Upper-tropospheric forcing of the intensification rates of tropical cyclones Flo and Ed based on TCM-90 observations.

Transcription

1 Calhun: The NPS Institutinal Archive Theses and Dissertatins Thesis Cllectin Upper-trpspheric frcing f the intensificatin rates f trpical cyclnes Fl and Ed based n TCM-90 bservatins. Rucker, Jeff H. Mnterey, Califrnia. Naval Pstgraduate Schl

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6 Unclassified SECURITY CLASSIFICATIN F THIS PAGE REPRT DCUMENTATIN PAGE 1a. REPRT SECURITY CLASSIFICATIN 1b. RESTRICTIVE MARKINGS Unclassified 2a. SECURITY CLASSIFICATIN AUTHRITY 3. DISTRIBUTIN/AVAILABILITY F REPRT Apprved fr public release; distributin is unlimited. 2b. DECLASSIFICATIN/DWNGRADING SCHEDULE 4 PERFRMING RGANIZATIN REPRT NUMBER(S) 5. MNITRING RGANIZATIN REPRT NUMBER(S) 6a. NAME F PERFRMING RGANIZATIN Naval Pstgraduate Schl 6c ADDRESS (City, State, and ZIP Cde) Mnterey, CA b. FFICE SYMBL (If applicable) 55 7a. NAME F MNITRING RGANIZATIN Naval Pstgraduate Schl 7b. ADDRESS (City, State, and ZIP Cde) Mnterey, CA a. NAME F FUNDING/SPNSRING RGANIZATIN 8b. FFICE SYMBL (If applicable) 9. PRCUREMENT INSTRUMENT IDENTIFICATIN NUMBER 8c. ADDRESS (City, State, and ZIP Cde) 10. SURCE F FUNDING NUMBERS Prgram Element N Priect N Task N Wrk Unit Accessin Number 1 1. TITLE (Include Security Classificatin) Upper-Trpspheric Frcing f the Intensificatin Rates ftrpical Cyclnes Fl and Ed Based n TCM-90 bservatins 12. PERSNAL AUTHR(S) Rucker, Jeff H. 13a. TYPE F REPRT Master's Thesis 16. SUPPLEMENTARY NTATIN 13b. TIME VERED Frm T 14 DATE F REPRT (year, mnth, day) June PAGE UNT The views expressed in this thesis are thse f the authr and d nt reflect the fficial plicy r psitin f the Department f Defense r the U.S. Gvernment. 17. SATI DES 1 8. SUBJECT TERMS (cntinue n reverse if necessary and identify by blck number) FIELD GRUP SUBGRUP _2H1 Envirnmental frcing f trpical cyclne intensity, Supertyphn Fl, Typhn Ed, TCM ABSTRACT (cntinue n reverse if necessary and identify by blck number) A case study f the upper-trpspheric frcing f the intensificatin rates f typhns Fl and Ed has been accmplished. High- reslutin, handdrawn streamline analyses f the 150, 200, 250, and 300 mb winds frm the final bservatinal data set f the Trpical Cyclne Mtin (TCM-90) field experiment were cnstructed fr 00, 06, 12, and 18 UTC during the perid 00 UTC 12 September thrugh 00 UTC 19 September. Three basic interactin mechanisms that appeared t have majr rles in upper-level frcing n Fl and Ed were defined. utflw layer changes were quantified thrugh calculatin f the mass divergence and eddy flux cnvergence f relative angular mmentum using a radial-band averaging technique at 200 mb. A nearly 1:1 qualitative relatinship was fund t exist between the develpment f ne r mre f the these mass/heat exprt mechanisms and crrespnding changes in the intensificatin rates. These calculatins frm the best-ever typhn regin data set strngly suggest that upper-level frcing leads the intensificatin prcess by h. 20. DISTRIBUTIN/AVAILABILITY F ABSTRACT Q UNCLASSIflED/UNLIMITE l"" SAME AS REPRT I"] TIC USERS 22a. NAME F RESPNSIBLE INDIVIDUAL Elsberry, Russell L. D FRM 1473, 84 MAR T ABSTRACT SECURITY CLASSIFICATIN Unclassified 22b. TELEPHNE (Include Area cde) (408) APR editin may be used until exhausted All ther editins are bslete 22c. FFICE SYMBL MR/ES SECURITY CLASSIFICATIN F THIS PAGE Unclassified

7 Apprved fr public release; distributin is unlimited. Upper-Trpspheric Frcing f the Intensificatin Rates f Trpical Cyclnes Fl and Ed based n TCM-90 bservatins by JeffH v Rucker Lieutenant, United States Navy B.S., United States Naval Academy, 1986 Submitted in partial fulfillment f the requirements fr the degree f MASTER F SCIENCE IN Meterlgy and Physical ceangraphy frm the NAVAL PSTGRADUATE SCHL June 1992 n

8 ABSTRACT A case study f the upper- trpspheric frcing f the intensificatin rates f typhns Fl and Ed has been accmplished. High- reslutin, hand-drawn streamline analyses f the 150, 200, 250, and 300 mb winds frm the final bservatinal data set f the Trpical Cyclne Mtin (TCM-90) field experiment were cnstructed fr 00, 06, 12, and 18 UTC during the perid 00 UTC 12 September thrugh 00 UTC 19 September Three basic interactin mechanisms that appeared t have majr rles in upper- level frcing n Fl and Ed were defined. utflw layer changes were quantified thrugh calculatin f the mass divergence and eddy flux cnvergence f relative angular mmentum using a radial -band averaging technique at 200 mb. A nearly 1:1 qualitative relatinship was fund t exist between the develpment f ne r mre f these mass/heat exprt mechanisms and crrespnding changes in the intensificatin rates. These calculatins frm the best -ever typhn regin data set strngly suggest that upper- level frcing leads the intensificatin prcess by h. in

9 . 6.1 TABLE F NTENTS I. INTRDUCTIN 1 A. BACKGRUND 1 B. GALS 6 IT. APPRACH 8 A. DATA SET 8 B. ANALYSIS STRATEGY Streamline analyses Crrelatin f upper- level frcing with intensificatin rates 16 3 Tls fr examining the vertical structure f cld lws 25 a. Temperature crss -sectins in space b. Time series f rawinsndes 34 C. LIMITATINS / QUALIFICATINS 3 6 III. SUPERTYPHN FL 39 A. RGANIZATIN AND DEVELPMENT T TRPICAL STRM Initial Develpment The TUTT cell reservir functin 43 B. INTENSIFICATIN T TYPHN Reservir functin effects 47 IV

