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IWAKUNI/KURE

TROPICAL CYCLONES

For the purpose of this study, any tropical cyclone approaching within 180 nmi of Iwakuni is considered to represent a threat to the Ports of Iwakuni and Kure. Table 1 contains a descriptive history of all tropical storms and typhoons passing within 180 nmi of Iwakuni during the 64-year period 1945-2008. All of the tropical cyclone statistics used in this report for storms passing within 180 nmi of Iwakuni are based on the SAIC generated data set used to compile Table 1.

Tropical cyclones which affect Iwakuni and Kure generally form in an area bounded by the latitudes 5°N and 30°N between longitudes 120°E and 165°E. The latitudinal boundaries shift poleward in the summer months and equatorward in winter in response to seasonal changes of the synoptic environment. It is possible for tropical cyclones to form during any month or season; however, those affecting the Japanese Islands, and hence, Iwakuni and Kure, are confined for the most part to the spring through fall months, with late summer and early fall being the most likely period for an occurrence.

An examination of Table 2 shows that June through October is the primary season for tropical cyclone activity at Iwakuni, with 90% (136 of 151) occurring during that period. Of those, 81% (122 of 151) occurred from July through September. Although storms have occurred as early as May (two tropical cyclones), the overwhelming majority of the storms are confined to the June through October period. August and September are the months of greatest activity, with 63% (95 of 151) occurring during those months. September is the month of greatest threat from typhoon-strength storms, as it has 46% (24 of 52) of the total typhoon occurrences.

Table 2 also shows the motion history of the 151 tropical storms and typhoons that passed within 180 nmi of Iwakuni during the 64-year period 1945-2008. The average movement for all storms when at CPA to the Port of Iwakuni is 033° at 21 kt. Average storm speeds are quite high at CPA and vary from 16 to 29 kt. June's 29 kt average speeds are the fastest.

Figure 29 shows the annual distribution in seven-day increments of the 151 storms by 7-day periods. The period of activity extends from 28 May until 27 October with a peak in September.

Figure 30 shows a chronology of the 151 tropical cyclones that passed within 180 nmi of the Iwakuni area in the 1945-2008 period. An average of 2.36 tropical cyclones per year passes within 180 nmi of Iwakuni. Historically, there were 52 tropical cyclones at typhoon strength while within the 180 nmi radius which equates to almost one (0.81) every year. Out of a total of 64 years there were only four years which had no tropical cyclones and a total of 11 years which had three or more tropical cyclones.

Figure 31 depicts the octants from which the 151 tropical cyclones in the data set approached the Port of Iwakuni. As the figure shows, a total of 97 (64.2%) approached from the southwest octant and 32 (21.2%) approached from the south octant. This was largely due to Japan's position relative to the primary tropical cyclone storm track, and Iwakuni's position just north of the primary storm recurvature area on the south coast of Japan. Note that the majority of the storms (129 of 151) make their approach from the south and southwest indicating that they have undergone recurvature and are therefore beginning to weaken even before reaching Japan. This weakening coupled with the mountainous topography surrounding the Hiroshima Bay region accounts for substantially reduced winds associated with tropical cyclones. It must be kept in mind that although the effects of storms are reduced quite effectively, destructive winds can and may occur in the Bay Region with any tropical cyclone. It should also be noted that the approach direction is determined at CPA, and may not represent the initial approach direction of the tropical cyclone toward the Port of Iwakuni.

Figure 32 and Figure 33 show the tracks of the 151 tropical cyclones split into two periods of time: July through September 15, and September 16 through June. Be aware that the total number of storms on the two track charts adds up to 162 because some of the tropical cyclones overlap. Both figures show more storms passing to the east than the west of the 180nm radius.

