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MURORAN

TROPICAL CYCLONES AFFECTING MURORAN

Tropical Cyclone Climatology at Muroran

Tropical cyclones which 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. The same genesis area is true for the storms affecting Muroran, but the northerly location of Muroran significantly reduces the number of tropical cyclones approaching within 180 nmi of the port. Many have lost or are rapidly losing their tropical characteristics and becoming extratropical by the time they reach CPA.

For the purpose of this study, any tropical storm or typhoon approaching within 180 nmi of Muroran is considered to represent a threat to the port. Table V-31 contains a descriptive history of all tropical storms and typhoons passing within 180 nmi of Muroran during the 47-year period 1945-1991. As can be noted from the table, comparatively few storms retaining tropical characteristics affect Muroran; only 39 storms approached within the 180 nmi radius of Muroran, as compared to the 115 storms that entered the 180 nmi threat radius around Yokosuka during the same period. Of the total number (39) of storms listed in the table, only 4 were of typhoon strength when at CPA to Muroran. All of the tropical cyclone statistics used in this report for storms passing within 180 nmi of Muroran are based on the SAIC generated data set used to compile Table V-31.

Historically, tropical cyclones have occurred in all months in the genesis area described in a preceding paragraph, but the primary tropical cyclone season for Muroran is more narrowly defined. As shown in Table V-32, a few tropical cyclones have affected the port as early as June and July (1 each month), and as late as December (1), but the primary tropical cyclone season for Muroran is during August and September. The same table shows the motion history of the 39 storms that passed within 180 nmi of Muroran during the 47-year period 1945 through 1991. It is interesting to note the relatively fast movement of the storms included in the table; those storms reaching CPA in September had an average speed of 39 kt while the average heading and speed of all storms entering the 180 nmi radius circle during the 47-year period was 042° at 35 kt. The fast movement is due to the influence of the upper-level westerlies as the tropical cyclone moves north and eastward toward higher latitudes.

During the 47-year period from 1945 through 1991 there were only 39 tropical storms and typhoons that met the 180 nmi threat criterion for Muroran, an average of less than one per year. Figure V-135 shows the monthly distribution of the 39 storms by 7-day periods. The period of peak activity extends from early August through September, but the occurrence of typhoon strength storms has been limited to the last 21 days of September.

Figure V-136 shows the annual distribution of tropical storms and typhoons passing within 180 nmi of Muroran during the 47-year period 1945 through 1991. As can be seen in the figure, there were few years in the 1950's and 1960's that had no occurrences, but since the early 1970's, there have been several years when no tropical storm or typhoon approached within 180 nmi of Muroran. No typhoon strength storm affected Muroran during the 1962-1991 period.

Figure V-137 presents a graphical depiction of the number of tropical cyclones versus CPA. The storm classification is based on the maximum wind near the storm center while that center was within 180 nmi of Muroran, not necessarily at CPA. The sloping lines represent mathematical fits to the data points for the various intensity classifications. The average radii of maximum wind for 34, 100, and 140 kt tropical cyclones at Muroran are 37.6, 36.0, and 33.1 nmi, respectively.

Figure V-138 depicts, on an 8-point compass, the octants from which the 39 tropical cyclones in the data set approached Muroran. Over one-half of the storms, 21 of 39, were moving from the southwest octant. Approximately one-fourth (9 of 39) were moving from the south octant, and an equal number were moving from the west octant. It should be noted that the approach direction is determined at CPA, and may not represent the initial approach direction of the tropical storm or typhoon toward Muroran.

Figures V-139 and V-140 provide information on the probability of remote tropical storms and/or typhoons passing within 180 nmi of Muroran and average time to CPA. The solid lines represent a 'percent threat' for any tropical cyclone location within the area depicted. The heavy dashed lines represent the approximate time in days for a system to reach Muroran. For example, in Figure V-139, during the months of July and August a tropical cyclone located at 30°N 135°E has approximately a 20% probability of passing within 180 nmi of Muroran and will reach Muroran in about 1-1/2 to 2 days.

