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Technical Report TR 78-02

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«. TITLE fond Subl/l/oJ

Extratropical Storm Evolution from Tropical Cyclones in the Western North Pacific Ocean

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Samson Brand and Charles P. Guard

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Naval Environmental Prediction Research Facility

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Tropical cyclones Typhoons Extratropical cyclones

s20. ABSTRACT (Conllnuo on revori* ./d» // n.c»..»rT *nd Identity by block number)

This Study examines 16 recurving tropical storms and typhoonjs that occurred in the western North Pacific Ocean in 1971, to determine the characteristics of the storms after they became extratropical. Many of the extratropical storms that evolved from tropical cyclones maintained destructive seas and winds as long as 5 days after the point of recurvature. Six of the 16 extratropical storms crossed the North Pacific and affected

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20. Abstract (continued)

the Aleutians or western North America. Some of the physical processes associated with the evolution of extratropical storms from tropical cyclones are also discussed.

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AD-A060 910 040200 (U) NAVAL ENVIRONMENTAL PREDICTION RESEARCH FACILITY MONTEREY CALIF

TI (6) Extratropical Storm Evolution from Tropical Cyclones in the Western North Pacific Ocean. (U) Technical rept., Brand,Samson Guard,Charles P. Jul 1978 25p NEPRF-TR-78-02 Unclassified report *Tropical cyclones, *North Pacific Ocean, *StormQ, Typhoons, Weather forecasting, Meteorology, Meteorological data. Wind, Temperature, Oceanographic data. Aerial reconnaissance, Data acquisition, Safety, Warning systems (U) Extratropical storms (U) This study examines 16 recurving tropical storms and typhoons that occurred in the western North Pacific Ocean in 1971, to determine the characteristics of the storms after they became extratropical. Many of the extratropical storms that evolved from tropical cyclones maintained destructive seas and winds as long as 5 days after the point of recurvature. Six of the 16 extratropical storms crossed the North Pacific and affected the Aleutians or western North America. Some of the physical processes associated with the evolution of extratropical storms from tropical cyclones are also discussed. (Author) (U) 01

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/.iBHAfiT TECHNtCAl REPORT SICHOH aAVAL POSTGHADOATE 3CHOr>- ''OimOTt, OOffOJiNIA 93!>- ■

NAVENVPREDRSCHFAC TECHNICAL REPORT TR 7802

EXTRATROPICAL STORM EVOLUTION FROM TROPICAL CYCLONES IN THE WESTERN NORTH PACIFIC OCEAN

SAMSON BRAND AND CHARLES P. GUARD

JULY 1978

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DIPA^TMENT OF THg NAVY sinscTOR NavitL, ©eessswsssAssM"^ AND MBTeogsoLoov

Code 32:St 5605 Ser 920

S7 JUL 1978 From: Director, Naval Oceanography and Meteorology To: Distribution

Subj: NAVENVPREDRSCHFAC Technical Report 78-02; forwarding of

End: (1) "Extratropical Storm Evolution from Tropical Cyclones in the Western North Pacific Ocean," NAVENVPREDRSCHFAC TR 78-02

1. Enclosure (1) examines 16 recurving tropical storms and typhoons that occurred in the western North Pacific Ocean in 1971 to determine the characteristics of these storms after they became extratropical. These analyses of storm behavior should prove helpful to weather forecasters responsible for ship routing and issuing warnings of storm-related strong winds and high seas.

