MONTHLY -

REVIEW

VOLUME 92, NUMBER 2 FEBRUARY 4 964

BECBS OF WURRl STRUCUURE: NE SUGGESTED BY AND SWOJECU ERCUWY OBSERVATIO ROBERT W. FETT’ Air Weather Service Member, National Weather Satellite Center, U.S. Weather Bureau, Washington, D.C.

ABSTRACT Satellite photographs of hurricanes reveal the complete storm in relationship to its environmcnt. In many instances “outer convective bands” or “pre-hurricane squall lnes” appear to partially “ring” the storm. These bands are usually separated from the rim of thc high cloud shield by a relatively clear channel, visible and distinct for long distances along the circumference. This channel may be formed through the action of a major subsiding branch of the hurricane’s circulation. Many photographs also reveal extensive areas of convective, cirrus-producing cloudiness in the wake of the storm. This cloudiness appears to be intimately associated with that of the hurricane, and strongly influenced by outflow effects. Time cross.section analyses of hurricanes Carla and Anna suggest peripheral subsidence. The area of possible subsidence, in both instances, occurred under an upper shear line, where air diverging in the outflow layer from the hurricane converged with air emanating from the subtropical High. A jet stream, in the case of , appeared in the region of the subsident annular zone, at the edge of the high cloud shield. This high speed current curved anticyclonically along the northern quadrants of the storm-then, on being directed southward, split into two main branches. The eastern branch curved cyclonically into a trailing vortex, apparent as a cold Low at 200 mb. The western branch continued southward, in alignment with the curvature of cloud streaks forming the cirrus “tail” of hurricane Carla. Thcse features appear to be typical of many hurricanes in certain stages of development. Model considerations, employing these features, wiBh a discussion of ramifications, are suggested in this paper.

1. INTRODUCTION over the entire southern boundary of . Other satellite pictures, notttbly a TIROS I11 view of TIROS 111 photographs of hurricane Anna (fig. 1) on on September 16, 1961 (fig. 3) and a July 22, 1961 and Project Mercury photographs of TJROS V view of typhoon Riith on August 15, 1962 (fig. 2) on September 13, 1961, reveal the (fig. 4), have shown marked annular zones along the presence of a relatively clear annular zone2 along the northern boundaries of storm cloudiness. The quadrant periphery of the high cloud shield of each storm. In the or quadrank of occurrence of this feature (30 not appear Mercury photographs, traces of the lower cloud field can to be related in any obvious or casily applied manner to still be seen in the annular zone. Low clouds again direction of storm movement, but the feature is seen appear under the edge of the high cloud shield, visible frequently enough to justify the impression that it may through breaks in the upper cloudiness. It is of particular be an important manifestation of the hurricane’s interest that row alignment appears to be maintained circulation. parallel to that previously established in outlying areas. Another noteworthy feature appearing on the photo- The implication is that fairly intense subsidence occurred, graphs is the presence of n curved line of intensified in a narrow zone at the edge of the high cloud shield, and was imposed on the existing lower-level flow. It will be 2 The term “annular zone” or “annulus”, IIS used in this paper, refers to the relatively notnd +’i-t’I suggestion of a subsident zone is apparent clear or cloud-free strip (especially free of loner clouds) adjacent to tho rim of thc high cloud shield of the hurricane, along any portion of its circumference. It will he shown 1 Capt., USAF that this is an aiiniklar zoiie ojsnbsidence. 43

Unauthenticated | Downloaded 10/10/21 05:21 AM UTC 44 MONTHLY WEATHER REVIEW Vol. 92, No. P

FIGUREl.-TIlZOS I11 view of hurricane Anna at approsiiiiatcly 1500 GMT, July 22, lDG1. View is to the north-northwest. Storm was moving westward toward lower central portion of horizon.

convection at the outer edge of the iinnulus and paralleling cloudiness dong the northeastern edge of this storm. the curvature of the rim of the high cloud shield along the Cirrus cloudiness, from the high cloud shield, extends over edges of the storms. When the Mercury photographs the annular zone, merging with the cloudiness of the (fig. 2) were viewed stereoscopiciilly, cloud tops of the convective band. As to the question of proper inter- outer convective biind were determined to be at an altitude pretation of the convective nature ol this cloudiness, this of 15000-16000 ft., well above the other more suppressed interpretation may be cleduced from many ol the pub- cloud lines. Other TIROS hurricane and typhoon photo- lished satellite studies now avtiilable [a]. In the case of graphs suggest that these lines at times develop to full figure 5, convective cloudiness was dso verified through squall line intensity. Figuie 5 is n TIROS V picture of surface reports at Okioawa, located just on the north side typhoon Sarah, taken on Augnst 16, 1962, which shows of the band [7]. However, it is true that cirrus bands very clewrl; the development of n line of convective have been reported at the very edge of the hurricane

Unauthenticated | Downloaded 10/10/21 05:21 AM UTC February 1964 Robert W. Fett 45

FIGURE2.-A mosaic of NASA Project Mercury photographs of hurricane Debbie (tiorthwcst quadrant), taken at approsimately 1415 OMT, September 13, 1961. Storm vas riioririg toward the ~iorthcast.

