Ports.com

SASEBO

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 Sasebo 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 Sasebo during the 61-year period 1945-2005. All of the tropical cyclone statistics used in this report for storms passing within 180 nmi of Sasebo are based on the SAIC generated data set used to compile Table V-1.

Although the primary tropical cyclone season for Sasebo is from June to October, Table V-2 shows that Sasebo 's season extends from May through November. However, only two storms were recorded during May and one storm was recorded during November throughout the 61-year period 1945-2005. Table V-2 also shows the motion history of the 149 tropical storms and typhoons that passed within 180 nmi of Sasebo during the period 1945-2005. August had the greatest tropical cyclone activity, with 50 of 149 storms (33.6 %). July and September also had a high number of storms with 31 (20.8 %) in July and 37 (24.8 %) in September. Of the 149 storms in the data set, 58 were of typhoon strength (≥64 kt) within 180 nmi of Sasebo at CPA. Note that 46 of the 58 typhoons occurred in August and September. The average movement for the storms was 026° at 19 kt at CPA.

It should be noted that the monthly average storm speed at CPA given in Table V-2 is relatively high, especially for the storms occurring in June. As shown in Figures A-2 through A-25, most June tropical cyclones that pass close to Sasebo recurve between 20°N and 30°N before accelerating to the northeast. Because Sasebo is north of the normal latitudinal recurvature range, many storms passing within 180 nmi of the Port have already recurved and are accelerating northeastward at their CPA.

Figure V-42 shows the monthly distribution of the 149 storms by 7-day periods. The period of peak activity extends from mid-July through September, but a secondary peak of tropical storm activity occurs during the last half of June.

Figure V-43 shows a chronology of the 149 tropical cyclones that passed within 180 nmi of the Sasebo area in the 1945-2005 period. An average of 2.44 tropical cyclones per year pass within 180 nmi of Sasebo. Thirty-Nine percent of these (.95 per-year, or 1 every 1.1 years) were at typhoon strength while within the 180 nmi radius. There have been only 6 years since 1945 when no tropical cyclone entered Sasebo 's 180 nmi threat radius. In 2004, eight tropical storms and typhoons passed within 180 nmi and were a signifigant threat!

Figure V-44 depicts the octants from which the 149 tropical cyclones in the data set approached Sasebo. Over 88% entering the 180 nmi threat radius of Sasebo approached from the southeast through west direction. Fifty five percent of the storms alone approached from the south-southwest.

Figure V-45 depicts the primary threat axis of the 149 tropical storms and typhoons that passed within 180 nmi of Sasebo 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 68 tropical storms and typhoons that crossed the dashed arc before entering Sasebo 's 180 nmi radius. The mean transit time from the arc to CPA for these 68 storms was 76 hours. The normal movement path for a tropical cyclone that has reached the latitude of Sasebo 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 Sasebo. The minimum transit time was 14 hours for Typhoon Marie, 1954 and the maximum was 242 hours for Typhoon Kirk, 1975.

Figure V-46 , Figure V-47 , and Figure V-48 show the tracks of the 149 tropical cyclones split into three periods of time: July 1-July 31, August 1-September 30, and October 1-June 30. 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 149 tropical cyclones that entered Sasebo's 180 nmi radius can be erratic, but for the most part present an overall consistent track throughout the entire tropical cyclone season.

Figure V-49 , Figure V-50 , and Figure V-51 are the statistical summaries of threat probability based on tropical cyclone tracks for the period 1945-2005. The summary coincides with the tracks presented in Figure V-46 , Figure V-47 , and Figure V-48. The data is presented with solid thin lines representing "percent threat" for the 180 nmi radius circle surrounding Sasebo. The solid dashed lines represent approximate approach times to Sasebo based on the climatological speeds of movement. In Figure V-50 for example, a tropical cyclone located near 26°N 133°E has approximately a 40 % probability of passing within 180 nmi of Sasebo during the August through September period. In addition, this tropical cyclone would reach Sasebo in about 1 1/2-2 days if the speed remains close to the climatological normal for tropical cyclones passing within 180 nmi of Sasebo. Of the three time periods examined, the threat axis appears to shift dramatically from the early season to late season tropical cyclone threat.