10 . t-y M\iUA LiBHAMT NAVAL PbTGHADUATE SCHL MNTEREY. CALIFRNIA Upper- level anticyclne influences The TUTT cell flywheel exprt mechanism.. 60 C. INTENSIFICATIN T SUPERTYPHN Initial interactin with the subtrpical ridge The vacuum exprt mechanism 72 Effects f multiple interactin channels. 77 D. INTENSIFICATIN T MAXIMUM STRENGTH Cmplex interactin with CI and the subtrpical ridge TUTT cell interactin effects 92 3 Emergence f vacuum effect as dminant exprt mechanism 93 E. INITIAL WEAKENING Lss f the flywheel mechanism Resurgence f the reservir functin IV. TYPHN ED 119 A. DEVELPMENT T TYPHN Slw evlutin perid Intensificatin perid Mature stage 13 6 B. INTENSIFICATIN T MAXIMUM STRENGTH 141 C. INITIAL WEAKENING Apprach f landfall 147

11 2. Resurgence f a dual utflw channel pattern 147 V. SUMMARY AND NCSINS 150 A. GALS AND METHD 150 B. SUMMARY F INTERACTIN MECHANISMS The TUTT cell reservir functin The TUTT cell flywheel mechanism The vacuum exprt mechanism 161 C. SUMMARY F THE INTENSIFICATIN PRCESSES Supertyphn Fl Typhn Ed 167 D. NCSINS 170 VI. REMMENDATINS 176 LIST F REFERENCES 180 INITIAL DISTRIBUTIN LIST 183 VI

12 LIST F FIGURES Fig. 1. Psitins f upper- level cld lws CI, C2, C3, C4 (slid lines) each 6 h based n hand analyses f streamlines. Best tracks f trpical cyclnes Fl and Ed (ATCR 1990) are indicated by dashed lines. Map times are indicated in the frm DAY/UTC 15 Fig. 2. Best track infrmatin fr Supertyphn Fl frm ATCR (199 0) 18 Fig. 3a. Average mass flux acrss 6 lat. radius (kt) at 200 mb fr Supertyphn Fl. Bld smthed line is subjective best -fit t the mass flux calculatins indicated at each map time by the dt. Intensity estimates are frm ATCR 1990 (slid thin line) and the intensity change (dashed) is ver each 6 h perid. 21 Fig. 3b. As in (a), except fr Typhn Ed. 22 Fig. 4a. Eddy flux cnvergence (EFC) f relative angular mmentum (kt 2 ) acrss 6 lat. radius at 200 mb fr Supertyphn Fl. Bld smthed line is subjective best -fit t the EFC calculatins indicated at each map time by the dt. Intensity and intensity change are as in Fig Fig. 4b. As in (a), except fr Typhn Ed 24 Fig. 5a. Bulk vertical wind shear (kt) between mean 200 mb wind speed at 6 lat. radius and strm speed fr vn

13 Supertyphn Fl. Bld smthed line is subjective best fit t the bulk vertical shear calculatins indicated at each map time by the dt. Intensity and intensity change are as in Fig Fig. 5b. As in (a), except fr Typhn Ed 27 Fig. 6. Statistics f layer-mean virtual temperature difference ( C) between (a) clear satellite temperature retrievals and degraded rawinsndes, and (b) cludy and partly cludy satellite temperature retrievals and degraded rawinsndes. Clcatin threshld is ±100 km and ±6 h. Dashed line is bias and slid line is RMS difference ( C) 31 Fig. 7. (a) Crss- sectin f satellite temperature deviatins frm standard atmspheric values thrugh western prtin f upper- level cld lw CI at 06 UTC 14 September. Abscissa is retrieval psitin number and rdinate is pressure level in mb. Cld lw CI is indicated by the regin f minimum temperature deviatin (CI). (b) rientatin f crss -sectin (bld slid line) in (a). Psitins f Fl and Ed are marked by the trpical cyclne symbls 33 Fig. 8. Time series f upper-air winds (kt; full barb equals 10 kt) at Marcus Island (WM statin number 47991; 24.0 N, 154 E) frm 00 UTC 13 September thrugh 12 UTC 18 September. Dashed line marks the signature f cld lw CI as it passed suth f the statin n a viii

14 westward track. Cld lw CI was at clsest pint f apprach (CPA) t the statin at 12 UTC 15 September 3 5 Fig mb streamline analysis fr 00 UTC 12 September. bservatin symbls are as fllws:, rawinsnde; a, drpwinsnde;, cnventinal aircraft (AIREP) ; *, NASA DC8 research aircraft ; x, peratinal clud- track wind; E3, reprcessed clud-drift wind. Dashed lines are istachs f trpical cyclne utflw jets and synptic -scale jets in 20 kt increments beginning at 40 kt. Trpical cyclne psitins are marked by the strm symbl and upper- level cld lws are labeled CI, C2, C3 etc 40 Fig. 10. As in Fig. 9., except fr 12 UTC 12 September 41 Fig. 11. Schematic f the TUTT cell reservir functin, (a) 200 mb flw pattern indicating cnvergence f utflw frm trpical cyclne int cld lws labeled C. (b) Tpgraphy f 200 mb surface riented Nrth (left) - Suth (right) with utflw int weak TUTT cell t the nrth and strng utflw t the suth, (c) Tpgraphy f 200 mb surface riented West - (left) East (right) with enhanced utflw frm trpical cyclne int strng TUTT cell t the east 44 Fig. 12. As in Fig. 9, except fr 00 UTC 13 September. 46 IX

15 -, Fig. 13. Infrared satellite imagery fr 2332 UTC 12 September. Upper- level cld lws are labeled CI, C2 C3 etc 48 Fig. 14. As in Fig. 9, except fr 00 UTC 14 September. 50 Fig. 15. As in Fig. 13, except fr 2333 UTC 13 September 51 Fig. 16. (a) Rawinsnde temperature deviatins ( C) frm standard atmsphere values and winds (kt) alng crss sectin thrugh suthern prtin f upper- level cld lw C2 at 18 UTC 13 September. Ship designatr at psitins f upper-air sundings are indicated. Cld lw C2 is in the regin f minimum temperature deviatin labeled C2. (b) Ship psitins fr upper-air sundings used in (a) 54 Fig. 17. As in Fig. 9, except fr 12 UTC 14 September. 56 Fig. 18. As in Fig. 9, except at 250 mb fr 12 UTC 14 September 58 Fig. 19. Schematic f the TUTT cell flywheel mechanism, (a) 200 mb flw pattern indicating large regin f upper-level diffluence n sutheastern periphery f TUTT cell (C). (b) 200 mb surface riented West (left) - East (right) with enhanced utflw int and arund TUTT cell t the east f the trpical cyclne Fig. 20. As in Fig. 13, except fr 1233 UTC 14 September 62

