Statistical Considerations of Pressure Oscillations Occurring near the Center

by

Sadao Yoshizumi

Meteorological Research Institute,

(Received November 30, 1973)

Abstract

The aim of this paper is to make a statistical investigation of the pressure oscillations occurring near the center of a typhoon. An analysis of the barograms recorded on the Southwest Islands, Japan, during the decade from 1956 to 1965 shows that the occurrence frequency of the oscillations with the maximum double amplitude exceeding 3 mb is approximately 10%, and the frequency of those with the maximum amplitude of 2 mb or larger is about 30%. In order to get a clue as to the relationship of pressure oscillations to an elliptical , the reported frequency of an elliptical eye is also examined in this paper, based on the data obtained from flight observations. The reported frequency of an elliptical eye is estimated to be about 10%, and is considered to be fairly independent of the central pressure. Considerable agreement of the occurrence frequencies of both pressure oscillations and an elliptical eye suggests that pressure oscillations are closely related with an elliptical eye. From an examination of reports on an eye shape in cases where pressure oscillations were recorded, it is inferred that some of oscillations are associated with an elliptical eye.

1. Introduction It is well known that a barogram during the passage of a often shows oscillatory pressure variations with various periods, superimposed on the V-shaped trace accompanied by the storm system. Included among the pressure oscillations are pressure ripples with the period of 10 min to one hour and gusty oscillations with the period of 1 sec to 5 min (FUJITA, 1952). JORDAN (1962) names the former 'the Deppermann type' and the latter 'the Gherzi type' after the early investigators.

Whereas gusty or Gherzi-type oscillation is widely known to be the pressure fluc- tuation due to the aerodynamic effect of strong gusty winds (FUJITA, 1952), the Deppermann type has not been fully understood yet. Recently, MITSUTA and YOSHIZUMI (1968) have reported a pronounced pressure oscillation recorded at Miyakojima Weath- er Station in the Southwest Islands on September 5, 1966. This recorded oscillation lasted over half a day with the period of about one hour and had the maximum double amplitude of about 10 mb. In their later research (1973) it is pointed out that the pressure oscillation is assoicated with the counterclockwise rotation of the elliptical eye of a typhoon.

The occurrence frequency of this type of oscillation was investigated by DEP- PERMANN (1939). According to him, about one-third of barograms which recorded a minimum pressure of 740 mmHg (986.6 mb) and below showed a pressure vari- ation with the period of 10 to 30 min. However, since DEPPERMANN (1939) supposed some of them to be due to a disturbance such as thunderstorms, the pressure vari- ations treated by him may have included non-oscillatory perturbations accompanied by outer rain bands which surround the eye, as is clear from the investigations by LIGDA (1955), USHIJIMA (1958), TATEHIRA (1691) and others.

In this paper, a statistical consideration will be made on the pressure oscillations of Deppermann type that occur near the typhoon center. For this purpose an exami- nation will be made of the barograms recorded in the Southwest Islands, Japan, during the decade from 1956 to 1965. Furthermore, the occurrence frequency of an elliptical eye will be estimated by using the data on the eye configuration obtained from the flight observations. This estimation may give a clue for seeing whether this type of oscillations is related with the elliptical eye, like the case reported by MITSUTA and YOSHIZUMI (1973).

2. Occurrence frequency of pressure oscillations

The Southwest Islands, Japan, is a region over which tropical cyclones pass with very high frequency. It is in this region that the spectacular pressure oscil- 1974Statistical Considerations of Pressure Oscillations in Typhoon3 lations reported by JORDAN (1962) and MITSUTA and YOSHIZUMI (1968, 1973) were observed. In view of this fact, it is hoped that many other examples of pressure oscillations can be found out on the past records of barographs in this region, through they may probably be less remarkable. In this sense, this region is most suitable for the investigation of the occurrence frequency of pressure oscillations occuring near the typoon center.

The weather stations relevant to our investigation are (47936) , Miyako- jima (47927), Ishigakijima (47918) and Yonagunijima (47912) (see Fig. 1) . The barograms to be investigated are those recorded from 1956 to 1965, except for those recorded at the Yonagunijima station which was founded in November, 1956.

