APRIL 1998 NOTES AND CORRESPONDENCE 1091 Statistical Analysis of the Characteristics of Severe Typhoons Hitting the Japanese Main Islands TAKESHI FUJII General Education and Research Center, Kyoto Sangyo University, Kyoto, Japan 5 May 1997 and 19 July 1997 ABSTRACT Characteristics of 51 severe typhoons hitting the Japanese main islands with central pressure equal to or less than 980 hPa during the period 1955±94 were analyzed by an objective method using hourly station observation during typhoon passages. Position of a typhoon center, central pressure depth Dp, and radius of the maximum wind rm, were obtained at hourly intervals after landfall on the main islands of Japan. The pressure pro®le of severe typhoons used in this analysis was chosen from formulas presented in previous papers, namely the same as one used by the U.S. Army Corps of Engineers for hurricanes hitting Florida. Coastlines of the main islands were divided into three sections: areas A, B, and C extending from west to east. Statistical analyses of parameters were made for each area. At time of landfall, the maximum value of Dp was 83.2 hPa for area A, 85.2 hPa for area B, and 47.8 hPa for area C. The differences in return period of Dp among areas are considered to be caused by the SST distribution off the Paci®c coast. On average, typhoons making landfall in area C have larger rm and speed, and display a more eastward component of translation than those in the other two areas. The differences of speed and direction among areas and months can be explained to be caused by variation of the synoptic-scale air current at the 500-hPa level. 1. Introduction suta and Fujii 1986). The 51 severe typhoons making landfall on the Japanese main islands in this period In most parts of western Japan, severe natural dis- with the central pressure equal to or less than 980 hPa asters have mainly been caused by typhoons. In 1959, were analyzed by ®tting to the pressure pro®le for- Typhoon Vera (Isewan) caused a high storm surge and mula by Schloemer. more than 5000 deaths. Recently, Typhoon Mireille of 1991 damaged about 680 000 wooden houses, and losses paid by insurance amounted to 600 billion yen 2. Experimental formula for pressure pro®le of (about 5 billion U.S.). Statistical images of such se- a typhoon vere typhoons hitting Japan are ®gured out in this Mature stage tropical cyclones are characterized by paper. their concentric pressure patterns. The pressure dis- The author has presented an objective analysis tribution can be given by one radial pressure pro®le. method of pressure patterns of typhoons ®tting a pre- The formula chosen to represent typhoons hitting the scribed pressure formula (Fujii 1974). Using this Japanese islands by a previous study (Mitsuta et al. method, the author and collaborators chose the best 1979) is one proposed by Schloemer (1954), namely, ®t formula for tropical cyclones hitting the Japanese main islands from various formulas presented previ- 1 ously (Mitsuta et al. 1979), to be the one used by the p 5 pc 1Dp exp 2 , (1) U.S. Corps of Engineers. (Schloemer 1954) for hur- 12x ricanes hitting Florida. where p is the sea level pressure at the radial distance In this study, pressure patterns of typhoons making r, pc is the pressure at a typhoon center, Dp 5 p` 2 landfall on the Japanese main islands were reanalyzed p c ( p` is peripheral pressure) de®ned as the central for a 40-yr period from 1955 to 1994 including those pressure depth, and x 5 r/r m (r m is radius of the max- analyzed in previous studies (Mitsuta et al. 1979; Mit- imum wind speed). Holland (1980) has extended this equation by add- ing another parameter, B, to represent small-scale tropical cyclones in Australia: Corresponding author address: Takeshi Fujii, General Education and Research Center, Kyoto Sangyo University, Kamigamo, Kita, 1 Kyoto 603-8555, Japan. p 5 pc 1Dp exp 2 . (2) E-mail: [email protected] 12xB q 1998 American Meteorological Society Unauthenticated | Downloaded 09/24/21 02:19 PM UTC 1092 MONTHLY WEATHER REVIEW VOLUME 126 This formula was applied to the estimation of the estimates of the typhoon center (f, l ), r m , Dp, and storm surge in Bangladesh by Hubbert et al. (1991) s p , were determined. and Flather (1994). Hubbert et al. also presented an In this study, the distance increment was reduced experimental relation representing increase of B with from 0.028 used in a previous study (Mitsuta and Fujii decreasing p c . 