Small-Scale Spiral Bands Observed in Hurricanes Andrew ...

VOLUME 126 JULY 1998M O N T H L Y W E A T H E R R E V I E W

q 1998 American Meteorological Society 1749

Small-Scale Spiral Bands Observed in Hurricanes Andrew, Hugo, and Erin

ROBERT GALL, JOHN TUTTLE, AND PETER HILDEBRAND

National Center for Atmospheric Research, Boulder, Colorado

(Manuscript received 7 July 1997, in final form 10 October 1997)

ABSTRACT

Analysis of radar data taken from the three intense hurricanes that passed close to WSR-57 and WSR-88Dradar sites at their point of landfall illustrate small-scale spiral bands that are frequently observed within 100km or so of the hurricane center. Faintly visible in the radar reflectivity images, these bands have scales of 10km across the band and can extend for 100 km as they spiral outward in a clockwise fashion. They appear tomove around the hurricane with speeds close to the tangential wind at the level of the bands and are characterizedby enhanced updrafts with higher equivalent potential temperature in the regions of elevated reflectivity. Theyinduce wind speed variations of at least 8 m s21 across the bands. The authors suggest that these small-scalehurricane spiral bands are similar to boundary layer rolls although they extend through depths of 5–6 km, whichis more than would be expected for rolls in the boundary layer near the sea. The data presented here are notsufficient to completely describe the structure of the spiral bands, so their role in hurricane dynamics is notknown.

1. Introduction

Recent observations of three landfalling hurricanes,Hugo, Andrew, and Erin, have provided measurementsof ubiquitous, small-scale spiral bands. These obser-vations include the passage of Hurricane Hugo over theCharleston, South Carolina, WSR-57 radar on 22 Sep-tember 1989; Hurricane Andrew just south of the Mi-ami, Florida, WSR-57 radar on 24 August 1992; andHurricane Erin close to the Mobile, Alabama, WSR-88D on 3 August 1995. In each case, continuous radarcoverage of the hurricane was provided for over an hourprior to landfall, and in each case, the hurricanes con-tained small-scale spiral bands that have not been de-scribed in detail in the literature previously. These spi-rals appear to be distinct from the rainbands that havebeen described from radar studies for decades (Senn andHiser 1959). The observations of the landfall of Hur-ricane Erin also provided measurements of radar radialvelocity perturbations in the small-scale hurricane spiralbands.

A growing body of evidence suggests that importantsmall-scale circulations and features can be found in theinner hurricane rainbands and near the eyewall of ahurricane, and that many of these features are relatedto the overall hurricane dynamics. Radar, satellite, visualobservations, hurricane damage analyses, and theoret-ical studies indicate a variety of small-scale circulations.

Corresponding author address: Robert Gall, National Center forAtmospheric Research, P.O. Box 3000, Boulder, CO 80307.E-mail: [email protected]

These include closed circulation features smaller thanthe eye itself—with a diameter perhaps one-third of theeye diameter—that form within the eyewall and extendinto the eye (e.g., Black and Marks 1991); polygonaleyewalls (e.g., Lewis and Hawkins 1982); small-scalewavelike features (scale about 5 km) just inside theeyewall, that may result from convection or the strongshear in the region (e.g., Bluestein and Marks 1987);and small-scale spiral features, with possible similarityto boundary layer rolls and with a radial wavelengthabout 10 km, that appear in the inner rainband regionof the hurricane (e.g., Fung 1977; Shapiro 1983; Tuttleand Gall 1995).

The role of hurricane rainbands in hurricane ener-getics has been examined by Lewis and Hawkins (1982),Willoughby et al. (1984), Guinn and Schubert (1993),and others. The earlier works identified the polygonalnature of hurricane eyewalls and the differing nature ofinner and outer rainbands. They noted an inner corerainband region in which the rainbands and the air ro-tated with the hurricane, an outer region in which am-bient air moved through and past the hurricane, and astationary band complex—composed of a principalband, secondary bands, and a connecting band—thatseparated these two regions.

Willoughby (1978a,b), Lewis and Hawkins (1982),Chimonas and Hauser (1997), and others postulated thatthe inner rainband structure was related to horizontallypropagating gravity waves and that the polygonal natureof the eyewall and of these rainbands was related tointerference patterns on these waves. Alternative hy-potheses suggest that the inner rainbands are produced

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by dynamic instability of boundary layer flow structures[Fung (1977), who postulated the bands had similarityto the boundary layer secondary circulations as de-scribed by Faller and Kaylor (1966), Lilly (1966), andothers], or by the effects of a moving low pressure pat-tern (Shapiro 1983).

More recently, Guinn and Schubert (1993) and Mont-gomery and Kallenbach (1997) suggested the hurricanecould be regarded as a potential vorticity (PV) maximumthat ingests smaller PV maxima as it goes along. Theypostulate that the inner bands result from the breakingof PV (Rossby) waves, plus mergers of the hurricanevortex with patches of high–PV air; air that is quicklyengulfed and wrapped around the vortex to form a rain-band. These studies did not describe the presence of theof the type of small-scale spiral bands that are reportedupon in this paper.

