INFORMATION TO USERS

This manuscript has been reproduced trom the· microfilm master. UMI films the text directly from the original or copy submitted. Thus, 'some thesis and dissertation copies are in typewriter face, while others may be from any type of computer printer.

The quality of this reproduction is dependent upon the quality of the copY submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleedthrough. substandard margins, and improper alignment can adversely affect reproduction.

ln the unlikely event that the author did not send UMI a complete manuscript and there are missing pages, these will be noted. Also. if unauthorized copyright material had to be removed. a note will indicate the deletion.

Oversize materials (e.g., maps, drawings, charts) are reproduced by seetioning the original, beginning at the upper left-hand corner and continuing from left to right in equal sections with small overlaps.

Photographs included in the original manuscript have been reproduced xerographically in this copy. Higher quality 6D x 9- black and white photographie prints are available for any photographs or illustrations appearing in this copy for an additional charge. Contact UMI d!rectly to arder.

Bell & Howell Information and Leaming 300 North Zeeb Road, Ann Arbor. MI 48106-1346 USA 800-521-0600

NOTE TO USERS

This reproduction is the best copy available.

• A STUDY OF TROPICAL TO EXTRATROPICAL CYCLONE TRANSITION IN THE ~t WESTERN NORTH ATLANTIC OCEAN, 1963-1996

by Christopher T. Fogarty

Department of Atmospheric and Oceanic Sciences McGill University ~Iontreal, Quebec

August 1999

A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfilment of the requirements of the degree of Master ofScience.

©C.T.Fogrurty1999

• National Library Bibliothèque nationale 1+1 of Canada du Canada Acquisitions and Acquisitions et Bibliographie Services services bibliographiques 395 Wellington Street 395. rue Wellington Ottawa ON K1 A ON4 Ottawa ON K1A ON4 canada canada VOUf file Votre ,.hiftH!QI

aur file Notre fefèlfHlœ

The author has granted a non­ L'auteur a accordé une licence non exclusive licence allowing the exclusive pennettant à la National Library ofCanada to Bibliothèque nationale du Canada de reproduce, loan, distribute or sell reproduire, prêter, distribuer ou copies ofthis thesis in microform, vendre des copies de cette thèse sous paper or electronic fonnats. la fonne de microfiche/film, de reproduction sur papier ou sur fonnat électronique.

The author retains ownership ofthe L'auteur conserve la propriété du copyright in this thesis. Neither the droit d'auteur qui protège cette thèse. thesis nor substantial extracts frOID it Ni la thèse ni des extraits substantiels may be printed or otherwise de celle-ci ne doivent être imprimés reproduced without the author's ou autrement reproduits sans son penmSSlon. autorisation.

0-612-55057-5

Canada • ABSTRACT The transformation of 45 tropical cyclones into extratropical cyclones over the western Nàhh Atlantic Ocean between 1963 and 1996 is studied. Cases are selected from the National Hurricane Center's "best track" archive. National Centers. for Environmental Prediction (NeEP) reanalyses of geopotential height data are used to construct a synoptic-dynamic climatology of extratropical transition. or "ET". The Forecast Systems Laboratory (FSL) upper-air archive of six near-track stations is used to produce sounding composites.

Primary results of the study follow.

1. A statistically-significant lOOO-SOO-hPa warm anomaly (with respect to the 1963-96 climatology) persists for the one-week period prior to the passage of the tropical systems into the Canadian Maritime provinces.

2. A northwestward extension of the surface subtropical anticyclone exists over the Canadian Atlantic Provinces during the two-day period prior to the arrivai of the cyclone.

3. The tropical cyclone's warm core and conditionally-unstable tropical airrnass are maintained after transition.

4. The presence of quasi-geostrophic forcing for ascent, typically seen in extratropical cyclones, is observed during periods in which the systems are still classified as tropical cyclones. This forcing for ascent continues during the extratropical transformation, and typically occurs ahead and to the left of the storm track.

• li RÉsUMÉ

• Ce travail est le résultat de l'étude de quarante-cinq transitions de dépressions tropicales ~~ dépressions extratropicales ayant eu lieu dans l'ouest de l'océan Atlantique nord entre 1963 et 1996. Les cas ont été choisis parmi les trajectoires lissées des archives du National Hurricane Center. Les réanalyses des hauteurs géopbtentielles ont été utilisées pour produire une climatologie synoptique et pour analyser le forçage quasi­ géostrophique iors des transitions extratropicaies (ET). Les données àu Foret.:ast Systems Laboratory (FSL) provenant de six sites de lancement localisés près des trajectoires ont été utilisées pour produire des composites de radiosondage.

Cette étude a révélé que

1. une crête anormale de l'épaisseur géopotentielle 1000-500 hPa se forme au-dessus du centre de l'Amérique du nord au moins une semaine avant la transition;

2. deux jours avant l'arrivée de la dépression, l'anticyclone subtropical stétend au-dessus des provinces maritimes du Canada;

3. le noyau chaud de la dépression tropicale est maintenu après la transition;

4. il Ya augmentation du forçage quasi-géostrophique pendant la transition et le forçage ascendant tend à s'exercer habituellement à l'avant et sur la gauche de la trajectoire.

• iii • TABLE OF CONTENTS

Abstract 0:, ii Résumé Hi Table of Contents iv Acknowledgments vi

Chapter 1 -Introduction 1 1.1 Extratropical Transition 1 1.2 ~Iotivation 2 1.3 Thesis Objectives 4

Chapter 2 - National Hurricane Center Archive and Case List 5 2.1 National Hurricane Center Archive 5 2.2 Case List 5

Chapter 3 - NCEP Composite and Diagnostic Reanalyses 18 3.1 The NCEP Reanalysis Data Set 18 3.2 Synoptic Climatology of ET 18 3.2.1 Data Compositing Procedures 18 3.2.2 Analysis ofSynoptic Climatology 19 3.3 Diagnostics of Quasi-geostrophic Forcing 25 3.3.1 Computational Procedure 2S 3.3.2 Examples ofExtratropical Transition 26 3.3.3 Example ofET Rainfall Pattern 28 3.3.4 Example ofET Cloud Pattern 29 • iv Chapter 4 • Upper-air Data Set and Sounding climatology 35 4.1 The Data Set 35 • 4.2 Sable Island Sounding Climatology 36

'p Chapter 5 • Tropospheric Structure During Transition 40 S.l Sounding Data Compositing Procedures 40 5.2 Results of Sounding Composites 44 5.3 Analysis of Convective Instability 52 5.4 Analysis of Thermal Advection 55

Chapter 6 • Summary and Conclusions 58 References 60

• v • ACKNOWLEDGMENTS Th~-, completion of this project would not have been possible without assistance from colleagues and friends. 1 would like to thank my supervisor Dr. John Gyakum for bis guidance throughout this study. 1feel we work very weIl as a teani, both contributing new ideas and techniques on the topic of extratropical transition. This was a very interesting project, and 1 thank Dr. Gyakum for allowing me to pursue my interest into scientific research. His enthusiasm and interest in this area of meteorological research provided me with more incentive to work hard to complete aIl of what we set out ta do, and ta achieve this, the final product.

It is certainly difficult ta name all those who have helped in any way - big or

small- throughout the course of my research, however, 1wish ta thank my colleagues in Dr. Gyakum's syooptic meteorology research group for their help over the past year. Marco Carrera, Rick Danielson and Werner Wintels were always willing to help with computer-related questions. 1 thank Werner and Rick for those many hsynoptic discussions" in the computer lab that spawned sorne of my research ideas. 1 especially would like to thank Ayrton Moraes for bis tutoring of a course that 1 needed to pass to advance to the research stage and Dr. Owen Hertzman at Dalhousie University for persuading me to continue with research. We appreciate Steve Miller's assistance from Environment Canada for providing us with the daily weather summaries.

1 would like to thank my colleagues, friends and family for their understanding during the difficult times, particularly during the early stages of research. My family has been with me all the way, helping in any way they cano

• vi • 1. Introduction 1.1 Extratropical Transition

Extratropical transition (ET) is defined as the transformation of a tropical cyclone (TC) into an extratropical cyclone. The process invalves an intrusion of drier air inta the innèc <...irculalion, jncn~asing asymnlèLry, and progressive 10s5 of the distinctive üpper• level circulation and warm-core structure of a TC (Sinclair 1993a). Brand and Guard (1979) define ET as a process by which a TCls primary energy source changes from ~.,'. latent-heat release to baroclinic processes. ET usually occurs when a tropical cyclone moves from the low latitudes into the mid-latitudes where it often "recurves" upon interaction with the westerlies. Recurvature is said to have occurred if the tropical cyclone'spath changes from a westward heading to an eastward heading while maintaining sorne poleward motion. Interaction with the westerlies implies that the tropical cyclone'spath has been influenced by the mid- to upper-tropospheric winds. and its structure changed owing to the presence of quasi-geostrophic forcing (e.g.. Bosart and Lackmann 1995).

The transition of a tropical cyclone into an extratropical cyclone often begins over the subtropical latitudes, 25° ta 35° in both hemispheres. ETs are most commonly found in the western North Atlantic Ocean off the coast of North America, western North Pacific Ocean off China and lapan, and western South Pacifie Ocean off New Zealand and Australia (Sinclair 1993b). This study focuses on ET in the western North Atlantic Ocean.

ET systems onglnating as tropical cyclones and undergoing transition to an extratropical cyclone can last for over two weeks. They often form as tropical waves off the coast of Africa and move westward to the coast of the United States, and eventually travel back across the Atlantic as strong extratropical storms that strike western Europe (Browning and Vaughan 1998). These systems often spend most of their time in the • tropical phase owing to weak steering flow environments in the tropics. In an observational study of western North Pacifie transition cyclones, Brand and Guard (1979) found that the time for transition ta occur varies from one to three days. The extratropical • phase of these cyclones was found to be about four or five days.

·-1 The extratropical system almost a1ways retains sorne tropical characteristics well after transition. This is evidenced by the large quantities of rainfall measured over Ireland and the United Kingdom from storms that were once hurricanes (Browning and Vaughan 1998). The original tropical cyclone seems to act as a seed for moisture in these extratropical storms. allowing for enhanced cyclogenesis in cases of re-intensification (a deepening of at least 15 hPa of the cyclone's central pressure after it has become extratropical (Klein et al. 1998».

The meteorological community has not yet accepted a general conceptual model of extratropical transition. Matano and Sekioka (1971) describe two observed transformational processes (complex and compound transition) for western North Pacific tropical cyclones that do not dissipate over land or water. Abraham and Parkes (1998) express the need for a conceptual model based on upper-level forcing and flow patterns. A conceptual model proposed by Klein et al. (1998) describes the behavior of the extratropical cyclone after transition (Le. does the extratropical cyclone dissipate, moderately re-intensify or deeply re-intensify?).

1.2 Motivation

Growing up in Nova Scotia, Canada, 1 often tracked hurricanes that would move northward from the tropics and affect our weather. 1 was intrigued by the fact that a once powerful tropical storm could travel so far north and still retain sorne tropical characteristics. This childhood interest bas fueled my ambition to research these types of storms. Many of the storms in this thesis were witnessed and documented when 1 was youager, making this work of special interest to me. • Meteorologists have always had difficulty forecasting the track and intensity of 2 tropical cyclones undergoing transition since Numerical Weather Prediction models designed to forecast weather in the mid-latitudes have difficulty when a storm of tropical • origin moves northward (Sinclair 1993a). The most common problem arises in precipitation forecasting, since the transformed cyclone can possess significantly more p moisture than a typical mid-latitude storm. Mariners are particularly interested in them because of the high winds and rough seas produced. Many of these sto'rms strike or graze the coastal United States (U.S.) and Canada causing millions of dollars in coastal property damage from heavy wave and wind action (DiMego and Bosart 1982; Bosart and Lackmann 1995).

