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SOME NEW ASPECTS OF THE CLIMATOLOGY OF TYPHOONS OVER THE WEST PACIFIC OCEAN Yi-hui Ding* Department of Atmospheric Science Colorado State University Fort Collins, Colorado 80523 Lian-shau Chen Central Meteorological Bureau Beijing, People’s Republic of China ABSTRACT Based on about 30 years of typhoon data over the West Pacific and the coastal regions of China, somestatisticalaspectsoftheclimatologyof typhoons have been studied. Special emphasis is placed on studying those climatological features which may be related to the potential typhoon- caused damages of the coastal regions of China. 1. INTRODUCTION Many researchers have studied the climatology of typhoons over the West Pacific1’2’3y4’5*6’7’8. These studies provide very useful climatological background for further understanding the synoptic and dynamic aspects of typhoon formation and motion and improving the forecasting of typhoons. Recent- ly, the Central Meteorological Bureau of China, Beijing, edited and published the annual summary for the 30-year period of 1949-1978 in which many radar data in the coastal regions of China, satel- lite data and other typhoon data had been incorpor- ated. It became feasible to study the structure, formation and motion of typhoons on a climatologi- cal basis. The statistics of typhoons this paper presents are a part of the efforts in this aspect. In China, traditionally, the tropical cyclone is classified in three categories according to the sustained wind force of typhoons: severe typhoon (Ts) , with the maximum wind speed exceeding 32.7 mf s, typhoon (T) , with the maximum wind in the range from 17.2 to 32.6 mfs and tropical depression (D), with the maximum wind being less than 17.1 m/s. Basically, this classification is similar to that used in the United States for hurricanes. 2. SOME STATISTICAL ASPECTS OF THE GENESIS FREQUENCY AND STRENGTH OF TYPHOONS Tables 1 and 2 show the frequency of typhoon formation over the West Pacific (westward of 180O) by year and by month for about a 30-year period 1949-1978, respectively. The following facts should be noted: (1) Theannual cyclone (D+T+Ts) and 28.0, respectively. average numbers of tropical typhoons (T+Ts) are 37.3 and The annual average typhoon number over the West Pacific is 28. The variability of typhoon frequency is very large, ranging from a minimum of 18 per year within the 30-year period on record, to a maximum of 40. The annual percent departure from the mean typhoon number is also shown in Table 1. Its mean value is about 17%, which is 9% higher than that of the global number of tropical cyclones over the last 20 years, according to Gray’s’ sta- tistics. The annual percent departures from the mean range from +43% in 1967 to -36% in 1977. Therefore,inextremeyearstheannualtyphoon number may depart from its long-term mean by one- third to one-half of that mean number. This vari- ability is quite large. (2) Thevariabilityoftyphoonnumberby seasonandbymonthisalsoquitelarge. For example, the variation of typhoon number for the summer season (July-September) has ranged from 26 in 1967 to 7 in 1971, with a seasonal mean of 15. In July the typhoon number varies from 1 to 8, the monthly mean being 6.3. In August, the variation is from 3 to 10, with a monthly mean of 5.8; in September, from 2 to 9, with a monthly mean of 5.1. (3) In spring (April, May and June) and autumn (October and November) the interannual variation of the number of typhoons is very large. As examples of extremely low or high numbers of typhoon occurrences in spring we can consider the years 1954, 1958, 1959, 1969, 1971 and 1975, for autumn the years 1951, 1966, 1967, and 1976. Also in winter, especially in December, a high variabil- ity of typhoon formation is observed. For example, in 1952 and 1963 the typhoon activity was quite unusual. In February the typhoon number is gen- erally at a minimum. We hope to study the causes of unusual typhoon activity within and outside the main typhoon season in more detail. (4) In some years, for example in 1949, 1954, 1959, 1969, 1971 and 1975 none, or only a few, typhoons occurred in the first half of these years *Visiting scientist from the Institute of Atmospheric Physics, Academia Sinica, Beijing, People’s Republic of China. a47 CH1685-7/81/0000-0847 $00.75 0 1981 IEEE