Depending on the mechanism that forces these hur-ricane features and on their structure, these featurescould have an important role in modulating the intensityof the hurricane (e.g., Guinn and Schubert 1993; Mont-gomery and Kallenbach 1997; Chimonas and Hauser1997). These features could also introduce variabilitiesin the eyewall wind field—perhaps at locations of in-tersection between the eyewall and rainbands or spiralbands—such that small-scale regions may have consid-erably enhanced winds (e.g., Lewis and Hawkins 1982;Wakimoto and Black 1994). There may also be variousconvective features within the eyewall such as down-bursts and tornadoes; however, other than the hints fromdamage patterns, (e.g., Wakimoto and Black 1994),there is little direct evidence of this. Such a small-scalearea of considerably enhanced winds was suggested asthe cause of the 15 September 1989 turbulent incidentthat occurred when the National Oceanic and Atmo-spheric Administration (NOAA) P3 instrumented re-search aircraft entered the eye of Hugo (Black andMarks 1991).

There has never been a careful quantitative exami-nation of the structure of many of the above-mentionedfeatures. Questions such as their role in radial transportof tangential momentum or how they modulate the spa-tial variation of the wind speeds have not been deter-mined. A number of theoretical studies suggest that spi-ral structures may transport significant amounts of mo-mentum. Montgomery and Kallenbach (1997) discussedthe impact of Rossby-like spiral waves. Chimonas andHauser (1997) discussed gravity wave–like spirals andsuggested that momentum transport by such wavescould limit the rotational intensity of certain types ofvortices. Also, Emanuel (1997) hypothesizes that inten-sification of hurricanes is indirectly accelerated by tur-bulent stresses in the eyewall, which causes subsidencein the eye. Small-scale convective spiral bands adjacentto the eyewall may play a similar role in eyewall dy-namics.

Hurricanes Hugo, Andrew, and Erin were intense andsymmetric at landfall, and their passage, very close to

land-based radars, provided high temporal and spatialresolution measurements of radar reflectivity. This pro-vided an opportunity to search for additional evidenceof some of the small-scale features described above. Inparticular, in this paper we will concentrate on the small-scale spiral bands that were immediately noticeable inthe radar data as we began the analysis. These spiralbands are distinctly different than the more familiar,intense, asymmetric hurricane rainbands both in theirspatial structure and in their symmetry about the eye.The fact that these hurricanes were intense and sym-metric makes the examination of the small-scale spiralssimpler and more definitive than if the hurricanes wereweak or very irregular.

The data we will present will describe the existenceand nature of these spiral bands. We will suggest thatthe bands apparent in our data have similarities with theroll vortices that develop as an instability in the clearconvective boundary layer. The bands described in thisstudy, however, are deeper than classic boundary layerdisturbances. Based on the present data, and the lack ofa clear theory of their origin, we are presently unableto determine what role these spiral bands may play inhurricane dynamics. Other small-scale features are alsoapparent in the data and they will be the subject of futurepapers.

2. The data

The hurricane measurements used in this analysisconsisted of WSR-57 and WSR-88D radar reflectivitydata from Hurricanes Hugo, Andrew, and Erin; and insitu data from the NOAA WP-3D aircraft flight throughthe Hurricane Hugo eye and from a United States AirForce (USAF) C-130 reconnaissance aircraft flightthrough Hurricane Andrew.

Our primary data are radar reflectivity measurementsof Hurricane Hugo, taken by the Charleston, South Car-olina, WSR-57 radar between 0230 and 0430 UTC1 22September 1989, and of Hurricane Andrew, taken bythe Miami, Florida, WSR-57 radar between 0600 and0800 24 August 1992. During these observations, theWSR-57 radars operated at an elevation angle of 0.58,providing range and azimuthal resolutions about 1.0 kmand 18, respectively, with a scan update every minute.In each case the data were digitally recorded with aNOAA Hurricane Research Division processor and re-cording system providing a near-continuous record oflow-level reflectivity data for several hours. TheCharleston radar operated throughout the passage ofHugo. The Miami radar failed at the time of the passageof Andrew’s eyewall near the site (the antenna blewdown), but very good data were collected prior to that

1 All times in this paper will be in UTC.


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time. In both cases we examined approximately 1 h ofdata.

Observations of Hurricane Erin were provided by theMobile, Alabama, WSR-88D radar between 1530 and1730 3 August 1995. Erin passed about 100 km to theeast of the radar as the storm moved northward acrossthe Florida panhandle. The radar scanned at a fixed setof elevation angles, completing a volume every 6 minwith an azimuthal resolution of 18 and range gate spac-ings of 1.0 and 0.25 km for reflectivity and velocity,respectively. As with the Hugo and Andrew observa-tions, the Erin data included observations during thefinal hour prior to landfall.

