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KAGOSHIMA

TROPICAL CYCLONES AFFECTING KAGOSHIMA

Tropical Cyclone Climatology For Kagoshima

Climatology indicates that the island of Kyushu has been affected by tropical cyclones from April through December. The majority, however, that pose a threat to Kagoshima (any tropical cyclone approaching within 180 n mi of Kagoshima Harbor is defined as a "threat" for the purpose of this study) occur during the months of June-October. Figure V-74 gives the frequency distribution during the months of "threat" occurrences by 5-day periods. This summary is based on data for the 28-year period, 1947-1974. Note that the maximum number occur during August and September.

Figure V-75 depicts, on an 8-point compass, the "threat" tropical cyclones according to the octant from which they approached Kagoshima. The circled numbers indicate the total that approached from an individual octant. The count for an octant of approach includes both recurving and non-recurving tropical cyclones. (See paragraphs 3 of Chapter I for a description of recurving tropical cyclones.) Note that a majority of these approach from the south-southeast and south-southwestern octants. A more detailed inspection of the 85 tracks revealed that 20 (22%) did not recurve prior to passing the closest point of approach to Kagoshima.

Table V-18 indicates that, of the 85 tropical cyclones that posed a threat to Kagoshima during the years 1947-1974 (June-October), 53% passed to the east of Kagoshima, 36% passed to the west and 11% passed in the immediate vicinity of the port. The fact that the majority of the "threat" tropical cyclones pass to the east, implies that Kagoshima is placed more often in the left or "navigable" semicircle where the wind and seas are less intense.

Figures V-76, V-77, V-78, V-79, to V-80 represent an analysis of the estimated probability for any tropical cyclone approaching within 180 n mi of Kagoshima. The solid lines represent the probability of coming within 180 n mi of Kagoshima for any storm location. The dashed lines represent the approximate time in days for a system to reach Kagoshima, computed from typical speeds of movement for tropical cyclones affecting Kagoshima (Table V-l9). For example, in Figure V-76, a tropical cyclone located at 25°N, 130°E has a 60% probability of passing within 180 n mi of Kagoshima and it will reach Kagoshima in about l l/2-2 days.

Note the significant shift in direction from which tropical cyclones approach the Kagoshima area (Figures V-76, V-77, V-78, V-79, to V-80). In June the "threat" is generally from the southwest whereas, in July and August it is more to the south and southeast, then becomes more southerly in September, and then south to southwesterly in October.

Wind And Topographic Effect

A total of 54 tropical cyclones approached within 180 n mi of Kagoshima in the 19-year period 1956-1974 during the months June-October, or about 3.1 a year. Table V-20 groups the tropical cyclones by strong (22 kt) and gale force (34 kt) wind intensities (based on hourly wind data) that they produced at Kagoshima. Tropical cyclone activity in the Kagoshima area is maximal during the months of August and September and these individual monthly values are also shown.

It can be discerned from Table V-20 that 21 (39%) of the total 54 tropical cyclones for the period June-October (1956-1974) resulted in winds of 34 kt or greater at Kagoshima. However, of 22 tropical cyclones tracked in August, 10 (45%) resulted in winds of 34 kt or greater.

An observation station for the Japanese Meteorological Agency is located in the downtown area of Kagoshima near the harbor facilities (see Figure V-73). The wind instrument is located on top of the station in such a manner as to be unobstructed from any nearby buildings or trees. During the period 1947-1974, the highest recorded wind gust in Kagoshima was 100 kt on 29 September 1955. This easterly gust was attributed to Typhoon Louise which passed 30 n mi to the west of Kagoshima on 29 September 1955. The duration of gale force wind (excess of 33 kt) was 5 hours during this storm.

Winds in Kagoshima Bay are significantly influenced by the surrounding topography and the geographical features of the bay itself. The extent of this influence is dependent on the direction of approach of the storm and the passage relative to the Kagoshima area. From an analysis of the tropical cyclones that affected Kagoshima it is apparent that tropical cyclones that result in gale force winds at Kagoshima can pass to the east or west of Kagoshima or in some instances the center of the storm passes over the immediate area. The basic difference in effect is the direction and strength of the resultant wind in the area.