16 Fig. 21. Visible channel satellite imagery fr 2332 UTC 14 September. Upper- level cld lws are labeled CI, C2 etc. 64 Fig. 22. As in Fig. 9, except fr 00 UTC 15 September. 67 Fig. 23. As in Fig. 9, except fr 12 UTC 15 September. 71 Fig. 24. As in Fig. 9, except fr 18 UTC 15 September. 73 Fig. 25. Schematic f the vacuum exprt mechanism, (a) 200 mb flw pattern depicting develpment f strng trpical cyclne utflw jet acrss a weakness in the subtrpical ridge and int the subtrpical jet. (b) Tpgraphy f 200 mb surface riented Nrth (left) - Suth (right) depicting enhanced utflw due t steeper nrth- suth pressure gradient n nrthern side f subtrpical ridge, (c) Tpgraphy f 200 mb surface riented Nrth (left) - Suth (right) depicting enhanced utflw acrss a break in the subtrpical ridge 74 Fig. 26. As in Fig. 13, except fr 2032 UTC 15 September 78 Fig. 27. As in Fig. 9, except fr 00 UTC 16 September. 80 Fig. 28. (a) As in Fig. 7a, except fr cld lw CI at 00 UTC 16 September, (b) As in Fig. 7b, except fr crsssectin in (a) 82 Fig. 29. As in Fig. 8, except fr ship EREH (20.0 N, 126 E) frm 00 UTC 13 September thrugh 06 UTC 18 XI

17 September. Cld lw C2 tracked westward and passed ver the ship arund 12 UTC 15 September Fig. 30. As in Fig. 9, except fr 06 UTC 16 September. 86 Fig. 31. As in Fig. 9, except fr 12 UTC 16 September. 87 Fig. 32. As in Fig. 9, except fr 00 UTC 17 September. 90 Fig. 33. As in Fig. 13, except fr 2332 UTC 16 September 91 Fig. 34. As in Fig. 9, except fr 06 UTC 17 September. 95 Fig. 35. As in Fig. 13, except fr 0533 UTC 17 September 96 Fig. 36. (a) As in Fig. 7a, except fr 06 UTC 17 September, (b) As in Fig. 7b, except fr crss-sectin in (a) 98 Fig. 37. As in Fig. 9, except fr 12 UTC 17 September. 99 Fig. 38. As in Fig. 13, except fr 1232 UTC 17 September 101 Fig. 39. (a) As in Fig. 7a, except fr 18 UTC 17 September, (b) As in Fig. 7b, except fr crss -sectin in (a) 103 Fig. 40. As in Fig. 9, except fr 18 UTC 17 September. 104 Fig. 41. As in Fig. 13, except fr 2132 UTC 17 September 106 Fig. 42. As in Fig. 9, except fr 00 UTC 18 September. 108 Fig. 43. As in Fig. 13, except fr 2332 UTC 17 September '. 109 Fig. 44. As in Fig. 9, except fr 06 UTC 18 September. Ill xii

18 Fig. 45. As in Fig. 8, except fr Chi Chi Jima (27.0 N, E; WM statin number 47971) frm 00 UTC 16 September thrugh 12 UTC 18 September. Cld lw CI passed suth and west f the statin n an increasingly west t nrthwesterly track Fig. 46. As in Fig. 9, except fr 12 UTC 18 September. 114 Fig. 47. As in Fig. 9, except fr 18 UTC 18 September. 116 Fig. 48. (a) As in Fig. 7a, except fr 12 UTC 18 September, (b) As in Fig. 7b, except fr crss-sectin in (a) 117 Fig. 49. As in Fig. 13, except fr 1832 UTC 18 September. 118 Fig. 50. Best track infrmatin fr Typhn Ed frm ATCR (1990) 121 Fig. 51. As in Fig. 9, except fr 06 UTC 13 September. 123 Fig. 52. As in Fig. 13, except fr 0633 UTC 13 September 125 Fig. 53. As in Fig. 9, except fr 12 UTC 13 September. 128 Fig. 54. As in Fig. 9, except fr 150 mb at 00 UTC 14 September 129 Fig. 55. As in Fig. 9, except fr 150 mb at 06 UTC 14 September 130 Fig. 56. As in Fig. 9, except fr 150 mb at 12 UTC 13 September 132 Fig. 57. As in Fig. 13, except fr 1234 UTC 13 September 134 xiii

19 , Fig. 58. As in Fig. 9, except fr 18 UTC 14 September. 139 Fig. 59. As in Fig. 13, except fr 1932 UTC 14 September 140 Fig. 60. As in Fig. 9, except fr 250 mb at 18 UTC 15 September 144 Fig. 61. Timeline f the intensificatin f Supertyphn Fl. " + " ("-" ) within the timeline indicates times when the intensificatin rate increased (decreased) and "TS", "TY", "STY" indicate the times at which Fl attained trpical strm, typhn, and supertyphn intensity respectively. Lines belw the timeline depict when each f the three interactin mechanisms was perating, with the thickness indicating the relative strength f the mechanism 152 Fig. 62. As in Fig. 61, except fr Typhn Ed 153 xiv

20 ACKNWLEDGEMENTS I wuld like t thank Mr. Pat Harr fr his gracius help thrughut the curse f this thesis. He prvided the charts n which all f the analyses were made and was always willing t answer my questins with a fresh perspective. I wuld als like t thank LCDR Dave Titley fr his help with the cmputer prgramming requirements f the study and fr graciusly taking the time t review prtins f the text. Special thanks g t my secnd reader, LCDR Gerge Dunnavan, wh was exceedingly helpful frm the very beginning - frm answering my numerus questins t reviewing the text while it was in prgress. Finally, I wuld especially like t thank my advisr, Prf. Russell Elsberry, fr prviding me with his insight, mtivatin, and patience that started me ff and kept me ging. Withut his help this study wuld nt have been pssible. xv