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Page 1: [IEEE OCEANS 81 - Boston, Ma, USA (1981.09.16-1981.09.18)] OCEANS 81 - Some New Aspects of the Climatology of Typhoons over the West Pacific Ocean

SOME NEW ASPECTS OF THE CLIMATOLOGY OF TYPHOONS OVER THE WEST PACIFIC OCEAN

Yi-hui Ding* Department of Atmospheric Science

Colorado State University Fort Collins, Colorado 80523

Lian-shau Chen Central Meteorological Bureau

Beijing, People’s Republic of China

ABSTRACT

Based on about 30 years of typhoon data over the West Pacific and the coastal regions of China, some statistical aspects of the climatology of typhoons have been studied. Special emphasis is placed on studying those climatological features which may be related to the potential typhoon- caused damages of the coastal regions of China.

1. INTRODUCTION

Many researchers have studied the climatology of typhoons over the West Pacific1’2’3y4’5*6’7’8. These studies provide very useful climatological background for further understanding the synoptic and dynamic aspects of typhoon formation and motion and improving the forecasting of typhoons. Recent- ly, the Central Meteorological Bureau of China, Beijing, edited and published the annual summary for the 30-year period of 1949-1978 in which many radar data in the coastal regions of China, satel- lite data and other typhoon data had been incorpor- ated. It became feasible to study the structure, formation and motion of typhoons on a climatologi- cal basis. The statistics of typhoons this paper presents are a part of the efforts in this aspect.

In China, traditionally, the tropical cyclone is classified in three categories according to the sustained wind force of typhoons: severe typhoon (Ts) , with the maximum wind speed exceeding 32.7 mf s , typhoon (T) , with the maximum wind in the range from 17.2 to 32.6 mfs and tropical depression (D), with the maximum wind being less than 17.1 m/s. Basically, this classification is similar to that used in the United States for hurricanes.

2. SOME STATISTICAL ASPECTS OF THE GENESIS FREQUENCY AND STRENGTH OF TYPHOONS

Tables 1 and 2 show the frequency of typhoon formation over the West Pacific (westward of 180O) by year and by month for about a 30-year period 1949-1978, respectively. The following facts should be noted:

(1) The annual cyclone (D+T+Ts) and 28.0, respectively.

average numbers of tropical typhoons (T+Ts) are 37.3 and

The annual average typhoon number over the West Pacific is 28. The variability of typhoon frequency is very large, ranging from a minimum of 18 per year within the 30-year period on record, to a maximum of 4 0 . The annual percent departure from the mean typhoon number is also shown in Table 1. Its mean value is about 17%, which is 9% higher than that of the global number of tropical cyclones over the last 20 years, according to Gray’s’ sta- tistics. The annual percent departures from the mean range from +43% in 1967 to -36% in 1977. Therefore, in extreme years the annual typhoon number may depart from its long-term mean by one- third to one-half of that mean number. This vari- ability is quite large.

(2) The variability of typhoon number by season and by month is also quite large. For example, the variation of typhoon number for the summer season (July-September) has ranged from 26 in 1967 to 7 in 1971, with a seasonal mean of 15. In July the typhoon number varies from 1 to 8, the monthly mean being 6.3. In August, the variation is from 3 to 10, with a monthly mean of 5.8; in September, from 2 to 9, with a monthly mean of 5.1.

(3) In spring (April, May and June) and autumn (October and November) the interannual variation of the number of typhoons is very large. As examples of extremely low or high numbers of typhoon occurrences in spring we can consider the years 1954, 1958, 1959, 1969, 1971 and 1975, for autumn the years 1951, 1966, 1967, and 1976. Also in winter, especially in December, a high variabil- ity o f typhoon formation is observed. For example, in 1952 and 1963 the typhoon activity was quite unusual. In February the typhoon number is gen- erally at a minimum. We hope to study the causes of unusual typhoon activity within and outside the main typhoon season in more detail.

