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YOKOHAMA

TROPICAL CYCLONES AFFECTING YOKOHAMA

Tropical Cyclone Climatology at Yokohama

For the purpose of this evaluation, any tropical storm or typhoon approaching within 180 nmi of Yokohama is considered to represent a threat to the port. Tables V-48A and V-48B, contain a descriptive history of all topical storms and typhoons passing within 180 nmi of Yokohama during the 48-year period 1945-1992. All of the tropical cyclone statistics used in this report for storms passing within 180 nmi of Yokohama are based on the data set used to compile Tables V-48A and V-48B.

Tropical cyclones which affect Japan generally form in the area bounded by 5ºN to 30ºN and 120ºE to 165ºE. In response to the seasonal changes of the synoptic environment, the latitudinal boundaries shift poleward during the summer months and then equatorward in winter. Considering the entire western North Pacific basin, about two-thirds of the tropical cyclones reach at least typhoon intensity at some point in their life cycle.

Tropical cyclones are nurtured by a warm marine environment. In this basin maximum storm intensity typically occurs between 20ºN and 25ºN where sea-surface temperatures average near 84ºF (29ºC) during the month of August. After recurvature into the westerlies and the association with a colder environment, tropical cyclones lose their tropical characteristics. In this situation, the size of the circulation usually expands, the speed of the maximum wind decreases, the translational (forward) speed of motion increases and the distribution of winds, rainfall and temperature becomes increasingly asymmetric. However, since Yokohama is located just north of the normal recurvature latitude, recurved tropical cyclones passing within 180 nmi of the port still retain many of their tropical characteristics.

Although tropical cyclones have occurred during all months in the genesis area described in the preceding paragraphs, the primary tropical cyclone season for Yokohama is from June through October. As shown in Table V-49, since 1945 storms have passed within 180 nmi of Yokohama as early as May and as late as November. None have been recorded during the months of December through April. Approximately 28% (31 of 112) of all tropical storms passing within 180 nmi were of at least minimum typhoon strength (64 kt) when at their closest point of approach (CPA) to Yokohama. None of the May and November storms were of typhoon strength when at their CPA to Yokohama.

Table V-49 also shows the motion history of the 112 tropical storms and typhoons which passed within 180 nmi of Yokohama during the period 1945-1992. The average movement for all storms when at CPA to Yokohama is 041 degrees at 27 kt.

The average storm speed at CPA to Yokohama ranges from 18 kt in August to 36 kt in October. As shown in Appendix A, most tropical cyclones that pass close to Yokohama recurve between 20ºN and 30ºN and then accelerate northeastward. Because Yokohama is north of the normal latitudinal recurvature range, most storms passing within 180 nmi of the port have already recurved and are moving northeastward under the influence of upper level westerlies.

Figure V-201 depicts the monthly distribution of all 112 storms that passed within 180 nmi of Yokohama during the period 1945-1992. Occurrence is shown in 7-day increments. The figure shows that the period of peak occurrence is from early August through mid-to-late October.

During the period from 1945 through 1992 there were 112 tropical storms and typhoons that met the 180 nmi threat criterion for Yokohama, an average of over two per year. Figure V-202 depicts the actual number of storms occurring each year during that 48-year period. The designation as a tropical storm or typhoon is based on the intensity of the storm at the time of CPA to Yokohama. It can be seen that most years had at least one occurrence, and some had as many as five storms. One year, 1990, had six storms pass within 180 nmi. Only seven of the 48 years shown had no occurrences. Considering that Yokohama lies in a region where tropical cyclone activity is a relatively common occurrence, an extremely atypical situation occurred in the three-year period 1986 through 1988, when no tropical storms or typhoons passed within 180 nmi of the port.

Figure V-203 depicts, on an 8-point compass, the octants from which the 112 tropical cyclones in the data set approached Yokohama. As the figure shows, almost 70% of the stones approached from the southwest. 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 Yokohama.

