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YOKOSUKA

TROPICAL CYCLONES

Tropical cyclones that affect Japan generally form in the area bounded by 5°N to 30°N and 120°E to 165°E. The latitudinal boundaries shift poleward during the summer months and then equatorward in winter in response to the seasonal changes of the synoptic environment.

For the purpose of this study, any tropical storm or typhoon approaching within 180 nmi of Yokosuka is considered to represent a threat to the Port. Table V-1 contains a descriptive history of all tropical storms and typhoons passing within 180 nmi of Yokosuka during the 61-year period 1945-2005. All of the tropical cyclone statistics used in this report for storms passing within 180 nmi of Yokosuka are based on the SAIC generated data set used to compile Table V-1.

The primary tropical cyclone season for Yokosuka is from June to October, although Table V-2 shows that Yokosuka 's season extends from May through November. However, only two storms were recorded during May and three storms were recorded during November throughout the 61-year period 1945-2005. Table V-2 also shows the motion history of the 153 tropical storms and typhoons that passed within 180 nmi of Yokosuka during the period 1945-2005. August and September had the greatest tropical cyclone activity, with 48 of 153 storms (31.4 %) in September and with 43 of 153 storms (28.1%) in August. Of the 153 storms in the data set, 54 were of typhoon strength (≥ 64 kt) within 180 nmi of Yokosuka at CPA. Note that the ratio of tropical storms to typhoons that occurred in September is equal: 24 to 24. The average movement for the storms at CPA is 044° at 26 kt.

It should be noted that the average storm speed at CPA given in Table V-2 is relatively high, ranging from 18 kt in August to 35 kt in October. The reason lies in the location of Yokosuka . As shown in Figure A-2 through A-25 most tropical cyclones that pass close to Yokosuka recurve between 20 and 30 degrees north latitude and then accelerate to the northeast. Because Yokosuka is north of the normal latitudinal recurvature range, most storms passing within 180 nmi of the Port have already recurved and are accelerating northeastward at their CPA.

Figure V-36 shows the monthly distribution of the 153 storms by 7-day periods. The period of activity extends from late May until 30 November.

Figure V-37 shows a chronology of the 153 tropical cyclones that passed within 180 nmi of the Yokosuka area in the 1945-2005 period. An average of 2.51 tropical cyclones per year pass within 180 nmi of Yokosuka . Thirty-five percent of these (.89 per-year, or 1 every 1.1 years) were at typhoon strength while within the 180 nmi radius. There have been only 7 years since 1945 when no tropical cyclone entered Yokosuka 's 180 nmi threat radius. 1946,1968, 1976, 1984, 1986, 1987 and 1999 had no storms passing within 180 nmi of Yokosuka .

Figure V-38 depicts the octants from which the 153 tropical cyclones in the data set approached Yokosuka . Over 95 percent of the storms entering the 180 nmi threat radius of Yokosuka approach from the south, southwest or west octants. 71.9% alone approach from the southwest.

Figure V-39 depicts the primary threat axis of the 153 tropical storms and typhoons that passed within 180 nmi of Yokosuka during the period 1945-2005. The shaded arrow, pointing to the average location of CPA to the site, is a composite track zone for the 112 tropical storms and typhoons that crossed the dashed arc before entering Yokosuka 's 180 nmi radius. The mean transit time from the arc to CPA for these 112 storms was 82 hours. The normal movement path for a tropical cyclone that has reached the latitude of Yokosuka is recurvature toward the northeast. The inset in the figure shows the variation in approach times for the tropical cyclones traveling within the track envelope, and illustrates the variable speed of movement of tropical cyclones that threaten Yokosuka . The minimum transit time was 28 hours for Faye 2, 1965 and the maximum was 273 hours for Ellen, 1975.

Figure V-40 , Figure V-41 , and Figure V-42 show the tracks of the 153 tropical cyclones split into three periods of time: July 16-August 31, September 1-October 31, and November 1-July 15. Be aware that the total number of storms on the three track charts adds up to 161 because some of the tropical cyclones overlap. As shown in the figures, the movement of the 153 tropical cyclones that entered Yokosuka 's 180 nmi radius can be erratic (particularly in the 16 July - 31 August period) but for the most part present an overall consistent track throughout the entire tropical cyclone season.