(4) In some years, for example in 1949, 1954, 1959, 1969, 1971 and 1975 none, or only a few, typhoons occurred in the first half of these years

*Visiting scientist from the Institute of Atmospheric Physics, Academia Sinica, Beijing, People’s Republic of China.

a47

CH1685-7/81/0000-0847 $00.75 0 1981 IEEE

Page 2: [IEEE OCEANS 81 - Boston, Ma, USA (1981.09.16-1981.09.18)] OCEANS 81 - Some New Aspects of the Climatology of Typhoons over the West Pacific Ocean

Table 1 Frequency of tropical cyclones over the northwest Pacific (including the South China Sea) by year and category.

l- West Pacific (including South China Sea) I South China Sea t - T

- 12 17 5

10 9 8 8 5 4

11 7 8 13 8 5 13 13 11 15 6 8 15 11 8

11 21 8 8

278 - - >.9 -

- T

- 2 2 1 2 4 4 1 2 1 5 2 2 5 3 2 3 5 1 0 2 2 5 2 2 5 5 2 0

72 - -

- 2.6

- tS

- 0 0 0 1 0 0 2 0 1 1 0 2 2 1 0 3 1 4 1 1 0 1 0 5 4 0 1 3

34 - - 1.2 -

D

- 6 10 10 5 8 10 9 14 5 3 8

10 10 5 8 4 9 9 13 9 10 21 15 6 5 4 7 4

237 - - 8.5 -

Ts

__ 15 15 15 21 17 15 20 18 18 22 17 22

24 20

20 23 19 24 25 23

12 14

25 23 13 16 15 17

T+Ts

- 27 32 20 31 26 23 28 23 22 33 24 30 33 32 25

32 36

40 35

29 22 27 36

24 31

37 23 25

T+Ts

- 2 2 1 3 4 4 3 2 2 6 2 4 7 4 2 6 6 5 1 3 2 6 2 7 9 5 3 3

DtTtTr

- 3 6 4 4 7 8 7

1 1 4 8 7 10 9 7 7 6

10 8 6 5 5 12 7 8 12 5 5 4

D*T+Ts

34

37 37 9 27 2

2 32 36

5 40 6 43 2 37 3

5 40 33

0 41 4

1949 1950 1951 1952 1953 1954 1955 1956 1957 1958 1959

1961 1960

1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976

44

32 2 38 5 53 3

29 2 30 0 41 3 29 1 37 5 51 6 48 3

1

528 __ 18.9 __

806 __ 28.8 -

195 - 7.0 __

Total

Average

Table 2 Frequency of typhoon formation over the northwest Pacific Ocean by year and month.

