TropicalCyclonesAffectingPusan

TropicalCyclonesAffectingPusan


PUSAN

TROPICAL CYCLONES AFFECTING PUSAN

For the purpose of this study, any tropical cyclone approaching within 180 nmi of Pusan is considered to represent a threat to the Port. Table 1 contains a descriptive history of all tropical storms and typhoons passing within 180 nmi of Pusan during the 63-year period 1945-2007. All of the tropical cyclone statistics used in this report for storms passing within 180 nmi of Pusan are based on the SAIC generated data set used to compile Table 1.

Tropical cyclones which affect Korea generally have the same genesis area as those affecting Japan: 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. Most of the storms affecting Pusan move northwestward from the genesis area to the East China Sea or western Philippine Sea south of the island of Kyushu, Japan before recurving north or northeastward to within 180 nmi of Pusan. Note that two typhoons (Typhoon Gilda in July 1974 and Typhoon Megi in August 2004) each passed within 10 nmi of Pusan when at CPA, but both passed to the east of Pusan. The tropical cyclones that have the most impact to the Pusan area are those tropical cyclones that pass just to the west, such as Typhoon Sarah (1959) and Typhoon Maemi (2003). These are the most dangerous for Pusan.

An examination of Table 2 shows that June through September is the primary season for tropical cyclone activity at Pusan, with 96% (108 of 112) occurring during that period. Of those, 88% (98 of 112) occurred from July through September. Although storms have occurred as early as May (one tropical storm on May 28th) and as late as October (three tropical cyclones), the overwhelming majority of the storms are confined to the June through September period. August is the month of greatest activity, with 45% (50 of 112) occurring during that month. August is also the month of greatest threat from typhoon-strength storms, as it has 52% (16 of 31) of the total typhoon occurrences.

Table 2 also shows the motion history of the 112 tropical storms and typhoons that passed within 180 nmi of Pusan during the 63-year period 1945-2007. The average movement for all storms when at CPA to the Port of Pusan is 028° at 20 kt. Average storm speeds are quite high at CPA and vary from 16 to 27 kt. June's 27 kt average speeds are the fastest.

Figure 17 shows the annual distribution in seven-day increments of the 112 storms by 7-day periods. The period of activity extends from 28 May until 11 October with peak in mid-August.

Figure 18 shows a chronology of the 112 tropical cyclones that passed within 180 nmi of the Pusan area in the 1945-2007 period. An average of 1.78 tropical cyclones per year passes within 180 nmi of Pusan. Historically, there were 31 tropical cyclones at typhoon strength while within the 180 nmi radius which equates to one every 2 years. Out of a total of 63 years there were only eight years which had no tropical cyclones and a total of 15 years which had three or four tropical cyclones.

Figure 19 depicts the octants from which the 112 tropical cyclones in the data set approached the Port of Pusan. As the figure shows, a total of 73 (65.2%) approached from the south-southwest octant. This was largely due to Korea's position relative to the primary tropical cyclone storm track, and Pusan's position just north of the primary storm recurvature area on the south coast of Korea. Ninety-two of the 112 storms (82.2%) were moving in a direction toward the northeast quadrant when at CPA which indicates that they had already recurved. Seventeen of the 112 storms were still moving towards the northwest quadrant when at CPA. It should be noted that the approach direction is determined at CPA, and may not represent the initial approach direction of the tropical cyclone toward the Port of Pusan.

Figure 20 and Figure 21 show the tracks of the 112 tropical cyclones split into two periods of time: July and August, and September through June. Be aware that the total number of storms on the two track charts adds up to 119 because some of the tropical cyclones overlap. A comparison of Figure 20 and Figure 21 shows that there is a distinct difference in the number of storms that pass east and west of the Port. During the September through June period there are a greater percentage of tropical cyclones, particularly typhoons, which pass to the east of the Port.

Figure 22 and Figure 23 are the statistical summaries of threat probability based on tropical cyclone tracks for the period 1945-2007. The summaries coincide with the tracks presented in Figure 20 and Figure 21. The data is presented with solid thin lines representing "percent threat" for the 180 nmi radius circle surrounding Pusan. The solid dashed lines represent approximate approach times to Pusan based on the climatological speeds of movement. In Figure 22 for example, a tropical cyclone located near 25°N 140°E has approximately a 20 % probability of passing within 180 nmi of Pusan during the July and August period. In addition, this tropical cyclone would reach the Port of Pusan in 3-4 days if the speed remains close to the climatological normal for tropical cyclones passing within 180 nmi of the Port of Pusan. A comparison of Figure 22 and Figure 23 shows that there is a significant difference in threat axes according to time of year. The months of July and August (Figure 22) have an axis that extends southward from Pusan to about 28°N near the Ryukyu Islands before slowly turning southeastward to the more tropical latitudes. The threat axis for the period from September through June, however, extends southwestward from Pusan to the east coast of the People's Republic of China at about 28°N before turning southeastward to the tropics.

Figure 24 depicts the primary threat axis of the 112 tropical storms and typhoons that passed within 180 nmi of Pusan during the period 1945-2007. The shaded arrow, pointing to the average location of CPA to the site, is a composite track zone for the 62 tropical storms and typhoons that crossed the dashed arc before entering Pusan's 180 nmi radius. The mean transit time from the arc to CPA for these 62 storms was 77 hours. However, the inset in the figure depicts a wide variation from the mean. The minimum transit time was 26 hours for Tropical Cyclone Ellis (1989) and the maximum was 236 hours for Typhoon Kirk(1996).

Figure 25 shows the tracks of the 112 tropical cyclones that passed within 180 nmi of Pusan over the years 1945-2007 (all months combined). Circular markers along the track indicate positions of tropical cyclones 72 hours before reaching the closest point of approach to Pusan. Note that only two of the tracks did not extend back 72 hours.