Because of climatological considerations, there are preferred areas of the western North Pacific basin from which tropical cyclones eventually affect Yokohama. However, there are some tropical cyclones which, even though they traverse these preferred areas, do not affect Yokohama. Also, as might be expected, there are seasonal shifts to these preferred areas.

Figures V-204, V-205, through V-206 address the probability of tropical cyclones affecting Yokohama. Using a grid system, a tabulation was made of the total number of tropical cyclones passing through a given grid area regardless of whether they eventually passed within 180 nmi of Yokohama. A further tabulation was made of those storms which did eventually pass within that distance from Yokohama. After smoothing, the two tabulations were converted into probabilities and contours were drawn to connect points of equal probability.

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 Yokohama. For example, as shown in Figure V-204, during the months of July and August a tropical cyclone located at 25ºN 145ºE has an approximate 40% probability of passing within 180 nmi of Yokohama and would reach Yokohama in about three to four days.

The time-to-travel lines on Figures V-204, V-205, through V-206 represent average speed of advance (SOA) and are based only on those tropical cyclones that passed within 180 nmi of Yokohama. Because there are large variations in SOA at these latitudes, these average SOA's should only be used as general information. Planning actions for individual cyclones should be based on the current conditions and forecasts. Further, because most tropical cyclones that pass near Yokohama have recurved (SOA typically is slower during recurvature), the average SOA's reflected in the figures will likely be slower than the values that represent all of the tropical cyclones (non-recurvers and recurvers) passing through this region.

The average time-to-travel lines on each figure show that the speed of movement of tropical cyclones differs at different latitudes. For example, in Figure V-205 an average storm approaching within 180 nmi of Yokohama during September will travel about 360 nmi during the approximate two day period when it is between 4½-6 and 3-4 days away from Yokohama and moving northwest along the primary threat axis. During the period when it is between 3-4 and 1½-2 days away, an average storm slows during recurvature and will travel about 300 nmi. However, when an average storm is less than 1½-2 days away, it begins to accelerate after recurvature and will travel about 700 nmi during that period. Figures V-204 and V-206 show similar changes in speed of movement for other months of the year. However, the early season storms shown in Figure V-204 exhibit a less dramatic increase in speed of movement in the last two days of travel.

A comparison of Figures V-204, V-205, through V-206 shows that there are significant differences in the threat axes depicted by the three figures. During the months of July and August (Figure V-204) the threat axis extends south from Yokohama to about 30ºN before turning southeastward to a >40% maximum centered about 20ºN 160ºE. During the month of September (Figure V-205) the threat axis extends southwest from Yokohama to approximately 33ºN, and then gradually turns southeastward to a >40% maximum centered about 14ºN 150ºE. For the period of October through June (Figure V-206) the threat axis extends sharply southwestward from Yokohama to a position near 26ºN 130ºE before turning southeastward to lower latitudes.

Wind and Topographical Effects

Winds are measured for the Port of Yokohama at the Yokohama Regional Meteorological Station. It is located approximately 1.5 nmi south of the southeast end of Mizuho Wharf (Figure V-198). The ground elevation of the station is 128 ft (39 m) above sea level. The anemometer is located an additional 64 ft (19.6 m) above ground at the station, giving a total elevation of 192 ft (58.6 m) above sea level. Local port authorities state the measured winds are not influenced by buildings or terrain, and are representative of winds in the harbor. However, wind data contained in Table V-50 (Yokohama Regional meteorological Station) and Table V-51 (Tokyo International Airport) indicate that there may be a significant difference between the winds reported by the Meteorological Station and actual conditions experienced in more exposed areas of the harbor. The data contained in Tables V-50 and V-51 have been selected from observations recorded during the passage of the tropical cyclones listed in each table.