circulation with an ripparen t absence of any cotivective Historically, of course, n1;uiy witness accounts hiLve cloudiness, as for e re 4. With a good testified to tlie pre\dence of pre-liui*ricnne divergence. viewing niigle and f:th there is no difficulty Uiiusually clew d:Lys followed by 2% sudden ons1:tugtt t of in differentiating bet cloudiriess and the hurricane coriditioris as the “bitr” of the storin ptissed oyer convective band or ure 6 is a mosaic the station, Iitive been widely quoted in literature. In n showing typhoon Carmen 011 August 15, 1963. A cirrus pre-satellite study, the nppeimuice of a11 absolutely cletir band of the type commonly associated with a jet stream and persistent strip along tlie southwestern edge of hurri- appears along the nortliern edge of this st01-m.~Note cane Daisy, in whit presuinably would be tlie annular the much I’ainter reflectivity of this band as opposed to area, mtls noted by Malkus, Ronne, and Chaffee [IO]. In tlie convective bands iietirer the center of the storm md this stiiiie study :I persistent band of cumulus congestus also those farther to the north. The characteristic was found 260 n. mi. west of the hurricane center orieritecl texture ol‘ cirrus as opposed to the mucli brighter globular north-south. This corresponds very closely with the patterns of cumulonimbus make it an easy mutter to locatioii and orientation of the btuid appearing 011 the differentiate between the two. Figure 7 is an unusually picture of hurricnne Debbie (fig. 2). Nntioiial Hurricane good example showing a pi-e-hurricane squall line ns- Research Project filriis of hurricwe Cleo, 1958, nlso show sociated with tropical storm Thelma 011 August 21, 1962. R narrow, cloudless strip at tlie edge of tlie high cloud sltield Again there is no difficulty in deducing tlie coiivective of this storm. A line of towering cumulus appears just nature of this cloucli ness. outmarcl from this strip. Radar studies hiire slio\vu that

3 See scctioii 3 regarding implications of {.hejet stream in this locatlon. LLpre-h~rricanesquid1 lines” frequently developed in pe-

Unauthenticated | Downloaded 10/10/21 05:21 AM UTC 46 MONTHLY WEATHER REVIEW Vol. 92, No. 2

uultK tjAND

NNULAR ZONE

FIGURE3.-A mosaic and nephanalysis of a TIROS I11 pass over hurricane Esther on September 16, 1961. The Great Lakes arc clearly visible in the top portion of the mosaic. The storm mas moving toward the west-iiorthwest. ripheral areas. Serin, Hiser, and Low [13] have shown that These are important aspects which suggest that n major t,hese lines are typically located 100 to 400 mi. in advance feature of the storm's general circulation may be involved, of the center of the storm. These lines are not always causing continual regeneration of the cloud elements which characterized by the same spiral curvature as the interior make up the band, arid ninintilin it for long periods in bands, according to their account, and, also unlike the the same relative position. The TIROS observcitions, interior bands, individual echoes usually show little showing outer convective bands in close proximity to the tangential motion relative to the storm centers. subsident annular zones, suggest that the two may be

Unauthenticated | Downloaded 10/10/21 05:21 AM UTC February 1964 Robort W. Fett 47

FIGURE4.-A mosaic and nephanalysis of a TIIEOS V pass over typhoon Ruth on August 15, 1962. A surface analysis for 0000 GMT, August 15, is also shoivn. The storin was moving toward the northwest.

related. Such a relationship has certainly iierer before It is the object of this paper to present a more complete been proposed. In fact, the idea of major subsidence in interpretation of the new satellite evidence, by relating the the area of a hurricane poses certain problems which TIROS photographs to conventional information avail- Riehl [I I] has considered in some detail. He showed that able. Relationships suggested in this comparison will be explored in a preliminary and primarily descriptive man- a hurricane circulation could not exist as ti closed system. ner. For this purpose, time cross-section and allied Subsiding effects, in a closed system, could account for analyses will be presented for hurricanes Cnrla and Anna the cloudless annular zone at the periphery of the storm. of 1961. The niajor point of his argument, howerer, was that dry adiabatic warming iri the subsidelit branch of this system 2. FORMATION OF HURRICANE CARLA would soon destroy t,he solenoidal field necessary for AND RELATIONSHIP TO UPPER SHEAR ZONE hurricane tnaintainmce, tind bring the circulation to a Carla formed froiii a perturbation which had its origin halt. His argument does not Rppear to preclude the in the equatorial-trough zone. The National Meteoro- possibility that an important subsiding branch of the logical Center's Northern Hemisphere Surfwe Chart of hurricane circulation could exist without inrolving the 1800 GMT, September 2, 1961 (fig. 8) shows the position of implications of ti closed system. This has been suggested this zone at approximately 12ON., oriented generally in at least one paper [5] interpreting the TIROS I photo- east-west. Also apparent on this chart is the warm gr:tph of tt hurricane near New Zenland. The model High pushing strongly into the southeastern hurricane circultition proposed by Bergeron [ 11 also incor- United States and extending into the Gulf of Mexico. ponrtes $1 major subsident branch at the edge of the storm. This High extended well into the upper atmosphere and I

Unauthenticated | Downloaded 10/10/21 05:21 AM UTC 48 MONTHLY WEATHER REVIEW Vol. 92, No. 2