2. 1 Selection procedure of barograms to be examined Since our interest is focused on the pressure oscillatoins near the typhoon center, the barograms to be investigated are selected by the following procedure. Firstly, an object of investigation is confined to the barograms that indicate the minimum pres- sure below 980 mb, which is available from reports in the Bulletin of Ryukyu Weather Bureau. This procedure means that barograms at the time of which is weak or pass far away from the weather station are rejected from the investigation. Secondly, the tracks of typhoons which produce minimum pressure less than 980 mb were traced by using the data of their positions listed in the Geophysical Review published by the Japan Meteorological Agency. When it is judged from the drawn tracks that the typhoon does not approach within 50 km or so of the station with the minimum pressure below 980 mb recorded at it, the barogram at the station is again excluded. 2. 2 Result During the decade from 1956 to 1965 the number of cases in which the recorded minimum pressure was lower than 980 mb is 12 at Naha, 8 at Miyakoijma, 6 at Ishi- gakijima, and 5 at Yonagunijima, respectively, the total number being 31 for the four weather stations. Of 31 cases, 18 are judged to be associated with the typhoons that passed within 50 km or so of the stations (see Table 1).

For 18 cases selected above, barograms were collected and examined. Of 18 cases, 6 cases show oscillatory pressure variations with the maximum double amplitude of 2 mb or larger (Figs. 2-7). Brief description of these pressure oscillations will be made here. Detailed analyses will be made of some of them in another paper, together with some other examples.

Case 1 (Fig. 2) : This case was observed at Naha on September 8, 1956, during the (No. 5612) and has already been reported by JORDAN (1962). The recorded pressure oscillation has a period of about 20 min and a maximum double amplitude of about 6.5 mb. It is certain from the records of wind and rainfall that it occurred near the margin of the eye of the typhoon Emma, as JORDAN (1962) described. Case 2 (Fig. 3) : The second case was recorded at Miyakojima on September 25, 1957, during the passage of the typhoon Faye (No. 5714) . At the bottom of the trace the barograph shows a definite pressure variation with a period of about 15 min and a maximum amplitude of about 3.5 mb. There are some reports from the north- ern part of this island that stars could be seen during the calm that night, while at Miyakojima Weather Station situated in the southern part no star was to be seen, though the winds weakened for about 20 minabout the time of the pressure mini- mum (Geophysical Review, Sept. 1957) . It is therefore considered that the pressure variation was observed in the vicinity of the eye.

Case 3 (Fig. 4) : The pressure oscillation recorded at Ishigakijima on July 15, 1959, occurred during the typhoon Billie (No. 5905). As will be seen clearly in Fig. 4, the pressure varies with a period of about 30 min from 0730 to 0900 JST. The maximum total amplitude is about 2.5 mb.

The smoothed track of the typhoon, which was drawn based on the position data in the Geophysical Review, indicates that the typhoon did not approach within about 80 km of Ishigakijima. At the time when the pressure variation occurred, wind speed did not decrease remarkably. However, it may be considered that the typhoon passed closer to the station since its central surface pressure was estimated to be 970 mb and the observed minimum pressure at the station was 974.6 mb (the value 6 1974 in Table 1 is affected by shorter-period fluctuation). In fact, a report of the typhoon location from aircraft reconnaissance made at 0600 JST on the 15th suggests that the typhoon may have made its way closer to the station. It may therefore be con- sidered that the pressure variation was observed just outside its eye.

Case 4. (Fig. 5) : In this case Miyakojima Weather Station entered into the eye of the (No. 5914) on September 15, 1959. Superimposed on an almost flat variation within the eye, the periodic pressure variation was recorded with a period of about 15 min and the maximum double amplitude of about 2 mb. Although it is difficult to discern due to the rapid fall and rise of pressure, the pressure minima seem to be recorded at about 1845 an d1950 JST.

Case 5 (Fig. 6) : At Ishigakijima, during the passage of the typhoon Gloria (No. 6314) on September 10 to 11, 1963, the pressure oscillation with the period of 20 to 30 min was observed on the trace of barograph fro m2100 JST on the 10th to 0100 JST on the 11th. As is evident from the flat trace from 22 to 24 JST, the weather station entered into the eye of the typoon.

Case 6 (Fig. 7) : The last case was observed at Miyakojima on July 4, 1964, during the passage of the typhoon Betty (No. 6405). The period of the observed pressure variation is about one hour and its maximum amplitude is about 2 mb. But the pressure variation has a less regular shape. The southern part of this isladn was reported to be swept by the eye.

