December 1973 Yasushi Mitsuta and Sadao Yoshizumi 475

Periodic Variations of Pressure, Wind and Rainfall Observed at Miyakojima during the Second Miyakojima

By Yasushi Mitsuta Disaster Prevention Research Institute, Kyoto University, Kyoto and Sadao Yoshizumi Meteorological Research Institute, (Manuscript received 17 July 1973)

Abstract

During the passage of the Second Miyakojima Typhoon the barograph at Miyakojima recorded a pronounced pressure oscillation with the period of about 50 min on September 5, 1966. The recorded oscillation extended over half a day and had a maximum double amplitude exceeding 10mb. Simultaneously, wind and rainfall intensity showed periodic variations with the approximately same period, too. It is shown that the periodic variations of these elements were associated with the counter- clockwise rotation of an elliptical revealed by radar observation.

which had a period of 20 to 40min and 1. Introduction maximum double amplitude of about 9mb. During the passage of a a Recently the most spectacular one of pressure barograph trace usually describes a smooth V- oscillations that have ever been known was shaped variation on the time scale of about half recorded at Miyakojima on September 5, 1966 a day. Pressure variations with various shorter during the passage of the Second Miyakojima periods are frequently found out to be superim- Typhoon (Typhoon 6618, Cora) (Mitsuta and posed on such a smooth variation. One with a Yoshizumi, 1968). The barograph described large amplitude has been known as "pumping" remarkably regular oscillation with the period of of the barometer (Tannehill, 1956). Fujita (1952) about 50min, which extended over half a day classified them into two groups, i.e., oscillatory and had a maximum double amplitude exceeding and non-oscillatory perturbations. 10mb. In addition, wind and rainfall intensity Oscillatory pressure perturbations include the also showed appreciably periodic variations. barometric ripples with the period of 10 min to In the present paper, a detailed analysis will one hour and the gusty oscillations with the be made of the periodic variations of meteoro- period of one sec to 5min. The latter is logical elements observed at Miyakojima during considered as the pressure fluctuation due to the the Second Miyakojima Typhoon. It will be aerodynamic effect of strong gusty winds (Fujita also shown that these variations have a definite 1952). However, the former type of pressure phase relation to each other and that they are oscillation has not been fully understood yet. associated with rotation of the elliptical eye of According to Deppermann (1939), about one-third the typhoon revealed by radar observation at the of barograms of tropical storms which recorded Miyakojima Weather Station. a minimum pressure of lower than 740mmHg (986.6mb) showed a pressure oscillation with the 2. Periodic variations of meteorological elements period of 10 to 30min. A spectacular example The Second Miyakojima Typhoon, which of such oscillation is that observed at brought about the pressure oscillation at during Typhoon Emma of 1956 (Jordan, 1962), Miyakojima, appeared as a weak tropical 476 Journal of the Meteorological Society of Japan Vol. 51, No. 6 depression to the northeast of Isl, on easily in Fig. 2 that the wind underwent periodic August 29, 1966. The depression developed into fluctuation in speed and direction both with nearly a typhoon on Aug. 31 and continued to move the same period as that of the pressure oscillation, on northwestward toward the Southwest Islands, and that the variation in wind speed began to its central pressure deepening. On the morning be seen at about the same time when the pressure of September 5 when it hit Miyakojima, it oscillation did. attained its maximum intensity of 918mb. The It is also evident from a regular stepped typhoon moved west-northwestward crossing the character of the rainfall curve (Fig. 3) that there East China Sea until it landed on the Continent occurred markedly periodic variation in rainfall of China on Sept. 7 and filled up rapidly. intensity at the Miyakojima Weather Station. Fig. 1 shows the barogram which was recorded The fact that the variations of these elements at the Miyakojima Weather Station during the have nearly the same period suggests that they typhoon on September 5, 1966. In this figure are closely related to each other. In order to pronounced pressure oscillations with the period make clear the relationship, Figure 4 was of about one hour stands out clearly from the prepared. In this figure a smoothed curve gradual variation that should be associated with represents the running mean of 11 readings at the typhoon passage. The pressure oscillation is 10-min intervals, i.e., the average value over very much more spectacular than that reported about two periods, and a deviation is one from by Jordan (1962) in the amplitude and the the smoothed value thus estimated. Winds were duration. It lasted for a long time of about read out at the middle points of broadly 12 hours from 05 to 17 JST on the 5 th. fluctuating traces. In Fig. 2 is reproduced the wind record at It is clear in Fig. 4 that the pressure oscillation Miyakojima. During the occurrence of the pressure oscillation wind direction generally veered from the northeast to the south and wind speed did not decrease to calm, indicating that the center of the typhoon approached from the southeast and passed on the southwest of this island toward the northwest. It can be seen

Fig. 2. The wind record at the Miyakojima Weather Station on September 4-5, 1966. Wind speed recording range is switched from 60m/sec into 120 Fig. 1. The barogram recorded at the Miyakojima m/sec full scale at 0420 JST on the Weather Station on September 5, 1966. 5th.

