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NUMAZU

TROPICAL CYCLONES AFFECTING THE NUMAZU OPERATING AREA

Tropical Cyclone Climatology for Numazu

Tropical cyclones which affect the Numazu Operating Area generally form in an area bounded by the latitudes 5°N and 30°N and the longitudes 120°E and 165°E. The latitudinal boundaries shift poleward during the summer months and then equatorward in winter in response to the seasonal location of the southern boundary of the prevailing easterlies.

In the genesis area mentioned above, typhoons have occurred in all months but, with rare exceptions, those affecting the main Japanese Islands are confined to the period May to November. Late summer and early autumn are the likeliest seasons. Size and intensity of the storms vary widely. The majority of those that pose a "threat" to the area (any tropical cyclone approaching within 180 n mi of Numazu is defined as a "threat" for the purpose of this study) occur during the months June-October. Figure V-20 gives the frequency distribution of threat occurrences by 5-day periods. This summary of 84 tropical cyclones is based on data for the 28-year period, June-October 1947-1974. Note that the maximum number occur during August and September.

Figure V-21 displays the "threat" of tropical cyclones according to the octant from which they approached the 180 n mi radius threat area. The circled numbers indicate the total that approached from an individual octant. The figure count for an octant of approach includes both recurving and nonrecurving tropical cyclones. (See Chapter I, paragraph 3 for description of recurving tropical cyclones.) The adjacent numbers express this as a percentage. It is evident that a majority of these approach from the southwestern quadrant. A more detailed inspection of the sample of 84 tracks reveals that 11 (13%) did not recurve before passing the closest point of approach (CPA) to the Numazu Operating Area.

Table V-4 indicates that of the 84 tropical cyclones that posed a "threat" to the Numazu Operating Area during the years 1947-1974, 46% passed to the east of Numazu, 42% passed to the west and 12% passed in the immediate vicinity (within 20 n mi) of the area. The apparent majority of the "threat" tropical cyclones passing to the west or in the immediate vicinity implies that the Numazu Operating Area is placed quite often in the right or "dangerous" semicircle where the winds and seas are more intense.

Figures V-22, V-23, V-24, V-25, through V-26 represent an analysis of the estimated "threat" probability for any tropical cyclone as it approaches the Numazu Operating Area. The solid lines represent the probability of a system within an isoline coming within 180 n mi of the Numazu Operating Area. The dashed lines represent the approximate time in days for a system to reach Numazu based on typical speeds of movement of tropical cyclones affecting Numazu (Table V-5). For example, in Figure V-22, a tropical cyclone located at 27°N, 140°E has a 60% probability of passing within 180 n mi of the Numazu Operating Area and it will reach Numazu in about one day.

The speeds in Table V-5 were derived by considering that as tropical cyclones recurve, their forward speed characteristically, but not always, slows during the recurvature period. It should be expected that the system will subsequently accelerate rapidly toward the north or northeast. Speeds of 20 to 30 kt are common and speeds as great as 50 kt have been observed.

Wind And Topographical Effects

A total of 50 tropical cyclones passed within 180 n mi of the Numazu Operating Area in the 19-year period 1956-1974 during the months June-October, or about 2.6 per year. Table V-6 groups the 50 tropical cyclones by strong (22 kt) and gale force (34 kt) wind intensities (based on hourly wind data) that they produced at Mishima. Tropical cyclone activity within 180 n mi of the Numazu Operating Area is at a maximum during the months of August and September and these individual monthly values are also shown.

It can be discerned from Table V-6 that 25 (50%) of the total 50 tropical cyclones for the period June-October (1947-1974) resulted in winds of 34 kt or greater at Mishima. However, note that of the 16 tropical cyclones in September, 10 (63%) of these resulted in winds of 34 kt or greater.

The observation station at Mishima is located approximately 3 n mi northeast of the city of Numazu. The wind instrument is located on top of the station in a residential section of the city. There is no appreciable difference in the elevation of the station and that of Numazu, both being located in a flat coastal plain lying between the mountainous ridge running south into the Izu Peninsula and Fujiyama to the northwest. The observed wind is fairly representative of that at the Numazu Harbor where the observation station had been previously located. However, during the period 1964-1973, the highest recorded wind gust in Numazu was 97 kt on 25 September 1966 while at Mishima the wind gust was recorded at 82 kt (also the highest recorded during the period). This southeasterly gust was attributed to Typhoon Ida which passed 30 n mi to the west of Numazu on 25 September 1966. During this particular typhoon, sustained winds were recorded in excess of 34 kt for 5 hours.