From an examination of 18 barograms with the minimum pressure below 980 mb during the typhoons which approached within about 50 km of the station, it was shown above that the occurrence frequency of pressure oscillations with the maximum double amplitude larger than 3 mb is only about 10 percent. It is about 30 percent that showed pressure oscillations with the maximum amplitude of 2 mb or larger. The estimated frequency is comparable to that obtained by DEPPERMANN (1939) .

3. Occurrence frequency of an elliptical eye MITSUTA and YOSHIZUMI (1973) state that a very pronounced pressure oscillation recorded at Miyakojima on September 5, 1966, resulted from the rotation of an elliptical isobar pattern associated with the elliptical eye of the typhoon Cora (No.

Table 2. Statistics on the eye configuration of typhoons with their central pressure of 984 mb or lower during the five-year period from 1961 to 1965. 1974Statistical Considerations of Pressure Oscillations in Typhoon9 6618). It is therefore of interest to examine how frequently typhoons have an elliptical eye. Of course, it is not yet confirmed whether or not pressure oscillations of this type are all produced by a disturbance of azimuthal wave number two around the eye, as indicated by the rotation of the elliptical eye. They may be produced by a disturbance of larger wave number and it is possible that, if non-rotating, even an elliptical eye does not bring about the pressure oscillation. At any rate, however, the occurrence frequency of the elliptical eye is expected to indicate to some extent the frequency of teh pressure oscillations.

For this reason the occurrence frequency of the elliptical eye was examined by making use of the data on the eye obtained by the flight observations, which are listed in the Geophysical Review. Table 2 indicates the result of the statistics on the eye configuration of typhoons with their central surface pressure of 984 mb or below during the period from 1961 to 1965. In this table, when two different diameters of 10S. YoshizumiVol. XXV No. 1 the eye are reported without any reference to its shape, the eye is classified as elliptical.

As is evident from the statistics, 10 percent of 564 reconnaissance reports showed the eye to be elliptical. It is noteworthy that the reported frequency obtained above is of the same order as the occurrence frequency of the pressure oscillation ex- amined in the previous section. Estimated frequency of relatively pronounced pressure oscillations with the maximum double amplitude larger than 3 mb is about 10 per- cent, and those with the maximum amplitude of 2 mb or larger about 30 percent. This fair agreement in both frequencies seems to support the supposition that the pressure oscillations near the typhoon center are closely related with an elliptical eye.

Let us now look over the eye configuration in the 18 cases treated in the previous section. Among 12 cases where pressure osccillations cannot be recognized, there are 11 cases where no reports can be found of the ellipitical eye within 12 hours of the time of closest approach of the typhoon to the stations. For a remaining case no information about the eye is available for the period under consideration.

Among six cases where pressure oscillations can be recognized, there are two cases (Cases 5 and 6) where reports of the elliptical eye can be found within 12 hours of the time of the cloest approach of the typhoon. It is possible in these two cases that the eye was elliptical during the occurrence of pres- sure oscillations, though its ellipticity might be small. In this respect it should be noted that even in the most pronounced pressure oscillation with the double ampli- tude exceeding 10 mb reported by MITSUTA and YOSHIZUMI (1973) , the major radius of the eye was 27 km and the minor one 18 km and thus its observed ellipticity is about 0.17 (= (the major radius—the minor radius) /the major radius) . In four other cases there are no reports of the elliptical eye within the period under consideration. We can therefore conclude that some of the pressure oscillations are associated with an elliptical eye, while other pressure oscillations may be produced by an eye with small ellipticity or by a disturbance with larger azimuthal wave number.

JORDAN (1961) describes that elliptical eyes are often reported in weaker storms but in deep storms the asymmetries are usually very small and it is rare when a shape other than circular is reported. From,1961 to 1965, the number of obser- vations of the elliptical eye is greater in higher central pressure and smaller in lower central pressure. More than half of the above estimated value of 10 percent are included within the range of central pressure between 984 and 965 mb. It must, however, be taken into account that the total number of observations tends to de- crease with lowering central surface pressure mainly because of the low occurrence frequency of the low central pressure. Fig. 8 illustrates the dependency of the relative reported frequency of the elliptical eye on the central surface pressure. It seems that this figure shows a decreasing tendency of the frequency with lowering central pressure. However, this need not be considered as significant since the number of the samplings is very small. It should rather be said that the frequency does not depend on the central pressure.