Fig. 3. The rainfall record at the Miyakojima Weather Station on September 5, 1966. December 1973 Yasushi Mitsuta and Sadao Yoshizumi 477

Fig. 4. Detailed change of meteorological elements at Miyakojima.

shows the two amplitude maxima of about *5mb pressure is falling, wind deflects northward, that at about two and a half hours before and after is, the radial component of the disturbed wind the time of smoothed pressure minimum at 1100 is inward. While the pressure is rising, on the JST. As the smoothed pressure decreases, that other hand, wind deflects southward, that is, the is, the typhoon approaches toward Miyakojima, disturbed radial component is outward. Such the pressure oscillation increases in amplitude phase relation of wind direction was kept up to the first maximum at about 0830 JST and with the pressure variation until 1500 JST. But then decreases to a minimum of *2mb at about it cannot be confirmed whether such relation 1100 JST. Once the smoothed pressure rises, was still maintained thereafter because unfortu- the oscillation begins to grow again until its nately the wind-mill type anemometer was blown amplitude reaches the second maximum at about down by strong winds about 1530 JST and hence 1330 JST. Thereafter it decays gradually. the traces for one hour before that time seems As can be seen from Fig. 4, it took 640 minutes to be unreliable due to the swaying of the wind from 0615 to 1655 JST for the pressure oscilla- vane. tion to undergo 12 periods completely. Therefore, The phase of the variation of wind speed to the averaged period of the pressure oscillation is that of pressure turned reverses two times at estimated to be 53min. Relation between the about 0930 and 1230 JST. Wind speed shows a pressure variation and the changes of other variation out of phase with the pressure oscilla- elements is also seen in this figure. While the tion until about 0930 JST; that is, stronger winds 478 Journal of the Meteorological Society of Japan Vol. 51, No. 6 blow at the time of the pressure minima and weaker winds at the the pressure maxima. During the period from 0930 to 1230 JST the phase of wind speed is inphase and wind speed is stronger when the pressure deviation is positive and weaker when the pressure deviation is negative. At about 1230 JST the wind speed comes to have a reverse phase with the pressure. The wind speed trace is also not available owing to the anemometer blown off. The rainfall intensity showed a periodic variation similar to the wind speed; that is, from about 0930 to 1230 JST rain was heavier when the pressure was higher and lighter when the pressure was lower. Before and after that period the relation between both variables was Fig. 5. Radar photograph taken at opposite and the maximum intensity of rainfall Miyakojima at 0453 JST on and the minimum of the pressure coincided in September 5, 1966. occurrence time. However, the rainfall record from 1300 to 1500 JST indicates a constant (1962, 1964) reported that the configuration of intensity. According to the staffs of the weather the eye changed from circular to elliptical and station, it is said that some obstacles fallen in then the major axis rotated in the cases of some the receiving funnel of the rain gauge prevented hurricanes. In a few Imakado (1966) the precipitating water from flowing smoothly found an elliptical eye rotating counterclockwise into the gauge. at a angular speed of one rotation per two to In the uppermost part of Fig. 4, the position six hours. of the typhoon center relative to the Miyakojima It is very interesting that the Second Miyakojima Weather Station determined by Mitsuta and Typhoon had an elliptical eye while the pressure Yoshizumi (1968) is shown. During the occurrence oscillation was occurring. Fig. 5 is a radar of the pressure oscillation the center was located photograph taken at Miyakojima at 0453 JST, within 50km from the station. The estimated Sept. 5, which reveal that the eye of the typhoon distance between the center and the station was was elliptical. In addition, its major axis of the about 25km at the times when the amplitude of elliptical eye was rotating counterclockwise at an the pressure variation showed the maximum, and angular speed of half a rotation per about one about 22km when the phase changes of wind hour, as shown in Fig. 6. Unfortunately radar speed and rainfall intensity occurred. observation could not be helped stopping at 0630 As shown in the lowermost part of Fig. 4, JST since strong winds made it difficult to scan the tidal record draws alternate rising and the radar antenna. Consequently we have no lowering of the sea level with the period of continuous information on the size and the about 50min. Its double amplitude, which did configuration of the eye after that time. not exceed 30cm by 1000 JST, increased However, some other discrete data are available, gradually toward a maximum of 80cm at 1400 which were obtained by five weather recon- JST and then decayed rapidly. Further description naissance flights from 0700 on the 5th to 1102 will not be given in the present paper, though JST on the 6th. All flight reports after 063& it is an interesting phenomenon to reveal the JST show that the eye of the typhoon was mechanism of the high tide caused by the elliptical (see Table 1). It may be therefore typhoon. inferred from these that the eye of the Second Miyakojima Typhoon remained elliptical during 3. Rotation of an elliptical eye of the typhoon the period of one day and a half. Radar observations often reveal that the It is noteworthy that the rotation period of tropical cyclone has not a circular but an the eye is about twice that of the pressure elliptical eye rotating counterclockwise. Sadowski oscillation. The rate of rotation is estimated to December 1973 Yasushi Mitsuta and Sadao Yoshizumi 479