CELE

FIGURE5.--A mosaic aiicl ncpliaiialysis of a TIROS V pass over t,yphooii Sarah 011 August 16, 1962. The storm was moving northward. diverging winds from a center orcr the United States were trough. An Atlantic shear line of this type, in the same evident at the 200-mb. level. The equntorial trough, in region,- has been described by Rielil .-1121. this instance, clisappenred rapidly with height, and was In :I nitinner siniilnr to tliitt described by Riehl, as thc replwed by a High :it upper elevations. Winds also hurricane niovcd northward, the shear line at upper ele- diverged from this center doft. The 200-nib. nnallysis of 1-ations moved iiorthwnrd, maintaining a position appros- 0000 GMT, September 3, 1961 (fig. 9), shows n shear line iniately 10' 1:Ltitude in advance of the surface center.' orer centrid Cuba, oriented northeast-southwest. Here the diverging winds from the High to the north clashed 4 Mo\ement of shear lincs in ad\ancc of typhoons Sarah and Ruth, 1962, has also been docurncntotl in ,mother report [i]See also shear line develogmcnt in J, study by Frank with those cmnnnting from the High above the equatorial [SI

Unauthenticated | Downloaded 10/10/21 05:21 AM UTC February 1964 Robert W. Fett 49

FIGURE6.-A TIROS VI1 mosaic showing typhoon Carinen at 0223 GMT, August 15, 1963.

Progressire locations of this line at upper levels and the since the anticyclone alof t orer the equatorial trough surface track of C:Lrla, in its various stnges, we shown in provided IL ready-made mechanism for the evacuhon of figure 10. Apparently the position of the slienr line was any lower-level inflow. strongly influenced by the moreinent of' the hurricane or vice versa. The shear line, in fact, may hare resulted as 3. TIME CROSS-SECTIONS AND ALLIED storm outflow eflects were impressed upon the existing ANALYSES OF HURRICANE CARLA circulation. The earlier stnges certainly provided es- In order to determine whether subsidence could be tremely favorable conditions for. hurricane format,ion detected along the periphery of the high cloud shield,

Unauthenticated | Downloaded 10/10/21 05:21 AM UTC 50 MONTHLY WEATHER REVIEW Vol. 92, No. 2

FIGURE7.-A TIROS V mosaic shon-ing tropical storm Thelma at 0203 GMT, August 21, 1962.

tinie cross-sections were prepared for some of the stations tions. The westerly belt appears to overlie a large por- along the path of the storm. tion of the hurricane circulation, and its center position The first station considered, which esperienced passage corresponds very closely to the time of closest approach of of the high cloud shield of hurricane Carla, was Havana, Carla to Havana, at 1800 GMT, September 7. Below the Cuba (fig. 11). As shown by the Havana cross-section westerly belt, at either end, appreciable dryness is indi- on September 3, 1961, and by the observations for the cated by dew point spreads in excess of 15" C. This is preceding days, deep easterlies prevailed over the station also shown, more reliably, in the specific humidity analysis at all levels for several days prior to September 4, 1961. of figure 12. Coincident with this dryness, surface reports Winds aloft are missing on September 4 but by 0000 GMT, indicated generally clear to scattered cloudiness conditions. September 5, winds in the layer from 300 mb. to at least But inward, toward the storm center, generally moist and 150 mb. had shifted from a northeasterly to a south- overcast conditions prevailed. The moist soundings, it westerly direction, indicating passage of an upper shear will be noted, did not merely reflect the presence of heavy zone at some time prior to that observation. The line precipitation areas. Similarly, the dry areas, with dew at upper elevations in figure 11 denotes the 360"-180" point spreads in excess of 15" C. were certainly unusual, isogon separating easterly from westerly flow. The and a wide departure from the LLnornialtropical atmo- leading edge of the westerlies, on the cross-section, corre- sphere". Evidently the major hurricane cloudiness was sponds to the position of the shear line indicated over confined within the limits prescribed by the dry zones at Havana by 0000 GMT, September 4, in figure 10. This is either end, and the westerlies aloft. the niost probable time of passage based on continuity The coincidence of such dryness with the arri\-al of an considerations and correspondence with other observa- upper shear zone, suggests that this zone niight also have

Unauthenticated | Downloaded 10/10/21 05:21 AM UTC February 1964 Robert W. Fett 51

FIGURE9.-200-1nb. streamline analysis for 0000 Grim, September 3, 1961. Shear line is drawn in heavy black ovcr Cuba.