Supplementing the radar data, flight-level data werecollected by the NOAA P3 in Hugo. During its flightthrough the hurricane at an altitude of about 3.5 km,the aircraft transected a number of the inner bands mea-suring the three wind components, pressure, tempera-ture, dewpoint, and cloud liquid water.

A careful examination of the reflectivity in all threehurricanes reveals small-scale spiral structures spiralingoutward from the eyewall, occurring mostly betweenthe eyewall and the larger outer rainbands. Furthermore,the single Doppler velocity information from the Mobileradar indicate that these spiral features are associatedwith the banded structure in the velocity field as well.Together, these data provide a fairly detailed picture ofthe structure and evolution of these inner spirals, butthe reader must keep in mind that the data being ex-amined were not collected with the purpose of definingthe structure of the spirals. Most of the data were col-lected for other purposes, such as operational forecast-ing and warnings, and therefore the picture of the spiralsthat we will try to describe must be assembled fromseveral data sources and hurricanes, none of which initself provides a complete representation of the spiral’sstructure, motion, and evolution.

3. Analysis

a. Preliminary analysis

Radar images of Hurricanes Hugo and Andrew areshown in Fig. 1, at times when the hurricanes were justoffshore and near the radar sites. The subsequent hur-ricane tracks were such that storm center of Hugo passeddirectly over the radar while the northern edge of An-drew’s eyewall passed a few miles south of the radar.The hurricane eyes were well defined in both storms.Of the two storms, Andrew had a very symmetric, smalleye of about 30-km diameter, which contained thestrongest reflectivities within 80 km of the eye. Hugowas somewhat less symmetric, had a larger eye (about50-km diameter) and had stronger rainbands near theeye with reflectivities nearly as high as those in theeyewall.

Significant spiral structures on several scales are ev-ident in both hurricanes (Fig. 1). The familiar rainbands

are perhaps more evident in Hugo than Andrew. Lessobvious, however, are a number of smaller bands spi-raling outward from near the eyewall. These are mostprominent in the east and south quadrants of the hur-ricanes; a number of them are denoted by arrows in Fig.1. In order to enhance the presence of the small-scalespiral structures we show a perturbation reflectivity fieldfor Andrew in Fig. 2. The perturbation field is obtainedby averaging the reflectivity with a seven-gate-by-sev-en-beam triangular filter and then subtracting the av-eraged reflectivity from the original. This was done foreach frame, for both Hurricanes Hugo and Andrew, aswell as for the WSR-88D data from Erin. An exampleof this difference field is shown in Fig. 2. Faintly evidentin this figure are spiral structures that have scales con-siderably smaller than those usually described for thefamiliar rainbands. These spiral bands are even moreapparent when a time sequence of the images are playedback in an animation loop.

An analysis of the perturbation fields, including care-ful analysis of animation loops, reveals the followingproperties of the small-scale spiral structures.

1) They spiral out from the storm center in a clockwisefashion.

2) The scale across the structures is of the order of 10km.

3) They appear to extend around the storm for dis-tances, along the spiral, of up to 100 km.

4) From the animation, they appear to move with thetangential wind.

5) Individual bands can be followed for periods of atleast 1 h.

6) The bands form an angle of perhaps 108 with circlesabout the center of the hurricane.

7) Along a fixed radius from the hurricane center theywould appear to move outward.

8) The variation in reflectivity across the bands is about10 dBZ.

The spiral bands were observed in all three of the hur-ricanes we analyzed and in each case the propertiesnoted above were apparent. We will further demonstrateeach of these properties in the discussion below.

b. A correlation analysis

Because animation is impossible to display in a printpublication we use a simple correlation analysis tech-nique to enhance the spiral structures even further. Thetechnique is similar to that described in Fankhauser etal. (1995), where it was used to examine the structureof horizontal rolls in the boundary layer in Florida. Here,we correlate a cosine function with the reflectivity dataas illustrated in Fig. 3. The cosine function has the formcos(2px/l), where we define a coordinate relative to aparticular range–azimuth point and where x is takenacross the band and y along the band (the function isindependent of y). Starting at some range–azimuth grid




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FIG. 1. Reflectivity from the Charleston, South Carolina, WSR-57 radar during Hurricane Hugo at 0656 UTC 22 September 1989, upperpanel, and the Miami, Florida, WSR-57 radar during Hurricane Andrew at 0320 UTC 24 August 1992, lower panel. Reflectivity in dBZ isindicated by the scale on the right of each panel, the axes are labeled in kilometers from the radar site and the circles around the radar sitein the images indicate the altitude of the beam in kilometers. Arrows on the figure indicate a couple of the bands that are the subject of thispaper.

point, we perform the correlation calculation over onewavelength l in the x direction and for some specifiedlength in the y direction. We then proceed to each gridpoint in the same manner, building up a field of cor-relation values for the entire scan. At each grid pointthe direction of the y axis is determined by rotating theaxis through a number of discrete angles and findingthe orientation of maximum correlation. It should beintuitive that in the presence of bands, the maximumcorrelation would occur when the y axis is parallel tothe along-band axis.