If the tropical cyclone passes to the east of Kagoshima, the path will generally be over water and the winds will be primarily northeasterly. While there will be some interaction with the Kyushu Mountains (see Figure V-54) to decrease the intensity of the winds, local topography becomes significant in its effect on northeasterly winds. The mountains on the northwestern side of the bay that rise to nearly 2000 ft and Sakurajima which rises to 3655 ft tend to direct and funnel winds from the northeastern quadrant into the narrow region of the Kagoshima Harbor area as can be seen in Figures V-72 and V-73. An example of this was Typhoon Helen which had a CPA of 40 n mi to the east-southeast of Kagoshima on 24 September 1966. During this particular typhoon, wind gusts were recorded up to 78 kt from the northeast.

In the case of tropical cyclones passing to the west of Kagoshima, the path is also over water in its approach to the area, thus retaining much of its strength before striking Kyushu. From Figures V-54 and V-72 it is evident that the protection offered by surrounding topography is of little assistance in decreasing the intensity of the storm as it makes its first encounter with land. The long broad expanse of Kagoshima Bay allows practically uninterrupted flow from the southeastern quadrant. These factors, in addition to the bay being placed in the "dangerous" semicircle, makes a western passage extremely dangerous. An example of this case was Typhoon Babs which had a CPA of 150 n mi to the west-northwest of Kagoshima on 16 August 1956. Typhoon Babs produced wind gusts of up to 72 kt from the south-southeast.

Figures V-81, V-82, through V-83 show the average maximum wind gust associated with the tropical cyclones studied, and the direction from which it originated as recorded at Kagoshima during the period 1947-1974. An evaluation of these figures show that winds from tropical cyclones passing to the west come primarily from the southeasterly direction and tend to be more intense than those from tracks of storms passing to the east. In those cases, the winds may come from any direction but for the most part come from the northeast or northwest. Occasionally, a tropical cyclone will pass in the immediate vicinity of Kagoshima. In the period 1947-1974, ten such storms tracked in such a manner with all but one producing gale force winds or stronger. Under such circumstances, there is no discernible pattern as to prevailing direction from which the strongest winds originate. The proximity of the storm's center when passing in the immediate vicinity, however, is indicative of force. Of the 10 tropical cyclones studied, the average maximum wind gust was 58 kt.

Figure V-84 shows the position of "threat" tropical cyclone centers when strong winds (22 kt) were first and last recorded at Kagoshima. A number of storms gave Kagoshima 22 kt winds when they were 300 n mi to the south of the city. Figure V-85 shows tropical cyclone center positions when gale force (34 kt) winds were first and last recorded at Kagoshima. It can be ascertained from this figure that winds 34 kt generally do not begin until the storm is about 180 n mi away. Notice the preponderance of storms that generate strong or gale force winds originate to the south and west of Kagoshima and that no gale force winds occurred when the storms moved north of 34°N.

The most severe threat to the harbor occurs when a tropical cyclone approaches from the southwest and passes west of Kagoshima within 50 n mi. In this case, winds will flow from the southeast unimpeded the entire length of Kagoshima Bay focusing on the narrow harbor area.

Wave Action

The geographical location of Kagoshima Harbor is such that regardless of whether a tropical cyclone passes east or west of the port, ships anchored in the outer harbor area will experience considerable wave action. Tropical cyclones passing to the east will generate winds from the northeastern quadrant. In this area there is an unbroken fetch of 12 n mi in the northern portion of Kagoshima Bay. In a similar manner, the fetch to the south-southeast of the harbor is approximately 28 n mi over which winds generated by a tropical cyclone passing to the west can flow unimpeded.

Tables V-21A, and V-21B show the wind speed required to generate various wave heights in Kagoshima Harbor based on the direction and length of fetch that the wind blows over.

During the same period of study that produced Tables V-21A, and V-21B, additional data shows the frequency distribution of wave height as can be seen in Table V-22.

It should be noted that these tables reflect weather conditions that occurred throughout the entire year for 10 complete years, and included 35 tropical cyclones.

Because of the configuration of the entrance to Kagoshima Bay, swell generated by the storm centers is effectively intercepted. Thus the sea state inside the bay and in the Kagoshima Harbor area is dependent solely on local wind conditions.

Storm Surge

Storm surge may be defined as an abnormal rise of the sea along a shore as the result of the winds of a storm and the pressure drop. 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. Kagoshima Harbor is in the dangerous semicircle when a tropical cyclone passes to the west of the area. The surge effect is most evident in the shallow waters of large inland bays that open to the south (Miyazaki, 1974).

Conversations with officials of the Kagoshima Harbor Office and Japanese Maritime Safety Agency indicate that the harbor area is not adversely affected by storm surges.

Source: http://www.nrlmry.navy.mil/port_studies/thh-nc/japan/kagoshim/text/sect4.htm