CHILUNG (KEELUNG)

3.4 TROPICAL CYCLONES AFFECTING CHILUNG

3.4.1 Tropical Cyclone Climatology for Chilung

Severe tropical cyclones can occur during any month of the year; however, the majority of those that pose a potential threat to Chilung (any tropical cyclone approaching within 180 n mi is considered a "threat") occur during the period June through early October. Figure III-14 gives the monthly summary, by 5-day periods, based on data for the 24 years 1949-1972. Note that the threat level is fairly constant for the months June through September. Approximately 70% of the tropical cyclones that pass within 180 n mi of Chilung pass to the south, 30% to the north.

In Figure III-15 the threat storms are displayed as a function of the compass octant from which they approached Chilung. The circled numbers indicate the number that approached from an individual octant, while the numbers in parentheses indicate the percent of the total that approached from that octant. Note that the majority approach from the south-southeast.

Figures III-16, III-17, III-18, III-19, through III-20 present the percentage of tropical cyclones that have passed within 180 n mi of Chilung (can be interpreted as probability of threat) for the months June-October. The dashed lines represent approximate approach times to Chilung based on an approach speed of 8-12 kt. For example, in Figure III-16 a storm located at 130°E and 15°N has about a 40% probability of passing within 180 n mi of Chilung and, if its speed remains in the 8-12 kt range it will reach Chilung in approximately 3-4 days (the faster the speed of an individual storm the shorter the time required to reach Chilung). The average speed of typhoons approaching Taiwan has been determined to be 10-11 kt (Brand and Blelloch, 1973).

In the 24 years 1949-1972 (considering the months of June through October) an average of slightly more than 3 tropical cyclones per year passed within 180 n mi of Chilung (83 total). The largest number that occurred in any single year was six (1959 and 1966). In Table III-2 the 83 storms that passed within 180 n mi are grouped according to their effect at Chilung. It can be seen that of the total of 83, only 22 storms (approximately 27%) resulted in winds of 34 kt or greater.

For a more detailed meteorological evaluation of Chilung harbor as a typhoon haven the reader is referred to Brown, 1974.

3.4.2 Effects of Topography

From analysis of the storm tracks, it is apparent that those tropical cyclones that result in gales at Chilung fall into two general categories: those that pass north of Chilung (within 180 n mi) and those that pass south of Chilung crossing Taiwan. (Note that in no case did gales result if the track was south of Taiwan.) Because Chilung is protected on three sides by mountains and open to the north, northerly winds pose the greatest threat to the harbor. This is not to say that only storms passing north of Chilung are significant -- any storm whose circulation results in a northerly component of wind at Chilung can be expected to create problems for ships in the harbor. An example of this was Typhoon Nadine (July, 1971). Nadine passed south of Chilung crossing Taiwan during the period 25 to 27 July. Late on 25 July, the winds at Chilung were recorded at 60 kt from the ENE, and wave heights near the outer harbor exceeded 40 ft.

Figure III-21 shows the position of tropical cyclone centers (1949-1972) when strong winds ( 22 kt) were first and last recorded at Chilung. It is apparent that a significant number of the storms begin to affect Chilung when they are still over 200 n mi away. Note that storms passing to the south of Taiwan do not significantly affect Chilung. Figure III-22 shows the tropical cyclone center positions when gale force (34 kt) winds were first and last recorded at Chilung. It can be seen that 34-kt winds generally do not begin until the storm is about 100 n mi away.