A comparison of the preceding figures reveals some differences in threat axes according to the time of year. During July, the threat axis extends south southeastward from Sasebo through approximately 19°N 137°E into the more tropical latitudes. By September, the axis near Sasebo extends south-southwestward to about 24°N 128°E before turning southeastward to an area near 13°N 164°E. The axis for the period October through June extends southwestward across the East China Sea to a position just east of Taiwan before turning southeastward into the lower latitudes.

Figure V-52 shows the tracks of the 149 tropical cyclones that passed within 180 nmi of Sasebo over the years 1945-2004 (all months combined). Circular markers along the track indicate positions of tropical cyclones 72 hours before reaching the closest point of approach to Sasebo. Note that only 6 of the 149 tracks did not extend back 72 hours.

Wind

Current wind speed and direction at Port Operations (Building 100) can be located via the following web site: http://www.npmodsasebo.navy.mil/obs/Davis_WX/current_Monitor.htm

Another Davis instrument is mounted on Building 200. For measuring strong southwest winds, the Building 100 sensor is the most representative of what ships might experience in the Inner Harbor.

Northerly winds prevail throughout the year, however, southerly winds occur during the summer.

The average local wind speeds range from 6-10 kt. Sasebo Ko is well protected on all sides by mountainous terrain. A significant exception to this is a valley on the west side of the Harbor just south of India and Juliet Basins near Kawatoni Cove. During moderate to strong westerly flow funneling can produce gale force winds in the vicinity of the valley.

Occasional periods of high winds and choppy seas occur in the region throughout the year. Storm force winds (48-63 kt) are very rare, and gale force winds (34-47 kt) occur mainly during the transition seasons of spring and fall or with the passage of a strong front.

Most of the information in the following indented paragraphs was derived from the port study completed in 1992 but has relevance to this port study.

A total of 39 tropical cyclones approached within 180 nmi of Sasebo in the 17-year period from 1975-1991. Of the 39 tropical cyclones, 10 (26%) resulted in sustained strong winds (≥22 kt) and only 2 resulted in sustained gale force winds (≥34 kt).

An analysis of the tracks of the tropical cyclones that produced winds of ≥22 kt or ≥34 kt at Sasebo shows that storms that pass east or west of Sasebo produce essentially the same wind threat. Of the storms cited as causing strong or gale force winds, one-half passed east of Sasebo and one-half passed to the west. Other factors must be considered, however. If the tropical cyclone path is to the east across Kyushu , the storm will lose some of its intensity over land. Additionally, the Kyushu Mountains and local topography (see Figure V-1 and Figure V-41 ) will provide protection from the winds of the tropical cyclone. The winds that reach the Port will have a strong northerly component, and wave motion in and near India and Juliet Basins will be minimal.

An early 1970's visit to Sasebo and subsequent analysis of the tropical cyclone threat to the Port produced the following conclusions.

1. All U. S. Naval vessels considered Sasebo Harbor a safe typhoon haven during the passage of Typhoon Bess east of the Harbor. (Typhoon Bess had a CPA of 78 nmi northeast of Sasebo , with center winds of 45 kt.) No damage was reported by any of the ships.
2. Winds reported by ships in the Harbor (maximum of 48 kt) tended to be higher than those reported by land based units. Ships in the southern part of the Harbor at typhoon anchorage reported winds near the maximum while ships in the northern part of the Harbor reported winds close to 40 kt.
3. The use of a second anchor dropped under foot to reduce yawing, and steaming to the anchor/mooring buoy to reduce the strain on the chain, are highly recommended.
4. The wet drydocks may provide shelter for smaller ships. (Note: 1992 discussions with port personnel indicated that there was the danger of holing a hull if the ship had any lateral motion due to the "step down" sides of the drydocks.)

In the case of a tropical cyclone passing west of Sasebo , the path is primarily over water. It is evident that the protection offered by topography to the south and west is much less than to the north and east of Sasebo. In addition, tropical cyclone passage to the west places Sasebo in the "dangerous" right hand semicircle, thus subjecting the Harbor to higher wind velocities. The passage of Typhoon Gilda in July 1974 is cited as an example. Gilda had a CPA of 80 nmi northwest of Sasebo, with a center wind of 53 kt, and produced winds of 45 kt as compared to winds of 38 kt with Typhoon Bess (cited above) that passed to the east. The following conclusions were reached as a result of the storm.