An interest in ET also exists in Western Europe. where severa! transition systems arrived as extratropica! storms bringing large quantities of rainfall (Browning and Vaughan 1998). Sometimes the extratropical system can cause as much damage as a tropical storm or hurricane but over a much larger area. This owes to the fact that the horizontal extent of the storm's wind and precipitation fields tend to expand considerably (Sinclair 1993a; Brand and Guurd 1979). By the time the storm reaches Europe it has had more time to tap available potential energy from the mid-latitude baroclinic zone. It is rare for a tropical cyclone to directly strike Europe.

Little is known about the structure and dynamics of ET cyclones during transition. Sorne case studies have been conducted for famous storms (e.g. Tropical Storm Agnes in 1972: DiMego and Bosart (1982); Hurricane David in 1979: Bosart and Lackmann 1995). However, these studies did not concentrate on the dynarnics of ET. Their purpose was rnainly to study the influence of quasi-geostrophic forcing on the observed precipitation patterns witnessed over the continent during the extratropical phase. Interest in ET is, however, increasing. Many new papers on the topie have been completed during recent years and attention is now being focussed on producing a conceptual model of ET by diagnosing and studying transition using reanalysis data sets (Hart 1998). • 3 1.3 Thesis Objectives • Our study airns to find information about the tropospheric structure and meteorological processes of extratropical transition, which can be used to aid in weather forecasting',' particularly over the ocean and coastal regions off the eastern coasts of Canada and the V.S. Specifie goals are explained below.

Thè pritnary ObjèClivè is to construct à synoptk dinlatolùgy of ET èll1pluyillg a sample of 45 cases of transition cyclones in the western North Atlantic Ocean between 1963 and 1996. Using geopotential height data from the National Centers for Environmental Prediction (NCEP) 2.50 x 2.5 0 latitude-longitude reanalysis we will forrn a synoptic climatology of sea level pressure (SLP), 500-hPa geopotential height, and 1000­ 500-hPa thickness over North America, northwestern Europe, and the western North Atlantic Ocean. The climatology will allow us to diagnose synoptic-scale weather patterns associated with transitioning tropical cyclones.

Our second goal is to use the Forecast Systems Laboratory (FSL) sounding database to construct sounding composites by focussing our attention on the time period of transition. The data from six near-track sounding stations situated along the east coasts of Canada and the U.S. are used to sample the surrounding environments and inner structures of transitioning cyclones. A sounding climatology is produced for Sable Island (43.9~, 600W) and is used as a reference for the composites. We use the results to observe tropospheric changes in temperature, humidity, and winds.

Finally, we study how quasi-geostrophic forcing in the mid-latitudes affects the translUon process. Analyses of quasi-geostrophic forcing will he conducted using a Q vector representation of the frictionless and adiabatic omega equation (Hoskins and Davies 1978). Computations of the Qvector divergence field will be performed for a few selected cases of transition. • 4 • 2. National Hurricane Center Archive and Case List

2.1 National Hurricane Center Archive

Tne National Hurricane Center (NHC) "bèSllrul:k" (a SUbjèdivdy :,cl1uulhc:ù paLh depicting TC motion based on aIl available data; Maher 1999) data archive (1886-1996) was the source for tropical storm statistics in the Atlantic Ocean Basin. Our data set of 45 cases was chosen from this archive to investigate ET (1963-1996 inclusive). The data consist of latitude and longitude of the storm's center. central SLP in hPa, maximum sustained wind speed in knots, and status of the storm (e.g. hurricane, extratropical). The NHC data are obtained from reconnaissance aireraft dropsondes, surface buoy and ship data and satellite estimates of tropical cyclone intensity (Dvorak 1975).

2.2 Case List

Based on the NHC best track data, 45 tropical cyclones during the 34-year period from 1963 ta 1996 tracked along or just off the eastern coast of the United States and Canada passing through the 6° latitude x 6° longitude box centered on 40~, 690W (see Fig. 2.11). A storm is considered to have passed through the box if at least one of the six­ hourly positions was within or on the border of the box. A reference time "ta" is chosen as the time when the cyclone is nearest the center of the box. Thirteen cases were extended beyond the NHC period of U'acking using the NCEP reanalysis data, 20 using manual surface analyses from the National Meteorological Center (NMC), and 12 were not extended. During the extended NCEP period, the systems are extratropical since the NHC follows the entire traclcs of the cyclones until they at least lose their tropical characteristics. The NCEP track extensions were detennined by zooming in on the • cyclone centers using the General Meteorological Package (GEMPAK) display of SLP at 5 0.5-hPa resolution every six hours (Koch et al. 1983). The systems were tracked until we could no longer resolve the cyclone centers at O.5-hPa resolution. In the case of NMC­ '. extended traclcs the cyclone was tracked for as long as it existed in the analysis. The beginning~ completed 0:,tracks are displayed in Fig. 2.1 for all 45 cases. A plot of transition and ending of each cyclone is displayed in Fig. 2.2. The transition lime denoted by "T" on the map was chosen as the time of the last tropical stage of the cyclone. Figures 2.3 and 2.4 show storm tracks for the NMC- and NCEP-extended cases respectively.

The complete storm list is shawn in table 2.1 with location and stage of each cyclone at the reference time toP In our analyses and discussions, we refer ta four stages of a cyclone as shown in table 2.1. These stages are: tropical depression (TD), tropical storm (TS), hurricane (H), and extratropical (XT). A TD is any tropical cyclone in which

the maximum sustained surface wind speed (using a l-minute average) is 17 m S·l or less. A TS is any tropical cyclone in which the maximum sustained surface wind speed is 1 between 18 and 32 fi 5- • Maximum sustained surface winds must be 33 m S·I or greater for the tropical cyclone ta be classified as a hurricane. During the XT stage, baroc1inic forcing is presumed ta be the dominant source of energy.

Central sea level pressure traces for the 20 NMC-extended cases between 1975 and 1996 are displayed in Figs. 2.5 through 2.9 with estimated forward speed of the cyclone and storm track. The forward speed was estimated from the straight-line distance between consecutive track positions during the previous six hours. Central storm pressures from the NHC best track archive were available every six hours after 1974, but ooly sporadically before that time. National Meteorological Center (NMC) manual surface analyses were used to obtain the central SLPs beyond those of the NHC. The NCEP data were rendered insufficient for the central SLP analyses because the pressures were round to he much greater than in the manual analyses. Note that the NMC segments of the tracks, unlike the NHC segments, are not smoothed.

The official season for TC activity within the Atlantic Ocean Basin begins on June lh • 1st and ends on November 30 • The majority of TCs experience ET during August and 6 September (in comparison to the peak of general Atlantic tropic.J activity), as observed in Fig. 2.10 (a) of ET cyclone frequency based on our collection of storms. Another • observation is that there were ETs during every month of the hurricane season, with a slight skewness ofevents towards the summer months ofJune, July and August. ':of

From the NHC and NCEP track data we found the lifetime of ET cyclones appears to vary considerably. They cao last anywhere from a couple days to over three weeks (see Fig. 2.10 (b». The mean lifetime of our set is about 10 or Il duys, with cyclones spending an average of 7 days in the tropieal (TOt TS or H) stage and 3 to 4 days as extratropical systems. Twenty percent of the systems existed for two or more weeks. Hurricane Dora in 1964 moved slowly through the tropies and eventually stalled for three days in the Labrador Sea as an XT storm. The lifetime of the Dora case was 22 duys from lh lh August 28 ta September 19 • If Dora had eventually tracked back across the Atlantic as most systems do, she ceuld have lived even longer, perhaps 30 days. It is no surprise that on average ET cyclones have a longer lifetime than mest purely extratropical cyclones. A similar analysis using NMC-extended trades instead of NeE? (20 cases in Figs. 2.5 - 2.9) reveaJ very similar results.

• 7 .~ ... '5N • 70N 65N 60N S5N SON 45N .OH 35N 30N {l'- 25N 20N l5N 10N 100W

fig. 2.1 Tracks of 45 ET events based on NHC, N~C and NCEP data. The 6 degree latitude-longitude referenee box employed in the selecetion of cases is indicated, centered on 40.0 N, 69.0 W.

[ 75N...,.-...... ~~----~------~------. 70N \. . ..~. :::E~Z? .[,: EEE S5N ~ t,... [ SON ' . '[ E [ 4SN E 40N 35N 30N .. 25N 1 20N 1 l5N .... - -1 -8, . 1 10rOOW 90W 80W 70W 60W 50W 40W 30E

Fig. 2.2 Location markers for beginning. transition and ending for 011 45 ET cases. The 6 degree latitude-longitude reference box employed in the selecetion of cases is • indicated. centered on 40.0 N, 69.0 W. 8 ,,~ 75N -or-~~-----~------==r-~------...... , 70N • 65N SON S5N SON ~5N ~ON & . ~ Z~ (~ ~ ::: .",/ 1 -v~ 25N ~\N~"'"rop [l5Slon..' 20M .\ 9 .r~QISI rm~ . 15N ~.~~Hurriçanv' ) ~fxlrttttop~,al \ 1ON +--.,...... ,...... ,~...... ,r----r-~..;w.~~.....,...... --r-"~--r-.-.-...,.-~..,...-o..--f. t. 100W 90W 80W 70W SOW 50W 40W 30W 20W 10W 0 10E 20E 30E

Fig. 2.3 Tracks for the 20 NHC/NUC cases. The 6 degree latitude-longitude reference box employed in the selecetion of cases is indicaled, cenfered al 40.0 N, 69.0 W.

75N~~~~----~------~r------' 70N S5N SON S5N SON 45N

~ON~---" 35N 30N 25N 20N 1SN 1ON +--...... ~~...... ~-...-.- ...... --::p..;IIlimln~- .....--...... -'-l~---w...-...-...... --f 100W 90W 30r

Fig. 2.4 Tracks for the 13 NHC/NCEP cases. The 6 degree latitude-longitude reference box employed in the selecetion of cases is • indicated. centtrad at 40.0 N. 69.0 W. 9 Storm Name t(O) (YY MM DO HH) Location Stage

Ginny 6310 2912 40.8N, 67.2W H • Dora .. 64091418 40.6N,68.1W XT Gladys 64092400 39.2N, 69.0W H , unnameeJ .. 65061706 38.4N, 69.4W XT Doria .. 67091512 38.0N, 68.9W H unnamed .. 68 09 16 18 37.3N, 66.5W TS Gladys 6810 21 00 38.6N,S8.3W H Anna .. 69080312 39.0N,69.5W TS Blanche 69081200 38.6N,68.0W H Camille •• 690821 12 37.3N,68.4W TS Gerda •• 6909 0918 40.1N,69.9W H unnamed .. 70081806 37.0N,72.5W TS Arlene 71 07 0612 39.6N,68.3W TS Beth •• 71 081512 39.7N,67.2W H Heidi •• 71 0914 06 39.4N,69.3W TS Carrie 72 09 0318 40.6N,70.2W TS Alfa •• 73 08 01 OS 40.5N,69.4W TD Dolly 74090500 40.8N,67.5W TS Amy· 750702 00 37.4N,66.7W TS Blanche • 75 07 2800 39.3N,67.2W H Gladys • 7510 02 18 37.8N,67.0W H élla • 78090406 38.0N, 66.0W H Charley • 80 08 23 06 38.9N,66.7W H Cindy • 81 0803 06 37AN, 66.2W TD Dennis • 81 0821 06 37.8N,68.0W TS unnamed • 81111612 4O.0N,69.5W TS unnamed • 82062000 39.5N,70.0W TS Diana • 84 09 15 12 38.5N, 70.3W TS Ana • 85071800 37.2N,66.8W TS Chartey • 860819 06 4OAN,69.1W TS Alberto • 8808 07 06 40.0N, 70.8W TD Bertha • 90 07 31 06 37.5N, 66.5W H UI/· 90101412 40.0N, 67.5W TS Ana • 91 070406 37.1N,67.8W TS Bob • 91 081918 41AN,71.4W H unnamed 91 1031 00 40.0N,68.5W XT Emily • 93090200 39.0N,68.5W H Allison 95060718 39.8N, 69.2W XT Barry .. 95 07 09 00 38.7N,66.0W TS Felix or. 95082112 39.0N,66.1W TS Arthur 960621 00 37.3N,70.3W TD Bertha 96071400 42.1 N, 71 .9W TS Edouard • 9609 02 06 39.8N, 69.4W H Hortense • 96 091412 38.SN,67.1W H Joseohlne 9610 09 06 41 .ON. 71.0W XT

