core. It is expected that future analyses ofthe large and unique dataset collected in HurricaneIsabel will serve to elucidate much of the fundamen-tal dynamics and thermodynamics of the hurricane eye.

ACKNOWLEDGMENTS. We are grateful to Ray Zehr(NOAA/NESDIS/RAMM Team) and John Knaff (CSU/CIRA) for directing the SRSO on GOES-12 during Isabel’s

lifetime and for providing us with preliminary animations.Scott Bachmeier (UW—Madison/CIMSS) created the ani-mations available at the link provided above. We wouldalso like to thank Mike Black, Sim Aberson, Chris Velden,and Ed Zipser for their helpful input.

Additional satellite imagery can be found at:• http://cimss.ssec.wisc.edu/tropic/isabel 2003.html• http://www.cira.colostate.edu/ramm/rmsdsol/isabel-

web.html• http://rapidfire.sci.gsfc.nasa.gov/gallery/?search=isabel

FOR FURTHER READINGEmanuel, K. A., 1997: Some aspects of hurricane inner-

core dynamics and energetics. J. Atmos. Sci., 54,1014–1026.

Fletcher, R. D., J. R. Smith, and R. C. Bundgaard, 1961:Superior photographic reconnaissance of tropical cy-clones. Weatherwise, 14, 102–109.

Kossin, J. P., B. D. McNoldy, and W. H. Schubert, 2002:Vortical swirls in hurricane eye clouds. Mon. Wea.Rev., 130, 3144–3149.

——, and W. H. Schubert, 2001: Mesovortices, polygo-nal flow patterns, and rapid pressure falls in hurri-cane-like vortices. J. Atmos. Sci., 58, 2196–2209.

——, and ——, 2003: Diffusion versus advective rear-rangement of a circular vortex sheet. J. Atmos. Sci.,60, 586–589.

Montgomery, M. T., V. A. Vladimirov, and P. V.Denissenko, 2002: An experimental study on hurri-cane mesovortices. J. Fluid Mech., 471, 1–32.