QflRelABD

Distribution List: SNDL Part 1 SNDL Activitv

CINCLANTFLT CINCPACFLT Fleet Commanders COMNAVSURFLANT COMNAVSURFPAC COMPHIBGRULANT COMPHIBGRUPAC (Group 1 only) , Surface Group PAC (WESTPAC only) Aircraft Carrier (CV, CVN) Amphibious Command Ship (LCC) Amphibious Assault Ship (LHA), (LPH) Destroyer Tender LANT (AD) (Puget Sound only) Deep Submergence Support Ship (AGDS) Misc. Command Ship (AGF) Training Aircraft Carrier (CVT) Unified Commands (CINCPAC only)

SNDL Al A2A A3 A4A

B2

C4F7

Jft sft it ^

E3A E3B E3C rB38 FDl FD2 FD3 FD4 FD5 FD6 FD7 FF5 FF38 FKAIA FKA6A12 FKQ5 FTl FT2 FT35 FT 73 FT75 FT78 V5

Special Assto to the Asst= SECNAV (R&D) Chief of Naval Research (Code 734) Chief of Naval Operations (CP-952) Chief of Naval liaterial (tlAT-034)

Special Agencies» Staffs, Boards & Conmiittses (NSA, Ft. Mead£ only) Weather Service Environraental Detachments (Agana, Atsugi, Bermuda, Cecil Field, Charleston, Corpus Christi, Cubi Pt,, Guantanamo Bay, Key West, Kingsville, Mayport, Whiting Field, Midway, Misawa, New Orleans, Roosevelt Roads, Oceana, Diego Garcia, Kadena) Laboratory, Office of Naval Research (Code 2620) Office of Naval Research Branch Offices Ocean Research & Development Activity Oceanographic System Pacific Oceanographer of the Navy Naval Oceanographic Office Naval Oceanography and Meteorology Fleet Numerical Weather Central Fleet Weather Central Fleet Weather Facility Naval Weather Service Facilities Safety Center Naval Academy Air Systems Command Headquarters (Air 370, Air 55) Ocean Systems Center Space Systems Activity Chief of Naval Education and Training Chief of Naval Air Training Amphibious School Naval Postgraduate School Naval War College Education and Training Program Develop. Center Marine Corps Air Station

Officer in Charge Service School Command Great Lakes Detachment Chanute Stop 71 Chanute AFB, XL 6t»68

Commandant of the Marine Corps *ATTN -ASL-44 Navy Department Washington, DC 20380

Headquarters, Air Weather Service Scott AFB, IL 62225

National Weather Service Room 307 World Weather Bldg. 5200 Auth Rd. Camp Springs, MD 20023

U.S. Department of Commerce National Oceanic and Atmospheric Administration Rockville, MD 20852

National Hurricane Center P.O. Box 248286 Coral Gables, FL 32124

Commander Pcicific Missile Range Point Mugu, CA 93042

CONTENTS

1. Introduction 1

2. Discussion of Results 3

2.1 Tropical Cyclone Transformation 3 2.2 Statistical Analysis 3 2.3 Recurved Tropical Cyclones 11 2.4 Extratropical Stage Transition 13

3. Future Research 18

Acknowledgments 19

References 20

1 INTRODUCTION

The purpose of this study is to provide to the operational

forecaster an analysis of the characteristics and behavior of

extratropical storms that have evolved from tropical cyclones.

Used in conjunction with conventional prediction techniques, this

information should be useful to ship routers as well as to

weather centrals responsible for issuing warnings of storm-related

strong winds and high seas. Qr^cJ A tropical cyclone is identified as becoming "extratropical"

when it loses its tropical characteristics. The term implies both

northward displacement from the tropics and the conversion of the

cyclone's primary energy source from latent-heat release to baro-

clinic processes; it does not carry any implication regarding the

storm's strength or size. As part of the U.S. Fleet Weather Central (FWC) Guam, the

Joint Typhoon Warning Center (JTWC) is responsible for issuing

tropical cyclone warnings in the western North Pacific. When a

storm becomes extratropical, this responsibility passes to FWC

Guam, which issues all gale and storm warnings for systems of

nontropical character. The reclassification from tropical to extratropical status

is based on a number of factors including thermal and convective

structure, inflow/outflow patterns at both the gradient and upper

tropospheric levels, and the general synoptic situation. The

warnings of JTWC and FWC Guam typically are coordinated 12-24

hours prior to the storm's reclassification to ensure uninter-

rupted forecasting service to operational users.