Table 1.1 List oC cases tr:Iàed by the NHC and NMC(." by 1bc NHC and NCEP (--). and solely by 1bc NHC (00 ->- Refermee timc -t(0)" dcDotes 1bc lime wbel11be storm is ocare:st40-N. 69~. Stagcs are: H• Hum:aœ. TS • Tropial Storm. 'ID • Tropil::a1 DepresaoD. XT· Ex1rauopical as defiDed inthe cext. • 10 P(hPa) AMY 1975 S(mlS) 1020 __------"T3S 1010 30 1~ ~ • =t:=~S~~ç::=~It:JQ20 970 15 960 10 9SO .J---.--..---~:...... ---'olt.H__\litH__I+1HH 9040 5 930 0 N ~ ~ ~ 8 8 ~ ~ N 8 ~ ~ ; j ~ 1~ ~ ~ ~ 1~ ~ ~ TIME (MMlDDIHH) a b

P(hPa) BLANCHE 1975 s(mls) 102t] 35 1010 30 1~ ~ 990 ~----~:::______:ltfl"'!==-----i 900 20 970 1S 960 10 :: t;~~:::::::::::=::::==!=tr.:l5 930 0 ~~~~8~~~~~8~8~i ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ § o 0 000 0 0 0 0 0 0 0 0 0 0 c TIME (MMlDD/HH)

P(hPa) GLADYS 1975 S(mlS) 1020 35 1010 30 1~ ~ 99O-f----:'---,....~.___--I_-\-_+H ~ 20 970 15 960 10 950+--~--t'...-~ __o:::::::::-~'t-f---irt-l 9040 5 930 0 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ i ~ 1~ i ï1~ ~ ~ ~ ~ e TIME (MMIODIHH)

P(hPa) ELlA 1978 S(mlS) 1020 _------__,. 35 1010 30 1~ ~ 990 I---~------_Hr____~ 980 20 970 15 960 10 950 I------..,Pt.t:-----P~---~ 940 5 930 0 1ii;Iii 11 g TIME (MMlDOIHH)

Fig. 2.5 Left panels: Central sea level pressure traces, P (heavy), and forward speed, s (light), of (a) Amy, Cc) Blanche, (e) Gladys, and (g) Ella before and after transition based on six-hourly NHC and NMC track positions. Composite reference time (t(O» and transition time (ET) are shawn below the pressure trace• RÏ1mt panels: NHClNMC.::J)roduced storm tracIcs for the entire life cycle of (h) Amy, (dl Blanche, (f) Gladys, • ana (h) Ella. Rectangle otFCape Cod denotes the t(O) reference box. Il P(hPa) CHARLEY 1980 s(mls) 1020 ~,..------t+----:13O__------rr----r35 1010 1000 ---;;:~--I::.--_I:I!IfIII~~~25 • 990 -I-__ 9BO -I----=---~~_t__...._-""20 970 -I------;I':....-..a.-,If--l.-----+ 15 960 10 950 t-::=-=-b""""---::;;=:::j~---....w-T15 940 930 ~ ..-...... _---_~_~O t4 i i § 1 a b

P(hPa) CINDY 1981 s(mls) 1020 35 1010 - 30 1000 - '-. -1(0) 25 1 • 990 20 980 .-. 970 15 .- 1 "' 960 ..,,- 1 10 950 '- ~ v 5 940 - 930 o N ~ i N N ~ ~ i0 1 0 0 i 1110 C TIME (MMlOOIHH) d

P(hPa} DENNIS 1981 s(mls) 1020 35 1010 30 1000 990 25 980 20 970 15 960 10 950 940 5 Q30 0 ~ ; ~ ; ~ ; § ; ~ ; ~ ~ 1 ~ ~ ~ §1;~ § ~ i i ~ ~ ~ ~ ~ § TIME (MMIOOIHH) e f

P(hPa} unnamed 1981 s(mls) 1020 35 1010 30 1000 1 ...... ~. 25 990 .--'~I 980 ...... 20 -"'i'IO\ 970 15 i60 .-/\. / 10 950 !MO -" 5 930 o ~ ~ ~ ~ ~ ~ ~ ~ ~ 5 ~ ~ ;; ~ ~ ~ ~ ! 1 1 1 ! ~ 1 g TIME (MMlDDIHH) h

Fig.2.6 As in Fig. 2.5 but for Charley (a and b), Cindy (c and d), Dennis (e and f), and Subtropical Storm • #3 - 1981 (g and hl. 12 P(hPa) unnamed 1982 s(mls) 1020 ,....------r 35 1010 30 1000 25 990 f-~-~~---~ --..,I!!-9 • 980 ...... 20 970 15 960 la 950 ~--.ll.o'_--~v_.:.bo'_-~.,.",....~I-Hi 940 5 930 a 1i 11111111 1 a TIME (MMIODIHH) b

P(hPa) DIANA 1984 s(mls) 1020 .,....------r35 lOlO 30 1000 25 99O.f--...... ;;;;~--_I_~1JIII!!E:.__1...... ,,..._-__1 980 20 970 15 960 ~--_+_#_--._I__JoJI---~"-"___i 950 10 940 5 930 0

c d

P(hPa) ANA 1985 s(mls) 1020 .,....------__ 35 1010 30 1000 +---~:::IiIE"""=~_ _,,,...l8Ill....~ 990 25 980 20 910 15 960 la 950+----f''-----.x...;'1---~ 940 5 930 a

e f

P(hPa) CHARLEY 1986 s(mls) 1020 r-....------,. 35 lOlO 30 1000 I---~-__'_=.llli<_-___,,_-_.l~ 990 ~ 980 20 970 15 960 10 9SO f------7""Il;:-=A:."""""",....-J.:...... -----::l~r_--11 940 5 930 0 ~ ~ ~ !::t ~ ~ ~ ï ~ ~ N l:t ~ ~ ~ ; ~ ~ ~ 1i ~ i ~ 1i 1i g TtME (MMIOOIHH)

Fig. 2.7 As in Fig. 2.5 but for Subtropical Stonn #1 - 1982 (a and b). Diana (c and d). Ana (e and f)t and • Charley (g and hl. 13 P(hPa) ALBERTO 1988 s(mls) 1020 35 1010 ...... 30 1000 1(0) - 25 990 • 980 20 970 -"""- 15 960 , 10 950 - - -- r 5 ~ - --- o 930 IUUIIIII!U TIME (MMlDDIHH) a b

BERTHA 1990 s(mls)

.fI!!!!!!~---~------__:0iII'1-3O__------y35

~----~ .....=-----___:~--125 20

t======:s~~=~ 15 +------.....;,:,;;:....--7f/t---.::t 10 ~~=_=_~ .....__....;;;;;;...... c;...--=::::=-1IC...-_r__r_-,5....._ ...... -...... 0 ~ ~ ~ ~ ~ ~ ~ ~ ~ ;::: ~ ~ ~ ~ § § o 0 0 0 0 1i i TIME (MMIODIHH) d

P{hPa) UU 1990 S(mlS) 1020 .,.------,- 35 1010 30

1= C~~~~s;;:;:j~~~~~i:tt25 980 20 970 15 960 10 9SO 5 9

P(hPa) ANA 1991 s(mls) 1020 35 1010 30 1000 25 990 980 20 970 15 960 10 ./".O1/D1/1 71. 950 5 ...... -.,v'7""'a.~~'l2Z "- M) 930 0 ~1~S~8~8~8~B~8~ § I~~~~~~~~~~~~ g nME (MMlDDIHH) h • Fig. 2.8 As in Fig. 2.5 but for Alberto (a and b)t Bertha (c and d), Lili (e and f)t and Ana (g and h). 14 P(hPa) BOB 1991 s(mls)

1020 ~==:::::::::=:::e;:;;;;;;~~3S 1010 ~ 3O 1~ ~ 990 +-~_"""''---''K.- -I • 980 20 970 15 960 10 950 +-~:RJ#tr-----\'-----=-'f'~H 940 5 ~ 0 § ~ ~ ~ § ~ ~ ~ § ~ ~ ~ § § i i ~ i § ~ § § §§ ~ ~ ~ ~ TIME (MMlDDIHH) a b

P(hPa) EMILY 1993 s(mls) 1020 ...... _-=------~35 1010 30 1~ ~ 990 +----or------.,r----I 980 20 970 15 960 10 950 +,.,~------F-~..---..-.l 940 5 m 0 ~ § ~ ~ ~ i § ~ ~ ~ i ~ ~ ~ ~ § § § ~ § ~ ~ ~ 1 TIME (MMIODIHH) c d

P(hPa) EDOUARD 1996 s(mls) 1020 ...... ------~35 1010 30 1000 t-----::~------__:=___,H 990 25 980 20 970 15 960 t----t---.cA---'!J!!.-A-----:J1.H 9!O 10 940 5 ~ 0 ~ ai ~ 1i 1III 1Iii 1; e TIME (MMIOOIHH) r

P(hPa) HO~TENSE 1996 s(mls) 1020 -r------~ 35 10101000 ~~~~;:~======:;jç 30 990 25 980 20 970 15 960 10 950 f-'lIl:::~~--::::~---_1k_:1..I_-=-.iIH 940 5 ~ 0 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ III III 1~ j i ~ i i g TlME (MMIOOIHH) h • Fig. 2.9 As in Fig. 2.5 but for Bob (a and b), Emily Cc and d), Edouard (e and 0, and Hortense (g and hl. 15 • Monthly Number of Transitions (45 cases, 1963-1996) 16 14 II) 12 enC1J ca u 10 -...0 8 .cCD E 6 :::3 z 4 2 0 J J A S 0 N a Month

Ouratlon of ET Cyclones (45 cases, 1963-1996)

12 Mean Durations: 10 Tropical Phase: 7 days en Extratropical Phase: 3-4 days CDrn 8 ca u -..0 6 CD .Q E :s 4- Z 2

0 1-3 6-9 9-12 12-15 15-18 18..21 21·24 b Range ln days (lncludes lower Iimlt)

Fig. 2.10 (a) Monthly number ofexttatropical transitions during the Atlantic Basin hurricane season. The months are June through November. (b) Duration in days of all • cyclones studied. from the beginning oftheir tropical to the end oftheir extratropical phases. 16 ',. • r •

.....

liJ "

"45 NORTH ATLANTIC' 4" OCEAN····

3 .' ::i So~i,j~'tf.~twork

('

5 ~?'-~~tH-A'ME~

~ ~ ~. .~11 ",2·5 . -• !...!L...: -90 ~85 _ 0 l 7 5 7 ri-- -1- 6 /--5 '5 ( "/5 0 _ ..t "\ -40 -l" - 3 0 Fig. 2.11Locator map for references in the text. loset: upper-air sites used in chapter 5; reference box used for synoptic climiltology in chapter 3. • 3. NCEP Composite and Diagnostic Reanalyses ., 3.1 The NCEP Reanalysis Data Set

Six-hourly (0000, 0600, 1200, 1800 UTC) gcopotcntial hcight data (1000, 850, 500 hPa) on a 2.5 0 latitude x 2.5 0 longitude grid (144 x 73 global grid points) were obtained from the National Centers for Environmental Prediction (NCEP) reanalysis Grid Point Data Set (Kalnay et al. 1996). These data were used to develop a synoptic climatology of ET and to produce diagnostics ofquasi-geostrophic forcing.