Fig. 6. Radar echo distribution during the period from 0445 to 0631 JST on September 5, 1966. The stippled area shows where radar echo is confused.

be 180 deg per 58 min from radar photographs 0630 JST when the pressure minimum was taken from 0459 to 0631 JST. Furthermore, we observed, the major axis was oriented NNW-SSE can find out an interesting fact when we look and the station was situated on the side of the over the correspondence between the phase of major axis. the pressure oscillation and the orientation of The above-mentioned facts lead us to the the elliptical eye during the period from 0500 to inference that the pressure deviation has a 0630 JST (see Figs. 1 and 6). At 0600 JST negative sign on the side of the major axis and when the pressure maximum was observed, the a positive sign on the side of the minor axis. major axis of the eye was oriented ENE-WSW If such correspondence was hold after 0630 JST, and the Miyakojima Weather Station was situated the rotation rate of the eye relative to the earth on the side of the minor axis. At 0530 and can be evaluated from the period of the pressure 480 Journal of the Meteorological Society of Japan Vol. 51, No. 6 oscillation by taking account of azimuth change Thus the rotation rate relative to the earth is of the typhoon center to the station due to its estimated to be 180deg per 57min (=64 min movement. During the 640-min period from 180 deg/(360 deg*6-142deg)). This estimated* 0615 to 1655 JST, the pressure variation value is in good agreement with the rate deter- underwent 12 periods, which should correspond mined from radar photographs, 180deg per to 6 rotations (=360deg*6) and the azimuth 58 min. change of the typhoon center was 142deg. By the use of the rotation rate obtained above, the orientation of the major axis of the elliptical Table 1. The configuration and the size of the eye can be fixed for the period from 0500 to eye reported by aircraft reconnaissance 1800 JST. The fixed orientation is illustrated at 30-min intervals in Fig. 7. For the period during which radar observation was not made, the estimated orientation can be compared with that obtained by three flights, as listed in Table 1. Reconnaissance aircraft reported that the major axis was oriented E to W at 0700, NE to SW at 1100 and NW to SE at 1800 JST, while the estimated orientation, shown in Fig. 7, at corresponding time is 074 to 254deg, 038 to 218 deg and 154 to 334 deg (measured clockwise from

Fig. 7. The path of the Second Miyakojima Typhoon and the orientation of its elliptical eye. Small figures on the left indicate the orientation of the black part of the eye measured clockwise from the north. December 1973 Yasushi Mitsuta and Sadao Yoshizumi 481