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22 I. INTRDUCTIN A. BACKGRUND Trpical cyclne intensificatin at the mst fundamental level is a result f cnvective instability prcesses driven by the energy surces f ceanic heat and misture fluxes and the latent heat f cndensatin. Studies (e.g., Miller 1958; Emanuel 1988) suggest that sea- surface temperature (SST) establishes a maximum ptential intensity that the trpical cyclnes may attain, which tends t supprt the view that internal and lcal prcesses such as mist cnvectin and air/sea interactin dminate the intensificatin prcess. Numerical mdeling studies (yama 1982; Emanuel 1986), which have histrically treated the intensifying strm as existing in an inert envirnment, have further shwn that an intense trpical cyclne can be maintained nly thrugh the additin f heat by cean surface fluxes. Clearly, the prcesses f mist cnvectin and air/sea interactin lay the fundatin fr the intensificatin prcess. Interactins between the trpical cyclne and the envirnment have als been shwn t play a significant rle in the intensificatin prcess. Mtivatin fr many f these studies has been the desire t explain why nly a few trpical cyclnes attain their maximum ptential intensity. Merrill

23 (1988) addressed this issue directly and argued that since the maximum intensity is rarely reached, the envirnment generally must exert a negative influence that cntributes t a weakening f the strm. Cnversely, ther studies (Riehl 1950; Miller 1958; Sadler 1976, 1978) have shwn that envirnmental interactins can cntribute t intensificatin f the trpical cyclne. ne basic idea is that the subsidence assciated with the upper- level utflw must nt ccur in the immediate envirnment f the strm (Riehl 1950). Subsidence in the nearstrm envirnment wuld weaken and eventually destry the hrizntal temperature gradient needed t intensify the strm. Hlland and Merrill (1984) speculated that as the trpical cyclne intensifies, the anticyclnic asymmetric utflw at the upper levels must increase and becme less inertially stable. As described by Hlland (1987), decreasing inertial stability in the utflw layer results in less resistance t envirnmental frcing. Thus, the upper trpsphere is cnsidered t be the favred layer fr interactins between the trpical cyclne and the envirnment t influence intensificatin. The bservatinal studies f Miller (1958) and Sadler (1978) are cnsistent with the ideas f Riehl (1950) and Hlland and Merrill (1984) since they relate the intensificatin t interactins with the upper- trpspheric envirnment. Using analyses f 200 mb geptential heights cmpsitied frm five mature trpical cyclnes, Miller (1958)

24 -, fund that the strms deepened as an upper- level trugh shifted inward tward the center f the utflw regime. The interactin mechanism is similar t that described by Hlland (1987), in which a strng utflw channel t the midlatitude westerlies develps cncurrently with the apprach f an upper- level trugh frm the west. Sadler (1978) fund a similar result, and argued that the large-scale circulatin f the upper trpsphere and the existence f multiple utflw channels play a majr rle in trpical cyclne intensificatin. He described the intensificatin f fur typhns in the western Nrth Pacific that develped a strng utflw channel t the nrth int the Trpical Upper Trpspheric Trugh (TUTT) alng with an utflw channel int the upper- level equatrial easterlies t the suth. Rapid intensificatin ccurred when an especially vigrus utflw channel t the nrth develped in cnjunctin with the prper psitining f the strm relative t intense, cyclnic cells within the TUTT. Cnsistent with the ideas f Riehl (1950) Sadler (1978) reasned that the existence f these strng utflw channels permitted the efficient remval f mass and heat away frm the central cnvective regin f the strm such that intensificatin was likely t ccur. Mrever, the decay f the efficient utflw channel and the crrespnding lss f the dual utflw pattern resulted in the weakening f an intense trpical cyclne.

25 Significant evidence exists therefre that interactins between the envirnment and the trpical cyclne, especially thse that ccur within the upper- trpspheric utflw layer where the inertial stability is favrably lw, can play a majr rle in determining the rate f intensificatin. As described by Mlinari and Vllar (1989), the rle f envirnmental frcing may be best cnsidered as ne f mdulating the ultimate intensity f the strm. Whereas lack f upper- level interactin may result in cnditins unfavrable fr intensificatin, significant upper- level interactin may prduce much mre rapid intensificatin than wuld nrmally ccur in islatin (Sadler 1978). Such upperlevel frcing cannt prduce an intensity beynd the upper bund set by the SST, because in such a case the cean fluxes culd nt supprt the strm. Since trpical cyclnes seldm develp t their maximum ptential intensity, frecasting the ultimate intensity f a deepening strm may well depend mre n the ability t frecast the upper- level frcing than n the knwledge f the SST within a presumably near-unifrm warm ceanic regin. Frecasting such upper- level frcing requires an understanding and quantificatin f the physical mechanisms that have been described in bservatinal studies such as Miller (1958) and Sadler (1978). Pfeffer (1958) prpsed that the rle f envirnmental interactins in trpical cyclne intensificatin culd be quantified by the calculatin f the hrizntal transprt f

26 - angular mmentum by azimuthal eddies in the cyclne envirnment system. Subsequent cmpsiting and mdeling studies (McBride and Zehr 1981; Pfeffer and Challa 1981) have shwn that the utflw layers f develping strms are characterized by large inward eddy fluxes f cyclnic angular mmentum, while such fluxes were weaker in the nn- develping strms. As described by Hlland (1987), fluxes f eddy cyclnic angular mmentum prvide a ptential mechanism fr intensificatin by enhancing the utflw f the strm. This idea was verified by the case study f Hurricane Elena (1985) in which Mlinari and Vllar (19 89) fund that utflw- layer eddy angular mmentum fluxes were crrelated with the changes in intensity. n the ther hand, Merrill (1988) fund n significant relatinship between such fluxes and crrespnding intensity changes in a 5 year cmpsite f Atlantic hurricanes. Mre recently, DeMaria et al. (1991) calculated the eddy flux cnvergence (EFC) f relative angular mmentum at 2 00 mb fr the named strms during tw Atlantic hurricane seasns. They fund nly a small (but statistically significant) psitive crrelatin between EFC and intensity changes. They suggested that ther envirnmental factrs such as the influences f SST and vertical shear nt accunted fr in their study may have als played a rle. Studies f eddy mmentum frcing have therefre added significantly t the understanding and quantificatin f the physical mechanisms that have been hypthesized t relate