3.2 Synoptic Climatology of ET

3.2.1 Data Compositing Procedures

A reference time (10) is chosen as the time (based on six-hourly position~) when the cyclone is nearest the center of a 6° latitude by 6° longitude box centered on 40~, 69OW, southeast of Cape Cod, Massachusetts (see Fig. 2.11). The cyclone at this time may be either tropical or extratropical. This domain samples a region of most dense cyclone traific during our study period (see Figs. 2.1 and 2.11).

Composites of SLP, SOO-hPa geopotential height and IOOO-500-hPa geopotential thickness are computed over North America, northwestem Europe, and the North Atlantic Ocean using the NCEP gridded reanalysis data. The anomaly field is computed from the difference between the ET synoptic climatology field (45-case composite) and the long­ term climatology field (1963-1996) from to-4 days to to+3 days. The statistical significance of the anomaly field is determined from the Student's t-test (see e.g., • Wonacott and Wonacott 1990, ch. 8), where a "t-number" is formulated as 18 ± t =(U - jJ.).{N (3.1) • s where U is the sample mean for all cases at each grid point, J.l is the climatological mean, .., N is the sample size (45 in our study) and s represents the standard deviation of the

sample (S2 =variance). Larger values of t yield greater statistical significance.

3.2.2 Analysis ofSynoptic Climatology

Fig. 3.1 displays the synoptic climatology fields from ta-4 days to 10+3 days (we refer the reader ta Fig. 2.11 for the reference box). The contoured fields are SLP (heavy solid), 500-hPa height (light solid), and 1000-500-hPa thickness (light dashed). In CA) there is no indication of the tropical cyclone owing ta smearing in the compositing procedure. The most obvious feature is the subtropical anticyclone in the Atlantic Ocean. During the period leading up to 10 (A through E) the composite cyclone appears off the O.S. southeast coast and migrates northeastward. Geopotential height and thickness troughs foern over the central V.S. and move eastward, merging with the cyclone near ta. After merging with the mid-latitude flow (F through H), the cyclone races off to the northeast while the geopotential height and thickness troughs weaken.

Fig. 3.2 shows the SLP anomaly field from 10-4 days to 10+3 days. Early on (A to C) a significant trough-ridge pattern forms in the eastern North Atlantic, before the appearance of the composite ET cyclone. In CC) the composite ET cyclone appears as a significant 3-hPa negative anomaly east of Florida. AIso at this time a significant ridge anomaly develops over the Maritime Provinces, completing a trough-ridge-trough-ridge sequence between the southeastem U.S. and northwestem Europe. As the cyclone moves northeastward, the Maritime ridge anomaly moves over Newfoundland and spreads northward and southward (see E). After to+l day this ridge anomaly and composite ET cyclone disappear, yet an area of negatively anomalous SLPs persist in the eastem North • Atlantic offthe western coasts ofFrance and Spain. 19 Fig. 3.3 displays the 500-hPa height anomaly fields from 10-4 days to te+3 days. The contoured fields are 500-hPa geopotential height (light solid), anomaly of height • (thick solid and dashed), and statistical significance (95% light shading, 99% dark shading). A large area of positively anomalous heights (+43 to +88 m) appears over the 'p eastem half of Canada, Iceland and northwestem Europe throughout the time period CA through H). As 500-hPa ridging develops ahead of the surface cyclone (see D), the

eastem Canadian ridge anomaly (+77 m) becomes enhanced over the Maritime Provinces. In (E). the cyclone appears as a significant negative anomaly south of Cape Cod while the ridge anomaly (+88 m) intensifies over eastern Canada. In (F) through (H) this ridge

anomaly weakens over eastern Canada, but still exists at to+3 days. From (B) ta (G) we notice anomalous troughing (minimum of -60 m in CF» downstream from the ridge which is observed ta weaken after te+ 1 day.

To conclude the synoptic climatology we present IOOQ-SOQ-hPa thickness anomalies in Fig. 3.4 from lo-4 days to te+3 days. The contoured fields are IOOO-SOQ-hPa thickness (light solid), thickness anomaly (thick solid and dashed), and statistical significance (95% light shading, 99% dark shading). In (A) and (B) we observe a broad area of positively anomalous thickness (-+3°C) across central North America. This anomalous region is observed at least two weeks prior to te (not shown). In (C) and (D) an isolated anomaly (-+3.5°C) associated with development of a thermal ridge forms over western Quebec then moves eastward. AIso during this time an anomalous tmckness ridge (> + I.SoC) forros over Iceland and nonhwestern Europe. In CE) and CF) the Quebec anomaly is strongest over Atlantic Canada. In (G) and CH) the anomaly quickly weakens to -+2°C and merges with a broad positively anomalous area over central Canada. A depletion of the northwestem Europe anomaly ta less than + 1.5°C is aIso noticed after te (see E through H).

• 20 •

Fig. 3.1 Composite SLP (beavy solid contours), SOO-hPa geopotential height (thin solid contours) and l000-S00-hPa geopotential thickness (thin dashed contours) for A) t-96 hrs, B) t-72 hrs, C) t-48 hrs, D) t-24 hrs, E) t=OO hrs, F) t+24 hrs, G) t+48 hrs, and H) t+72 hrs. SLP contour interval is 4 hP~ SOO-hPa • height and lQOO-SOQ..hPa thickness fields al S-dam intervals 21 •

(AII-96IU' {BI 1-1~bn

(lU I-OObn IFII+Hhu

(a) 1.41bu tHll+7:zbn

Fig. 3.2 Composite SLP anomaIy (thick solid contours every 2 hPa) and SLP (thin solid contours every 4 hPa) for A) t-96 hrs. B) t-72 hrs. C) t48 hrs. D) t-24 hrs. E) t=OO hrs. F) t+24 hrs. G) t+48 hrs, and H) t+72 hrs. Shading denotes statisticaI significance of the anomaly fieid at 95% (1ight) and 99% (dark) con­ • fidence leveIs. P symbolizes a positive SLP anomaly and N a negative SLP anomaly. 22 •

...... "/ '.:------"- "L .6G • ~ll'e-:;'Q ~

(KI 1.71lln

Fig. 3.3 Composite 500-hPa geopotential height anomaly (thick solid contours every 30 m) and S()()"hPa geopotential height (thin soUd contours every 6 dam) for A) t-96 hrs, B) t-72 hrs, C) t48 hrs, D) t-24 hrs, E) t=OO hrs, F) t+24 bIs, G) t+48 hrs, and H) t+72 hrs. Shading denotes statistical significance ofthe anomaly field at 95% (light) and 99% (dark) confidence levels. P symbolizes a positive height anomaly • and N a negative height anomaly. 23 •

,0' . ,.,. .

"

.60 .'0 .40 .20

(AII.96bn (811-12hu

(CII·...bn

.., . ,. " .

·60 .SG .40 ·-30 'o ..• (El I-OOlln

Fig. 3.4 Composite lOOO-SQO-hPa geopotential thickness anomaly (thick solid contous every 30 m) and lQOO-SOO-hPa geopotential thickness (thin dashed contours every 6 dam) for A) t-96 bIs. B) t-72 hrs. C) t-48 brs. D} t-24 hrs, E) t=OO hrs. F) t+24 hrs. G} t+48 hrs. and H) t+72 hrs. Shading denotes statistical significance ofthe anomaly field at 95% (light) and 99% (dark) confidence levels. P symbolizes a positive • thickness anomaly and N a negative thickness anomaly. 24 • 3.3 Diagnostics of Quasi-geostrophic Forcing 3.3.1 Computational Procedure ·f

To study and diagnose the effects of quasi-geostrophic (QG) forcing on the extratropical transition process. \ve use the Q vectûc rèpre~entation oi the quasi­ geostrophic omega equation (Hoskins and Davies 1978; application: Bosart and Rogers

1995). The omega equation (see e.g. t Bluestein 1992 ch. 5) used to diagnose synoptic­ scaIe vertical motion neglecting the effects of diabatic heating and friction is

2 "2V +---,f} a } =---l-V,·Vf, a r t'7 (~r +[)]__Rd t'7'VM/(_V.·vt'7 T). ( p a OPM a dp ,~ p g (Jp 1 ); P (3.2)

The Qvector representation follows

(3.3)

where

Q =--Rd (aV-_.R V T-·vav, t'7 T ) =(Q Q) ap ax p' dY pIt 2· (3.4)

In the above equations,fo is the Coriolis parametert 0' is a static stability pararneter (-T[aln8/dp]) where e is potential temperature), co = DplDt represents the vertical motion, çg is the geostrophic vorticity, Ri is the universal gas constant for dry air, Vg • denotes the geostrophic wind vector, and T is temperature. 25 Using only the NCEP reanalysis height data at 850 and 500 hPa, we can compute fields of Q vector divergence (forcing for descent) and convergence (forcing for ascent). • Vg in 3.4 is given by

v =1...(_ az az) =(u v) (3.5) Il f ay ,ax g' K

and T in 3.4 is given by

T = Z500 - 2 850 g , (3.6) In(850) R 500 li

a fonn of the hypsometric equatian. Z is geopotential height. An average of 850- and 500-hPa heights is used in equation 3.5. 675 hPa thus becomes the reference level far Q. Equation 3.6 serves as an estimate of 675-hPa temperature. Since the tropical cyclonic circulation tends to be confined to the lower tropasphere we feel using 850- and 500-hPa data will allow us ta better represent the thermal advection component of quasi­ geostrophic forcing.

3.3.2 Examples ofExtratropical Transition

We observe various types of transition among cases in the sample. A strong mid­ latitude baroclinic zone (850-hPa temperature gradient of 5°C per 3° of latitude or more) and deep mid-Ievel long-wave trough accompanied many transitions (approximately 50%). Other cases involved the merging of tropical cyclone with only a weak baroclinic zone without a trough or surface front (approximately 25%). Rarer cases where the cyclone merged with a closed or cutoff mid-Ievel cyclone (at least one closed 500-hPa geopotential height contour based on 60-meter contour intervals), or moved into a large­ • scale ridging environment aIso appear in the case list. 26 The first ofour examples of ET is shown in Fig. 3.5 for Tropical Storm Josephine in October 1996 (see Pasch and Avila 1999). The contoured fields are 675-hPa Q vector • divergence (thick solid - negative. thick dashed - positive). SOO-hPa geopotential height (thin solid). and 675-hPa temperature (thin dashed) at 12-hour intervals from 18 hours ·of before ET ta 18 hours after ET. An ascentldescent forcing couplet is noticed in Fig.

3.S(A) 18 hours before transition. This forcing would resu1t in larger magnitudes of Cl) than for the same amount of forcing in the mid-latitudes awing to the 10wer statie stability of the subtropics. The weak baroclinicity within the storm environment allows for sorne thermal advection to the east and west of the cyclone, hence the presence of forcing far vertical motion. In (B). it becomes more apparent that the storm is merging with a mid­ leve1 long-wave traugh to the north. and that QG forcing is increasing. After transition and landfall on the Florida Panhand1e, Josephine races to the northeast accompanied by increasing QG forcing and a deepening of the trough. This deepening implies more cyclonic vorticity advection over the surface cyclone, which aids in its maintenance. or potential re-intensification. This intensification was abserved with Josephine as she moved along the eastern coast of the U.S. In (0) the magnitude of the forcing is very typical of many extratropical systems (e.g.• Sames and Colman 1993; Basart and Rogers 1995).