the north), respectively. This good agreement the pressure oscillation. The size is reported to supports the inference that the elliptical eye be 50*60km at 0600 JST by radar observation, continued to rotate as before 0630 JST*. 36*54km at 1100 JST and 47*67km at 1800 In the case of Typhoon Emma, Jordan (1962a) JST by flight observation (see Table 1 and Fig. discussed possible causes of this type of pressure 6). oscillation : 1) oscillatory motion or pulsation Fig. 8 shows the time change of the relative of the typhoon system, 2) dynamic pressure position of the Miyakojima Weather Station to effects by topography, 3) convective cells in the the center of the elliptical eye, which was eye wall, 4) wave disturbances propagating determined from Fig. 7. around the center and so on. He pointed out The distributions of meteorological elements that it is difficult to explain the pressure oscilla- were constructed based on the relative position tion in that case from 1)-3). Nevertheless, he of the station shown in Fig. 8. The pressure could not obtain enough evidence to derive a deviation distribution thus obtained is illustrated definite conclusion. in Fig. 9. The observed pressure deviation is In the present case, as mentioned above, radar negative in the quadrants with the major axis observation made until 0630 JST revealed that and positive in the quadrants with the minor the typhoon had an elliptical eye which was axis. Their extreme values are found out near rotating counterclockwise. From comparing the 25 km from the center. The pressure deviation rotation of the eye with the pressure oscillation is the departure from the concentric pressure it was inferred that the rotation of the elliptical distribution by definition, so that Fig. 9 means eye was associated with the pressure oscillation that within 50 km from the center the isobars and its one rotation produced two crests and are ellipses whose major and minor axes coincide two troughs on the barogram. This inference is with those of the eye seen by the radar. The supported by flight observations made after pressure deviation field shown in Fig. 9 does not 0630 JST. mean that the typhoon has two pressure centers since the minimum pressure at trough is lower 4. Structure in the vicinity of the elliptical eye at lower smoothed pressure, i.e., decreases toward In the previous section it was shown that the the eye center, as evident from Fig. 1. pressure oscillation was associated with the The distribution of the 10-min rate of rotation of the elliptical eye. In the present precipitation is depicted in Fig. 10, where an section we shall try to describe the structure in area with the rate of 2.5 mm/10 min or more is the vicinity of the elliptical eye. With the hatched. It is evident from this figure that the assumption of the steady state, the transformation rainfall area is of an elliptical ring whose width from the temporal to the spacial change will is wider on the side of the major axis and make it possible to depict the distribution of narrower on the side of the minor axis, similar meteorological elements on the basis of observed to that shown in Fig. 6. The orientation of its values at the Miyakojima Weather Station only. major axis is coincident with that derived from The assumption of the steady state does not the pressure oscillation. Such good agreement seem to be unreasonable since the typhoon supports the inference that the pressure showed little change in the central pressure and oscillation was associated with the rotation of a slight decrease in the size of the eye during the elliptical eye. The elliptical ring of maximum intensity, as seen in Fig. 10, has a longer * Data given in Tabel 1 were taken from teletype diameter of 55 km and a shorter one of 40km. data received at the Ryukyu Weather Bureau. This estimated size is in good agreement with According to Annual Typhoon .Rerort of 1966 by that reported by flight observation at 1100 JST. U.S. Fleet Weather Central/Joint Typhoon From comparison between Figs. 9 and 10 it Warning Center, Guam, the orientation of the can be easily understood that the phase relation major axis is NW-SE at 2225 JST on the 4th and at 0700 JST on the 5th. Thus this report of rainfall intensity to pressure variation was means that the discrepancy between the reported reversed at the distance of about 22km from orientation is about 45 degrees at 0700 JST on the center. Rainfall is more intense in the areas the 5th. But this discrepancy is not serious for of positive pressure deviation or in the direction the above inference. of the minor axis inside the minor radius and in 482 Journal of the Meteorological Society of Japan Vol. 51, No. 6

Fig. 8. Position of the Miyakojima Weather Station relativeto the elliptical eye during the period from 0500 to 1700 JST on Sept. 5. The black part of the eye shown in Fig. 7 is oriented upward.

Fig. 9. Distribution of pressure deviation. The black part of the eye shown in Fig. 7 is oriented upward. December 1973 Yasushi Mitsuta and Sadao Yoshizumi 483

Fig. 10. Distribution of rainfall intensity (in mm/10 min). The black part of the eye shown in Fig. 7 is oriented upward.