Figure V-43 , Figure V-44 , and Figure V-45 is the statistical summary of threat probability based on tropical cyclone tracks for the period 1945-2005. The summary coincides with the tracks presented in Figure V-40 , Figure V-41 , and Figure V-42 . The data is presented with solid thin lines representing "percent threat" for the 180 nmi radius circle surrounding Yokosuka . The solid dashed lines represent approximate approach times to Yokosuka based on the climatological speeds of movement. In Figure V-44 for example, a tropical cyclone located near 23°N 128°E has approximately a 40 % probability of passing within 180 nmi of Yokosuka during the September through October period. In addition, this tropical cyclone would reach Yokosuka in 3-4 days if the speed remains close to the climatological normal for tropical cyclones passing within 180 nmi of Yokosuka. Of the three time periods examined, the threat axis appears the most focused in the 1 Sept - 31 October time period.

Wind

Note that when sustained winds exceed 30 kt, ships generally will not be permitted to tie up or depart from the piers and pilots will not be available. Under these conditions, Naval ships entering the Port of Yokosuka will generally be required to go to anchorage. Additionally, cranes cannot be used when winds exceed 24 kt and ships cannot enter or depart from a drydock when winds are at or exceed 20 kt.

The observation station for the Naval Pacific Meteorology and Oceanography Center Yokosuka is located on top of a 175 ft hill at FLEACTS Yokosuka, and the wind instrument is another 55 ft above the station; the observed wind velocity is about 10 kt greater than that observed in the harbor, but otherwise is representative. An ASOS located at sea level in the vicinity of the heliport, but not near the piers is more representative of winds experienced by ships at the piers.

An anemometer installed on the roof of the Port Operations building provides readings that are generally 10-12 kt less than those experienced by ships at the ends of the exposed piers such as Harbor Master Pier.

A Davis Vantage Pro Weather Station exposed to the northerly winds is operational near the degaussing range and could be networked with additional sensors at other pier locations and NPMOC to provide accurate measurements of severe weather effects on Naval assets and provide decision makers with real time conditions. As of the Site Visit in November 2005, NPMOC and FLEACTS were investigating networked sensors.

The remaining paragraphs in Para 2.7.1 are carried over from the previous port study completed in 1992 but are still relevant is showing the effect of storms passing to the east or west of the Port and the expected direction of the winds.

The data contained in Table V-3 has been selected from observations recorded at NPMOC Yokosuka (Previously NAVOCEANCOMFAC) during the passage of the tropical cyclones listed in the table.

A total of 91 tropical storms or typhoons passed within 180 nmi of Yokosuka during the period August 1953 through December 1991, the period during which surface observations for Yokosuka were available. Of those storms, 69 (76%) caused sustained winds of ≥22 kt at the port, and 29 (32%) caused sustained winds of at least gale force, ≥34 kt. Whether or not the individual storm passed east or west of the port seems to make little difference in wind speeds at the port. Thirty-seven of the 69 storms (54%) causing winds of ≥22 kt passed on the seaward (east) side of Yokosuka while 32 (46%) passed to the west. Storms passing on the seaward side of Yokosuka brought winds of ≥34 kt to the area in 52% of the cases (15 of 29), while those passing on the west comprised 48% (14 of 29).

The basic difference between an east passage and a west passage is the direction of the resultant wind on the harbor. If the tropical cyclone passes to the west of Yokosuka , the winds will generally be from the south. For a passage to the west, the storm must necessarily cross the mountain ranges of Honshu (see Figure V-35 ). An example of this was Typhoon Bess (August, 1982) that had a CPA of 126 nmi west of Yokosuka . Bess pounded the harbor with gusts to 80 kt from the southeast and sustained gale force winds (≥34 kt) for almost 9 hours.

Note: The July/August period is when storms passing CPA to Yokosuka are moving their slowest (Table V-2); Thus, strong winds will affect the harbor for longer time periods during those months.

If the tropical cyclone passes east of Yokosuka , the path will be over water and the winds at the Port will be generally northerly. An example of this was Typhoon Gay (October 1981). The storm had a CPA of 24 nmi southeast of Yokosuka , and brought sustained winds of 40 kt with gusts to 60 kt to the port area.

Figure V-46 shows the track segments for the 69 tropical cyclones that correspond to periods of sustained winds ≥22 kt at Yokosuka . Twenty-two of the 91 storms did not cause sustained winds of ≥22 kt. Similarly, Figure V-47 shows the positions of the centers of the 29 tropical cyclones that brought sustained winds ≥34 kt to Yokosuka . Sixty-two of the 91 storms did not cause sustained ≥34 kt winds.

In Figure V-46 only 9 of the 69 storms that caused winds of ≥22 kt did so while the storm center was south of 30°N. All of the storms causing winds of ≥34 kt (see Figure V-47 ) were north of 32°N before the onset of the wind at Yokosuka .