Figure 3.6 shows the transition of Hurricane Allison in June of 1995 (see Lawrence and Mayfield 1998). The contoured fields are as in Fig. 3.5. Allison followed a path similar to Josephine's. but note the much weaker magnitudes of forcing. and the slower forward motion. There are no distinct synoptic-scale troughs affecting Allison. The small short-wave trough over the storm is directly related to the surface tropical system. Magnitudes of QG forcing are less than half of those seen for Josephine in 1996. It is interesting to note. however. that Allison did re-intensify as an extratropical system once she moved over the Atlantic Ocean off Cape Hatteras. The warm, moist atmosphere most likely aided this re-intensification over the Gulf Stream. In general. however, tropical cyclones merging with weak baroclinic zones are unlikely to undergo • re-intensification. 27 Figure 3.7 displays the QG forcing associated with the transition of Hurricane Gladys in 1968. The contoured fields are as in Fig. 3.5. Only two out of 45 events • underwent this type of transition - the other was Hurricane Bertha in August 1990 (see Mayfield aqd Lawrence 1991). A short-wave trough is seen over the Great Lakes in (A) while Gladys is still a hurricane, not yet merged with the mid-latitude flow. The merger begins in (B) as the short-wave trough approaches from the west. Àfter transition, the trough becomes eut off and rapidly inherits the cyclone. The QG forcing couplet is observed to rotate cyclonically about the surface cyclone position. From this analysis one may conclude that there was heavy rainfall over the province of New Brunswick and the state of Maine during the transition of Gladys. This type of transition has received the most attention in the past. One example is Tropical Storm Agnes (not in our case list) in 1972 (DiMego and Bosart 1982) when a deepening trough became eut off and captured much of the moisture associated with Agnes off the East Coast of the V.S. Extensive flooding resulted, accentuated by the rugged terrain of the Appalachian Mountains in the eastem V.S.

Another less common type of transition is shawn in Fig. 3.8 for that of Hurricane Edouard in September 1996 (Pasch and Avila 1999). This type of transition occurs when the tropical cyclone moves into a ridging environment (e.g. at 500 hPa). There are no

troughs influencing the motion of the storm. These cases often stall or drift slowly [0 the east, as was the case with Edouard (see Fig. 2.9 (e) and (1). Although this type of transition often results in rapid dissipation of the cyclone after transition, locally heavy amounts of precipitation may fall owing ta the slow motion of the storm. There are significant values of QG forcing for ascent even before transition (see B), over Nova Scotia, where rainfalls exceeded 200 mm. After transition, forcing became weak owing ta the lack of upper-Ievel cyclonic vorticity advection.

3.3.3 Example ofETRainfall Pattern

Figure 3.9 shows the QG forcing and rainfall accumulation patterns associated • with Hurricane Blanche in July of 1975 (Hebert 1976). The contoured fields are the same 28 as those in the previous analysis. In (E), 12-hour rainfall accumulations (in mm) are shawn for select observing sites in the Maritime Provinces during the passage of Blanche. • Note in (A), (B) and (C) how the synoptic-scale forcing for vertical motion tends to be located to .the., northwest of the storm (left of the track). This skewness in large scale forcing seems to show up in the rainfall patterns throughout the Maritime Provinces. East of the storm track there appears to he a lack of moisture and hence small accumulations are noted in Halifax, and Sydney, Nova Scotia. Such tendencies for heaviest amounts of precipitation to be to the left of the storm track were observed for other events as weIl.

3.3.4 Example ofET Cloud Pattern

Ta show the relation between cloud and QG forcing field we present Hurricane Bob of August 1991 (Pasch and Avila 1992) in Fig. 3.10. The contoured fields are as before. Panel (A) corresponds with panel (E), (B) with (F), and so on. The cloud imagery is a grayscale of Advanced Very High-Resolution Radiometer (AVHRR) imagery (Gallagher and Schubert 1998). The cloud does not perfectly match up with the ascent field because much of the cloud may be owing to sub grid scale convection and cirrus cloudiness. Still, the major cloudy regions correspond ta Q vector convergence, for example, to the north and east of Bob. The familiar features (e.g. Sekioka 1970) associated with transition are the development of warm and cold fronts whose presence are suggested in Bob's cloud field, a displacement ofconvection to the north and west and clearing south of the eye or cyclone center (other cases were analyzed but are not shown here). The clear area wrapping around ta the south of the storm is evidenced in the forcing field with descent to the south and west. By 1800 UTC on August 19th the cloud field appears ta be reminiscent of extratropical cloud patterns with developed fronts and displaced c10udiness north ofthe center. Transition was not declared by the NHC unti118 hours later (see Fig. 2.9 (a) and (b». Judging by the large magnitudes in QG forcing (up 16 1 3 to ± 22 x 10- hPa- s- ) during NHC's tropical phase we feel the transition of Bob was declared too late. Such values of QG forcing are comparable ta mid-latitude cyclones • (Bames and Colman 1993; Bosart and Rogers 1995). 29 Fig. 3.5 QG forcing during extratropical transition of Tropical Storm Josephine. 2aV· Q field (thick contours) every 3.0 x 10. 16 hPa-ls·J (thick solid « 0): forcing for • ascent; thick dashed (> 0): forcing for descent) for A) 18 hrs before transition, B) 6 hrs before tran~ition, C) 6 hrs after transition, and D) 18 hrs after transition. 500-hPa geopotential field every 60 m is light solid. 850-500-hPa layer mean temperature is light dashed field every 5 Kelvin. The reference Ievel for the 20V .Q field 'is 675 hPa. Surface

cyclone is marked with conventional syrnbol (see Fig. 2.1). Ti me~ ~hown are of the format yymmddlhh.

• 30 •

(4) OI.(Q) US IIP. 'De·U IIh.I •••J '''007111 -

ICI OhIQ) US IlP. IDe·U IlPa·I.,.J "'001/11 (DI 01.(1.1) 675 IlP. IGe·II Il''.I.••J 96I0191QO Fig. J.5 Tropical Stonn Josephine - October 1996 (see previous page for details)

CA) OI.(Q) I1S IaP. '''·U Ilh.....J 151115/11 III Dh(Q) US lira lDe·16 Ilr.·...·J '5UO""

(C) DI..(Q) 115 IlP. II•• " Ilh·l••-J 95..../11 (Dl DI"CQ) 615 IlP...... Ilh·l.,·J ,seU1111 • Fig. 3.6 As in Fig. 3.5 but for Hurricane Allison - June 1995 31 •

(AI D.... CQ) "5 Il'. 10.·1' Il,.·I••·J 611011111

. ;90" . '.:n \·...10 :JO --- ICI D.... tQ) 615 Il'. 10..&6 IlPa.I.••J 611011/11 101 DhlQI '15 la"~ th." Il,••I.••J UllIlZ/DD Fig. 3.7 As in Fig. 3.5 but for Hurricane Gladys - October 1968

(41 O.... CQ) 615 IlPa Ih.t' Il'...... J ".,U/Il CI) DhCQ) '15 Il'. 10.·16 IlPa·I ••·J "atU/OO

(C) Dh(Q) 675 Il,. Ih·l' .., ...... J ""IJ/U CD) Dh(Q) 675 IlPa lh·l' "Pa·I ••·J "nu/oo • Fig.3.8 As in Fig. 3.5 but for Hurricane Edouard - September 1996 32 •

\~

la, lDhlQI "5 IIPa 10.·.. IIr.·I ••·J 150711/11

(CI IDh(QI '75 IIP. 18••'. IIP•• I .••J 150111/1. IDI lDhlU' "5 "1". 10.·U b ...·I..d H01U/Da

53N ...-~~------~--.. ~~~ H~R 1'[BLANCHE 50N .:.. . ~ 49N . ~ 48N 47N 46N 45N 44N :~~ -:-...... : .. : / :.. .. d Tropiço'l 'Depression' 41 N ',' .. ~.... :..."':.1. ...:, ~ ]Hr~!~O~ .S~o~ .. , : ~ . ~ WTlcane, . 40N . .. ' : .. + ExtrQ~opical 39N lWlfHL (inin) 0600 lHROUQ{ 1800 ure' 28~ Jt1 75 .. 38N ~_"""-- __-_--._.....-_-r-"""'" (E) 72W 70W 68W 66W 64W 62W 60W 58W 56W 54W

Fig.3.9 Panels (A) through (D) are as described in Fig. 3.5 but for Hurricane Blanche - July 1975, for A) 750728/06, B) 750728/12, C) 750728118, and D) 750729/00. E) Total rainfall (mm) over the 12-hr period shawn at varions locations throughout the Maritime Provinces. Upper case letters A, B and C along the • track segment in E correspond to the times in panels(A), (B) and (C). 33 •

(Al lDheQl 61S IIP. U •• U IIP.·I ••·j 'IUI"U CI, lDhlQI US IIP. 1...·1. IIP••I ••·j 11011"11

eCI lDheQI US IIP. 1a..U IIP.·I •••j 'IU10/00 ID, lDI .. eQl "S IIP. 10.·1. IlP•• I ••·J 1I0.10/DA

Fig. 3.10 Panels (A) through (D) are as described in Fig. 3.5 but for Hurricane Bob - August 1991. for A) 910819/1~ B) 910819/18, C) 910820/00, and D) 910820106. Panels (E) through (H): AVHRR imagery • showing cloud pattern (white regions) corresponding ta prdlels (A) through (0). 34 • 4 Upper-air Data Set and Sounding Climatology 4.1 The'., Data Set

In this chapter we use the Foreeust Systems Laboratory (FSL) sounding database to 5tudy temperature, moisture and winds during ET. The database ..:onsist5 of tht nltrgtr of two sounding data sets: National CUmate Data Center (NCDC) data, and Global Telecommunication System (GTS) data from the National Severe Storms Foreeast Center. Two major differenees exist between the NCDC and GTS databases. There are more significant wind and temperature levels in the NCDe data owing to slight differences in the signifieant level eriterion, and aIso because the NCDe data has calculated values of temperature and wind at least every 50 hPa below 200 hPa, and every 25 hPa above 200 hPa. Secondly, GTS wind levels are PPBB levels - "regionally fixed and significant" whereas NeDe uses one·minute balloon ascent intervals to interpolate winds to thermodynamieally signifieant levels.

When the same sounding is arehived in both data formats, they are merged. Two soundings are eonsidered the same if their times match, they have the same WBAN (Weather Bureau, Army, and Navy) number, and if surfaee pressures and temperatures are within 1 hPa and loe respeetively. Before merging, the data sets are checked far hydrostatic eonsistency, and often geopatential height and temperature corrections must be made. Gross error ehecks are aIso performed sa as to provide a database that ean he used with Httle intervention by the researcher. Details on the database can be found in the technical memorandum by Schwartz and Govett (1992).

• 35 • 4.2 Sable Island Sounding Climatology Our first task in analyzing the three-dimensional tropospheric structure of ET cyclones is, to create a reference troposphere or "sounding climatologyu. Sable Island (YSA) is chosen for the climatology because it is close to many of the cyclone tracks. Radiosonde data from the FSL was collected from January 1958 to June 1993 at the Atmospheric Environment Service (AES) of Environment Canada weather station located on the western end of the island at 43.9° N, 60.0° W. There were 25520 out of a possible 25930 (every 0000 UTe and 1200 UTe) soundings archived at the FSL for YSA during the above time periode Therefore, only 410 soundings were not archived over the 36­ year periode Table 4.1 shows the annual breakdown of archived soundings at the FSL for YSA.