Figure 4 shows the result of applying the correlationanalysis to the reflectivity data (the total reflectivity, notthe perturbation described in the preceding section) ofHugo using different values of l. Note that as l isincreased, larger scales are emphasized. The highly cor-related structure on the right side of the lower-rightpanel, calculated for a wavelength l of 40 km, empha-sizes the large rainband that was about 100 km fromthe center of Hugo and just outside the domain shownin Fig. 1. As l is decreased, the smaller-scale spiralfeatures are emphasized and are best seen when l isabout 10–15 km. When even smaller values of l areused, the fields eventually look random. In all subse-quent calculations we use a value of 10 km for l, whichis the approximate average scale of the spiral featuresof interest here. The length along the y axis over whichthe correlation was calculated was arbitrarily set to 20km; short enough so that the spirals are essentially linearover that length.

It should be noted that a similar analysis was triedwith the input reflectivity replaced with a random field.The result was a correlation field that was also random.Thus the analysis technique brings out features that ap-pear in the data and are not artifacts of the correlationanalysis process. Further evidence is shown in Fig. 5where the correlation field is overlaid on reflectivity inan analysis of Andrew data. A careful comparison re-veals that the spiral features in the correlation field cor-respond to spiral features in the original unmodifiedreflectivity field. While the spirals shown in the corre-lation fields represent only minor reflectivity variations,they are apparent in the original data. Also note that thespiral features have symmetry with respect to the centerof the hurricane, not with respect to the radar. This isevidence that they are not artifacts of the radar.

4. Discussion of the small-scale spiral bandsa. Radar analysis of Hugo and Andrew

The correlation fields for Hurricanes Andrew andHugo, at 10-min intervals over a period of 30 min, are

shown in Figs. 6 and 7, respectively. These figures dem-onstrate that the spiral structures maintain their identitythrough at least half-hour intervals. They also can beseen to slowly evolve in structure and to rotate in acounterclockwise direction around the storm. At a fixedtime, they spiral outward with respect to the center inthe clockwise direction with an angle of approximately108 with circles about the hurricane center, on the av-erage, though a wide variety of intersection angles isapparent. We will say more about the intersection anglelater. The structure, time behavior, and scale of the spi-rals is similar in both Hugo and Andrew.

In Hugo (Fig. 7) the spirals are somewhat more ap-parent than in Andrew, an expected result, since theHugo spirals had higher reflectivity than the Andrewspirals. In Andrew (Fig. 6), the correlation field appearsrandom over the landmass of Florida, to the left of theradar site, while in Hugo (Fig. 7), the spirals are apparentthroughout the storm, even over land. This differencebetween Hugo and Andrew may be related to the largerground clutter pattern of the Miami radar (Fig. 1). TheMIA radar was located on top of a tall building, thuscausing the ground clutter to extend to longer ranges.

Figure 8a shows time–radius plots of the correlationvalues along a fixed azimuth from the center of Andrew.These were taken along azimuth 1358 in a frame ofreference moving with the storm radar reflectivity cen-ter. The stripes represent the intersection of bands ofhigh correlation shown in the earlier figures with thisparticular azimuth. When the stripes are vertical, theband they represent is stationary with respect to thecenter. When they are slanted outward toward the topof the figure, the band is propagating outward with re-spect to the center along this radius and when they slantinward, they are propagating inward. The band nearestthe center is the eyewall, which remains at nearly con-stant radius over the 30 min shown in this figure. A fewof the bands near the center are also stationary withrespect to the center. Beyond a radius of about 30 km,however, band propagation is outward at about 10 ms21, out to approximately 100 km.

Figure 8b shows the same time–radius analysis asin Fig. 8a, but in a frame of reference rotating withthe hurricane tangential winds. The initial position ofthe azimuth is the same as Fig. 8a, but as time pro-gresses each analysis point on the original azimuthrotates about the eye according to the tangential windat that radius. Tangential wind as a function of radiusfrom the eye center was determined using data col-lected by a Air Force C130 reconnaissance aircraft.



FIG. 2. Same as Fig. 1 except showing the perturbation reflectivity for Andrew computed from the total reflectivity of Fig. 1. See text foran explanation of perturbation reflectivity. Perturbation reflectivity in dBZ is indicated by the scale on the right of each panel, the axes arelabeled in kilometers from the radar site, and the circles around the radar site in the images indicate the altitude of the beam in kilometers.

Averages of the tangential wind calculated with respectto the moving radar reflectivity center of the hurricanefor legs flown by the C130 near the time of Fig. 8 wereused. The C130 was flying at an altitude of about 3.0km, about at midheight with respect to the depth ofthe band. We will present more about the vertical extentof the bands later in this section.