Winds in Suruga Bay are greatly influenced by the surrounding topography and geographical features of the bay itself. The extent of this influence is dictated by the direction of approach of the storm and the passage relative to the Numazu Operating Area. From an analysis of the tropical cyclone tracks that affected Numazu, it is apparent that tropical cyclones that result in gale force winds or greater at Mishima can pass to the east or west of Mishima or in some instances the center of the storm passes over the immediate area. The basic difference between the passages is the direction of the resulting winds in the area.

If the tropical cyclone passes to the west of the Numazu Operating Area, the winds will be predominantly from the southwest. For a passage to the west, the storm must necessarily cross the mountain ranges of Honshu. An example of this was Typhoon Vera (September 1959) which had a CPA of 110 n mi to the northwest of Mishima. The typhoon pounded the area with gusts of 68 kt from the southwest and sustained gale force winds for a 7-hour period.

If the tropical cyclone passes to the east of Mishima, the path will generally be over water and the winds will be primarily northeasterly. An example of this was Typhoon Ida (September 1958) which had a CPA of 30 n mi to the southeast of Mishima. As a result, the area experienced gusts of 64 kt from the north-northeast.

Occasionally, a tropical cyclone will pass in the vicinity of Suruga Bay. In the 28-year period (1947-1974), ten tropical cyclones tracked in such a manner with 7 of them bringing gale force winds to the area. Under such circumstances there is no discernable pattern of a prevailing direction from which the strongest winds originate. Nor is the proximity of a storm's center indicative of force. Of the 10 tropical cyclones tracked, the maximum wind gust recorded ranged from 20 kt to 85 kt.

Figure V-27 shows the position of "threat" tropical cyclone centers when strong winds (22 kt) were first and last recorded at Mishima. A number of storms gave Mishima 22 kt winds when they were 300 n mi from the city with a predominant number of occurrences to the south and east of the Numazu Operating Area. Note also that strong winds were still being generated by a few storms when the storm centers were as far north as the island of Hokkaido. Figure V-28 shows tropical cyclone center positions when gale force (34 kt) winds were first and last recorded at Mishima. 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 gale force winds are south-southeast and northwest from Mishima and that those storm centers that track to the northwest of the area produce gale force winds of longer duration.

Wind And Swell Wave Action

The combination of the extreme depth and geographical orientation of Suruga Bay makes the area susceptable to extreme wave action -- often on short notice. Because of the depth of the bay, wave activity in the open areas of the bay is to be considered similar to that of the open ocean (under certain circumstances). Incoming wind and swell wave energy does not begin to come under the shoaling effect until it reaches the extreme northern reaches of the bay.

The northeast-southwest geographical orientation of Suruga Bay makes it extremely vulnerable to the wind and wave condition of the ocean and provides an unimpeded region for winds from the southwest quadrant to flow. The Numazu Operating Area is located in the northern reaches of the bay in extremely deep water and fully exposed to the aforementioned southwesterly flow of wind. The result is that the operating area is placed at the focal point of a considerable amount of incoming wave energy -- produced both locally and at great distances.

The beach along the northern side of the bay can be considered steep and is composed of pea gravel. From Figure V-l9 it can be seen that the bottom profile drops off rapidly to over 200 m in less than 1 n mi. The Japanese government has built a massive sea wall (approximately 50 ft high) that stretches along the entire northern coast of Suruga Bay and on the southwestern flank of the Numazu Harbor.

Maximum wave action generated by winds will occur when tropical cyclones pass to the west of the Numazu Operating Area. The resultant winds from the southwestern quadrant will flow unimpeded the entire length of the Suruga Bay, and depending on the size of the storm and the area over which the generating wind blows (fetch), produce wave heights typically associated with such winds encountered in the open ocean.