The wind and weather information listed in the tables include data from the Yokohama Regional Meteorological Station (35º26'N 139º39'E) for the 20-year period 1973 through 1992, and data from Tokyo International Airport (35º33'N 139º46'E) for the same period. As previously stated, the Yokohama site is located approximately 1.5 nmi south of the southeast end of Mizuho Wharf. Between 1985 and 1989 (exact date uncertain) the location of the Tokyo International Airport site changed to 35º33N 139º47E, a move of less than 1 nmi. The Tokyo International Airport sites are located just northeast of the port of Kawasaki, which places them approximately 7.5 to 8 nmi northeast of the southeast end of Mizuho Wharf (Figure V-198). From available charts, the more recent airport site appears to be situated on a low-lying, reclaimed portion of the bay. Station elevation at the airport sites changed from 6.6 ft (2 m) to 9.8 ft (3 m) in 1974, from 9.8 ft (3 m) to 19.7 ft (6 m) in 1981, and from 19.7 ft (6 m) to 26.3 ft (8 m) at some point between 1985 and 1989. Information on the elevation of the wind measuring equipment above ground level is not available.

Tables V-50 and V-51 contain observational data reported by the two sites during passages of selected tropical cyclones. An examination of the data contained in the tables reveals a significant difference in observed wind speeds at the two sites when data for the same passing storms are considered. In all cases, the winds reported by the Tokyo Airport are stronger than those reported by the Yokohama Regional Meteorological Station. The amount of the difference varies with each storm. On average, Tokyo International Airport's reported sustained winds are about 50% stronger than the sustained speeds reported by the Yokohama site. The majority of wind directions at the two sites are in close agreement.

The observed weather and maximum wind velocities listed in Tables V-50 and V-51 did not necessarily occur at the same time; the data in the tables represent the strongest wind and most severe weather conditions recorded during the entire passage of each storm. Wind gust data were not included in the Yokohama Regional Meteorological Station observations.

A total of 34 tropical storms or typhoons passed within 180 nmi of the Port of Yokohama during the period July 1973 through June 1992, the period during which surface observations for Yokohama are available. Of those storms, nine (26%) caused sustained winds of 22 kt at the Yokohama Regional Meteorological Station. Only one (3%), Typhoon Tip in 1979, caused winds of 34 kt. For the same period, however, 18 (53%) caused sustained winds of 22 kt at the Tokyo International Airport, and nine (26%) caused winds of 34 kt.

Seven of the nine storms causing sustained 22 kt winds at the Yokohama Regional Meteorological Station passed west of the harbor; two storms passed east. Typhoon Tip was the only storm bringing 34 kt winds to the station, and Tip passed west of the port. Of the 18 storms bringing sustained 22 kt winds to Tokyo International Airport, ten passed west of Yokohama, and eight passed to the east. Of the nine storms causing winds of 34 kt at the airport, six passed west of Yokohama and three passed to the east.

In the harbor, the direction of the resultant wind is the basic difference between a storm passing east of the harbor and one passing to the west. If the tropical cyclone passes west of Yokohama, the wind will generally be from the south. An example of this was Typhoon Tip (October 1979) which had a CPA of 68 nmi northwest of Yokohama. Tip caused southerly winds of 47 kt with gusts to 77 kt at Tokyo International Airport and 34 kt at the Yokohama Regional Meteorological Station.

If the tropical cyclone passes east of Yokohama, the path will be over water and the wind in the harbor will be generally northerly. An example of this was Typhoon Gay in October 1981. Gay brought north-northeasterly winds of 40 kt with gusts to 57 kt to Tokyo International Airport while the Yokohama Regional Meteorological Station recorded 23 kt.

The beginning and end points of the arrows in Figure V-207 give the positions of 23 tropical cyclone centers when sustained winds of 22 kt began and ended at Yokohama or Tokyo International Airport. Eleven of the 34 storms that passed within 180 nmi during the period 1973 through 1992 did not cause winds of 22 kt. Winds 22 kt began (and, incidentally, ended) in only three instances when the storm center was south of 32ºN.

Figure V-208 shows the positions of six tropical cyclone centers when sustained winds of 34 kt began and ended at Yokohama or Tokyo International Airport. None of the winds 34 kt began before the storm center was north of 34ºN.