Figure 34 and Figure 35 are the statistical summaries of threat probability based on tropical cyclone tracks for the period 1945-2008. The summaries coincide with the tracks presented in Figure 32 and Figure 33. The data is presented with solid thin lines representing "percent threat" for the 180 nmi radius circle surrounding Iwakuni. The solid dashed lines represent approximate approach times to Iwakuni based on the climatological speeds of movement. In Figure 34 for example, a tropical cyclone located near 28°N 138°E has approximately a 30 % probability of passing within 180 nmi of Iwakuni during the July through September 15 period. In addition, this tropical cyclone would reach the Port of Iwakuni in 1½ - 2 days if the speed remains close to the climatological normal for tropical cyclones passing within 180 nmi of the Port of Iwakuni. A comparison of Figure 34 and Figure 35 shows that there is a significant difference in threat axes according to time of year. The months of July, August, and early September (Figure 34) have an axis that extends southward from Iwakuni to about 28°N before slowly turning southeastward to the more tropical latitudes. The threat axis for the period from September 16 through June, however, extends southwestward from Iwakuni to just past Okinawa at about 26°N before turning southeastward to the tropics.

Figure 36 depicts the primary threat axis of the 151 tropical storms and typhoons that passed within 180 nmi of Iwakuni during the period 1945-2008. The shaded arrow, pointing to the average location of CPA to the site, is a composite track zone for the 72 tropical storms and typhoons that crossed the dashed arc before entering Iwakuni's 180 nmi radius. The mean transit time from the arc to CPA for these 72 storms was 77 hours. However, the inset in the figure depicts a wide variation from the mean.

Figure 37 shows the tracks of the 151 tropical cyclones that passed within 180 nmi of Iwakuni over the years 1945-2008 (all months combined). Circular markers along the track indicate positions of tropical cyclones 72 hours before reaching the closest point of approach to Iwakuni. Note that only 10 of the tracks did not extend back 72 hours.

WIND

The Iwakuni wind recorder is located west of the flightline, approximately 0.75 nmi (1500 yds) from the Inner Harbor. The measurements tend to mirror those experienced by ships in the Port.

Due to the counterclockwise rotation of tropical cyclones in the Northern Hemisphere, those storms passing to the west of Iwakuni can be expected to give southerly winds shifting to westerly. If the storm passes to the south and east, southeasterly winds would first be experienced, followed by easterly and/or northerly winds.

These observations, coupled with an analysis of tropical cyclone winds for the last port study during the period 1955-1973, indicate a bias for storms passing west of and within 120 nmi of Iwakuni to be those most likely to give strong winds up to and exceeding gale force.

Iwakuni Port Operations personnel stated during the Site Visit in October 2009 that north/south winds have the most potential to cause problems in the Port of Iwakuni, particularly when entering the mouth of the Harbor.

The following extract from Brand and Blelloch (1976) points out favorable sheltering aspects of Iwakuni and vicinity.

"The mountainous terrain of the islands of Honshu, Kyushu, and Shikoku, with elevations exceeding 3000 ft, would lead one to expect that the winds of a tropical cyclone would be greatly reduced before reaching the Hiroshima Bay region. This is, in fact, the case when storms pass either to the west or the east of the bay region.

When storms pass to the west, the wind will normally be reduced 35-50% while storms passing to the east will usually have their winds reduced approximately 60%. Also it appears that southerly winds coming off the Bongo Straits and Inland Sea have the path of least resistance to the Hiroshima Bay area. Generally this would be the case when a storm passes to the west."

Brand and Belloch stop short of recommending Iwakuni as a typhoon haven, but they do recommend the nearby Port of Kure and area typhoon anchorages. Jarrell (1988) in a Naval Environmental Prediction Research Facility report titled "Forecasting Aids for Setting Tropical Cyclone Conditions: Sasebo and Iwakuni", Japan surmised that the expectation of 50 kt winds should be sufficient to warrant evacuation of seaworthy vessels from Iwakuni.