Recurved tropical cyclones are very difficult to forecast.

Burroughs and Brand (1973) found that the average 24-hour movement

forecast errors for the JTWC 1961-69 typhoon forecasts, verifying

after the point of recurvature, were nearly 30% larger than the

average annual forecast errors. These errors were associated with

the increases in forward speed of movement after the point of

recurvature. Although the recurved tropical cyclone generally

weakens in intensity (maximum wind), it still remains a significant

meteorological phenomenon in its extratropical stage. For example,

for the typhoons to the east of the China mainland from 1971-75,

the average maximum wind at the point at which the storms became -1 extratropical was 54 kt (27.8 ms"'). Typhoon Agnes (September,

1974) had a maximum wind of 100 kt (51.5 ms"^) when it became

extratropical at 34°N.

■2-

2. DISCUSSION OF RESULTS

2.1 TROPICAL CYCLONE TRANSFORMATION

The transformation of a tropical cyclone into an extratropical

cyclone can occur in various ways. For example, Sekioka (1970,

1972a and b) and Matano and Sekioka (1971a and b) describe two

observed transformational processes for western North Pacific

tropical cyclones that do not dissipate over land or water:

(1) "Complex" transformation -- a pre-existing front meets a

tropical cyclone; a new extratropical cyclone is induced on the

front and becomes the dominant system. The process makes it appear

that the tropical cyclone has been transformed into an extratropical

cyclone (see Figure la).

(2) "Compound" transformation -- a pre-existing extratropical

cyclone approaches and merges with a tropical cyclone and usurps

the tropical vortex. The tropical system appears to have been

transformed into an extratropical cyclone (Figure lb).

Tropical cyclones can also decay and dissipate over water

without first becoming extratropical (Figure Ic). (This study does

not address tropical cyclones that dissipate over a land mass.)

2.2 STATISTICAL ANALYSIS

Of the 16 tropical storms and typhoons in the western North

Pacific in 1971 that fit the categories cited above, 11 were

"complex," 3 were "compound" and 2 were "dissipators." Table 1

lists these tropical storms and typhoons by individual categories

and Figure 2 shows their tracks. Several of the tropical cyclones

could be followed completely across the Pacific and three of the

storms affected the ocean regions north of 55N.

Table 2 shows the average times that the 16 tropical storms

and typhoons existed in the extratropical stage as well as their

times in the recurvature-to-extratropical and the before-recurva-

ture periods. The storms existed longer in the extratropical

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-6-

stage (4.4 days) than in the period before recurvature (3.9 days)

One extratropical storm that evolved from a tropical cyclone

existed longer than 9 days.

Table 2. Average duration by stages of the 16 tropical storms and typhoons examined for the 1971 season. Standard deviations and extremes are also given.

Number of Days in Stage

Before Recurvature

Recurvature to Extratropical Extratropi cal

Mean • 3.9 1 .5 4.4

Standard Deviati on

2.8 0.9 2.7

(Min) 1 .0 0 . 3 0.7 Extremes

(Max) 8.9 5.5 9.3

The average distances traveled by the storms during the

indicated stages are shown in Table 3. The average distance in

the extratropical stage was 2644 n mi (4900 km) with a maximum

value of 5640 n mi (10452 km).l

Table 3. Average distances traversed by stages of the 16 tropical storms and typhoons examined for the 1971 season. Standard deviations and extremes are also given.

Distances Traversed (n mi/km)

Before Recurvature

Recurvature to Extratropi cal Extratropi cal

Mean 836/1549 574/1064 2644/4900 : — "1

Standard Devi ati on

549/1017 300/556 1540/2654

(Min) 145/269 160/297 585/1084

ExLremes (Max) 2025/3753 1200/2224 5640/10452

Track information and other meteorological/oceanographic data were reduced from the analyses of Fleet Weather Central, Pearl Harbor; the National Meteorological Center; Fleet Weather Central, Guam; Optimum Track Ship Routing, Fleet Numerical Weather Central; and the 1971 Annual Typhoon Report (U.S. FWC/JTWC, 1971). 7