Year 1949 1950 1951 1952 1953 1954 1955 1956 1957 1958 1959 1960

1962 1961

1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976

1978 1977

Aver Total

Fmm 30 years X Deviation

+I4 -4

-29 +I1

-18 -7

0

Jan. 1 0 0 0 0 0 1 0 2 1 0 0 1 0 0 0 2 0 0 0 1 0 1 1 0 1 1 1 0 0 14

Feb. 0 0 0 0 1 0 1 0 0 0 1 0 0 1 0 0 1 0 1 1 0 1 0 0 0 0 0 1 0 0 9

0 . 3

Mar. 0 0 1 0 0 1 1 1 0 0 1 0 1 0 0 0 1 0 2 0 1 0 1 0 0 1 0 0 0 0 12

0.4

Apr

1 2 0 0 0 1 2 1 1 1 1 0 1 1 0 1 1 1 1 1 0 3 0 0 1 0 2 0 1

24

Hay June July 0 - 1

Aug

9 3 5 6 6 7 5 4 5 6 10 6 7 3 6 6 10 9 8 4 6 5 5 6 7 5 4 3 6

175

Sept. Oct 3

Nov. 4 3 1 3 4 3 1 3 3 3 3 1 2 3 0 5 2 3 3 5 3 4 2 2 3 3 3 1 2 2 80

2.7

Dec. 2 2 2 5 2 1 1 1 0 2 2 1 1 2 3 1 0 1 1 0 1 0 0 3 1 2 1 2 1 0

41

1.4

lotal 27 32

31 20

26 23 28 23 22 33 24 30 33 32 25 36 32 35 40 29 22 27 36

24 31

31 21 24

26 18

841

28.0

5 5 2 3 2 1 6 3 1 7 2 3 5 7 5 7 5 4 8 3 3 4 7 6 7 4 2 4 5 4

130

8 6 2 5 3 6 3 5 5 6 4 6 6 4 5 7 7 9 9 3 5 4 6 6 3 3 4 4 3 5

152

5.1

I 2 2 4 3 0 2 2 1 3 0 3 3 0 4 2 2 1 1 2 0 3 3 3 0 3 0 2 1 3

56

3 4 6 4 4 4 1 4 3 4 4 5 5 4 6 2 4 4 5 3 5 4 4 4 5 5 1 3 5

118

3.9

1 0 1 1 0 0 1 2 0 1 3 2 0 2 3 2 1 1 0 0 4 1 0 1 0 2 0 0 30

+14 +25 +43 +4 -21 -4 +29 +11 - 14 +11 -25 -14 -36 -7

17 0.8 1.0 1.8 4.3 5.8 age 0.5

causing prolonged relatively calm periods over the West Pacific and the South China Sea, respectively. Pacific Ocean. Over the West Pacific, a single peak with an aver-

age number of 6 may be seen in August. The minimum (5) The average numbers of tropical cyclone number of typhoons occurs in February. In the

and typhoons over the South China Sea are 7 and typhoon season (June through October), on the 3 . 8 , respectively. average, 22 typhoons form which account for 75% of

the total typhoon number for the whole year. The Figures 1 and 2 show the monthly distributions activity of typhoons in the West Pacific culminates

of genesis frequency of tropical cyclones over the in July, August and September. Interestingly, the

848

Page 3: [IEEE OCEANS 81 - Boston, Ma, USA (1981.09.16-1981.09.18)] OCEANS 81 - Some New Aspects of the Climatology of Typhoons over the West Pacific Ocean

Fig. 1 Monthly frequency of tropical cyclones over the West Pac i f ic ( inc luding the South China Sea).

2 ....A\

Mom" .I*% FEB "An *PI1 MA" ,""E N L " LUG SEPI . O C T NO". DEC

Fig. 2 Monthly frequency of tropical cyclones over the South China Sea.

peak of the monthly distribution for the South China Sea appears i n September r a t h e r t h a n i n August. This may b e r e l a t e d t o t h e a c t i v i t y o f t h e Indian summer monsoon, s ince a s imi la r peak of formation frequency of tropical depressions over t h e Bay of Bengal also occurs in September and October.

The s i g n i f i c a n t changes i n t h e s i z e o r s c a l e and s t rength of typhoons are of ten observed and very much dependent on the season. Here, w e def ine an a r ea where a t l e a s t t h e wind force of number 6 in the Beaufor t sca le i s measured a s t h e s i z e o f a typhoon. Figure 3 shows t h a t t h e l a r g e s t s c a l e o f typhoons occurs in October with an average diameter of 8.6 latitude. During the major typhoon season (July, August and September), the size of typhoons i s r e l a t ive ly sma l l . This s t a t i s t i c is s i m i l a r t o

tha t ob ta ined by Brand . Figure 4 shows the annual d i s t r i b u t i o n of occurrence freguency of very severe typhoons with the maximum winds exceeding 50 m/s. The peak of the f requency d i s t r ibu t ion i s observed i n September with an average of 3 3 . 3 , which accounts for 25.6% of t h e t o t a l number f o r t h e whole year .

4

8

,A* I E S . M A R A P I L.*V ,"%E JUL" ALG SEP, O C T -0". OEC

Monthly d i s t r i b u t i o n of the diameter of the region of wind force of number 6 i n the Beaufort wind s c a l e .

