Statistical Considerations of Pressure Oscillations Occurring near the Typhoon Center by Sadao Yoshizumi Meteorological Research Institute, Tokyo (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 eye, 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 tropical cyclone 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 Naha (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 typhoons 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 typhoon Emma (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 typhoon Sarah (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.