FIGURE8.-NMC Northern Hemisphere surface chart for 1800 GMT, Scpteinher 2, 1961.

been one of appreciable convergence. The dryness, ac- cording to this argument, would have resulted from sub- sidence in response to the upper convergence. If this were true, then, under the shear zone, warming should be evident, depending on the subsident intensity. An tinitlysis of 24-hr. temperature changes (fig. 13) shows temperature incretises at most levels during the period of passttge of the leading edge of the shear zone.5 In par- ticular, the 24-hr. period ending 0600 GMT, September 5 showed temperature increases, through a deep layer below the shear line, to a maximum increase of 4’ C. At the same time, height rises in pressure surfaces occurred at upper elevations, in spite of steadily dropping surface pressure (figs. 11 and 12). All of these indications sug- gest that this was, indeed, a, region of fairly intense subsidence. It appears unreasonable to assunie that the dryness could have arrived over the station advectively, after ti long trajectory orer an oceanic area, and still have maintained the temperature distribution observed. FIGURE10.-Progressive locations of upper shear line in association Temperature increases similar to those observed, par- with the track of hurricane Carla. Asterisk designates 200-mb. ticularly in the lower and middle troposphere, according shear line position. All other positions are for the 300-nib. Icvel. to Dum and Miller [3], are “mostly dynamic in origin”. Hurricane stages: tropical depression (dcveloprnent) (dotted line), The timing of this particular temperature increase, during tropical storm stagc (dashed line), hurricanc stagc (solid line), a period of minimum cloudiness, and just prior to prob- and extratropical stage (line of crosscs). tible time of passage of the edge of Carla’s high cloud shield at 1200 GMT September 5, relates well to the type Another aspect, pointed up by the time-section and of conditions one might anticipate according to the associated.with the location of the shear zone over the TlROS views of the annular zone. General warming annulus of subsidence, appears to be of primary im- also occurred on September 12 and may again reflect portance. The thermal gradient aloft would have a it peripherul subsiderit tendency. tendency to be increased in this area. This would occur 5 Warming, prior to passage of the shear zone, on Septcml)cr 3 occurred after passage as warm, diverging, upper-level winds from the hurricane of a trough. Warming on September 7 occurred during the closest approach of Carla to IXavana, rcflccting central warm core characteristics. merged with the cooler air flowing outward from the 716-347-64-2

Unauthenticated | Downloaded 10/10/21 05:21 AM UTC 52 MONTHLY WEATHER REVIEW Vol. 92, No. 2

. .$ . +. . \$. .& w:::...... I. -..... e r ?, ...... , ...... 7 f ,<:I:::::' "" f . *y. . . r\. ' n .. *. +:.:::. E .[.'j?.( r. f i: I ?- ? y * ?. I!,_ e

Unauthenticated | Downloaded 10/10/21 05:21 AM UTC February 1964 Robert W. Fett 53

70 ...... - 60 '0'' I- ...... 0 0 ...... \ .... w I / / 50\, 50 ...... /.. ././. . .\...... c 4 / \ -\ .. +-.A. . +. .I.-/.+ .... I ./ "Y' ii/ ...... I. \ . ./, . . .\...... 'I' ./ \ . .\...... I ...... c .I' ....

.... S-. :I; ...... /...... +... ( A::. / 2,: I...... - ...... F/-?e? ...... -6.C ...... 7--./O,i .. .\' .- .... :I: . . ' \' ' . .... _____ TIME (Z po 12 )4_-12- DATE SEPT ZSEEI FIGURE12.-An analysis of specific humidity (mixing ratio) values, over Havana, Cuba, in gm. of watcr vapor per kgm. of dry air. 24-hr. height changcs were plotted in the middle of the 24-hr. period and analyzed (dashed lines) in 200-ft. increments.

......

.... '0 " v .+2+4'y,2 ' ' &&......

......

FIGURE13.-24-lir. tcrnpcraturc chaiigc analysis for Havana, Cuba. Isopleths arc drawn for point changes of 2°C.

subtropical High. Additional warming through subsid- production of a jet stream aloft. It is of interest, in ence adjacent to outer convective bands of nppreciable this respect, to note the rapid decrease in the speed of upward vertical motion would tend further to intensify the easterlies with height on Havanii's 1800 GMT sounding the thermtil gradient o\-er a rery mmow zone. Similar of September 5. This sounding was made shortly tifter mechnnisms have long been considered fnvorsble for the passage of the high cloud shield over the station. Wind

Unauthenticated | Downloaded 10/10/21 05:21 AM UTC 54 MONTHLY WEATHER REVIEW Vol. 92, No. 2

FIGURE14.-A composite TIROS I11 nephanalysis froin orbital passes 867 and 868. Pictures were taken at 2045 and 2226 GMT, September 10, 1961. Scattered cloud areas are inclicated by “S” and broken cloud areas by “B.” 200-nib. winds for 0000 GMT, September 11, 11361 are also showii

information suggests a strong westerly therinal wind and typhoon photos. The first concerns the development effect contributing to the production of a westerly 45-kt. of R trailing vortex at upper levels. An example of this wind at 150 mb. can be seen in figure 14 near 24’ N., 72’ W. This cold- Several days later, TJROS TJI was in a position to core vortex was only weakly apparent at lower levels. photograph the “Carla” area. Figure 14 shows a coin- It later developed to tropical storm intensity.6 The posite nephanalysis obtained from two TIROS TI1 pcisses second feature concerns the development of trailing con- over the area of hurricane Carla between 2045 and 2226 vective cloudiness extending toward the equator for long GMT on September 10, 1961. The 200-mb. winds for distances. An excellent example of this particular feature 0000 GMT, September 11, are plotted on this composite is shown in the TIROS V mosaic of typhoon Ruth (fig. 4). for comparison. Of immediate interest is the jet stream (See also fig. 6.) Similar cloudiness is indicated in the which curves anticyclonically at the very edge of the ,nephanalysis of hurricane Carlti. overcast cloudiness of Carla near what has been described If the developinent of the hurrictme jet, as described, as the annular zone of subsidence. (See also fig. 6.) The is typical, then it is interesting to consider the appearance derelopment of such a high-speed current at upper ele- of these features in relation to the concept of the con- vations remote from the center of the storni has been servntiori of potential \ orticity. With constant rorticity noted in other hurricane studies. Dunn and Miller [3] tincl no divergence, there would be a tendency for a determined that this effect appeared even in mean data trailing vortex to be generated at upper levels, east and gathered around hurricanes from the 12.5-16.0-km. level. generally south of the original storm center. This would The fact that the feature, at least in this instance, also occur because of the requirement for the relative rorticity occurred near the edge of hurricane cloudiness, is a matter to increase in compensation for the decreasing value of of great interest, and possibly of fundttnientltl importance. the Coriolis parameter obtained in the southward flow