"0"T" ,AH. FEB. "*R API I . " A " ,"HE JUL" AUB. SEPT. OCT. "0". DEC.

Fig . 4 Monthly d i s t r i b u t i o n o f t h e numbers o f occurrence time and percentage of severe typhoons (maximum wind 2 50 m/s). Counts are based on every 6-hour typhoon report.

An important problem i n t h e typhoon p r e d i c t i o n i s t o f o r e c a s t t h e change i n s t r e n g t h of typhoons b e f o r e t h e i r l a n d f a l l . I n terms o f t he cen t r a l maximum wind and minimum su r face p re s su re , t he changes i n s t r e n g t h of typhoons broadly f a l l i n t o two types: Type I which is character ized by a gradual decrease i n s t r e n g t h 36 hours before land- f a l l , and Type I1 which i s character ized by a p rogres s ive i n t ens i f i ca t ion 36 hours before land- f a l l . The s t a t i s t i c s i n d i c a t e t h a t most of t h e c a s e s f a l l i n t o Type I , e s p e c i a l l y i n t h e c o a s t a l region of North China (see Fig. 5) . The magnitudes of decrease or increase in the maximum and minimum pressure 6-12 hours before l andfa l l a re g iven in Table 3.

Table 4 g i v e s t h e s t a t i s t i c s o f t h e s c a l e and shape of the typhoon eye which is near ly coincident wi th the r ing o f maximum wind speed. The v a r i a t i o n of the diameter of typhoon eyes range from several km to severa l hundred km. Host of the typhoons assume the closed-eye shape with an average dia- meter of the eye of 41 km ( see F ig . 6 ) .

849

Page 4: [IEEE OCEANS 81 - Boston, Ma, USA (1981.09.16-1981.09.18)] OCEANS 81 - Some New Aspects of the Climatology of Typhoons over the West Pacific Ocean

Fig.

The spectrum of frequency of the diameter of typhoon the eye.

O U Y E I E * OF E I E 1t.l

6 The changes in strength of typhoons before landfall on the different coastal areas of China. Thin line denotes Type I, dashed line denotes Type 11, and thick line indicates the average condition. The numbers in parentheses are case numbers studied.

Table 3 Scale (km) of the different shapes of typhoon eyes.

Numbers of flaximum fliniom Diameter Diameter Diameter

Average

Circular

open 100

3. SOME STATISTICS OF THE TRACKS OF TYPHOONS

Among all coastal regions of the mainland of Asia, China is greatest affected by typhoons. During the major typhoon season, the number of typhoons landing on China is several times the numbers landing on the other Pacific countries. In July, August and September the typhoons landing on China account for 48% and 40% of the totals for the corresponding months, respectively. Secondly, the Philippines and Japan are very frequently hit by typhoons, especially in October, November, and December for the Philippines and in July and August for Japan. The same typhoons may go far northward and land on the far east territory of the Soviet Union.

In China, about 4 / 5 of all 30 provinces have been affected or hit by typhoons, but the most seriously affected province is Guandong (Canton) in South China, especially in the region to the west of the delta of the Pearl River. During the major typhoon season (July, August, and September) the percentage for Taiwan, Fujiamg and Zhejiang prov- inces are also very high, especially in July for Taiwan (53%). Along the coastal region of North China (about northward of 30'N) the cases of the direct landfall of typhoons are very seldom ob- served.

One cannot find two identical tracks even in the long-term records of typhoon paths. But we may classify the paths of typhoons based on similar, preferred tracks which are believed to represent certain types of major trends of movement of ty- phoons. Figure 7 schematically shows the classifi- cation of typhoon paths based on typhoon data

Table 4 Average magnitude of decrease in strength of typhoons 6 and 12 hours before landfall on the different coastal regions of China. AV is the change in the maximum wind speed and AP is change in central minimum surface pressure,

I China

Location Banan Fujiamg t o Guandong Taiwan Shanghai

-6 AV mls

ab A€' H I .