27 envirnmental frcing t trpical cyclne intensificatin. Nevertheless, a precise understanding f the interactin between the cnvective scale f the trpical cyclne and the synptic -scale frcing remains unclear. Such an understanding is hampered by the scarcity f detailed bservatins f the three-dimensinal structure f the trpical cyclne utflw layer. B. GALS The Trpical Cyclne Mtin (TCM-90) field experiment cnducted in the western Nrth Pacific frm August t September 1990 prvides a high density f bservatins frm varius data surces (Elsberry 1990; Elsberry et al. 1990; Harr et al. 1991). T better understand the interactins between the trpical cyclne and its envirnment, particular emphasis was placed n btaining bservatins f the utflw layers f the six target trpical cyclnes during the experiment. Prir data sets and the rutine peratinal data have histrically lacked the spatial and tempral reslutin necessary t fully describe the upper- level interactins that may play a rle in the intensificatin prcess. f particular interest during TCM-9 was the nearsimultaneus, yet cnsiderably different, develpments f tw trpical cyclnes ver the same regin f favrably warm cean. Trpical cyclne Ed, which was the first t develp, achieved nly slightly mre than minimum typhn intensity

28 . (65 kt), while Fl intensified rapidly and achieved supertyphn intensity (2 130 kt). The TCM-90 data set (Harr et al. 1991; see descriptin in next sectin) prvides an utstanding pprtunity t determine what rle upper- level frcing may have played in the develpment f these tw strms The hypthesis f this study is that the pst -prcessed TCM-90 data set prvides sufficient spatial and tempral reslutin t depict upper- trpspheric frcing n trpical cyclnes Fl and Ed, and that such frcing can be related t changes in the intensificatin rates f these strms. The gal f the study is t use this best -ever typhn regin data set t: (i) identify specific interactins between the upper- level envirnment and the trpical cyclnes; and (ii) crrelate changes in these interactins with bserved changes in the intensities f Fl and Ed.

29 ; II. APPRACH A. DATA SET The final TCM-90 data base is described in detail by Harr et al. (1991) and is cmprised f tw categries: (i) realtime data; and (ii) delayed data. Real-time data were cllected and prcessed during the field phases f the experiment in cnjunctin with the Navy peratinal Glbal Atmspheric Predictin System (NGAPS) at the Fleet Numerical ceangraphy Center (FNC), Mnterey CA. Real-time data include: 90 % f the rawinsnde bservatins; all f the pilt ballns; mst f the surface data (land, ship and buy) ; aircraft bservatins (mstly cnventinal aircraft reprts) satellite temperature sundings retrieved by the Natinal Envirnmental Satellite, Data and Infrmatin Service (NESDIS) ; and satellite clud- tracked winds (frm the Japanese Meterlgical Agency). Extensive quality cntrl prcedures that were applied t the real-time data are described in Baker (1991). Delayed data were cllected after the field phase and cnsist f bth bservatins that were nt received in realtime and bservatins that were reprcessed prir t being added t the data base. Delayed data include: radar wind prfiler data; surface synptic reprts (frm the USSR ships

30 . and Japanese islands) ; sme f the drifting buy data; flightlevel data and drpwinsndes frm the NASA DC8 research aircraft; reprcessed satellite clud-drift winds (manually prcessed by the University f Wiscnsin) ; and the remaining 10 % f rawinsnde sundings received later. While sme f the delayed data were subjected t sme type f quality cntrl at the surce, nne f these data underwent the vertical cnsistency checks assciated with the NGAPS quality cntrl prcess. Gradient -level and upper-level (200 mb) streamlines were analyzed peratinally at 00 and 12 UTC by the Jint Typhn Warning Center (JTWC), Guam staff based n real-time wind bservatins received at JTWC. Real-time satellite imagery and wrking best tracks frm the Autmated Trpical Cyclne Frecast (ATCF) system were als used in the preparatin f the peratinal analyses. Summaries f the Intensive bserving Perids (IPs) by the TCM-90 research team are prvided in Elsberry et al. (1990) Subjective interpretatins f the peratinal analyses, synptic discussins, hyptheses and research aspects, and ntes n the data cverage are included in these IP summaries. Summaries fr IPs 5, 6 and 7 (00 UTC 13 September - 12 UTC 14 September, 00 UTC 15 September - 12 UTC 16 September, and 00 UTC 17 September - 00 UTC 19 September respectively) describe trpical cyclnes Ed and Fl and suggest that upper- level interactin may have played a rle in

31 . the intensificatin f these tw strms. Fl develped t supertyphn intensity, while Ed reached nly slightly mre than minimum typhn strength, despite the nearly simultaneus develpment f the tw strms within the same regin. f specific interest was the apparent interactin between Fl and an adjacent large TUTT cell, whereas n strng frcing f Ed was evident A review f the peratinal JTWC streamline analyses and accmpanying hard- cpy satellite imagery fr IPs 5, 6 and 7 was accmplished as a first step. The gal was t determine whether the additinal real-time data prvided by the TCM-90 experiment were sufficient t characterize the upper- level interactins that induced intensity changes in the trpical cyclnes. While evidence f synptic-scale interactins was fund, the spatial and tempral (12 h) reslutin affrded by the real-time bservatins was insufficient t fully describe the upper- level frcing. Tracks f upper- level anticyclnes and cld lws were ften erratic due t a lack f time cntinuity. As a result f the spradic develpment and decay f these features as depicted n the peratinal charts, pssible interactins between these features and the trpical cyclnes were difficult t analyze. In particular, crrelatins between changes in the upper- level circulatin pattern and crrespnding changes in the intensities f Fl and Ed were incnclusive. This preliminary analysis indicated that a detailed examinatin f upper- level frcing n the 10

32 intensificatin rates f the tw strms wuld require the additinal spatial and tempral reslutin affrded by the delayed data in the final TCM-9 data base. B. ANALYSIS STRATEGY 1. Streamline analyses High- reslutin, hand-drawn streamline analyses f the 150, 200, 250 and 300 mb winds frm the final TCM-90 bservatinal data set were cnstructed at 00, 06, 12 and 18 UTC fr the perid 00 UTC 12 September thrugh 00 UTC 19 September. Istachs als were analyzed in limited regins f the trpical cyclne utflw and synptic -scale jets. Cnsistent with generally accepted guidelines, the hierarchy f reprts used in cnstructing the streamlines was (in rder f decreasing significance) : (i) real-time upper-air statin sundings, including pilt balln reprts frm the U.S. Pacific Islands; (ii) delayed upper- air statin sundings; (iii) NASA DC8 flight -level winds; (iv) drpwindsndes ; (v) reprcessed satellite clud-drift winds; (vi) real-time cnventinal aircraft reprts; and (vii) peratinal cludtrack winds. Real-time upper- air sundings were cnsidered t be mre representative f the atmspheric state than were the delayed rawinsndes because the real-time data were subjected t NGAPS quality cntrl prcedures. The reprcessed cluddrift winds were cnsidered t be mre accurate than peratinal clud-drift winds because the height assignment f 11