Figure 14 also reveals two additional features which 6 Frank’s [B] independent study of this unnamed tropical storm, involving streamline should be considered, in relation to cloud forms appearing analysis at the 200 mb level, established changcs of the vortex from cold core cyclonic rotation at 200 nih to \\’arm core anticyclonic rotation at the same level The initial in the case of hurricane Carla and other TIROS liurrictirie upper level natore of tho vortex aas established,

Unauthenticated | Downloaded 10/10/21 05:21 AM UTC February 1964 Robert W. Fett 55

Unauthenticated | Downloaded 10/10/21 05:21 AM UTC 56 MONTHLY WEATHER REVIEW Vol. 92, No. 2 of the jet stream current. The appearance of tin upper- 3. The esistence of a slieiir zone at upper elevations was level vortes trailing Ciirla (fig. 14) nnd in another esaniple found preceding arrival of the high cloud shield and (fig. 3) apparently in the process of formation southeast generally overlying the entire hurricane circulation. The of hurricane Esther, is in ngreemeiit with this argument. shoar zone appetired to move with the hurricane and in Flow toward the south or southwest with anticyclonic fact to define the radial limit of thc outflow of the storm. shear would necessarily be divergent, since an ticyclonic 4. Jndic:Ltions of very dry air lit the edge of the hurricane rorticity would be acquired in the southwnrd movement circuliition and in tlie annular area, versus estreniely of the air mass. The upper-level divergence occurring moist air under the high cloud shield, were clearly evident, in an outflow layer of this type, probably at least 10,000 particularly in the Havana cross-section. ft. thick, \vould certainly be fiiror:ible for the generation 5. Evidence for subsidence idong the periphcry of the of underlying convective activity. Rielil [12] in fact high cloud shield was indicated, ovcr Havana, by tempera- suggests that the area 011 the western margin of a, south- ture increases in the midtroposphere, height rises of ward flowing 200-nib. current is a f:ivor:ible one for pressure surfaces at upper elcvations, and indic:itions of , because of the pressure falls such falling surfiice pressure. a flow would induce “at all levels beneath”. Viewed in 6. Winds changed with hcight from generally steady this light the convective cloudiness trailing Carla (fig. easterlies prior to passagc of the high cloud shield, to 14), Ruth (fig. 4), and Carinen (fig. 6), is especially easterlies decreasing in speed with height, and finally to interesting.’ Direction of upper-level flow can sometimes high-spced westerlies at the edge of the high cloud shield. be determined in the TIRQS pictures by noting the directioji of the shetiring iinvil tops and the cirrus fringes. Southward flow appears to be iiidicnted south of typhoon 4. TIME CROSS-SECTION FOR SWAN ISLAND AND Ruth in figure 4. Southwestward flow doft is clearly COMPARATIVE DISCUSSION OF HURRICANE ANNA revealed in the cirrus striations of the cumulonimbus cloudiness to the south of Carmen (fig. 6). In the case The course of hurricane Anna through the Caribbean, of hurricane Carla the 200-nib. analysis reveals th%t in an iirea of good synoptic coreriige, suggested that this southward flow dso existed, at least east of 90’ W., in would be a promising storm to esnmine for possible agreement with the alignment of cloud streaks noted in recurrence of the characteristics previously discussed. the nephannlysis. In nddition, the TIROS coverage, at the time, was es- This concludcs, with one exception, the major discussion cellent (fig. I). associnting features apparent in the analyses of hurricanc Hurricane Amin apparently formed out of an easterly Carla, with clinracteristics suggested by the satellite wave development, reaching hurricane intensity during photog~aphs. The time cross-section analysis for Havana July 20 and 21, 1961. Etirly stages of development did not clerwly show tlie development of an “outer con- observed by TlROS 111 have been described by Fritz [9]. vcctivc band” or a “pre-hurricane squall line”. A line of Figure 16 shows the track of this storm as it iiio\-ed west- this type did dcvelop later, however, and is quite nppiirent u7iird from the into the Yucatan Peninsula. in a cross-section analysis for Burrwood, La. (fig. 15). This siime figure shows progressive locations of an advaiice This analysis shows a pronounced squdl line effect, shortly shear line el-idelit at the 200-nib. level. Features of flow after passage of the upper sheiir zone. Notc, in pnrticular, tissociiited with this shear line are cleiirly re\-ealed in the the pronounced dryness of the sounding at 0000 GAIT, analysis for this level at 1200 GMT July 21, 1961 (fig. 17). September 8, following squall line passage. In addition, It can be seen in this figure that hurricane Anna was capped the development of 60-kt. winds, at upper elevations above by a strorig anticyclone. Winds iit the periphery of this the squall line, is certainly a itlatter of grent interest circulation, 300 to 500 mi. out, exhibited a tendency to be rclated to the previous discussion. Burrwood, inciden- nearly tangential to a circle concenhic with the storm tally, was nearly 300 n. mi. from the storm’s center, even center. The shear line at this level was evident, passing at time of closest approach of the storm to the station northeast-southwest through eastern Cuba and past during September 9 and 10. Jamaica to Costa Rica. Winds of highest speed appeared We may suniniarize the relevant features found in thc Over Stin Juan, tilong the northern edge of the various analyses of Carla as follows: storm, corresponding, in this instance, to the right rear 1. Less cloudy conditions esisted for at least 4s hr. quadrant of the hurricane. prior to arrival of the high cloud shield, followed by Cloudiness from hurricane Anna covered a much generdly overcast conditions in the hurricsne circulntion, snialler area than in tlie instance of Carla and cross- with clearing again to the rear. sections showing effects of Anna’s passage appear some- 2. Evidence appeared for a trough or squall line of what compressed horizontally, when compared to those intensified convection in advance of the high cloud shield, of Cda (fig. 18). Nevertheless, many feiitures are indicated on the Burrwood cross-section.