WIND

The locations of weather stations in the vicinity of Pusan (Busan) and Chinhae (Jinhae) are shown in Figure 26. Pusan's meteorological observation station is located on an east-facing hillside on the west side of Pusan's North Inner Harbor. Local authorities state that winds observed at the station are not necessarily representative of those at the Port because of the influence of the hills west and north of the station. Because of the AWS location near adjacent hills west of the station and generally mountainous topography north of the Port, local harbor personnel consider winds at this station to be representative of winds at the Port only from southerly directions.

The Busan Vessel Traffic Service (VTS) Center located on Jodo Island has an anemometer but the wind measurements are not representative of those in the Port/anchorage areas. Based on discussions with the VTS personnel, if the wind is from the southwest at 15 kt as measured at the VTS Center, a vessel can expect about 20 kt in the South Outer Harbor anchorage areas. If the wind is from the northeast, 20 kt at the VTS Center would equate to approximately 15 kt in the South Outer Harbor anchorage areas.

The Korean Meteorological Association and Busan Regional Maritime Affairs and Port Office have Severe Weather Alerts Criteria for issuing inshore and offshore advisories and warnings (Figure 27). Wind and Wave Advisories are issued with sustained winds 27.2-38.9 kt (14-20 m/s) for a duration of 3 hours on the sea surface or a wave height greater than 9.8-13.1 ft (3-4 m). Wind and Wave Warnings are issued with sustained winds 40.8 kt (21 m/s) or greater for a duration of 3 hours on the sea surface or a wave height greater than 16.4 ft (5 m). The Busan Regional Maritime Affairs and Port (Fisheries) Office will provide this info to the Busan VTS who will pass the information to vessels initially on VHF Channel 16 and subsequently on VHF Channel 12.

Note that the southeast through south direction is open to the sea from Pusan. The remainder of the directions, from southwest clockwise through east, has significant topographical barriers to wind flow. Consequently, any storm passing west of the Harbor would normally pose the greatest wind threat to Pusan.

The ROKN Port Operations stated during the Site Visit in May 2009 that all ship and small boat operations normally cease in the ROKN Base Harbor with winds ≥ 25 kt or seas ≥ 1.5 m.

Tropical cyclones passing to the West of Pusan are the most dangerous for the Port and Harbors.

Typhoon Maemi in September 2003 caused extensive damage in the Port of Pusan with estimated sustained winds of at least 80 kt. Newspaper reports stated, "The typhoon mauled South Korea's Main Port of Pusan. The storm knocked down 1,000-tonne cranes and tossed boats against each other. At least 82 vessels sank in huge seas and fishing boats were stacked like driftwood on shore roads." "Packing record winds of 135 mph, the typhoon lifted shipping containers in the air, toppled gigantic cranes and flipped an evacuated cruise ship on its side in Busan, South Korea's second largest city and the worst-hit urban area." Previously, in 1959 Typhoon Sarah with 55 kt winds observed in the Bay caused several Republic of Korea Navy vessels to run aground. The tracks for Typhoon Maemi and Typhoon Sarah are shown in Figure 1.

WAVES

With winds from east and south, heavy seas may run in the Outer Harbors, but the Inner Harbor is afforded some protection by the breakwaters at the entrances.

The maximum wave heights that can be expected with typhoon strength winds in Pusan's North Harbor are given in Table 3 This information has been extracted from a prior evaluation of the Port of Pusan.

As Table 3 indicates, wave motion in the North Inner Harbor is limited due to the short fetch. However, the North Outer Harbor has a limited fetch only for northerly winds; south to southeasterly winds can bring waves as high as 12 ft (3.7 m) to the Harbor due to the long fetch south of the Harbor.

The pilots indicated during the Site Visit in May 2009 that the Port has experienced as much as 9.8-16.4 ft (3-5 m) during winter storms and 26.2-32.8 ft (8-10 m) in extreme typhoon conditions in the South Outer Harbor anchorage areas.

The wave heights presented in Table 3 are intended as a guide only. Specific storms may generate waves that vary from those listed in the table.

STORM SURGE

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 cyclone is more intense in the dangerous semicircle, the region of most intense winds.

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. The dome height is related to local pressure (i.e., a barometric effect dependent on the intensity of the storm) and to wind stress on the water caused by local winds. Other significant contributing factors are storm speed, direction of approach, bottom topography, and coincidence with astronomical tide.

The worst combination of circumstances (Harris, 1963, and Pore and Barrientos, 1976) would include:
(1) An intense storm approaching perpendicular to the coast with the Harbor within 30 nmi to the right of the storm's track.
(2) Broad, shallow, slowly shoaling bathymetry.
(3) Coincidence with high astronomical tide.

The North Harbor at Pusan is susceptible to storm surge when the area experiences prolonged southerly winds of ≥ 35 kt. Past occurrences have resulted in increased water levels of up to 10 ft (3 m). A previous evaluation mentioned serious damage that resulted from Typhoon Sarah as the storm moved onshore just west of Pusan in September 1959. Typhoon Sarah had center winds of 98 kt and was moving 034° 24 kt when at its CPA to Pusan of 319° 12 nmi. Typhoon Maemi was the most significant typhoon in recent times to seriously impact Pusan with wind, waves, and storm surge. As stated in newspaper reports dated September 14, 2003, "Tidal waves heaved an evacuated cruise liner onto its side on a beach in Pusan, South Korea's second-largest city." "The typhoon landed when the tide was full, causing bigger damage."

Source: http://www.nrlmry.navy.mil/port_studies/thh-nc/korea/pusan/text/tropical_cyclones.htm