1. U. S. Naval vessels involved considered Sasebo Harbor as a safe typhoon haven during Typhoon Gilda's passage west of Sasebo.
2. India Basin , berths 8 and 9 (see Figure V-4) can be used as a shelter from a typhoon passing to the west of Sasebo. Maximum seas reported in India Basin during Gilda's passage were 5 ft confused. [However, see Sections 5.8.2 and 5.8.3 for more recent views on the safety of the berths during typhoon passage.]
3. All radars should be used under conditions of restricted visibility to obtain accurate fixes.

It must be pointed out that all of the wind data evaluations cited in this report are based on the assumption that the "threat" tropical cyclone was solely responsible for the winds produced at Sasebo. This assumption does not take into account other extratropical synoptic features existing at the time that could "bias" the winds observed at Sasebo. That is, tropical cyclones transitioning to extratropical cyclones as cold air enters the storm structure.It is important to note that at least two ships dragged anchor because of winds exceeding 85 kt from the northeast from Typhoon Tokage in October 2004. Typhoon Tokage's CPA was over 100 nmi ESE of Sasebo.

Also, all or a portion of the hourly wind data analyzed during the 17-year period may have originated with the Japanese Weather Bureau. Since the anemometer of their station does not extend above a hill just south of the station, the southerly winds used may be less than those experienced by ships in the Harbor.

The most severe threat to the harbor occurs when a tropical cyclone approaches from the southwest and passes west of Sasebo , within 50 nmi or so. In this case, the elevations of Hario Island (see Figure V-2) serve as a good wind barrier for ships at typhoon anchorage at Ebisu Bay. When tropical cyclones pass to the east of Sasebo, the elevated terrain to the north and east of the Harbor provides excellent protection (see Figure V-41). In this case, maximum winds can be expected from the north. For a more complete description of the effects of tropical cyclones on Sasebo Harbor, See Rudolph, 1975.

Waves

The orientation of the bay limits the buildup of significant wave heights within the Harbor except when a northerly or southerly component exists. This is by far the largest fetch area provided by Sasebo Ko stretching over 6 nmi from north to south. Northerly gale force or stronger winds that are sustained for over 12 hours can produce waves in excess of 6 ft (1.8 m) in Yokose and other areas of south Sasebo Ko.

Sasebo Harbor experiences its highest waves when a typhoon passes to the west since this places Sasebo in the right or "dangerous" semicircle of the typhoon. The greater relative wind (wind velocity plus storm movement) in this area generates higher waves. Because the shape of the Harbor reduces the fetch, the effects of the typhoon-related sea are minimized. However, India Basin is constructed so that northerly moving waves pass through the basin entrance, reflect off of the face of the north quay wall (berths 4, 5, and 6) and impact ships moored to berths 1 and 9. This is known as the bathtub effect and the impact on ships moored to berths 1 and 9 can be significant as the ships bounce up and down erratically in the sloshing water.

The maximum wave heights that can be expected with typhoon strength winds (≥64 kt) are given in Table V-3.

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. Storm surge is caused by wind stress on the water surface and the effects of atmospheric pressure reduction. Also, tides may act to either increase or decrease this rise in water level. 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 Sasebo 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 opening southward along the south coast of Japan (from Miyazaki , 1974). The height of the storm surge in a given port on a south facing coast is dependent on the tropical cyclone track. If the track is to the west of the port, the peak surge will be large; the opposite is true for a track to the east of the port.

Of 10 tropical cyclones that passed to the west of Sasebo over a 5-year period, the maximum tidal height over the normal tide was 1.3 feet. Since the tidal range for Sasebo Harbor is 10 to 12 feet, this relatively small storm surge would be significant only if it coincided with a high spring tide and large waves. These three factors did coincide in July 1974 with the passage of Typhoon Gilda (CPA of 80 nmi west-northwest of Sasebo). Large amounts of water were forced over the southern walls in both Juliet and India basins. However, even under these extreme conditions, India Basin was considered a "safe, adequate haven" by the commanding officer of USS SACRAMENTO (AOE-1) berthed at India Basin berths 7 and 8 during Typhoon Gilda's passage.

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