33 . the peratinal winds is knwn t be suspect (Harr et al 1991). By cntrast, a careful height assignment culd be dne at the University f Wiscnsin after the field experiment by using the additinal upper-air bservatins frm TCM-90 and the ther internatinal experiments. Fur levels were analyzed t achieve vertical cnsistency and t increase the hrizntal reslutin at adjacent levels in data- sparse regins. The existence f reprcessed cluddrift winds at all fur pressure levels was f particular significance. Since these bservatins require cluds as tracers, and since the strng trpical cyclne utflw jets are typically marked by cirrus streamers, changes in the maximum utflw layer may be represented by changes in the level f maximum density f clud-drift wind bservatins. These bservatins were critical t assess the vertical structure f the utflw layer. The bjective f this multilayer analysis apprach was t describe the vertical structure, r tpgraphy, f the upper- level flw pattern. Fr example, the vertical structure f adjacent synptic -scale features such as cld lws may play a rle in upper- level frcing f the trpical cyclne and thus must be cnsidered. Hard- cpies f the satellite imagery were used extensively during the cnstructin f these analyses. This imagery prvides a means f independently verifying the bservatins in regins where the circulatin was revealed in the clud patterns, and as a means f identifying and lcating 12

34 circulatin centers f features such as cld lws. Imagery was particularly useful in describing the rientatin f the upper- level anticyclnes that were revealed by the clear regins that had upper- level cnvergence and subsidence versus regins with high cluds (upper- level divergence and deep cnvectin) that marked the utdraft circulatins. In general, the wind bservatins near the center f the trpical cyclne are a reflectin f bth the hrizntal mtin f the air parcels and f previus vertical mtin. Fr example, if the bservatins nearest the trpical cyclne at 250 mb are lcated 100 km frm the strm center and suggest anticyclnic utflw, then the air parcels may have riginated as an utflw frm a higher level (e.g., between mb) within the strm. Thus, utflw at a certain level cannt necessarily be inferred based slely upn bservatins that are lcated mre than 100 km frm the strm center. This cncept f the utflw regime was invked nly in the subjective interpretatin f the analyses, because there was n way in which t represent vertical mtin with the hrizntal streamlines. Adhering t generally accepted guidelines, the streamline analyses were cnstructed by "drawing t the bservatins," and thus care must be taken when evaluating utflw at a certain level based n a literal interpretatin f the streamline directin relative t a circle abut the strm center. 13

35 Since the mtin f significant upper- level circulatin features relative t the trpical cyclnes was required t assess pssible interactins, cnsiderable emphasis was placed n defining the lcatins f the features at each map time. Wrking best tracks f the upper- level cld lws and anticyclnes were cnstructed simultaneusly with the cnstructin f the streamlines. Initial psitins f the varius circulatin features were estimated frm the first set f streamlines that mainly cnsisted f a cmparisn f the wind bservatins and the satellite imagery. Spatial and tempral cntinuity cncepts were then applied in an iterative fashin t adjust the feature centers n the analyses. This iterative streamline analysis technique was utilized in bth the hrizntal and vertical directins, as bservatins at adjacent levels culd ften be used t better lcate a circulatin feature. After several iteratins using all f the available data, best tracks were cnstructed fr each f the upper-level cld lws (Fig. 1). Best tracks fr the upperlevel anticyclnes were nt cnstructed because f the small displacements f these features during the perid f study. In summary, the streamline and istach hand analyses prduced here ffered many imprvements ver the peratinal charts and enabled the study f upper- level frcing n Fl and Ed. While the increased number f bservatins was certainly significant, the spatial reslutin was mst imprved by the fur layer, subjective iteratin analysis technique. Tempral 14

36 u cn u ft (D (B m " ^ d r <D E cn -^ U %( * * «5 «h nj g > - 2 ^ <D G G rh a a cn $ u cn - fd c cn -G u cu c ^ ^^ "8 cn H a u_, rd _ TJ cn cn >i 04 H T3 03 CD '^4 U U H U D H T3 -i- 1 fd E- 1 rh i_> -H \ cn cn T3 >< h cn cu c < fa Q -H Q 15

37 reslutin was similarly imprved thrugh the cnstructin f analyses each 6 h. With imprved cntinuity in bth time and space, the develpment, mtin and decay f the upper- level circulatin features culd be described with mre cnfidence. Mst imprtantly, the reslutin was imprved t the degree that upper- level interactins culd be identified, tracked, and crrelated with intensity changes. 2. Crrelatin f upper- level frcing with intensificatin rates The final set f streamline and istach analyses and the crrespnding satellite imagery were carefully studied, and a detailed, subjective meterlgical summary was cmpiled fr bth Fl and Ed. Several aspects f the upper- level circulatin pattern were examined: (i) changes in the rientatin and level f maximum utflw f the trpical cyclne utflw jets; (ii) changes in the strength and psitin f the subtrpical ridge and subtrpical jet; (iii) changes in the rientatin and apparent strength f the upperlevel lng- wave trugh; (iv) changes in the hrizntal extent and mtin f the TUTT cells; (v) develpment and decay f bth indraft and utdraft anticyclnes; (vi) changes in the synptic -scale, backgrund flw pattern; and (vii) indicatins f any direct interactin between the tw trpical cyclnes. Each f these aspects was cmpared independently and in cmbinatin t changes in the bserved intensities f the 16

38 trpical cyclnes as determined by JTWC (e.g., Fig. 2). f specific interest was the degree t which changes in the upper- level flw pattern culd accunt fr the develpment f Fl t supertyphn intensity while Ed achieved nly slightly mre than minimal typhn strength. As a result f the careful study, three basic interactins between the trpical cyclnes and the upper- trpspheric envirnment were identified and subjectively crrelated with changes in the intensificatin rates. These are described in detail in Chapter III. The intensificatin rates used here are based n the pststrm interpretatins by JTWC in the Annual Trpical Cyclne Reprt (ATCR). JTWC uses the Dvrak pattern recgnitin technique and available bservatins t estimate the minimum pressure and maximum sustained surface winds (ATCR 1990). nly in a few special cases, such as penetratin f the eyewall by the NASA DC8 research aircraft, are direct intensity measurements available in the western Nrth Pacific. Estimates f intensity are prvided each 6 h (Fig. 2). As described by Frank (1987), the accuracy f these intensity estimates is difficult t assess withut direct bservatins, and thus the ability t accurately define the intensity change ver a 6 h perid is smewhat questinable. Fr this reasn, the study is primarily cncerned with trends in the intensificatin rates. An intensity errr f ±5 kt was assumed in crrelating changes in the upper- level frcing t changes in the intensificatin rates. 17