(2)

(3)

(4)

The 16 recurving tropical storms and typhoons of 1971 were

closely examined to determine the day-to-day variations, after

the point of recurvature, of four parameters:

"^ (1) Intensity (maximum wind)

Speed of movement

Average radius to 30 kt (15 ms"^) wind

Average radius to 12 ft (4 m) seas

Three days after the point of recurvature, 14 of the 16

storms were still discernable from synoptic information. By the

fourth day, 11 were still noticeable and- by the fifth day, 9 of

the 16 still appeared. Figure 3 shows the mean day-to-day changes

of the four parameters for the 9 storms existing out to 5 days

after the point of recurvature.

It is interesting that such a large sample of the recurved

storms extended out to 4 or 5 days after recurvature, thus

presenting the severe maritime operational hazards indicated by

the parameters shown in Figure 3. The intensity (maximum wind)

decreases in the mean quite rapidly after the point of recurvature,

with the intensity of most typhoons decreasing more rapidly than

that of tropical storms. The speed of movement increases through-

out the period. The average radius to 30 kt (15 ms"^) wind

apparently does not decrease throughout the period, but appears

to oscillate near the mean. The average radius to 12 ft (4 m)

seas decreases, but the decrease is not significant.2

Notice that while the average maximum wind value decreases

sharply throughout the period, the extent of the area of 30 kt

(15 ms~ ) winds and 12 ft (4 m) seas remain as significant features.

A schematic depiction of the day-to-day variations of the azimuth-

ally averaged wind profiles for the storms existing to 5 days is

shown in Figure 4.

-,.. , '"'"!?^ intensity and speed changes were significant at the 1% level of a "t" test.

■8-

+2 +3 +4 DAY AFTER RECURVATURE

Figure 3. The mean day-to-day change in (a) intensity, (b) speed of movement, (c) radius to 30 kt (15 ms"-'-) wind, and (d) radius to 12 ft (4 m) seas for the 1971 tropical storms and typhoons examined that existed out to 5 days. The mean values of the parameters at the point of recurvature were: maximum wind, 78 kt (40.2 ms~l); speed, 10.6 kt (5.5 ms~l); radius to 30 kt (15 ms"!) wind, 171 n mi (317 km); and radius to 12 ft (4 m) seas, 307 n mi (569 km).

■9^

DISTANCE FROM CENTER (KM) 93 185 278 371

-31 52

50 100 150 200 DISTANCE FROM CENTER (N Ml)

Figure 4. Schematic presentation of day-to-day variation in the azimuthally averaged wind profiles for the recurved tropical storms and typhoons existing for 5 days after the point of recurvature. Profiles are shown based on a radius to maximum wind of 15 n mi (27.8 km).

-10-

2,3 REDURVED TROPICAL CYCLONES

A tropical cyclone's maximum wind will typically weaken soon

after the storm's recurvature (Riehl, 1972). The primary physical

processes associated with a tropical cyclone's weakening or decay

are those which degrade the release of latent heat, its main

energy-producing mechanism.

There are a number of environmental factors that can be

associated with a tropical cyclone's weakening: an increase in

vertical shear; decreased outflow aloft; the incursion of cool,

dry air; and the influence of cooler sea surface temperatures, 3

for example. The infrared images in Figure 5 show two periods

24 hours apart in the life of Typhoon June which demonstrate its

weakening. Figure 5a shows the storm just after recurvature when

it was entering a region of strong vertical shear and accelerating

rapidly to the northeast. Figure 5b shows the deep convection

reduced quite dramatically 24 hours later as the storm was

approaching the extratropical stage.

Perhaps the most noticeable weakening of a tropical cyclone

occurs with the incursion of cold, dry air at low levels or its

passage over cooler sea surfaces. In weaker tropical cyclones

the cold, dry air is normally observed to first penetrate the

center in the cold sector of front quadrants. In intense systems,

however, the first penetration of cold, dry air into the eye region

is often observed to occur in the warm sector.