~ rerealed which parallel those apparent in hurricane Carlti. 7 Since this paper was origiually submitted the case history of typhoon Vera, 19F2, 111 pnrticular, the band of westerlies tit upper elev at’ lolls has been documentcd 161. This storm aplieared to dcvelop and intensify under the diverging outflow in the soul hcastern quadrant of typhoon Ruth. froin 0000 GMT,July 22 until 1800 GMT, July 24, appears

Unauthenticated | Downloaded 10/10/21 05:21 AM UTC February 1964 Robert W. Fett 57

FIGURE16.--Progrcssivc 300-mb. locations of uppcr shcar line in association with the track of hurricanc Anna. Hurricane stages: FIGURE 17.-200-mb. aiialysis for 1200 GAIT, July 21, 1961. tropical storm stage (dashcd liiic), hurricane stage (solid line), Contours arc drawn in 100-ft. incrcmcnts. Surface position of and deprcssion (dissipation) stagc (starred line). hurricane Aiina and shcar line over Cuba (dashed line) arc shown at tinic of aiialysis. derived froin the storm circulntiorr, and is nearly centered followed by subsiding motion and warming at the edge over the time of closest approach of the storm to the of the high cloud shield. Passiige of' the rim of the high station. This was approximately 1500 GMT July 23. cloud shield over the station presumably occurred at ap- At either end of this band of westerlies, are pronouriced proximately 0900 GMT, July 22. At this tiiiie two- dry zones, with dew point spreads indicated in escess of tenths total sky coverage wiLs reported consisting of t~ 2OOC. A sharp trough is apparent, preceding the dry few low cumuli. zone. This trough passed the station between 1200 It is notable, in this cross-section, that the highest GMT, July 21 md 0000 GMT, July 22. The synoptic reports pressure reported was 1015.8 nib., and that this was during this period suggest the intensified convection one recorded approximately 3 hr. prior to passage of the high might expect exterior to the nnnular zone. The TIRQS cloud shield. This saine tendency, for highest pressure picture of hurricane Anna, sliown in figure 1, shows the to occur within 24 hr. of passage of the high cloud shield, outer convective band in the southwestern quadrant was noted in the New Zealmd hurricane study [5]. quite clearly. Swan Islnnd, at the time of this picture This mtiy be a reflection of the convergence aloft over the (1500 GMT, July 22, 1961) was located ~~pprosimi~tely120 annular zone and relates well to the numerous observations 11. mi. west of the rim of the high cloud shield. In figure rerifying pre-hurricane divergence. For stations directly 1 this position is just off the tip of the second cloud mass in adwnce of the storm, when this pattern is obtained, between the hurricane and the horizon near the top center pressure readings should show a tendency for rising of the horizon view. This positioni~igis in good agreement 24-lrr. pressure changes prior to passage of tlre high cloud with the location of general features indicated by the time shield, followed by falling 24-hr. pressure chtinges in the cross-section. Dew point spreads of less then 5OC transition zone and as the high cloud shield iiio~esover typify middle levels in the outer con\-ective band mea. the station. This pattern did occur for stations in This is followed by the extreniely dry air of the annular advance of the New Zedand hurricane, and also for zone. Florida stations in advance of hurricane Donna, 1960. The variation of wind with height, indicated on the This same tendency nppears evident in the 24-lx. pressure cross-section is of special interest. Prior to 1200 GMT, changes in the cross-section of Swan Tslancl (fig. 18). July 21, easterlies backed and increased in speed with which shows rising 24-hr. pressure tendencies during the height, particularly above the 500-inb. level, indicating period 1800 GMT, July 21 through 0600 GMT, July 22. cold air advection. However, in the suspected annular A final item concerns the wind field aloft. No noticenble zone, at 0000 GMT, July 22, easterlies in middle levels tendency for an especially high-speed wind, is tipparent veered and decreased in speed with height. From the along the forward (western) edge of this storm. This was 1200 GMT sounding, July 22, it can be seen that lower-level also true along the western edge of Carla. However, as easterlies changed to upper-level westerlies with speeds up in the case of Carla, higher speed winds do appear dong to 25 kt. These wind variations lend support to suggested the northern quadrants of the storm. This suggests upward verticd motion in the trough area with cooling tlrnt tlre thernial contrast between hurrictine outflow and

Unauthenticated | Downloaded 10/10/21 05:21 AM UTC 58 MONTHLY WEATHER REVIEW Vol. 92,No. 2

70000 ...... -.