39 E 100 NS L30 L US Fig. 2. Best track infrmatin fr Supertyphn Fl frm ATCR (1990). 18

40 . A quantitative measure f the upper- level frcing was determined thrugh the use f a radial -band averaging technique similar t that described by Chan and Gray (19 82) A circle f radius 6 deg. lat. was divided int 16 radial sectrs and was gegraphically riented (versus a strmfllwing rientatin) n the best track psitins f bth strms. The speed and directin f the average wind crssing this circle were estimated fr each f the radial sectrs at each map time based n the final set f 200 mb streamline and istach analyses. Radial and tangential wind cmpnents relative t the strm center were then calculated using the average wind fr each sectr at each map time. The 16 average winds and radial and tangential wind cmpnents were then used t calculate the directin and speed f the mean wind crssing the circle and the mean radial and tangential wind cmpnent fr each map time. As described by Hlland (1987), a radial distance f 6 deg. lat. frm the strm center is well within the anticyclnic utflw regime that is characterized by lw inertial stabilty and hence lw resistance t radial mtin. It is within this regime therefre that interactins between the trpical cyclne and the envirnment that may prduce intensity changes are mst likely. Fr this reasn, the magnitude f the average radial wind cmpnent at each map time was interpreted as a measure f the mass flux acrss the circle (mass divergence). Such changes in the mass divergence 19

41 were subjectively crrelated with bth the develpment f the three interactin mechanisms that were identified and the intensity estimates prvided by JTWC (ATCR 1990). Graphs f the mass flux at 200 mb fr the utflw regimes f Fl and Ed are shwn in Figs. 3a and 3b respectively. The 16 radial and tangential wind cmpnents were als used t calculate the eddy flux cnvergence (EFC) f relative angular mmentum at 2 00 mb fr bth Fl and Ed using the relatinship EFC = v r v e = v r v e - v x ' where v r and v 9 are the radial and tangential wind cmpnents respectively. As described by DeMaria et al. (1991), changes in EFC are related t the interactins between the trpical cyclne and the envirnment. Thus, the changes in EFC at 200 mb fr Fl and Ed were subjectively crrelated with the develpment f the three identified interactin mechanisms and the strm intensity estimates. Graphs f the EFC at 200 mb fr Fl and Ed are shwn in Figs. 4a and 4b respectively. Finally, the mean 200 mb wind speed and directin crssing the 6 deg. lat. circle at each map time were subtracted frm the crrespnding best track curse and speed f the strm (ATCR 1990). The resulting vectr difference was interpreted as a measure f the bulk vertical shear f the utflw layer 20

42 ',q cd jj n H 4J T3 CN "W CD C 05 M -U J ffl 03 ~ > en -H CD 71 C 03 S x3 ^ 3 8 M 73 -H ^1 cd 5 en T3 en a) 5-1 u 03 rh l*-l cn en 03 e id S 6 a cn H U 03 2^ cn g 3 03 u u ^ u E- e u U 03 CD 5-1 CD > cn 2>.c x a, 3 cn x: 03 m,h cd jj 4-1 u cd cn Cu cn a 03 a) e cn 5-i CD -h sz &4 U-l JJ T3 CD CD (t3 cn 03 cn t3 CD.U 03 CD 6 cn h c u 3 cn u cn c a CD CD J JJ a c H -H (M 'J/ PI) ALISN31NI. (M) A1Q SSVW in CM i CM «- i m eg 21

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45 T3 W c x: & >i U 4-4 4J a X 5 C Cn -H fa (M a/m) UNVII AlliJNiJiNl (H) A1I 4 JND1NI S 5 UK) (S/) 0d3 I T 3 CM 00 24

46 - relative t the lwer trpsphere winds that are advecting the strm. Large values f the magnitude f this bulk shear vectr therefre indicated perids when the strms may have experienced tendencies twards disrganizatin f the utflw layer, which wuld represent negative upper- level frcing n the intensificatin rates. Thus, the magnitude f the bulk shear vectr was an additinal factr cnsidered in the subjective crrelatin f the changes in the upper- level frcing with crrespnding changes in the intensificatin rates. Graphs depicting the bulk shear vectr at 200 mb fr Fl and Ed are shwn in Figs. 5a and 5b respectively. 3. Tls fr examining the vertical structure f cld lws Interpretatin f the final streamline and istach analyses suggested that the vertical structure f significant upper- level circulatin features such as cld lws was imprtant fr gaging the upper- level frcing n trpical cyclne intensificatin rates. Tw methds, r tls, were used t better define the vertical structure f the cld lws: (i) spatial crss -sectins frm rawinsndes and satellite derived temperature retrievals; and (ii) time series f selected rawinsndes. The gal was t determine whether these features penetrated dwn t the middle trpsphere. In additin, a mre cmplete descriptin f hw the vertical structure f these features changed during the perid f study 25

47 T3 t3 U T3 H (D (0 3 CQ Xi cn >i JJ CN S id 2 c CD JJ C JJ a u cu a-" a> C/J CD u ij 0) M -U -Q H T3 rd 0) en -a 0) a, 5 m CQ cu e H JJ a fd m S X! Cn a -h a3 Cu cu 03 JJ 03 T3 J JJ cn t cu ^i "j as >. a cn H jj T3 JJ 03 rcj J ft3 L SZ rh T3 JJ 3.(B tj) CD ^ cu a cn h c 03 cn a u jj H cn c CD JJ h CU a3 c a, cn cn u CI <J/I>1) (l>l) AJ.ISN31N! (M) UV3IIS "1V0I1U3A MTTI8 8 < ^r pj a) 3 26