Each sounding chosen for the climatology is converted to 25-hPa resolution using linear interpolation between data levels. The base level in the climatology is 975 hPa, allowing us to include aImost all credible soundings since only a small fraction had surface pressures below 975 hPa. The ceiling level is 100 hPa, which is aIsa the ceiling level of data contained in the FSL database. The climatologies are broken down into 48 periods. Each month has 4 periods, 2 for each synoptic hour (0000 UTC and 1200 UTe) th during the flI'St part (1 st to IS ) of the month, and 2 for each synoptic hour during the latter part (16th to last day) of the month.

Ta construct the climatology, we discard soundings containing too little dat~ and choose only those soundings satisfying the criterion discussed below. So as not to throw away too many soundings, we compute the temperature and wind climatologies separately. Befoee a sounding is considered, we require that there be dry bulb temperature (n, dew point temperature (Td) and wind data at the surface. Consideration of a particular sounding depends on pre-determined thresholds on density of temPerature and wind data. If the sounding contains data for at least every SO-hPa throughout the troposphere we consider it suitable for interpolation to 25-hPa resolution. To aid in • writing the algorithm for interpolation, we require T at 100 hPa. Since 100 hPa is a 36 mandatory leveI, T data were aImost invariably recorded at that level. Owing to limitations on moisture readings at eold temperatures, it is not praetieaI to choose aoly • soundings containing dew point readings as high as 100 hPa. Many soundings anly have reliable dew point data up ta 300 or 400 hPa depending on the time of year. In an effort ~, to avoid eliminating tao many soundings from the climatoLogy, a dew point data eeiling was ehosen according ta the time af year as follows:

1. 400 hPa far January, February and March 2. 350 hPa for April, May and June 3. 300 hPa for July, August and September 4. 350 hPa far Detober, November and December

We notice that wind data in the database appear sparse above 200 hPa, 50 200 hPu is chasen as the upper LeveL in the wind climatoLogy (with observations of wind at both the surface and 200 hPa). StipuLating that wind data be as high as 100 hPa eliminates a great number of the soundings. Table 4.2 shows the number of soundings comprising the sounding climatalogy for eaeh period.

An exampLe of a sounding climatalogy is shown in Fig. 4.1 for the September 1st .OO UTe) periadסס) ta 15 th

• 37 Number ofsoundings archived at Sable Island 1958 692(38) 1959 641(89) • 1960 687(45) 1961 687(43) 1962 724(6) 1963 725(5) 1964. 705(27) 1965 729(1) -1 1966 730(0) 1967 728(2) 1968 731(1) 1969 726(4) 1970 728(2) 1971 727(3) 1972 731( 1) 1973 730(0) 1974 730(0) 1975 730(0) 1976 732(0) 1977 730(0) 1978 730(0) 1979 729(0) 1980 712(20) 1981 727(3) 1982 729( 1) 1983 712(18) 1984 729(3) 1985 727(3) 1986 729( 1) 1987 728(2) 1988 732(0) 1989 730(0) 1990 723(7) 1991 712(18) 1992 684(48) 1993 344(18)

Table 4.1 Annua1 breakdown of soundings which were used to develop the Sable Island sounding climatology. The number of missing soundings is in parentheses. Total number of soundings archived: 25520; missing soundings: 410.

Number ofsoundings comprising the sounding climatology MONTH TIMEPERIOD lst-lSth (00 OTC) lst-15th (U OTC) 16th-eod (00 UTC) 16th-end (12 UTC) Month Total January 314 (182) 308 (174) 310 (185) 295 (181) 1227 (722) February 282 (191) 296 (196) 265 (139) 255 (162) 1098 (688) March 314 (205) 333 (224) 341 (225) 337 (223) 1325 (877) April 164 (212) 170 (218) 202 (237) 196 (227) 732 (894) May 271 (241) 274 (226) 350 (282) 381 (259) 1276 (1008) June 372 (222) 337 (244) 380 (242) 392 (246) 1481 (954) July 278 (217) 289 (243) 323 (269) 335 (264) 1225 (993) August 330 (245) 344 (236) 302 (238) 320 (264) 1296 (983) September 255 (236) 285 (241) 218 (223) 223 (229) 981 (929) Derober 331 (203) 336 (216) 353 (217) 342 (230) 1362 (866) November 275 (208) 285 (204) 225 (193) 222 (201) 1007 (806) Deeember 164 (179) 178 (191) 125 (210) 129 (197) 596 (777) 13606 (10497) Table 4.2 Breakdown ofsoundings for each ofthe 48 dry bulb temperature and dew point temperature climatologies~ and wind climatologies (in parentheses). "End" in the • headings denotes last clay ofthe month. 38 Temp (C) Wind 100, " • 32~..

UILJ 1 '\~ ~ , UILJ wU

~ ~ ~ ~ UL.J 1

"':J~

1000 , , \ .... .,. .. 1050 SABLE ISLAND 1TSA I\LI"ATOLOCY " LAT. .3 ~ q,. LON- 6i,ail SEP 1 th~ough 1S 0000 ure

Fig.4.1 Sample sounding climatology (1958-1993) for Sable Island. Nova Scotia (YSA). The time period oo urc September 1 through 15. Dashed lines are mixing ratios in glkg. Temperature lines runסס is for from lawer left ta upper right every 10 degrees Celsius. • 39 • 5. Tropospheric Structure During Transition 5.1 SOllnding Data Compositing Procedures

Table 5.1 summarizes the list of sounding stations used ta sample the atmosphere

during the: transition of 35 out of the: 45 e:vents (1963 la 1993j. Tne exciusion of i Û cases was necessary either because the transition phase of the case was too far from the sounding network or we did not have the data (cases after 1993).

Upper-air 3-letter Lat. / Lon. Sampling Station Identifier Period

Chatham, Massachusetts CHH 41.67° / -69.97° Nov. 1970 - Dec. 1993 Nantucket, Massachusetts ACK 41.25° / -70.07° Jan. 1958 - Nov. 1970 Sable Island, Nova Scotia YSA 43.90° / -60.00° Jan. 1958 - Jun. 1993 Shelbume, Nova Scotia WOS 43.72° / -65.25° Nov. 1972 - Dec. 1986 St. John's, Newfoundland YYT 47.67° / -52.75° May 1971-Dec. 1993 Yarmouth, Nova Scotia WQI 43.87° / -66.05° Feb. 1989 - Dec. 1993

Table 5.1 Upper·air stations involved in the sounding study. Column four shows the "sampling period" from which soundings were used for compositing. For geographical location of these stations, see Fig. 2.11.

Ta sample the troposphere within and around the ET cyclone, we define a horizontal circular domain whose center matches that of the cyclone's (see Fig. 5.1). The domain consists of two concentric circles, one with a radius of 10° of latitude and the other with a 5° latitude radius. Four straight Hnes drawn through the center of the low split the domain into 16 "compositing sectors". A natura! coordinate system is used where s is the cyclone's motion vector as shawn in Fig. 5.1. This vector is determined as the average of the motion vectors 6 hours before and 6 bours after the time of interest. We bring to the reader's attention that the closed circulation associated with the cyclone is typically contained within the ioner region of the domain (5° of latitude radius from the • domain center). 40 Following the technique of Rogers and BosaIt (1986), Fig. 5.2 shows the • distribution of soundings sampled for 4 time periods during extratropical transition (35 events). The tirnes are t-18 hours, t-6 hours, t+6 hours and t+18 hours where t represents ·r time of transition. The most heavily sampled time was just before transition at t-6 hours. The weakest sampling occurred at t+ 18 hours, a time when the majotity of cyclones are moving into the Atlantic far away from the sounding stations. Since the number of soundings in each of the 16 sectors is generally small (often less than 10) we composite soundings for groups ofsectors, hereafter referred to as "domains".

• 41 •

n 500 km

Fig. S.l Sectoring scheme used for the composite sounding study. The cyclone is located at the center ofthe domain and moves in the direction indicated by s in a natura! coordinate system. Inner circle radius is 5 degrees of latitude and outer circle radius is 10 degrees of latitude.

• 42 • Distribution ofsoundings sampled at each stage Tropical Tropical t-18hr ·r t-6hr

Extratropical Extratropical t+6br t+18hr

(lnner circle radius = 5 deg. lat.) (Outer circle radius =10 deg. lat.) 500 km

Fig. 5.2 Location ofsoundings sampled throughout the compositing domain at the 4 times • shown. t refers to time of transition. 43 • 5.2 Results of Sounding Composites

In ~1us section, the magnitudes of anomalies are shawn in parentheses. Where

abbreviated, T" denotes "dry bulb temperature anomaly" and Td:l denotes "dew point temperature anomalyll .

Figures 5.3 (a) and (b) show the pre-transition composites for the outer ring domain between 5° and 100 of latitude from the cyclone center. We refer to this region as ll the lIambient atmosphere . 18 hours before transition (see Ca» there exists a region with a maximum statistical significance of 99% for positively anomalous dry bulb and dew point temperatures in the lower levels below 800 hPa. Ca1culation of statistical significance is deterrnined using the Student's t-test (see equation 3.1). Elsewhere (above 800 hPa) the atmosphere is nearly representative of climatology. In (b), 6 hours prior to transition, the

lower troposphere (below 700 hPa) is moister than climatology CTd:l +2° to +4°C). Temperatures below 800 hPa are now close to climatology. Pre-transition winds throughout the troposphere are generally from the west-southwest with magnitudes comparable ta climatology.

Figures 5.3 (c) and (d) present the post-transition composites for the outer ring domain. In (c) we observe a layer of "drying-out" between 500 and 800 hPa. This dry layer also exists in (d) with a maximum significance of 95%. We find a lowering of the tropopause ta approximately 250 hPa from its pre-transition altitude near 200 hPa. Significant cooling, by approximately 3°C, is witnessed at 400 hPa between (c) and (d). It is interesting to note that winds below 400 hPa tend to be veering between (c) and (d) indicating the development of negative thermal advection.

Figures 5.4 (a) and (b) display the pre-transition composites for the inner circular domain whose radius is 5° of latitude from the cyclone center. This domain is representative of the true "composite cyclone". In (a) there exists a deep layer of very • significant (99%) warm dry bulb temperatures (+2° to +3°C) from about 700 bPa ta 350 44 hPa. Another warm region (Ta +3°C) shows up below 900 hPa. Dew point temperatures in (a) are greater than climatology (up to +7°C) throughout the troposphere. The entire • troposphere in Cb) is very significantly warm (Ta about +4°C) up to the tropopause level near 175 h!;a which is reminiscent of a tropical atmosphere. Dew point departures from climatologyas great as +6°C are very significant (99%)9 up to 475 hPa. Winds tend to be

southwesterly through much of the sounding in (a), but become about 2 m S-I stronger below 800 hPa in (b).

Figures 5.4 (c) and (d) show the post-transition composites for the inner circular domain. Note the persistence of significant warmth (T;} +3°C) between 875 and 475 hPa in (c), and high dew point temperatures (+5°C) below 675 hPa (99% significant). By 18 hours after transition (see (d» we still observe anomalous warmth (Ta +3°C), particularly below 600 hPa. The lower significance of 80% is owing to the smaller sample size. WeIl above normal dew point temperatures (up ta +7°C) persists in (d) below 800 hPa, while less significantly different dew points (0° to -3°C) appear abave 600 hPa. Wind patterns in (c) and (d) are much like those before transition.