Note that now the stripes are all nearly vertical, in-dicating that the bands are moving around the stormapproximately with the local tangential wind and that

the outward propagation noted in the upper panel is aresult of the fact that the spirals intersect circles aboutthe center at an angle. This angle can then be approx-imately calculated from the local tangential velocityand the outward propagation speed of the band. Usingvalues of the local tangential wind speed and an av-erage outward propagation speed, we calculate that thebands near 60 km from the center intersect circles aboutthe center with an angle of roughly 108. This angle, asnoted earlier, is variable over the domain and the 108


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FIG. 3. Cartoon illustrating the correlation analysis process and the procedure for computingthe maximum correlation at a given pixel. The heavy dashed line is rotated until the correlationis maximum. The numbers on the axes refer to pixel numbers.

presents an average. We could find no systematic vari-ation of the angle with radius from the hurricane center.

It should be noted here that the most that can be saidabout the motion of the bands is that they more or lessmove with the local tangential wind. The winds givenby the C130 may only roughly approximate the steeringmotion experienced by the band; thus, if there is a weakmotion of the band relative to the tangential wind, itcannot be convincingly shown by this crude analysis.A careful examination of Fig. 8 might even suggest thatsome of the bands are moving slightly slower than thetangential wind but that could very easily be an artifactof the analysis.

Further evidence of the bands can be seen in Fig. 9showing an RHI scan in Andrew taken toward the south-

east. Although it was taken about one hour later, it in-tersects many of the bands evident in Fig. 6. Clearlyshown in this section is the eye and eyewall that slantoutward with height. Just outside the eyewall a seriesof vertical bands are apparent that extend from the low-est level observed by the radar to a height of at least 6km. They are separated by about 10 km and have re-flectivity variations of about 10 dBZ. By comparisonwith the corresponding correlation plots from near thistime, we can show that most of these features are as-sociated with the spiral bands in the correlation plots.Thus the RHI provides indications that the bands extendthrough considerable depths of the storm and, at leastin reflectivity, are fairly uniform with height and arenearly vertical.



FIG. 4. Results of the correlation analysis of Hurricane Hugo data, using values of the wavelengthl 5 10–40 km. The shading indicates the value of the correlation function starting at value of0.2 and incrementing by 0.05. Range rings are labeled with altitude of the radar beam above sealevel. Axes are labeled in kilometers from the radar site.

b. In situ measurements from Hugo

The Air Force C-130 measurements in Andrew wereonly taken at 1-min intervals and thus are not useful forstudying small-scale features such as the spirals notedabove. On the other hand, the data from the NOAA WP-3D pass through Hugo were taken at 1-s intervals andtherefore have sufficient resolution in time to providesome in situ information on the properties of the bands.

Figure 10 shows the P3 track through Hugo super-imposed on the radar images near the time the P3 passedover the bands indicated by the arrows. The P3 wasflying at an altitude of about 3.5 km. The upper panelshows the inbound leg and the lower panel the outboundleg. Figure 11 is the time plot of tangential wind VT,radial velocity VR, vertical velocity w, cloud liquid waterQC, potential temperature u, and equivalent potentialtemperature uE measured by the NOAA WP-3D. Tan-gential wind and radial velocity were computed fromthe aircraft data with respect to the moving radar centerof the hurricane. The time when the P3 passed over eachof the bands indicated in Fig. 10 is shown as verticaldashed lines in Fig. 11. The numbering scheme for thebands is repeated in Fig. 11.

Each of the spiral bands is characterized by distinctvertical velocity maxima. The traces in Fig. 11 indicate

the presence of 18–1.58C maxima in uE, maxima in QC,and perhaps a slight maximum in potential temperaturecentered on each band. The relationship between VR andVT and the bands is very inconclusive. We tried a numberof ways to show a consistent relationship between thebands and these velocities determined from the aircraftdata but could not find any convincing relationship. Acareful examination of Fig. 10 indicates that some ofthe bands (4, 5, and 6) may be associated with positiveperturbations in VR and VT. Band 1 appears to be as-sociated with a strong negative perturbation in VR. Forband 2 we see no indication in the VR plot and for band3 perhaps a very slight positive perturbation. Bands 1and 2 might be associated with very slight positive per-turbations in VT, and for band 3 it is very difficult todetermine any sign of a VT perturbation with respect tothe band.

That the bands are characterized by elevated liquidwater content is perhaps not surprising since they arealso characterized by enhanced radar reflectivity. In ad-dition, they are regions of enhanced vertical velocityand equivalent potential temperature. They appear to beweakly convectively active regions with higher equiv-alent potential temperature air probably brought up fromthe boundary layer and are, in these respects, similar to


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FIG. 5. Correlation field from the correlation analysis of the Andrew data from Fig. 1, using l 5 10, white contours, superimposed onthe Fig. 1 radar reflectivity field. Contours start at 0.3 and increment by 0.1. Reflectivity scale same as Fig. 1.

convective boundary layer rolls. These bands also lacka well-defined tangential wind maximum and, therefore,they are not secondary eyewalls as seen in some hur-ricanes (Willoughby et al. 1982; Jorgensen 1984a,b).

c. Radar analysis from Erin

Figure 12 shows two radar reflectivity images fromthe WSR-88D data from Erin. The upper panel is thetotal reflectivity field and lower panel is the perturbationfield, computed in the same manner as was used for Fig.

