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 hurricane Carla, 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 hurricane Anna. Other satellite pictures, notttbly a TIROS I11 view of TIROS 111 photographs of hurricane Anna (fig. 1) on hurricane Esther 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 hurricane Debbie (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 eye 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.