4. Summary In this paper a statistical investigation was made of pressure oscillations occur- ring near the typhoon center on the basis of the barograms recorded when typhoons approached within about 50 km of the station in the Southwest Islands, Japan, during 1974Statistical Considerations of Pressure Oscillations in Typhoon11 the decade from 1956 to 1965. From an examination of 18 cases with the minimum pressure below 980 mb it was shown that the occurrence frequency of Deppermann- type pressure oscillations near the typhoon center is about 10 percent in the case of those with the maximum double amplitude exceeding 3 mb, and about 30 percent in the case of those with the maximum amplitude of 2 mb or lagrer.

In order to get a elude as to the relationship of the pressure oscillations with an elliptical eye, the reported frequency of an elliptical eye was examined based on the data concerning the eye configuration obtained from flight observations. In the case of the typhoons with the central pressure of 984 mb or below, the frequency is estimated at approximately 10 percent, being almost independent of the central pressure oscillations. Considerable agreement of the occurrence frequencies of both pressure oscillations and an elliptical eye seems to support the suposition that the pressure oscillations are closely related with an elliptical eye.

Nevertheless, in some cases where the pressure oscillations could be recognized, no reports of elliptical eye could be found within 12 hours of the time of the closest approach of the typhoon to the station. It is therefore concluded that some of the pressure oscillations are associated with the rotation of an elliptical eye. Others may be produced by an eye with small ellipticity or by a disturbance with larger azimuthal wave number around the eye. Acknowledgements---The present study was done in part on a research fund from the Ministry of Education while the author was at the Geophysical Institute, Kyoto University. The author would like to express his hearty thanks to Prof. R. Yamamoto of the Geophysical Institute, Kyoto University and Prof. Y. Mitsuta of the Disaster Prevention Research Institute, Kyoto University for their valuable suggestions. He is also indebted to the staff members of Ryukyu Weather Bureau for giving full facilities for data collection and to Miss H. Imai for drafting figures.

References

DEPPERMANN,C. E., 1939: Some characteristics of Phillipine typhoons. Bull. Amer. Met. Soc., 20, 303-307. FUJITA, T., 1952: Pressure distribution within typhoon. Geophys. Mag., 23, 437-452. JORDAN,C. L., 1961: Marked changes in the characteristics of the eye of intense typhoons between the deepening and filling stages. J. Met., 18, 779-789.

------, 1962: Surface pressure oscillations in tropical cyclones. National Hurricane Re- search Project Report No. 50, 39-50. LIGDA, M. G. H., 1955: Hurricane squall lines. Bull. Amer. Met. Soc., 36, 340-342. MITSUTA, Y., and S. YOSHIZUMI, 1968 ; Characteristics of the Second Miyakojima Typhoon. Bull. Disaster Prevention Res. Inst., Kyoto Univ., 18, Part 1, 15-34.

------, 1973: Periodic variations of pressure, wind and rainfall observed at Miyakojima during the Second Miyakojima Typhoon. J. Met. Soc. Japan, 51, 475-485. TATEHIRA, R., 1961: Radar and meso-scale analysis of rainbands in typhoon-----Case study of Typhoon Helen (No. 5821). J. Met. Res., 13, 264-279. (in Japanese.) USHIJIMA, T., 1958: Outer rain bands of typhoons. J. Met. Soc. Japan, 36, 1-10. 台風中心付近で現われる気圧振動の 統 計 的 研 究 吉 住 禎 夫

1956年 か ら1965年 まで の10年 間 に南 西 諸 島 で 記 録 され た 気 圧 自記 紙 を も とに,台 風 中心 付 近 に おけ る10分 か ら1時 間 程 度 の 周 期 を もつDeppermann型 気圧 振 動 の 出現 頻 度 を 調 査 した 。 最 大 全 振 幅 が3mbを 越 え る もの は 約10%,2mb以 上 の も の は約30%の 出現 頻 度 を もつ 。 Mitsuta and Yoshizumi(1973)に よ る1解 析 例 の 如 く,こ の 種 の 気圧 振動 が 長 円 形 の 台 風 眼 の 回 転 に 伴 う現 象 で あ るか ど うか を 推 測 す る手 掛 りと して,長 円 形 眼 の報 告 頻 度 を も調 査 した。1961年 か ら1965年 まで の5年 間 の長 円 形 眼 の 報 告 頻 度 は 約10%で あ り,中 心 気圧 に強 く依 存 しな い。

両 頻 度 が 同 じオ ー ダ ーで あ る こ と,お よび 気 圧振 動 の若 干 の観 測 値 の調 査 か ら,気 圧 振 動 の一 部 が 長 円形 眼 に 関 係 して い る もの と推 定 され る。