The effect of cold, dry air in degrading the tropical system

is dependent on how cold and dry it is. Storms passing near land

areas where cold outbreaks retain their character, fill much more

rapidly than do storms at great distances from these land masses,

where the cold, dry air has been significantly modified by the

ocean waters. The cooler ocean waters also have a dramatic

influence on the intensity of tropical cyclones. In the peak

typhoon season (July-October) for example, the south-north sea

surface temperature gradient near 40°N, is approximately 1°C per

degree latitude (Robinson and Bauer, 1971).

^All satellite data are taken from the Defense Meteorological Satellite Program (DMSP).

-11-

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-12-

2.4 EXTRATROPICAL STAGE TRANSITION

The physical processes associated with a tropical cyclone's

transition to the extratropical stage are those which produce a

baroclinic environment whose tropospheric thermal and pressure

patterns are favorable for extratropical cyclogenesis .

Tropical cyclones that undergo "compound" transitions generally

have favorable tropospheric support for extratropical cyclogenesis .

The resulting extratropical cyclone often undergoes further

deepening, which is probably a response to the increased baro-

clinicity afforded by the convective latent heat associated with

the tropical cyclone. The tropical cyclone linking with a frontal

zone ("complex" transition) may or may not have favorable tropo-

spheric support for extratropical cyclogenesis . If support is not

present, the decay process will probably be slow and the tropical

cyclone will ultimately dissipate. If support is present, the

transition will result in extratropical cyclogenesis .

Figure 6a illustrates both the result of a "compound" and an

impending "complex" transition. The northernmost storm is an intense

extratropical cyclone which formed through a "compound" transition

with Typhoon Tess (September 1975), merging with a nearly stationary

mid-latitude low pressure center to the east of Hokkaido, Japan,

approximately one day before this image was recorded. The resulting

extratropical cyclone was considerably deeper than either of the

merging cyclones. The southernmost storm is Typhoon Winnie, which

is undergoing a "complex" transformation and is just about to merge

with an approaching front. Figure 6b shows the two storms 24 hours

later. Typhoon Winnie is becoming extratropical as it merges with

the frontal system.

It should be noted that a tropical cyclone transition from •

a tropical to extratropical storm is not an instantaneous event.

The periods of change can vary from hours to days, during which

time the systems are hybrids that exhibit certain characteristics

of both stages.

13-

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-14-

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A system in transition has ^ery complex wind and temperature

structures and occasionally produces dramatic increases in maximum

winds. These increases occur because the storm's own mechanism

for producing kinetic energy from available potential energy

(derived primarily from latent heat release) is suddenly enhanced

by the baroclinic conversion,' in the surrounding environment, of

potential energy to kinetic energy as cold air encounters, moves

under, and lifts the warm moist air within the tropical cyclone

circulation.

Ocean areas in which the transition process occurs are

generally distant from reconnaissance bases, so detailed observa-

tions of transition are somewhat sparse. The transition of Typhoon

Hope (October 1964) in the western North Pacific, however, was

extensively observed by WB-50 aircraft of the 56th Weather

Reconnaissance Squadron as it became extratropical while accelerating

to the northeast following recurvature. Aircraft were in the storm

area for approximately eight hours of each observing day.

Figure 7 shows analyses of Typhoon Hope by the Joint Typhoon

Warning Center (FWC/JTWC, 1964) as cold air was entering the

storm system. The sizeable increases in surface wind speeds,

which appeared to last for about 12 hours, emphasize the opera-

tional hazards that could exist for vessels in such sectors during

a storm's transition stage. No reconnaissance was flown the

following day to observe duration or extent of strong winds, but

available surface data indicate maximum surface winds of 45 kt

(23 ms"^ ) .

Based on other observations of this type of phenomenon by

reconnaissance aircraft in the western North Pacific, it appears

that this sudden increase in wind over a short duration and over

a small area is common during the transition stage, especially

for early and late season tropical cyclones when cold air intrusions

into low latitudes are more common.