2. In Hurricane Erin, the spirals we have noted in Hugoand Andrew are clearly visible in the perturbation dataand completely surround the eye. The spacing betweenthe spirals over the water are about the same as in theearlier hurricanes, though they are somewhat moreclosely spaced over land, especially those nearest theradar. The reflectivity variations are about 7 dBZ. Theheight of the bands in RHI sections (not shown) fromErin is similar to that noted in Andrew though they arenot quite as deep.

The main information that we can add with the Erin



FIG. 6. The correlation fields for Andrew at 0657–0728 UTC. The upper left panel is the same as the overlay of Fig. 5. Contour intervalssame as Fig. 5. The axes are labeled in kilometers from the radar site. The bold line in lower-right panel indicates the fixed azimuth usedin the analysis of Fig. 8. Rings around the radar site indicate the altitude of the beam in kilometers.

data is some evidence of the velocity distribution withinthe small-scale spiral structures. Figure 13, lower panel,shows the radial velocity distribution provided by theMobile WSR-88D radar at the time of the images inFig. 12. The area shown in Fig. 13 is indicated by thebox in Fig. 12. For this panel we have subtracted spa-tially averaged velocity in the same way that the per-turbation reflectivities were computed in Figs. 2 and 12.Perturbation reflectivity is shown for this time in theupper panel. Also indicated on both panels are a seriesof dashed lines. These lines are at the same location inboth panels and are there to indicate the relative positionof the maxima of reflectivity and radial velocity.

The bands in the perturbation reflectivity field areclearly evident. They are somewhat closer together thanthe bands we have described for Hugo and Andrew; 6–8 km as opposed to 10 (or perhaps slightly more) km.Most of the spirals we have been discussing for Hugoand Andrew were over water. In Fig. 13, the bands areover land. It is not known if the difference in scale notedin Fig. 13 as compared to the earlier hurricanes is relatedto whether the bands are over ocean or over land, butthe appearance of the bands in this region of Erin isvery similar to that noted in the other two hurricanesand in other regions of Erin where the flow is over theocean. We also note, from RHI plots through the region


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FIG. 7. The correlation fields for Hugo at 0300–0331 UTC.

of Fig. 12 (not shown) that the vertical scale of the bandsin the region of Fig. 13 is less than in Andrew by abouta kilometer. If one can assume that the ratio of thevertical and horizontal scales for the bands should beapproximately constant, then the shallower bands oughtto be more closely spaced.

Although the perturbation velocity field is consider-ably noisier than the reflectivity field in Fig. 13, bandsof positive perturbation velocity that are highly corre-lated with the positive perturbations in the reflectivityare clearly apparent. Variations in the velocity acrossthe bands is on the order of 8 m s21. The box shownin Fig. 12b indicates that the location of the data in Fig.13 is directly east of the radar but northwest of the

hurricane center. There, the hurricane winds were fromthe northeast and the radial velocities, with respect tothe radar, were all toward the radar. The perturbationsassociated with the bands in this region were positive,indicating that the hurricane winds, at least with respectto the radar, were reduced under the high-reflectivitybands associated with the spirals.

In Erin the banded structure of the velocity data as-sociated with spiral structures was rare and was onlyobserved at the lowest elevation angle, and only nearthe radar, where the beam was roughly 300–400 mabove the surface. The bands shown in Fig. 13b are thebest example. These bands were over land and it ispossible that the spiral structures in this region could




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FIG. 9. RHI plot from the Miami radar during hurricane Andrew at 0835 UTC, at an azimuth of 1448. The reflectivityscale is indicated on the right in dBZ.

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FIG. 8. Time–radius plots of the correlation function starting at 0728 UTC in Andrew along a radius from the hurricane center at 1358.Solid lines indicate various radial propagation speeds. The top panel shows an analysis in a frame of reference moving with the storm centerand the bottom pane, in a frame of reference moving with the hurricane center and rotating with the hurricane tangential winds.

have had a different mechanism and therefore a differentstructure than the spirals we have been describing forHugo and Andrew, which were over water. However,the appearance of the spirals in the radar reflectivity inErin and their motion is similar to the spirals noted overwater in the other two hurricanes; therefore, we suspectthat the spirals in Erin near the radar are similar to thosewe have described for the other hurricanes. If the spiralsare somehow rooted in the boundary layer, then it wouldnot be surprising that some aspects of their structure,such as their spacing, might be different over land com-pared to over water since the boundary layer flows overwater and over land differ.

d. Additional properties of the bands

In section 4, a number of properties of the spiral bandswere listed. These were apparent in the initial prelim-inary analysis of the difference images and animationof those images. They were further confirmed in thesubsequent more detailed analysis. The analysis of thissection adds additional properties to the list. In the hur-ricanes observed

9) the bands travel outward along fixed radii from thehurricane center at 10 m s21;

10) they have a vertical extent of 5–7 km;11) regions of higher reflectivity associated with the

bands are characterized by enhanced upward ve-

locity, higher liquid water content, and higherequivalent potential temperature as compared to ad-jacent regions; and

12) wind speed variations associated with the bandsmay be on the order of 8 m s21 near the surfacewith weaker winds associated with the regions ofhigher reflectivity of the bands.