As can be seen in the preceding figures, there is little difference in the threat axes for tropical cyclones of July and August versus all other months for Muroran. There is a slight difference in the orientation of the axes southwest of Muroran, with the July and August storms tending to approach from a more southwesterly direction. After the threat axes cross southwestern Honshu, they both tend to turn near 30°N to the south and then southeastward toward lower latitudes. Of the 39 storms in the data set used to construct the figures, 4 passed over Korea and 2 passed through Tsushima Strait between Korea and Japan before crossing the Sea of Japan and entering the 180 nmi threat radius around Muroran. The 33 others crossed the main Japanese islands of Kyushu, Shikoku and/or Honshu before approaching Muroran. The July and August storms show a stronger tendency to follow a track over the Sea of Japan, while the storms occurring during the September through June period tended to follow the western coast of Honshu or travel over the land mass of Honshu while approaching the 180 nmi threat circle.

The track segments in Figure V-141 reflect the tropical cyclone positions when sustained winds of 22 kt were being experienced at Muroran. Only 3 of the 9 storms that passed within 180 nmi during the period of 1979 through 1991 resulted in sustained winds of 22 kt at Muroran. Note that none of the 9 storms caused sustained 34 kt winds at Muroran.

Wind and Topographical Effects

Because of the adjacent topography, the port of Muroran is adversely affected primarily by winds with a strong westerly component. Post-frontal winds that follow cold frontal passage during the winter months are the primary cause of strong westerly winds. However, a tropical cyclone that followed a track that would place it close to and east or northeast of Muroran could also cause strong westerly winds at the port.

Easterly winds also reach the port. Strong easterlies are not a common occurrence, but the 2 southermost storms whose track segments are represented in Figure V-141 brought easterly winds to Muroran. If a tropical cyclone followed a track that would place it close to and south or west of Muroran, strong easterly winds could result and impact port facilities.

Northerly winds do not pose a significant threat to the port because the mountainous terrain of Hokkaido serves as an effective barrier to northerly flow. If a weather system placed Hokkaido under strong northerly flow, the wind force would be diminished, and the direction at Muroran would likely have a strong westerly or easterly component.

Although not common, strong winds from the south quadrant are possible. A review of a 20-year record of maximum recorded winds at Muroran during the period 1971-1990 revealed 5 years where the year's maximum winds came from either south (2 occurrences) or south-southwest (3 occurrences). The strongest wind recorded during those five years was 46 kt (23 m/sec).

Local Weather Conditions

The data contained in Table V-33 has been selected from observations recorded at Muroran during the passage of the tropical cyclones listed in the table. It should be noted that no record of observations is available during the period 1945 through 1978.

Substantiating the comments by Muroran harbor personnel that the strongest winds at the port are caused by extra-tropical low pressure systems and associated frontal systems rather than tropical cyclones, a review of maximum wind speeds recorded at Muroran during the years 1971-1990 contained no events that coincided with the passing of a tropical cyclone near the area.

Wave Action

Waves are not a major problem at most of the port facilities. Wave generation in Uchiura Bay is fetch limited since the bay is only about 28 nmi across. As described above, the harbor entrance is open to the west, so any easterly-moving waves that are generated in Uchiura Bay can enter the outer harbor. If the waves contained enough energy, they could also pass through the narrower entrance to the inner harbors, and impact the port facilities on the east side of harbor Section 2, and in the northern part of Section 1. The Sakimori area, where U. S. Navy ships would likely moor would not be directly impacted.

The shallow-water wave generation tables contained in U. S. Army Coastal Engineering Research Center (1973) show that a 65 kt wind could raise waves of only 3 ft at Sakimori berths 1 and 2, given a 50 ft water depth and a fetch length of 4,000 yd (the longest distance across the outer harbor).

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. 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 which 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 configuration of Hokkaido Island in the vicinity of the harbor makes storm surges unlikely at Muroran because the harbor opens to the west away from the open sea area of potential storm surge buildup. The typical rapid movement of storms at this latitude further minimizes the storm surge threat. According to Muroran harbor personnel, there have been no instances of significant storm surge at Muroran.

Source: http://www.nrlmry.navy.mil/port_studies/thh-nc/japan/muroran/text/sect3.htm