Iwakuni Port Authorities confirmed that it was not safe for ships to remain in the Iwakuni Inner Harbor or at the Outer Harbor anchorages in the event of a strong tropical cyclone. Additionally, the Iwakuni weather personnel indicated that 80 kt sustained winds occurred in 1999 when Typhoon Bart passed 38 nmi to the west-northwest at CPA. This fact substantiates the need to sortie from Iwakuni with an approaching strong tropical cyclone.

The following paragraphs are excerpts from the previous port study that are still relevant today.

For the most part the line segments of Figure 38 represent less than 100 nmi which suggests that a tropical cyclone moving with a speed of 25 kt would give gale force winds at Iwakuni for less than 4 hours.

At Kure, it can be seen in Figure 39 and Figure 40 that the storm centers must be north of 31° N before winds exceeding 22 kt and north of 32° N before gale force winds are observed. Figure 39 also suggests that storms passing to the west may have a longer duration of winds in the 22-33 kt range, than storms which pass to the east.

The strongest winds recorded at Kure that could be associated with a tropical cyclone in the period 1937-1972 were in 1970, when Typhoon Anita passed directly over the city. The maximum sustained winds recorded were 51 kt from the northeast. It should be noted that stronger winds have been attributed to extratropical systems in the winter months.

The Kure weather office is located in the Kure Inner Harbor as shown in Figure 3. The winds recorded should be representative of those in the Inner Harbor but most probably are not representative of the winds in Area "F". When the winds are from the north or south, Area "F" would probably experience higher winds than the Inner Harbor. When the winds are from the west Area "F" should experience lighter winds than the Inner Harbor. However, regardless of wind direction or speed, Area "F" has been utilized and praised for its security and proclaimed as a safe location for vessels at Kure in the event of a strong tropical cyclone.

WAVES

The previous port study in 1976 lists the significant wave heights which could be observed at the Iwakuni Harbor anchorages (Table 3). Note in that the maximum wave heights at the Iwakuni Harbor anchorages (A through D) will occur when the winds are from the south (storm passage to the west). Table 4 lists the significant wave heights which could be observed in the Outer Harbor of Kure with winds of various speeds and directions. Observe in Table 4 that the maximum significant wave heights will occur when winds are from the north. Wave action in the Inner Harbor should be less than that in Outer Harbor.

Port Authorities and the JMSDF still consider these heights to be accurate in 2010. However, the wave heights presented in Table 3 and Table 4 are intended as a guide only. Specific storms may generate waves that vary from those listed in the table.

JMSDF Kure District Operations personnel indicated during the Site Visit in October 2009 that JMSDF ships sortied to Anchorage area "E" had experienced as much as ft (3 m) waves during tropical cyclone conditions. If the winds or waves reached a state where ships expected to drag anchor, the ships then would steam in the same general area until the threat had passed. The JMSDF emphasized that it was important to remain a safe distance from other ships when anchored in Anchorage area "E".

STORM SURGE

Storm surge may be visualized as a raised dome of water, moving with the storm, and centered a few miles to the right of its path. The dome height is related to local pressure (i.e., a barometric effect dependent on the intensity of the storm) and to wind stress on the water caused by local winds. Other significant contributing factors are storm speed, direction of approach, bottom topography, and coincidence with astronomical tide.

These surges are most pronounced along the south coast of the Japanese island chain where the bays are open to the Pacific Ocean. Due to its sheltered position within the Inland Sea, the numerous islands (notably Yashiro Island to the south), and the fact that winds are significantly reduced before reaching the Hiroshima Bay area, storm surge is negligible. Storm surge is also adjudged to be negligible in the Port of Kure and hence should produce no significant damage in that Port. These points were emphasized quite strongly in conversations with local harbor and weather authorities. Records are not kept on storm surge in Hiroshima Bay and no significant damage or incident has been attributed to this phenomenon.

Source: http://www.nrlmry.navy.mil/port_studies/thh-nc/japan/iwakuni_kure/text/tropical_cyclones.htm