For tropical cyclones passing to the east, as approximately 46% have done in the 28-year period 1947-1974, the effects of wind produced waves will be lessened by the northeasterly flow of winds interacting with the mountains of the Izu Peninsula. Under such circumstances however, wind waves generated over the shorter fetch will be of the high frequency type -- usually steep with a short time interval between successive crests.

Swell waves are characterized by long, smooth undulations of the sea surface. These waves result from storms located at great distances the coast and the time between successive crests may be quite large. Such waves seldom, if ever, break in deep water as in Saruga Bay and unless very high usually do not affect small craft operations while they are operating in the deep water. They do, of course, cause rolling and pitching of large vessels. However, swell waves are important in that upon reaching shallow water height increases markedly, perhaps by a factor of two or more. Thus when they reach a depth shallow enough to break, they give rise to immense surf which can or destruction to small craft may cause damage or destruction to small craft or harbor installations.

Since Suruga Bay opens to the southwest, it is exposed to swells arising from the tropical cyclone generating area in the lower latitudes. These swells with their incumbent high energy approach the coast at high speeds and in the case of a large offshore disturbance such as a typhoon, the swell will ordinarily arrive before the disturbance. This situation could hamper a ship's efforts in attempting to reach a typhoon haven ahead of the storm.

The two types of waves, wind and swell waves, usually exist simultaneously at any time in the open waters of Suruga Bay. Often times the swell are completely obscured by the wind waves generated by local wind conditions. It is only near the shoreline in the Numazu Operating Area, where the swell begins to peak to greater heights, is the observer made aware of their presence. In this area, where critical wave conditions result from swell generated by storms occurring at considerable distances, local wind conditions may be of little value in determining significant wave and surf characteristics. A consideration of the orientation of isobars on a weather map will reflect large scale wind patterns and permit an estimate of the extent of the generation area and, consequently, the length of the fetch and the direction of wave propagation.

Table V-7 is an example of Fleet Numerical Weather Central's Wave Refraction/Surf Prediction based on the bottom topography of the Numazu Operating Area, direction of the incoming wave energy, and the period of the wave. The result is a "surf coefficient" that is dependent on the angle of incidence of the wave energy ray with respect to the beach. This surf coefficient is the equivalent to the ratio of the shallow water wave height and wave height. With this coefficient, an observer located in the of the operating area can estimate the height of waves passing his apply the surf coefficient to determine the height of the surf at the beach. For example, if a wave from the southwest with a height estimated to be 5 ft high in the deeper water and a period (measured crest to crest) of 10 sec, the surf height would range from 5 ft to 10 ft with an average of 7 ft. It should be noted that independent studies have found that the average period of typhoon generated waves is approximately 8-12 sec.

Because of its geographical configuration, the above indicates that surf conditions near Numazu may be affected by distant storms' that pass to the south or southwest, even though they may pose no threat to the Numazu Operating Area or show tendency of recurving. The swell generated by these and other storms travel at speeds (kt) of three times their crest-to-crest period. (That is, a swell with a period of 12 sec will progress outward from the generating area at 36 kt.) Eventually the "family" of swell separates with the longer period swell outdistancing the shorter periods. Decay rates of swell energy varies according to the size of the generating area and strength of the wind over the fetch. Typical tropical cyclones of typhoon intensity can generate sufficient energy such that swell from these storms can be felt at distances of 800-1000 n mi from the center of the generating area. Thus, a typhoon hitting Taiwan can result in high surf at Numazu.

Storm Surge and Tides

Storm surges result when a tropical cyclone crosses a coastline. They are caused by an interaction between wind stress on the water, the sharp drop in atmospheric pressure, and the shallowness of the harbor or bay.

Ships operating in Suruga Bay should not normally notice such a surge due to the extreme depths of the bay. More evident would be the wind generated waves and swells originating from the tropical cyclone system itself.

Tidal ranges near the Numazu Harbor area are quite small -- less than 2 ft for maximum ranges. Therefore, any surge associated with a tropical cyclone would tend to have a significant effect close to the harbor entrance where the water depth becomes shallower (approximately 45 fathoms).

For a more detailed discussion on the effects of tropical cyclones on Numazu, see Wixom, 1975.

Source: http://www.nrlmry.navy.mil/port_studies/thh-nc/japan/numazu/text/sect2.htm