1986) to 0.018, and the interval of r m was reduced Using Eq. (2), Fujii and Mitsuta (1995) analyzed from 1 to 0.5 km. The outer boundary of the analysis pressure patterns of three typhoons, Mireille of 1991, region also extended from 200 to 250 km from the Yancy of 1993, and Orchid of 1994, that made landfall typhoon center. Data from approximately 20±40 sta- on the Japanese main islands in recent years with tions could be used at each hour. Results of the anal- strong intensity. Their results showed that pressure ysis of 51 severe typhoons over 40 years are sum- patterns can be approximated reasonably with B 5 marized in Table 1. Detailed hourly data for each ty- 1.0. This implies that pressure pro®les of intense ty- phoon are compiled in a database, which is available phoons reaching the Japanese main islands in mid- upon request. latitudes may be represented by Eq. (1) without any serious error. So, in this study Eq. (1) is used. 4. Statistical characteristics at time of landfall 3. Method of the objective analysis Most severe typhoons approach Japan from the The pressure distribution formula, Eq. (1), was ®t- southwest along the periphery of the Paci®c Ocean ted to an hourly sea level atmospheric pressure at over the warm Kuroshio Current. They are in a mature weather stations of the Japanese Meteorological stage, but some are already weakened after its peak. Agency (JMA) within about 250 km of the typhoon While the Japanese main islands extend from south- center by the least square method for each typhoon. southwest to north-northeast, typhoons hitting them In this process, the following weighting function, w r , are a little different in nature between the western and was applied in order to get a close ®tting to the ob- eastern parts of Japan. The Paci®c coasts of the main served sea level pressures near the typhoon center: islands of Japan were divided into three areasÐA, B, and CÐas shown in Fig. 1a. These were 20 out of 51 typhoons that made landfall in area A, 19 in area B, 100 , r . 10 km and 12 in area C. The time of landfall is de®ned as r w 5 (3) the ®rst hour after a typhoon pressure center crosses r the smoothed coastline as shown in Fig. 1a. Direction 10, r # 10 km. and speed of the typhoon movement at landfall are The ®tting process starts from a given central position de®ned as the vector difference between hourly ty- (latitude f and longitude l) and an initial rm. The ®rst phoon positions just before landfall and at landfall. estimation of Dp was computed by using the method of The distribution of sea surface temperature (SST) least squares. The initial central position, (f, l), can be in August is also shown in Fig. 1b, in relation to a rough estimate that may be the position of the weather comparison of typhoon intensities among areas. station showing the minimum pressure, and initial rm may also be 80 km, which is the average by the previous a. Central pressure depth, Dp study (Mitsuta and Fujii 1986). Root-mean-square error, sp, was computed from the difference between com- From the annual maximum values of Dp at time of puted sea level pressures and observed pressures at landfall, return periods are computed by the method weather stations, multiplied by the weighting function of Hazen (1930) for annual peak values for each area. wr shown in Eq. (3) as follows: However, these values cannot be compared among areas due to differences in the length of the coastline. 1/2 w (p 2 p )2 O r comp obs So, the widths of the areas looking from the averaged sp 5 , (4) w direction of typhoon translations, which is shown by O r an arrow in Fig. 1a, are 233 km in area A, 348 km where pcomp and pobs are computed and measured sea in area B, and 253 km in area C. Return periods of level pressures at each station. Dp are converted into the values per invading width With successive changes of r m at intervals of 0.5 of 100 km, which are shown in Fig. 2. km, s p is computed. Values of r m and Dp, are chosen Expected values for return periods of 50 years per as the set of values showing the minimum s p . Then, 100 km are 73 hPa in area A, 60 hPa in area B, and the typhoon center is shifted by 60.018 in latitude or 43 hPa in area C, and those for 25 years are 63, 57, longitude, and a second estimate of central position and 37 hPa, respectively. is chosen as the position that gives the minimum value Apparent discontinuities are seen from 52 to 64 hPa of s p after adjusting values of r m and Dp by the meth- for area A and from 37 to 51 hPa for area B.