At this point we do not have enough information fromthe radar data to determine the full circulation patternsthat exist within the bands.

5. Conclusions

The above analysis provides clear evidence for theexistence of relatively small-scale (;10 km) spiral fea-tures of rather deep vertical extent near the eyes of threehurricanes. We suspect that these small-scale spiral fea-tures are present in most intense hurricanes. Using theradar and in situ data, we have been able to describemany features of these small-scale spiral bands, but thequestion remains, what are they? At this point we canonly offer suggestions, since a definitive description oftheir structure is not available from the data at hand.

We know that the small-scale bands are on the orderof 5 km deep and have spatial scales on the order of10 km. The higher-reflectivity regions have stronger up-drafts and greater equivalent potential temperature thando regions of lower reflectivity. These small-scale bands



FIG. 10. NOAA P3 flight track superimposed on radar images from the Charleston, SouthCarolina, WSR-57 radar. The upper panel shows radar data at 0317 UTC, as the plane enteredthe storm from the right; the lower panel shows radar data at 0334 UTC, as the plane flew south,away from the storm center. Times along the track in UTC are indicated. The arrows refer tobands on the radar image that are referred to in Fig. 11. Reflectivity in dBZ is indicated by thescale on the right. Rings around the radar site indicate the altitude of the beam in kilometers.


JULY 1998 1763G A L L E T A L .

FIG. 11. Time series in situ measurements from the NOAA WP-3D as it entered and left the center ofHurricane Hugo along the track indicated in Fig. 10. Altitude of the aircraft was 3.5 km. The numbers onthe vertical velocity plot refer to the arrows along the track shown in Fig. 10. The vertical dashed linesaid in referring values of other quantities to the time of maxima in vertical velocity corresponding to passageover the bands. Shown are hurricane tangential velocity VT, radial velocity VR, vertical velocity w, cloudliquid water content QC, potential temperature u, and equivalent potential temperature uE.

appear to line up approximately with the low-level wind,where the radial component of the flow is part of thetotal wind. They move approximately with the meantangential wind throughout their depth. They exist nearthe center of the hurricane where the environment issaturated nearly everywhere and the lapse rates are neu-tral to moist ascent. It is our suggestion that these small-scale rainbands are similar to the rolls that exist in theplanetary boundary layer and other flows, that are drivenby the boundary layer shear in the presence of convec-tion. Such rolls often have 1:2 aspect ratios, such asdescribed here. We are confused by the depth of thethese hurricane bands, however, since they are muchdeeper than we would have expected for classic bound-

ary layer rolls. Perhaps the environment in the inner100 km or so of the hurricane is unstable enough, wetenough, and contains enough liquid water (so that weakdowndrafts associated with the band structure undergomoist adiabatic processes) and these properties holdthrough deep enough layers that deep structures typicalof boundary layer rolls could develop. Definitive an-swers concerning their mechanism would require mea-surements that define more of their structure, includingvelocity distributions within them. They should be ap-parent in numerical simulations of hurricanes if thosesimulations are nonhydrostatic and have at least 1-kmspatial resolution, and if the simulations use realisticturbulent heat, momentum, and moisture flux parame-



FIG. 12. Reflectivity, upper panel, and perturbation reflectivity, lower panel, for Hurricane Erin from the Mobile, Alabama, WSR-88Dradar on 3 August 1995. White square in the lower panel shows the position of the images in Fig. 13. Otherwise same as Figs. 1 and 2.


JULY 1998 1765G A L L E T A L .

FIG. 13. Perturbation reflectivity, upper panel, and radar radial velocity, lower panel, from the Mobile, Alabama, WSR-88D radar in theregion of the white square in Fig. 12. The time of these panels is the same as in Fig. 12. The scale on the upper panel is reflectivity in dBZ,the scale on the lower panel is radial velocity in meters per second. The white dashed lines are at the same position in both figures and areuseful for relating the relative position of bands in each figure. Otherwise as in Fig. 1.



terizations and microphysical schemes that are accurateand appropriate for the region within 100 km of thecenter of the hurricane. Most of the spiral’s circulationsoccur in regions of warm rain processes, Jorgensen etal. (1985). If these small-scale hurricane spiral bandsdo appear in numerical simulations, their structure,mechanism, and effect on the overall hurricane dynam-ics could be easily deduced.

Fung (1997) describes spiral structures within hur-ricanes that are driven by the instability of the Ekmanshear within the boundary layer. She then solves a globalproblem, where the spiral structures are fit to boundaryconditions at large radius. The result is a spiral structure,for the most unstable mode, that looks very much likethe spirals observed herein, and even have roughly theright radial scale. However, the spirals in her calcula-tions are nearly stationary with respect to the hurricanecenter; in our observations the spirals rotate with theflow around the hurricane center. Perhaps the reason forthe stationary solutions given by Fung is related to theprocess of fitting the solutions to the boundary condi-tions and considering the stability problem as a globalproblem. Her results are, however, tantalizingly closeto what are observed in the radar data. Other explana-tions for the spirals, including gravity waves and Ross-by-type waves, appear to be eliminated by the fact thatvertical velocity perturbations in the spirals are asso-ciated with warm temperature perturbations and the factthat the spirals appear to be moving with the tangentialwind.