Reconnaissance observations have also indicated that the

following are characteristics of tropical cyclones as they are

evolving into extratropical cyclones:

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16-

(1) Winds observed are in excess of the values that central

pressures would indicate. The maximum wind area does not always

relate to direction of storm movement (i.e., in right quadrant).

(2) There is a loss of cloud organization. The central area

of the storm fills with clouds with tops 6000-12,000 ft (1800-

3700 m). Clouds below 700 mb become stratiform. Remnants of the

wall cloud as observed by radar appear to rotate rapidly, and at

times the wall cloud completely disappears.

(3) There is a lack of torrential precipitation and many

times there is no precipitation.

(4) Light to moderate clear air turbulence (CAT) occurs over

a broad area surrounding the storm.

In a recent encounter with Typhoon Mary (December 1977) as It was becoming extratropical , a reconnaissance aircraft experienced a free fall of 2500 ft (760 m) in 4 sec near the storm center with an updraft immediately preceding it. In addition, a gust front (estimated surface winds of 80 kt/41 ms-1) was observed to pass near the storm center and hail was observed at the flight level of 700 mb.

17 -

3. FUTURE RESEARCH

This study has examined the characteristics and behavior of

extratropical storms that evolve from tropical cyclones. As is

true of many meteorological phenomenon at this scale, this evolu-

tionary process is a complex situation that is further complicated

by the lack of data for detailed studies. The fact that the storm

environment poses an operational hazard during the transition

period and many days afterward should encourage further synoptic

and dynamic examination of all the processes involved. Numerical

models capable of handling this scale could be used for diagnostic

examination of the rapid, short-duration intensification that

occurs. The results of such studies could give commanding officers

and ship routing officials a better understanding of the nature

and characteristics of storms at this stage in their life cycle,

as well as be of value to the weather centrals that issue warnings

of strong winds and high seas in the North Pacific Ocean.

-18-

ACKNOWLEDGMENTS

The authors express their appreciation to Mr. J. W.

Blelloch, Jr. of NEPRF for his assistance in data reduction. The

constructive comments of Dr. A. I. Weinstein and Mr. R. Fett of

NEPRF, and Captains F. Wells and R. Reamer of the USAF, are

gratefully acknowledged.

19-

REFERENCES

Burroughs, L. D. and S. Brand, 1973: Speed of tropical storms and typhoons after recurvature in the western North Pacific Ocean. J. Appl. Meteor. , 12, 3, 452-58.

Matano, H. and M. Sekioka, 1971a: On the synoptic structure of Typhoon Cora, 1969, as the compound system of tropical and extratropical cyclones. J. Met. Soc. Japan, 49, 282-95.

Matano, H. and M. Sekioka, 1971b: Some aspects of the extratropical transformation of a tropical cyclone. J. Met. Soc. Japan. 49, 736-743.

Riehl, H., 1972: Intensity of recurved typhoons 11 , 613-615.

J . Appl. Meteor

Robinson, M. K., and R. A. Bauer, 1971: Atlas of monthly sea surface and subsurface temperature and depth of the top of thermocline-North Pacific Ocean. Fleet Numerical Weather Central, Monterey, CA 93940, 96 pp.

Sekioka, M., 1970: On the behavior of cloud patterns as seen on satellite photographs in the transformation of a typhoon into an extratropical cyclone. J. Met. Soc. Japan, 48, 224-33.

Sekioka, M., 1972a: A kinematical consideration on behavior of a front within a typhoon area. Arch. Met. Geoph. Biokl., ser. A, 21, 1-12.

Sekioka, M., 1972b: Note on the extratropical transformation of a typhoon in relation with cold outbreaks. Arch. Met. Geoph. Biokl . , ser. A, 21 , 413-18.

U.S. Fleet Weather Central/Joint Typhoon Warning Center, 1964 and 1971: Annual typhoon report.

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