Fig. 11. Schematic illustration of the disturbed wind field. Its expected streamlines are indicated by solid lines with arrow. The black part of the eye shown in Fig. 7 is oriented upward. 484 Journal of the Meteorological Society of Japan Vol. 51, No. 6 the areas of negative pressure deviation outside Also, the procedure of transformation seems to the major radius. The phase relation is not be justified by the obtained reasonable distribu- definite in the area between major and minor tion of rainfall intensity. radii, as seen in Fig. 4. Fig. 11 is a schematic illustration of the Acknowledgements disturbed wind field. The expected streamlines The present study is based mainly on the data (solid lines with arrow) are composed of four which were collected for the investigation of the cells with its center at the radius of 22 km. Two disasters caused by the Second Miyakojima cells of the disturbed circulation on the major Typhoon directed by Prof. H. Ishizaki of the axis are of cyclonic sense and two ones on the Disaster Prevention Research Institute, Kyoto minor axis of anticyclonic sense. University under a Grant in Aid for Fundamental Scientific Research from the Ministry of Educa- 5. Summary tion. The authors are grateful to Prof. R. During the passage of the Second Miyakojima Yamamoto of the Geophysical Institute, Kyoto Typhoon the barograph at Miyakojima recorded University for valuable advice in the course of a pronounced pressure oscillation on September the present study and to the director of the 5, 1966. The recorded oscillation extended over Miyakojima Weather Station for supplying radar half a day from 0500 to 1700 JST with the photographs. They would also express their period of 53 min and had a maximum double thanks to Miss H. Imai for drafting some of amplitude of about 10 mb. Simultaneously wind figures. and rainfall intensity also showed remarkably periodic variations with the nearly same period. References Radar observation made at the Miyakojima Deppermann, C.E., 1939: Some characteristics of Weather Station until 0630 JST revealed that the Philippine typhoons. Bull. Amer. Meteor. Soc., typhoon had an elliptical eye which was rotating 20, 303-307. counterclockwise at the rate of half a rotation Fujita, T.T., 1952: Pressure distribution within typhoon. Geophys. Mag., 23, 437-452. per about one hour. The comparison between Hatakeyama, H., I. Imai and Y. Masuda, 1955: On the rotation of the eye and the pressure some radar observations of Typhoon "Lorna". oscillation indicated that the observed pressure Proc. UNESCO Symposium on typhoons, 121-128. deviation was negative on the side of the major Imakado, M., 1966: Detailed analysis of typhoon axis and positive on the side of the minor axis. structure by means of weather radars. Geophys. Flight observations suggest the persistence of Mag., 33, 99-106. such correspondence between them after 0630 Jordan, C.L., 1962a: Surface pressure oscillation in JST. tropical cyclones. National Hurricane Research Distributions of rainfall intensity, disturbed Project, Report No. 50, 39-50. 1962b: Surface-, pressure oscillations pressure and wind accompanied by the elliptical during a typhoon at Okinawa. Mon. Wea. Rev., eye was revealed by means of the transformation 90, 191-193. from the temporal to the spacial changes of their Mitsuta, Y., and S. Yoshizumi, 1968: Characteristics observed values at the Miyakojima Weather of the Second Miyakojima Typhoon. Bull. Station. Disaster Prevention Res. Inst., Kyoto Univ., 18, In the transformation the center of the elliptical Part 1, 15-34. eye was taken as the origin of the reference Sadowski, A., 1962: Radar analysis of hurricane coordinate. As Hatakeyama, Imai and Masuda Donna's recurvature. National Hurricane Research (1955) revealed, the centers of pressure and wind Project, Report No. 50, 63-69. field does not always coincide with that of the 1964: Evolution-, of hurricane Ginny as eye in a moving typhoon. Therefore, there may seen on WSR-57 radar at Charleston, S.C. Proc. be room to devise the procedure of trans- 11th Weather Radar Conf., 354-357. Tannehill, I.R., 1956: Hurricane. Princeton University formation. In the present case, however, the Press, Princeton, N.J., 308 pp. discrepancy between the centers appears to be small due to the slow movement of the typhoon. December 1973 Yasushi Mitsuta and Sadao Yoshizumi 485

第 二宮 古 島台風 通過 時宮 古 島に おいて 観測 され た 気圧 ・風 ・降 雨強 度 の周期 的変 動

光 田 寧 京都大学防災研究所 吉 住 禎 夫 気象研究所

第 二 宮 古 島 台 風 通 過 中 の1966年9月5日5時 か ら17時 ま で の12時 間 に わ た り,顕 著 な 気 圧 振 動 が 宮 古 島 に お い て記 録 され た.気 圧 振 動 の周 期 は約50分,最 大 全 振 幅 は10mbを 越 えた.同 時 に風 ・降 雨 強 度 も 気 圧 とほ ぼ 同周 期 の周 期 的変 動 を示 し た.こ れ ら要 素 の変 動 間 に 明確 な 位 相 関 係 の 存 在 が 認 め られ る. 宮 古 島 に お け る5日6時30分 まで 行 わ れ た レー ダ ー観 測 お よび そ の 後 の 飛 行 機 観 測 資 料 と の比 較 か ら,上 記 気 象 要 素 の 周期 的変 動 は,約1時 間 に半 回 転 の割 で 反 時 計 廻 りに 回 転 し てい た 長 円 形 の台 風 の眼 に 伴 うも の で あ る こ とが示 され る.宮 古 島 に お い て観 測 され た 気 象 要 素 の時 間 変 化 を 空 間 変 化 に変 換 す る こ とに よ り,長 円形 の 台 風 眼 周 辺 の 構 造 が 明 か に され る.