60 -60 -51 65000 7'cn' '5sin - -Tn

60000 ......

55000

50000

45000 ....

40000 .. '. . .. /

43/Ic -42f 35000 .. .. I' 33F -35 33f.. -38 .. . . \.-. 30000 '. \'

25000 ,,if- * -I/ 23000 ... .:

20000 I8000 16000 14000 12000 10000 8000 6000 4000 2000

24-HR fog -0-4 -0-6 +1_6 AP S U R F A C E

0 B S E R V A iN S TIME (Z )0000600 1200 1800 0000 0600 1200 1800 0000 0600 1200 1800 0000 0600 1200 1800 0000 0600 1200 1800 0000 0600 1200 180( DATE 20 JULY 1961 21 JULY 1961 22 JULY 1961 23 JULY 1961 24 JULY 1961 25 JULY 1961

FIGURElS.-Tirne cross-section for Swan Island. 24-hr. pressurc changes arc plotted in the middle of thc 24-hr. period. Dashcd horizontal lines indicate areas with dew point spreads of 5°C. or less. Dotted areas represent dew point spreads of 15'C. or grcater. Clear areas, geiierally below 30,000 ft., have dew point spreads between 5" and 15°C. The 360"-18O0 isogon is drawn separating casterly from westerly flow.

Unauthenticated | Downloaded 10/10/21 05:21 AM UTC m. February 1964 Koberl’ W. Fett 59

that of adjacent high pressure areas is perhaps of doniinmt southwiird flowing jet, fsvor developinen t of a secondary importance in tlie generation of higher speed winds. vortex trailing the original storm, beginning as a disturb- The cff ects of vertical motion, according to this view, ance of the upper air. In the instance of the vortex trail- would play an important, but only an assisting, role. ing hurricane Carla, contiuucd development to tropical We niay summarize the major cliaracteristics of the storm intensity resulted. The trailing area of convective, hurricane Anna analysis as follows: cirrus-producing cloudiness nitiy be initiated as a portion 1. Hurricane Anna wns preceded by a shear zone at of the southward flowing jet continues in this direction, upper levels, which appeared to inom with the storm. under the influence of horizontal divergence and near zero 2. A trough area of intensified convection appeared to vorticity. precede passnge of the edge of hurricane cloudiness. The preliminmy nitture of this study is certainly obvious 3. Major Iiurricane cloudiness tippeared limited to the and must be emphasized. Dettiiled observations includ- iirea between dry zones, at either end of a belt of upper ing raobs at least every three hours tire desirable for an westerlies, associated with the outflow layer. analysis of peripliertil effects. Tllis is necessary since the 4. The tendency for development of a high-speed wind annular zone, where the major subsidence is occurring, near the edge of storm cloudiness, at upper IeTTels, was may in certain areas be only 20 mi. in width, and may piiss noticeable along the northern quadrants of the storm. over a given station in a relntively short interval of time. 5. Tlierninl effects due to subsidence and advection, at Surface observations should be taken frequently enough, the edge of hui-ricane cloudiness, tire apparent in tlie and efforts should be intide, to insure recording of the time changes in wind yelocity with height. The pattern of passiige of the high cloud shield over the station. The nppeciring in the cross-section indicated easterly winds, existence and intensiCy of the outer convective bi~tids,as backing and increasing in speed with height, prior to viewed by ‘J’IROS, should be studied in relation to rdar passitge of the shear zone aloft. This was followed by photographs of the same features. The fact that ’J’IROS easterly winds veering and decreasing in speed with can view these bauds, since they lie at the periphery of the height in the annular zone, becoining westerly in the high cloud shield, is a matter of extreme importance. outflow layer of the hurricane. Tornadic activity, associated with hurricanes, has been shown to develop, primarily, in peripheral regions, pos- sibly in association with the prehurricane squall lines or 5. SUMMARY AND DISCUSSION OF FURTHER outer convective bands [3]. Satellite observations nom IMPLICATIONS permit views which show the locution of these bands, over TPROS and Project Mercury Iiurricane pliotograplis hundreds of miles, titid qualitative estimates of relative reveal at least two major features which appear to intensity are certainly possible. It should be noted that characterize inany storms at some time during their conditions in the annular zone, adjacent to these bands, clerelopment: (1) A relatively clear annular zone of fulfill several of the pre-conditions for tornado develop- subsidence appears, curving around the rini of the high ment listed by Ftiwbush and Miller [4]. Among these are: cloud shield over much of its circumference. (2) A line (I) a low-level temperature inversion ; (2) sharp vertical of intensified convection, or outer coiirective band, moisture strtitification; tmd (3) ti niirrow band of strong often appears, exterior to the annular zone. This line winds dof t. may vary in intensity and is not always continuous. It is extremely important now that specialized observw- A third feature, appearing in sollie storms as they tions be directed to the features observed so that results progress toward higher latitudes, is trailing, convective niay be foriiiulated in a quantita t’ive manner. cloudiness, generating much cirrus. This feature gen- erally appears directed toward the equator in both ACKNOWLEDGMENTS henlispheres. The author would like to express his appreciation to This study has reviewed certain arguments, which sug- Dr. Siginuncl Fritz and Mr. Lester Hubert, who suggested gest that the annular zone may be formed through an the initiation of the writing of this paper. Their sug- important subsident branch of the hurricane’s circulation. gestions, coninients, and criticisms spurred much of the Thermal effects of this subsidence, adjacent to areas of research involved. Additional discussions with Mr. An- upward vertical motion in tlie outer convective bands, drew Timchalk, Mr. Thomas Gray, Dr. Harry Johnson, may aid in the production of n high-speed band of winds, and Mr. Carl Erickson were very helpful. The aid of Mr. at upper elevations, curving tinticyclonically tangential to Thonins Babicki and h4r. Leonard Hatton in the prepara- the edge of hurricane cloudiness. Features of flow asso- tion of figures and that of Mrs. Margaret DiNenna in the ciated with shear lines over the annular zones are thought preparation of the innnuscript is gratefully acknowledged. to play a dominant role in the development of these winds. Maximum speed appears to result in the northern and REFERENCES eastern sections of the storm, near the 300-200-mb. level, 1. T. Bergeron, “The Problcm of Tropical Hurricancs,” Quarlerly where the horizontal temperature gradient is strongest. Journal of the Royal Meteorological Soctety, vol. 80, No. 344, Vorticity considerations applied to the eastern side of the April 1954, pp. 131-164. I 716347-64-3