Waves

Pilots indicated that the maximum wave height experienced inside the Harbor during a hurricane passage was 6.6 ft (2 m). Open ocean wave motion is negligible as the Port is almost surrounded by land and reef breaks. 9.8-16.4 ft (3-5 m) waves have been experienced outside the Harbor during typhoon passages but Pilots as well as data from meteorological reports indicate that higher waves are possible.

The Port of Yokosuka normally experiences very little wave motion as the result of a typhoon transiting the vicinity, but specific typhoon trajectory/strength combinations can cause problems. The amount of wave action will vary with the wind directions which result as the typhoon passes to the west or to the east of the port. The surrounding landmasses and the breakwater located near the entrance to Yokosuka Harbor are major factors in limiting the wave action in the port. Wave action will be greater for a northerly wind (typhoon passage east of Yokosuka ). The Port of Yokosuka is more-or-less open into Tokyo Bay in a north-northeasterly direction for a distance of about 25 nmi. Waves from a north-northeasterly direction must traverse various natural and man-made obstacles before reaching the Port; so much of the energy contained in the waves is depleted. Although the limited fetch area of Tokyo Bay reduces the wave heights that can be generated with a given wind speed, the distance is sufficient to produce waves that have the potential to enter the Port of Yokosuka and impact exposed facilities.

An example of the potential for damage from wave action occurred in November 1992, when Typhoon Kelsey passed south of Yokosuka . The northerly winds on the northwest side of the storm generated waves in Tokyo Bay that passed through the entrance to Yokosuka Harbor as 8 to 10 ft swell. Two AEGIS CG's that were moored to Harbor Master Pier were rolling about 5° at their berths but no significant damage was reported. A small open boat moored to the lee side of a JMSDF pier located in the Y-designated berths area in the southwest part of Yokosuka Harbor sunk after being swamped by waves washing over the pier. As shown in Figure V-7 , both of the affected piers are exposed to any wave energy that moves southward through the harbor entrance. Per discussion with the pilots and Port Ops during the Site Visit in November 2005, Harbor Master Pier West is exposed and vulnerable.

An estimate of the maximum wave height that may be encountered for typhoon force winds in the region of Yokosuka and Tokyo Bay is summarized in Table V-4. The major factors considered in this listing were the direction of tropical cyclone passage relative to Yokosuka , the direction and velocity of the resultant typhoon force winds, the length of fetch, a duration greater than 1 hour but less than 5 hours, and the location of obstacles in the path of the progressing waves. There is no single theoretical development for determining the actual growth of waves generated by winds over relatively shallow water ( U. S. Army Coastal Engineering Research Center, 1973 ).

The wave heights presented in Table V-4 are a guide only, and as was the case with the November 1992 storm discussed above, specific storms may generate waves in Tokyo Bay that pass through the entrance to the Port of Yokosuka with heights greater than those in the table. As of the Site Visit in November 2005, Port Operations and the pilots felt that these wave heights were representative of what they had experienced in Yokosuka and Tokyo Bay .

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 in the northern hemisphere. Storm surge is caused by wind stress on the water surface and the effects of atmospheric pressure reduction. The piling up of water on a coast ahead of a tropical storm or typhoon is more apparent in the dangerous semicircle, the region of most intense winds that is located to the right of the storm's direction of movement. The speed of the storm adds to the wind velocity generated by the mechanics of the storm itself. The port of Yokosuka will be placed in the dangerous semicircle when a typhoon passes to the west of the area.

The storm surge effect is most evident in the shallow waters of large inland bays open to the south coast of Japan ( Miyazaki , 1974 ). The surge forms to a large extent after entering the inland bays since the width of the continental shelf is generally narrow along the Japanese coast. Most of the surge occurs, therefore, at the inshore side of these bays, and not along the open coasts nor near the mouth of the bays. Peak surges of 7.6 ft (October 1919) and 7.3 ft (September 1938) were observed at the inshore (north) side of Tokyo Bay . They were the result of southerly winds caused by typhoons passing generally to the west of Tokyo Bay . Due to its sheltered position within Tokyo Bay and its location near the entrance, Yokosuka experiences little storm surge. Conversations with Port Operations personnel indicate that storm surges of about 3 ft have been felt within the harbor, but have not been a problem. Surges of this magnitude coupled with the normal tide range of 4 to 5 ft would not present any unusual difficulty to moored vessels if lines were tended.

Source: http://www.nrlmry.navy.mil/port_studies/thh-nc/japan/yokosuka/text/sect7.htm