Waves

The configuration of Tokyo Bay (Figure V-197) provides excellent protection to the Port of Yokohama from ocean swells. Miura Peninsula on the west side of the bay, and narrow Futtsu Point, which extends westward from the Boso Peninsula on the east side of the bay, prevent significant open-ocean wave motion from reaching Yokohama. Interviews with local harbor authorities reveal that, under 30 kt (15 m s-1) wind conditions, waves are limited to about 3.3 ft (1 m) within the harbor and 6.6 ft (2 m) in the anchorages.

The Guide to the Port of Yokohama, published by the Port of Yokohama Promotion Association in 1992, provides the following generalities regarding waves at Yokohama.

Under normal weather conditions, waves observed by the Yokohama District Weather Bureau are most frequently 1 ft to 1.6 ft (0.3 m to 0.5 m) high with a wind velocity of over 10 kt (5 m s-1). The annual occurrence of waves higher than 1.6 ft (0.5 m) is about 17 percent, while those exceeding 4.6 ft (1.4 m) are rarely observed. However, because this observation is related to only one-third of the wave heights observed, there is the possibility that waves exist with maximum heights which are 60-80 percent higher.

Under abnormal weather conditions, such as typhoon passages, waves have been observed on different occasions as follows: 10.5 ft (3.19 m), 10.5 ft (3.19 m), 8.5 ft (2.6 m), 7.2 ft (2.2 m) and 8.5 ft (2.6 m). The highest wave mentioned, 10.5 ft (3.19 m), occurred during Typhoon "Isewan," year not specified. The document goes on to state "The maximum height of waves during this typhoon was 60-80 percent higher than the average height under normal weather conditions. It is therefore possible for waves" 18 ft - 19.7 ft (5.5 m - 6.0 m) high "to occur once every 10-20 minutes, although this would not occur continuously."

Regarding the impact of ocean swell impacting the harbor, the document further states: "Even when there are 10-meter-high swells in the Pacific, they are only 10 cm high in the harbor, which is nearly insignificant."

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. The Port of Yokohama will be placed in the dangerous semicircle when a typhoon passes west of the area.

The storm surge effect is most evident in the shallow waters of large inland bays open to the south coast of Japan (Miyazaki, 1974). To a large extent, the surge forms after entering the inland bays because the width of the continental shelf is generally narrow along the Japanese coast. Most of the surge occurs, therefore, at the inshore end of these bays, and not along the open coasts nor near the mouth of the bays. Because Tokyo Bay is open to the south, a wind with a strong southerly component is necessary to cause a storm surge in the bay.

Local authorities state that the highest storm surge experienced at the Port of Yokohama is 10.67 ft (3.25 m), which occurred in October 1959. Because mean sea level at Yokohama is 3.77 ft (1.15 m) above the standard sea level height of zero, the value would translate to a water rise above mean sea level of 6.9ft (2.1 m).

As shown in Table V-52, a storm surge height of 4.63 ft (1.41 m) was observed at Tokyo during the passage of Typhoon Kitty on August 31, 1949. A graph included in the document entitled Meteorological and Oceanographic Characteristics for Safe Navigation of Ships on Tokyo Bay and Surrounding Waters that was published by the Tokyo Bay Marine Disaster Prevention Organization in 1993 compares the surge heights at Tokyo and Yokohama during Kitty's passage. The graph shows that Yokohama's maximum surge height was approximately 22% less than Tokyo's maximum surge height. As Tokyo is located farther north in Tokyo Bay than Yokohama it is to be expected that Tokyo would likely experience somewhat higher surge values (Figure V-209). The average percentage difference between the two locations for all storm surges is not known.

Figure V-209 has been adapted from the document referenced in the preceding paragraph and depicts the approximate relative difference in surge heights within the confines of Tokyo Bay. It shows that the greatest surge heights occur in the northern part of the bay.

Table V-53, adapted from Tokyo Bay Marine Disaster Prevention Organization (1993), lists the probabilities of occurrence of storm surges of the heights listed. It should be noted that the data are for Tokyo rather than Yokohama.

Source: http://www.nrlmry.navy.mil/port_studies/thh-nc/japan/yokohama/text/sect5.htm