Figures 5.5 (a) and (b) present the pre-transition composites for the left-of-track and right-of-track domains out to a radius of 100 of latitude from cyclone center. The right composite is dashed and the left composite is solid. Greatest differences between the composites in (a) exists in the lower troposphere (below 700 hPa) with 99% significant differences in both dew point and dry bulb temperatures (2 to 4°C). Increasing asymmetry develops between Ca) and (b) with both dew point and dry bulb temperatures wanner, by about 3°C, in the right semicircle everywhere below the tropopause. Significant warm advection is observed left of the storm track in Ca) as evidenced by the veering-with-height wind pattern below 600 hPa. A weaker wann advection pattern shows up to the right of the storm track. An interesting change in lower tropospheric winds (below 800 hPa) is noticeable in the left composite 6 hours before transition (see (h»). At this tinte there is strong cold advection, contrary to the warm advection found in • (a). 45 Figures 5.5 Cc) and (d) show the post-transition composites for the left-of-track and right-of-track domains. The trend of increasing asymmetry continues with very • significantly different profiles of dew point and dry bulb temperatures (3 0 ta 5°C) in (c) throughout.-,the troposphere. The tropopause appears to lower by about 50 hPa in the left composite, with no change in the tropopause level of the right composite. Less significant differences between dew point and dry bulb temperature profiles occue at 18 hours after transition in (d) owing to smaller sample sizes. Cold advection continues below 500 hPa in the left domain in (c), and through an even deeper layer (below 400 hPa) in (d).

Figures 5.6 Ca) and (b) present results of the pre-transition composites for front and rear semicircular domains out ta 100 of latitude from the cyclone center. The line dividing the domains is normal to the storm's track. The front composite is solid and the rear composite is dashed. Dry bulb temperature profiles differ by less than 1°C in Ca) 18

hours prior to transition. The troposphere is moister CTd up ta 4°C greater) in the front composite below 800 hPa and above 400 hPa. At 6 hours prior to transition differences between bry bulb temperature profiles are less than 1°C (see (b», while greater differences appear between dew point profiles (10 to 4°C). Warm advection is abserved in the front composite while cold advection appears in the rear composite in (a), a pattern that persists until 6 hours before transition in Cb).

Figures 5.6 Cc) and (d) show the post-transition composites far the front and rear composite domains. In (c) we see that dry bulb temperatures in the front composite are up to 2°C wanner than in the rear composite possibly owing to displaced convection (and thus latent heating) ahead of the storm as observed in chapter 3. Dew point temperatures are approximately 4°C greater in the front composite than rear composite throughout the troPQsphere with levels of significance near 95%. Finally in (d), dry bulb profiles are nearly equal throughout except above 250 hPa where dew points are about SoC colder in the front composite. The front composite appears quite moist above 700 hPa while the

rear composite is much drier owing to the differences in Td up to 7°C. Little difference in • moisture is observed below 700 hPa in (d). Thermal advection patterns are very sinillar 46 to the patterns observed before transition, with cold advection dominant in the rear • composite and wann advection in the front composite.

• 47 •

p hPa

:::t:;r.,.;:;_Ç;::::Z:~:Z:=~ ~:32~:L1t:t::J~~~~~ .. .. 't •• :O!POSITE • ~ll. ta CU"UaLOCT' 1lA90 Cll""ll.OCT • IlA!iI«O a b

p p hPa hPa

' .. loi "1 ! '''­ .... CQIdIQ$IT[ • Sl1ID o..l~AfaLllCT • OASI€II o..J!lATtl.1lCT • lIAHIl c d Fig.5.3 Composite soundings for the outer ring domain between 5 and lO degrees oflatitude radius from cyclone center. SoUd profile is the composite, dashed profile is the weighted Sable Island climatology. The times are a) 18 bours before ET, b) 6 hours before ET, c) 6 hours after ET and d) 18 hours after ET. The number ofsoundings in the composite is shawn at upper right. Short wind barb =2.5 mis. long barb = 5 mis and penant = 25 mis. Statistical significance for the difference between composite and climatology dry bulb temperatures is shawn for certain levels where the significance is a local maximum (similarly for • dew point temperature). 48 0=32 • j

p P hPa hPa

~::t:;'.';;1_~=Z:==~=~I 1:::;:;II~T:Gèt:jti=:tt~ttl l' .• :IJlPOSlTt • sa.ID :L1~UClll'T' !lAND :L1"UDLll'1' • CAND a b 0=26 '1 ~ p P hPa hPa

:::k,..~_~:::s;t:::~~~~u~...:5z~:CSéz:jti=~~jj ~IT[ • SClLID o.l"ATClCCT· llASIG a.llIAfDLlla 1.1• llASIG'" c d Fig. 5.4 Composite soundings for the inner circular domain whose radius is 5 degrees oflatitude from the cyclone center. SoUd profile is the composite. dashed is the weighted Sable Island climatology. The times are a) 18 hours before ET. b) 6 hours before ET. c) 6 hours alter ET and d) 18 hours after ET. The number ofsoundings in the composite is shown at upper right. Short wind barb =2.5 mis. long barb =5 mis and penant =25 mis. Statistical significance for the difference between composite and climatology dry bulb temperatures (at right) is shown for certain layers and levels where the significance is greatest (similarly for • dew point temperature). 49 •

P p hPa hPa

" Il A1Clfr·cr· flllCl • Cl'.il«~ a b

p p hPa hPa

I_I--+r...... ~,...... ~--:~:...,..~p;...:~~~~.....;:., Il'41,.. ­ 1.1 L' LUT-ar·nua - 5llLID RI CHf-Œ'-llUCI - Q.UIC c d Fig.5.5 Composite soundings for left-of-ttack domain (solid profile) and right-of-track domain (dashed profile for a) 18 hours before ET, b) 6 hours before ET, c) 6 hours after ET and d) 18 hours after ET. The number ofsoundings in cach composite is shown in the upper right corner above the corresponding wind barb axis. Short wind barb = 2.5 mis, full barb = 5 mis and penant = 2S mis. Statistical significance for the difference betwecn composites ofdry bulb tcmperatures (at right) is shawn for certain layers and levels • where the significance is greatest (similarly for dew point tcmperature). 50 •

p p hPa hPa

U"It---t'-r-~-';"~--'~~~~~l-J~..,L.l-""";:"-,o'1-~!A-l 1iWt &, .... 'IlQNr • sa.:O R[AII· JASICO R[AII • JA$I4(D a b

p p hPa hPa

....

c d Fig.5.6 Composite soundings for front semicircular domain (solid profile) and rear semicircular domain (dashed profile) for a) 18 hours before ET. b) 6 hours before ET. c) 6 hours after ET and d) 18 hours after ET. The number ofsoundings in each composite is shown in the upper right corner above the corresponding wind barb axis. Short wind barb =2.5 mis. full barb = 5 mis and penant =2S rn/s. Statistical significance for the difference between composites ofdry bulb temperature (at right) is shawn for certain layers and • levels where the significance is greatest (similarly for dew point temperatures). 51 • 5.3 Analysis of Convective Instability Vertical profiles of equivalent potential temperature, Se, and saturated equivalent

potential ;Jmperature, Ses, are computed in this section to assess convective and conditional instability during transition (see e.g. Holton 1992, chapter 9). The existence of a convective1y unstable layer in the lower troposphere after transition is found, suggesting an atmosphere which may be sensitive to quasigeostrophic forcing and thus the potential for re-intensification of the extratropical cyclone.

The potential temperature can be calculated knawing ambient dry bulb temperature T and pressure P according ta Rogers and Yau (1989) for example,

(5.1)

where 1C = 0.286 and Po = 100 kPa. The calculation of equivalent patential temperature follaws from Rogers and Yau:

(5.2)

ws(Td) is the saturation vapor pressure if the temperature of the air were equal to the dew point temperature. Tc is the condensation temperature, and is calculated iteratively from the following equation

(5.3)

8 3 • where A =2.53 X 10 kP~ B = 5.42 X 10 K and E = 0.622 are canstants. 52 The saturated equivalent potenùal is calculated under the assumpùon of a saturated • atmosphere. Saturated equivalent potential temperature is given by

'r () =e [2675Wj'(T)] (5.4) I!j' exp T

where T is the actual air temperature, not the temperature after adiabatic expansion to saturation as in 5.2.

Composite profiles of ec: and Sc:s for the inner circular domain (aIl soundings within 50 of latitude radius from cyclone) appear in Fig. 5.7. Climatology is shawn as the dashed profile along with the composite shown in solid. The same soundings used to construct composites in Fig. 5.4 have been used here. During the transition (a through d) the lower troposphere (below 850 hPa) tends to become more convectively and conditionally stable. In (b), (c) and (d) a convectively unstable layer between about 850 and 750 hPa becomes increasingly unstable. Increasing values of e~'i-ec: (from approximately l2°C in (a) to l6°C in (d» in the 8S0-S00-hPa layer signify a "drying-out" as noted in Fig. 5.4.

• 53 EPT and SEPT· 18 hrs Before ET EPT and SEPT· 6 hrs Before ET

350 350

1 1 • 450 Il 4SG ,• n=r3 , )f , 1 ) 550 550 • .1 ,.. 1 1 ,• ) 1 "éi15O . . Q. z:. c: • 1• • t , 7SO 750 .' ·• 1 1 ~ ,•• • ~ 850 • ·• ~ , 1 •• :( , 1 ... ~ ~ / ISO -t--+--+-...... -+--~--.,~--+--i - • ~. 1 / 305 310 315 320 325 3:JO 335 340 305 310 315 320 325 330 335 340 a EPT and SEPT (K) b EPT and SEPT (K)

EPT and SEPT· 6 hrs Alter ET EPT and SEPT ·18 hrs After ET 350 l/~ 4SO

1 n=1 ,9' •. 550 . ~ , . • ~ , 1 ,." ... . ,, ) ,1 t----+-.lt-+--~-+l_-+_I~-_I ! J 750 150 • , ,• , ~ ,i 150 • • •'"' •, • ,1 ". 1~ 850 • ,1 -• • ~~ 305 310 315 32D 325 33CI 3:15 340 305 310 315 320 325 330 335 340 C EPT and SEPT (1<) d EPT and SEPT (K)

Fig.5.7 Composite and climatology profiles ofequivalent potential temperature (EPT) and sat­ urated equivalent potential tcmperature (SEPT) for the inner domain (within sa latitude radius) for a) 18 hrs before ET, h) 6 hrs before ET, c) 6 hrs after ET, and d) 18 hrs after ET. Climatology is dashed (thick • EPT, thin - SEP'I) and composite is solid (thick - EPT, thin - SEP1). Number of • soundings composited is shawn at upper left. 54 • 5.4 Analysis ofThermal Advection Sounding composites give a qualitative analysis of thermal advection using wind dat~ but db not provide a quantitative measure of this quantity. In this section we compute the thermal advection for each sounding contained in the, lO-degree latitude radius composite domain for the 850-S00-hPa layer, and show that ET is dominated by large-scale warrn advection. Analyzing thermal advection provides one way of measuring how "extratropical" the ET cyclone is. The 850-S00-hPa layer is chosen because most of the sounding composites exhibited thermal advection confined to the lower troposphere. This layer is aIso consistent with the QG analysis in chapter 3 where we utilized 850-500-hPa layer mean temperature to compute Q vector divergence.

The following equations are used in the layer thermal advection computations.