We also do not know what role these small-scale hur-ricane spiral bands may play in hurricane dynamics.These spiral bands most likely include an upward fluxof heat and moisture and those fluxes occur throughsignificant depths. At this point, we do not know themagnitude of the heat and momentum transport, and wehave no information on the tangential momentum orangular momentum transport by the spirals, either hor-izontally or vertically. If they are responsible for angularmomentum transport, they could play a role in the dy-namics of the hurricane. For example, if they transportedsignificant amounts of angular momentum outward, theycould provide a mechanism that dissipates hurricane in-tensity, and vice versa. If they are transporting radialmomentum of the hurricane in the vertical, then theymay play a role in determining the inflow into the hur-ricane. Answers to the question of the impact of thespirals on hurricane dynamics will require a more thor-ough description of the circulations within the spiralsthan we are able to provide with our current analyses.Such a dataset may be provided by intercepting land-falling hurricanes with mobile Doppler radars andsounding systems or with suitably designed flight pat-terns for airborne radars.

Acknowledgments. The authors would like to thankPeter Dodge and Frank Marks of the Hurricane Research

Division of the National Oceanic and Atmospheric Ad-ministration for providing the radar and aircraft dataused in this analysis. The National Center for Atmo-spheric Research is supported by the National ScienceFoundation.

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Bluestein, H. W., and F. D. Marks Jr., 1987: On the structure of theeye-wall of Hurricane Diana (1984): Comparison of radar andvisual characteristics. Mon. Wea. Rev., 115, 2542–2552.

Chimonas, G., and H. M. Hauser, 1997: The transfer of angular mo-mentum from vorticies to gravity swirl waves. J. Atmos. Sci.,54, 1701–1711.

Emanuel, K. A., 1997: Some aspects of hurricane inner-core dynamicsand energetics. J. Atmos. Sci., 54, 1014–1026.

Faller, A. J., and R. E. Kaylor, 1966: A numerical study of the in-stability of the laminar Ekman boundary layer. J. Atmos. Sci.,23, 446–480.

Fankhauser, J. C., N. A. Crook, J. D. Tuttle, L. J. Miller, and C. G.Wade, 1995: Initiation of deep convection along boundary layerconvergence lines in a semi-tropical environment. Mon. Wea.Rev., 123, 291–313.

Fung, I. Y.-S., 1977: The organization of spiral rain bands in a hur-ricane. Ph.D. dissertation, Massachusetts Institute of Technol-ogy, 139 pp. [Available from Massachusetts Institute of Tech-nology, Dept. of Meteorology, Cambridge, MA 02139.]

Guinn, T. A., and W. H. Schubert, 1993: Hurricane spiral bands. J.Atmos. Sci., 50, 3380–3403.

Jorgensen, D. P., 1984a: Mesoscale and convective-scale character-istics of mature hurricanes. Part I: General observations by re-search aircraft. J. Atmos. Sci., 41, 1268–1285., 1984b: Mesoscale and convective-scale characteristics of ma-ture hurricanes. Part II: Inner core structure of Hurricane Allen(1980). J. Atmos. Sci., 41, 1287–1311., E. J. Zipser, and M. A. LeMone, 1985: Vertical motions inintense hurricanes. J. Atmos. Sci., 42, 839–856.

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Montgomery, M. T., and R. J. Kallenbach, 1997: A theory for vortexRossby-waves in its application to spiral bands and intensitychanges in hurricanes. Quart. J. Roy. Meteor. Soc., 123, 435–465.

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Tuttle, J. D., and R. L. Gall, 1995: Radar analysis of HurricanesAndrew and Hugo. Preprints, 21st Conf. on Hurricanes andTropical Meteorology, Miami, FL, Amer. Meteor. Soc., 608–610.

Wakimoto, R. M., and P. G. Black, 1994: Damage survey of HurricaneAndrew and its relationship to the eye-wall. Bull. Amer. Meteor.Soc., 75, 189–200.

Willoughby, H. E., 1978a: Possible mechanism for the formation ofhurricane rainbands. J. Atmos. Sci., 35, 838–848., 1978b: Vertical structure of hurricane rainbands and their in-teraction with the mean vortex. J. Atmos. Sci., 35, 849–858., J. A. Clos, and M. G. Shoreibah, 1982: Concentric eye walls,secondary wind maxima, and the evolution of the hurricane vor-tex. J. Atmos. Sci., 39, 395–411., F. D. Marks Jr., and R. J. Feinberg, 1984: Stationary and movingconvective bands in hurricanes. J. Atmos. Sci., 41, 3189–3211.


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