Unauthenticated | Downloaded 10/10/21 05:21 AM UTC 60 MONTHLY WEATHER REVIEW Vol. 92, No. 2

2. J. H. Conover, “Cloud Interpretation from Satellite Altitudes,” Utilizing TIROS Data,” Monthly Weather Review, vol. 91, Supplement 1 to Research Note 81, Air Force Cambridge NO. 8, Aug. 1963, pp. 355-366. Research Laboratory, May 1963. 9. S. Fritz, “Satellite Pictures and the Origin of Hurricane Anna,” 3. G. E. Dunn and B. I. Miller, Atlantic Hurricanes, Louisiana Monthly Weather Review, vol. 90, No. 12, Dec. 1962, pp. State University Press, Baton Rouge, La., 1960, 326 pp. 507-513. (See pp. 89-90.) 10. J. S. Malkus, C. Ronne, and M. Chaffee, “Cloud Patterns in Hurricane Daisy, 1958,” Woods Hole Oceanographic Institu- 4. E. F. Fawbush, R. C. Miller, and F. G. Starrett, “An Empirical tion, Reference No. 59-63, Technical Report No. 8, Woods Method of Forecasting Tornado Development, ” Bulletin of the American Meteorological Society, vol. 32, No. 1, Jan. 1951, Hole, Mass., Jan. 1960. (Unpublished manuscript.) 11. H. Riehl, “A Model of Hurricane Formation,” of pp. 1-9. Journal Applied Physics, vol. 21, No. 9, Sept. 1950, pp. 917-925. 5. R. W. Fett, “Hurricane Structure and the High Cloud Shield,” 12. H. Riehl, Tropical Meteorology, McGraw-Hill Book Co., Inc., Masters Thesis, St. Louis University, St. Louis, Mo., June , 1954, 392 pp. (see pp. 243-249). 1960. (Unpublished manuscript.) 13. H. V. Senn, H. W. Hiser, and E. F. Low, “Studies of the Evolu- 6. R. W. Fett, “Satellite Views of the Formative Stages of tion and Motion of Hurricane Spiral Bands and the Radar Typhoon Vera, 1962,” (manuscript submitted for publication). Echoes Which Form Them,” Marine Laboratory, University 7. R. W. Fett, “Details of Hurricane Structure Revealed in of Miami, Final Report, to U.S. Weather Bureau on Con- Satellite Photographs,” Proceedings of Third Technical Con- tract No. Cwb 9480, Coral Gables, Fla., Aug. 1959. ference on Hurricanes and Tropical Storms, Mexico City, June 1963 (to be published). [Manuscript received September 19, 1962; revised November 8. N. L. Frank, “Synoptic Case Study of Tropical Cyclogenesis 26, 1963.1

Correction Notice

Vol. 91, Nos. 10-12, p. 637: The sentences beginning at bottom of colum 1 should read: “In all figures the Q curve is a plot of increments of pressure corrected optical depth, in centimeters of precipitable water vapor, between successive pressure intervals. This plotted curve utilizes the numerical values of the radiation scale. It is interesting to see the optical depth increments decrease through the cloud, from 570 through 490 mb. in this example.” Figures 2 through 6 on pages 637-639: “mixing ratio Q” should be changed to “optical depth Q.”

Unauthenticated | Downloaded 10/10/21 05:21 AM UTC