Thermal wind (UT, VT) is defined as the difference between the geostrophic wind at two pressure levels where

UT = ugsoo - Ug850.·

VT =vgsoo - vgsso, (5.5)

The geostrophic wind (ug, vg) is estimated from the real wind at the indicated levels. We use the thermal wind relationships

u = _!i(ar )10(850) T f ay 500

v =R(aT \n(850) (5.6) T fax r 500

to find the layer mean thermal gradient VT between 850 and 500 hPa. Layer thermal advection is • 55 -V, .VTT (5.7) • where V, is the mean geostrophic wind in the 850-S00-hPa layer, which we estimate from the real (sounding) winds. 0:,

In a tropical atmosphere, horizontal temperature gradients are yery weak, and thus magnitudes of thermal advection are very weak, often less than ± 1°C / day. In the mid­ latitudes, there are stronger horizontal temperature gradients typically ± lue 1 degree of latitude during the warm season (June through September) as estimated from the NCEP c1imatology for the 8S0-S00-hPa layer. A typical ntid-Iatitude 850-S00-hPa mean

geostrophic wind estimated from NCEP climatology is 20 knots or 10 m S·I. The horizontal thermal gradient estimate becomes about ± 1 x 10.5 Oc / m. According ta 5.3, we estimate typical mid-latitude deep-Iayered thermal advection during the warm season of about ± 1 x 10-4 Oc / s or almost ± 100e / day. We can use these estimates as approximations for the baroclinicity of the cyclone's environment and hence ta what degree the cyclone is extratropical, where OOC / day signifies "tropical" and 1011C / day signifies "extratropical".

Fig. 5.8 displays scatter plots of 850-S00-hPa layer thermal advection versus radial distance of sounding from the cyclone center from 18 hours before transition ta 18 hours after transition, every 12 hours. Thermal advection was computed for all soundings containing winds at both 850 and 500 hPa inside the 10° latitude radius. In (a) evidence for the pri~sence of transition exists based on rather "non-tropical" magnitudes of

advection (greater than ±4°C / day). By 6 hours prior ta transition (see (b», a

considerable increase in warm advection to a mean of near 8°C 1day is observed - twice as strong as 12 hours earlier. By 18 hours after transition the mean warm advection has increased to 9°C 1day which is typical of mid-latitude quantities as estimated above. A much smaller increase of -4°C to -7°C 1 day in the mean cold advection component is found during transition. • 56 18 Hours Before ET 6 Hours Batere ET • 40.,-.------, 40~------_ rnMn • ...:us-Clday 30 30 0:, • i 20 -;;- 20 ~ • ~ ~ ••• E 10 10 •• • c c ,g o 1Jt-l!!!!~...... ~ 1i ti CD ~ > "ID "ID 'ii ·10 'ii ·10 e e ...ID .. z:CD .: f= ·20 ... ·20 • ·'11'".1--;· • -30 • rnHn.....O·Clday rmran.....S3·Clday -40 -.l -40 ...... ------• ..... Radiai distance tram cyclone (km) RadIai distance trom cyclone (km)

a b

6 Hours Alter ET 18 Hours Alter ET

40 ~------... 40 ~------.. n:Q2 n=28 rnean••8.2rClday 30 30 •

>:20 • ID

~ 10. .: •

,gc • •• • ~ • ... ->~ 0 +-----,-~ ---;:~~~.•-.l-.---~ 11 300" eo:. ~ ·1~ ~ "ii ·10 • Ê z:ID ... ·20 • • • me..,..."n·Clday -30 me."..a.I2·Clday

-40 ~-_.~------~ -40 ~------Radiai distance from cyclone (km) Radiai distance tram cyclone (km) c d

Fig. 5.8 Scatter plots of850-500 hPa layer thermal advection from all soundings located within a 100 latitude radius ofthe cyclone for a) 18 hrs before ET. b) 6 hrs before ET, c) 6 hrs after ET, and 18 hrs after ET. The number ofdata points (soundings) is shown in the upper lcft. Mean values • ofwarm advection (above abscissa) and cold advection (below abscissa) aIso shawn. 57 • 6. Summary and Conclusions A study of the transformation of 45 tropical cyclones into extratropical cyclones 0:, between 1963 and 1996 has been conducted for the western North Atlantic Ocean. A synoptic-dynamic climatology of extratropical transition (ET) cyclones was constructed from the National Centers for Environmental Prediction (NCEP) reanalyses of geopotential height data. The reanalyses were aIso used to diagnose quasi-geostrophic forcing during ET and were compared with typical precipitation and cloud patterns. Data from the Forecast Systems Laboratory (FSL) for six near-track upper-air stations were used to form sounding composites.

The mean lifetime of cyclones undergoing ET was found to be 10 to Il days (tropical period of seven days, extratropical period of three to four days). Longest-lived events were found to persist for over three weeks. ET cyclones typicaIly last longer than extratropical cyclones owing to the additional time in which the cyclone spends in the weak steering flow of the tropics. Brand and Guard (1979) determined a mean of five days for the extratropical phase of 14 West Pacifie transition cyclones.

Sorne principal resultsJrom the synoptic climatology follow.

1. A statistically-significant lOOO-SOO-hPa warm anomaly (with respect to the 1963-96 climatolagy) aver central and eastem North America persists for the one-week period prior ta the passage of the tropical systems into the Canadian Maritime provinces.

2. Anomalous sea level pressure and SOO-hPa geopotential height ridging over northwestem Europe foons at least four days prior to the passage of the tropical systems into the Canadian Maritime provinces.

3. An anomalous ridging pattern of the surface subtropical anticyclone occurs over • Atlantic Canada beginning two days prior to the arrivai of the cyclone. 58 4. A significant lOOO-SOO-hPa thickness ridge forms over central Canada three days • before the cyclone reached 40~ and intensifies as it travels eastward. o., S. A mobile SOO-hPa synoptic-scale trough often accompanies the transition over eastem North America.

Sorne principal resuils from the sounding compositesfollow.

1. The tropical cyclone's warm core and convectively-unstable tropical air mass are maintained after transition.

2. Significant drying-out of the troposphere in the 800-S00-hPa is observed during the transition process (commencing between 18 and six hours before transition).

3. Transition is accompanied by increasing asymmetry, most pronounced between regions to the left and right of the storm track.

4. Large-scale WarIn advection dominates cold advection before and after transition.

A key resultfrom the QGforcing analysesfollows.

The presence of quasi-geostrophic forcing for ascent, typically seen in extratropical cyclones, is observed during periods in which the systems are still classified as tropical cyclones. This forcing for ascent continues during the extratropical transformation, and typically occurs ahead and ta the left of the storm track.

Developing a better understanding of the horizontal and vertical structure of transitioning cyclones may lead to the creation of a more precise definition of extratropical transition. Ultimately, our objective has been to laya soUd foundation upon • which future work cao he conducted sa as to improve forecasting of these systems. 59 • References Abraham, ~1., and G. Parkes, 1998: The Transition of the USaxby Gale" into an Extratropical Storm. AMS 23rd Conference on Hurricanes and Tropical Meteorology: Extratropica/ Transition, 2, 795-797.

Bames, S. L., anà B. R. Coïman, i 993: Quasi-geostrophic Diagnosis oÏ Cyclogenesis Associated with a Cutoff Extratropical Cyclone - The Christmas 1987 Storm. Mon. Wea. Rev., 121, 1613-1633.

Bluestein, H.B., 1992: Synoptic-Dynamic Meteorology in Mid-latitudes. Vol. 1, Princip/es ofKinematics and Dynamics, Oxford University Press, 431 pp.

Bosart, L. F.. and E. Rogers, 1995: Incipient Explosive Marine Cyclogenesis: Coastal Development. Tel/us, 47A, 1-29.

Bosart, L. F., and G. M. Lackmann. 1995: Post-landfaH Tropical Cyclone Re- intensification in a Weakly Baroclinic Environment: A Case Study of Hurricane David (September 1979). Mon. Wea. Rev., 123, 3268-3291.

Brand, S., and C. P. Guard, 1979: An Observational Study of Extratropical Storms Evolved from Tropical Cyclones in the Western North Pacifie. J. Meteor. Soc. lapan, 57,479-482.

Browning, K. A., and G. Vaughan, 1998: Analysis of an Ex-tropical Cyclone after its Re­ intensification as a Warm-core ExtratropieaI Cyclone. Q. J. R.. Meteorol. Soc., 124, 2329-2356.

DiMego, G. 1., and L. F. Bosart, 1982: The Transformation ofTropical Storm Agnes into an Extratropical Cyclone. Part 1: The Observed Fields and Vertical Motion Computations. Mon. Wea. Rev., 110,385-411.

Dvo~ V. F., 1975: Tropical Cyclone Intensity Analysis and Forecasting from Satellite • Imagery. Mon. Wea. Rev., 103,420-430. 60 Gallagher, J., and A. Schubert, 1998: Sea Surface Temperature Satellite Image Archive. • http://dcz.gso.uri.edu/avhrr-archive/archive.html, 2 pp.

Hart, R. E:f 1998: Working Towards a Conceptual Model for Extratropical Transition: Diagnosing Transition and Post-Transition Intensity Change from Observations and Reanalyses. AMS 23rd Conference on Hurricanes and TropicaL Meteorology: Extratropical Transition, 2, S05-S08.

Hebert, P. J., 1976: Atlantic Hurricane Season of 1975. Mon. Wea. Rev., 104,453-465.

Holton, J. R., 1992: An Introduction to Dynamic Meteorology. Academic Press, 511 pp.

Hoskins, B. J., and H. C. Davies, 1978: A New Look at the arEquation. Quart. J. Roy. Meteor. Soc., 104,31-38.

Kalnay, E., and Coauthors, 1996: The NCEPINCAR 40-Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77, 437-471.

Klein, P. M., A.H. Patrick, and R.L. Elsberry, 1998: Tropical Cyclone and Mid-latitude Characteristics and Physical Mechanisms Contributing ta Extratropical Transition in the Western North Pacific. AMS 23rd Conference on Hurricanes and Tropical Meteorology: Extratropical Transition, 2, 799-S02.

Koch, S. E., and Coauthors, 1983: An Interactive Bames Objective Map Analysis Scheme for use with Satellite and Conventional Data. J. Cfimate Appt. Meteor., 22, 1487-1503.

Lawrence, M. B., and B. M. Mayfield, 1998: Atlantic Hurricane Season of 1995. Mon. Wea. Rev., 126, 1124-1151.

Maher, B. S., 1999: National Hurricane Center Web Page. http://www.nhc.noaa.gov, 2 pp.

Matanoy H., and M. Sekioka, 1971: Sorne Aspects of Extratropical Transformation of a • Tropical Cyclone. l. Meteor. Soc. lapan, 49, 736-743. 61 Mayfield, B. M., and M. B. Lawrence, 1991: Atlantic Hurricane Season of 1990. Mon. • Wea. Rev., 119,2014-2033.

Pasch, R. J.', and L. A. Avila, 1992: Atlantic Hurricane Season of 1991. Mon. Wea. Rev., 120, 2671-2687.

Pasch, R. J., and L. A. Avila, 1999: Atlantic Hurricane Season of 1996. Mon. Wea. Rev.. 127,581-610.

Rogers, E., and L. F. Bosart, 1986: An Investigation of Explosively Deepening Oceanic Cyclones. Mon. Wea. Rev., 114,702-718.

Rogers, R. R., and M. K. Vau, 1989: A Short Course in Cloud Pizysics. Pergamon Press. 293 pp.

Schwartz, B., and M. Govett, 1992: A Hydrostatically Consistent North American Radiosonde Database at the Forecast Systems Laboratory, 1946-present. NOAA Tech Memo., 81 pp.

Sekioka, M., 1970: uOn the Behavior of Cloud Patterns as Seen on Satellite Photographs in the Transformation of a Typhoon into an Extratropical Cyclone. J. Meteor. Soc. Japan,48,224-232.

Sinclair, M. R., 1993a: Synoptic-Scale Diagnosis of the Extratropical Transition of a Southwest Pacific Tropical Cyclone. Mon. ~Vea. Rev., 121,941-960.

---, 1993b: A Diagnostic Study of the Extratropical Precipitation Resulting From Tropical Cyclone Bola. Mon. Wea. Rev., 121,2690-2707.

Wonnacott, T.H, and R.J. Wonnacott, 1990: Introductory Statistics, Srh edition. John Wiley and Sons, 711 pp. • 62