-2

0 5 2

-4 -2 -4

3

-12 AV m l s

mb Ae A r .

-2

5 7 3 -1

-5 -3 -6

Shandong t o Liangning

- 1

0

-1

2

-3

3

-5

6

--

Page 5: [IEEE OCEANS 81 - Boston, Ma, USA (1981.09.16-1981.09.18)] OCEANS 81 - Some New Aspects of the Climatology of Typhoons over the West Pacific Ocean

Fig. 7 Schematic presentation of the classifi- cation of typhoon paths over the West Pacific.

during the period 1814-1974 (90 years). Seven typical paths have been obtained. Type I, which is characterized by recurving northward far away from land in the open ocean has the highest percentage. Type VI1 is very rarely observed and is character- ized by an abrupt westward turning at higher lati- tudes (2!j0-3OoN). The serious threat for China comes from Types 111-1, IV, V-I, VI, and VII, and especially from Types 111-1 and V-I, which show a sudden turning by a large angle during the track segment in the adjoining sea regions before land- fall so that the people in the hit area hardly have time to take any measures of preparedness.

Sometimes some typhoons do not move along the above-mentioned typical paths, but along very unusual or erratic paths. This phenomenon renders typhoon track prediction exceedingly difficult. Unit1 we learn much more about the reasons of the very complex paths of typhoons, we can hardly expect great improvement. Much research is being concentrated on this problem. Ten types of unusual paths of typhoons may be classified and are sche- matically shown on Fig. 8. Types 1 through 3 deal with the abrupt turning of the movement direction by a very large angle. Type 4 describes the rota- tion of binary typhoons. Type 5 demonstrates the irregular oscillation of typhoon paths. Types 6 and 7 show that the typhoons make one or several small loops in a limited area when they suddenly tend to be stagnant or slow down. Type 8 indicates the straight path without any oscillation or turn- ing and stalling during the whole period of life.

h + + +

+ +

t+ A +- Fig. 8 Schematic diagram of ten types of erratic

typhoon paths.

Types 9 and 10 show the sudden acceleration or deceleration of movement of typhoons. Statistics show that a typhoon may move along a certain er- ratic path with 28% of probability in July, August and September. Among them, the looping path is most probable.

ACKNOWLEDGMNTS

The authors are grateful to Dr. E.R. Reiter for his comments and suggestions. Thanks also go to Messrs. Y.S. Fang and S.M. Chen for their sta- tistical computations, and to Mrs. Bonnie Grantham for typing the manuscript. This research was performed under the National Science Foundation Grant No. ATM78-17835, Climate Dynamics Program, Atmospheric Science Division.

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REFERENCES

Gray, W.M., 1967: Global view of the origin of tropical disturbances and storms. @. @. g . , 96 ( lo) , 669-700. Tse, 1972: Typhoon Climatology. E, 321, 82-171.

Bell, G.J. and Kar-sing Tsui, 1973: Some typhoon soundings and their comparison with soundings in hurricanes. 2. m. Meteor., - 12 (1).

Brand, S . , 1972: Very large and very small typhoons of the western North Pacific Ocean. - J. E. E. of Japan, 50 ( 4 ) , 332-341.

-, 1973: Rapid intensification and low latitude weakening of tropical cyclones of the western North Pacific Ocean. J. w. Meteor., 12 ( l ) , 94-103.

- and J.W. Blelloch, 1973: Changes in the characteristics of typhoons crossing the Philippines. J . A m . Meteor., 11 (l), 104- 109.

- and -, 1974: Changes in the char- acteristics of typhoon crossing the island of Taiwan. w. e. Rev., 102, 708-713. Chen, Liang-shau and Yi-hui Ding, 1979: In- troduction to typhoons over the West Pacific Ocean. Science Press, Beijing, 491 pp.

Gray, W.M., 1979: Hurricanes: Their forma- tion, structure and likely role in the tropi- cal circulation. Meteorology over the tropical oceans. Edited by D.B. Shaw, Royal Meteorological Society, Brackwell, Britain.

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