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49 wuld enable a better understanding as t what rle the cld lws played in the develpment and/r decay f the three interactin mechanisms that were identified. a. Temperature crss -sectins in space Spatial crss -sectins f temperature prvided an independent means f identifying and quantifying the vertical structure f the cld lws. Tw surces f temperature data were used t cnstruct the crss -sectins. First, upper-air sundings frm the final TCM-9 bservatinal data base prvided high vertical reslutin f bserved atmspheric temperatures. Despite the imprved spatial and tempral reslutin prvided by the additinal TCM-90 upper-air statins, the rawinsnde sundings were still widely spaced. Thus, the ability t describe the structure f the cld lws depended n frtuitus tracks ver upper-air statins. The paucity f statins east f 140 E was particularly debilitating t the cnstructin f crss -sectins thrugh cld lws. An imprved data density was prvided by the real-time TCM-90 satellite temperature retrievals. These sundings were btained by the NAA-TVS and DMSP-SSM/T instruments. Thicknesses fr the standard atmspheric layers were prduced by NESDIS using a statistical retrieval methd. The hypsmetric equatin was used t cnvert the thicknesses t layer-mean virtual temperatures that culd then be used t 28

50 cnstruct a nine- layer, satellite-derived temperature sunding frm which crss -sectins were cnstructed. Since the gal f cnstructing the crss -sectins was t describe the vertical structure f the TUTT cell, the carse vertical reslutin in these satellite sundings was f sme cncern. The quality r accuracy f the retrieval thicknesses in the trpics als is unknwn. Thus, the satellite sundings first were cmpared with the TCM-90 rawinsndes, which were degraded t match the layer-mean values f the satellite sundings. The rawinsnde data were degraded by using the height infrmatin fr the same levels used t define the layer thicknesses in the satellite sundings. The measured thicknesses were then applied t the hypsmetric equatin t derive a layer-mean virtual temperature fr each f the nine pressure levels. Temperature differences between the layer-mean satellite and degraded rawinsnde virtual temperatures were cmputed fr all clcatins between 00 UTC 13 September t 00 UTC 19 September within the regin 5-30 N, E. The clcatin threshlds were ± 6 h and ± 100 km. Separate statistics were cmputed fr the clear, cludy and partly cludy retrievals (Table I). Statistics fr an additinal categry cnsisting f the partly cludy and cludy retrievals (CY/PC) were als cmputed. Since the differences between the cludy and partly cludy statistics were negligible, the CY/PC statistics are shwn with thse f the clear retrievals in 29

51 / rvfi Table I. Temperature differences ( C) between satellite retrievals and clcated rawinsndes during TCM-90. ;>rfssi:re : LsU RETRIEVAL TYPE * vre ~l CLEAR Number f samples Bias RMS difference CLUDY Number f samples Bias RMS dijerence PARTLY CLUDY -Number f samples Bias RMS difference CLUDY PARTLY CLUDY -Number f samples Bias RMS difference Figs. 6a and 6b respectively. This limited intercmparisn is in excellent agreement with Kelly et al. (1991), wh indicated that the satellite temperature bias fr all retrieval categries is typically less than 1 C. Mre imprtantly, the RMS difference between the satellite sundings and the layer-mean rawinsnde values is typically between 1-2 C. Largest differences ccurred abve 175 mb near the trpical trppause and between 775 mb and the surface. These results suggested that temperature anmalies f 2 C r mre, such as thse assciated with upper- level cld lws, might be detected by the satellite 30

52 1! i i E a ] E i 700 ] T( 8 C) T( 8 C) 2 i 3 4. b. Fig. 6. Statistics f layer-mean virtual temperature difference ( C) between (a) clear satellite temperature retrievals and degraded rawinsndes, and (b) cludy and partly cludy satellite temperature retrievals and degraded rawinsndes. Clcatin threshld is ±100 km and ±6 h. Dashed line is bias and slid line is RMS difference ( C). 31

53 . - sundings Temperature deviatin crss -sectins were cnstructed bysubtracting frm each the crrespnding (midlatitude) standard atmspheric temperature fr each pressure level. Such deviatins facilitate the identificatin f temperature anmalies n the spatial crss -sectins. As shwn in Fig. 7a, the trpical temperature deviatins are generally psitive relative t the standard atmsphere. Negative deviatins, r smaller psitive deviatins, are cld anmalies and hence shuld mark the cld signature f a TUTT cell. n the ther hand, regins f large psitive temperature deviatin are regins f abnrmally warm temperatures, and such warm anmalies match the signature f warm- cre systems such as trpical cyclnes. The 14 C temperature deviatin ispleth in Fig. 7a indicates a cld anmaly centered slightly nrth f sunding This matches fairly well the psitin f TUTT cell CI as shwn in Fig. 7b and suggests a vertical signature dwn t apprximately 500 mb. Similar temperature deviatin crss -sectins were cnstructed frm bth the rawinsnde and satellite sundings fr the upper- level lws shwn in Fig. 1 whenever the rientatin and density f the bservatins were favrable. The vertical structure infrmatin derived frm these crss sectins will be used belw in the interpretatin f the interactin between the cld lws and the trpical cyclnes. 32

54 i i '} w ^ C2 J,! ED n.0 ^ % n Fig. 7. (a) Crss -sectin f satellite temperature deviatins frm standard atmspheric values thrugh western prtin f upper- level cld lw CI at 06 UTC 14 September. Abscissa is retrieval psitin number and rdinate is pressure level in mb. Cld lw CI is indicated by the regin f minimum temperature deviatin (CI). (b) rientatin f crss-sectin (bld slid line) in (a). Psitins f Fl and Ed are marked by the trpical cyclne symbls. 33

55 Jb. Time series f rawinsndes Time series f rawinsndes were cnstructed fr upper-air statins when a cld lw passed (e.g., Fig. 8). The time series depicts the hrizntal and vertical structure as the circulatin feature (assumed t be in steady- state) passes the statin. In Fig. 8, a TUTT cell labelled CI in Fig. 1 was suth f the upper-air statin (Marcus Island) and mving westward. The signature f the cld lw was first detected at 12 UTC 14 September by the veering f the winds abve 400 mb. At the clsest pint f apprach (CPA), which was directly suth f the statin at abut 12 UTC 15 September, a strng cld- cre signature extended dwn t abut 500 mb. The upperlevel winds cntinued t veer thrugh 00 UTC 17 September as the TUTT cell tracked westward. The strng cyclnic flw n the western side f the cld lw is dented by the kt bservatins at mb frm 06 UTC t 12 UTC 16 September. The cell appeared t have maintained a significant vertical structure thrugh 12 UTC 17 September, and then was nt detected as it mved away frm the statin. These rawinsnde time series prvide a much better depictin f the vertical structure f the cld lw than d the temperature crss -sectins as in Fig. 7a. A majr disadvantage f this methd is that it requires a frtuitus passage f a cld lw near the upper-air statin. Given the paucity f upper-air statins in ' the trpics, the pprtunities t cnstruct a rawinsnde time series f winds 34