# An Evaluation of the Harbor of Yokosuka, Japan As a Typhoon Haven/Tropical Cyclones

2. TROPICAL CYCLONES
2.1LIFE CYCLE
An informative and convenient method to view the typhoon is to look at its evolution from a weak tropical disturbance to its dissipating stage. The evolution from a tropical disturbance to a mature typhoon and subsequent dissipation is usually a slow process but it can at times be very rapid. Several of the terms used to describe the life cycle are as follows (adapted from Crutcher and Quayle, 1974):
1. Tropical Disturbance
A tropical disturbance, the formative stage from which a typhoon may develop, is a weak low pressure area charaterized by below normal pressure, abnormal cloudiness, shower activity and weak cyclonic wind flow. The disturbance forms in the low latitudes over a warm sea surface. The fall of barometric pressure is generally a slow, gradual process to about 1000 millibars (mb). The strongest winds occur to the north of the developing vortex in the Northern Hemisphere.
2. Tropical Cyclone
A tropical cyclone is a cyclone originating over the tropical ocean. Weather fronts are not associated with the tropical cyclone as with temperate latitude cyclones and most of its energy is derived from an intricate system which, to a large extent, is dependent upon the sea-surface temperature (SST). The SST is usually greater than 26°C. In the Northern Hemisphere the wind rotates counterclockwise around the warm center core of low pressure.
3. Tropical Depression A tropical depression defines the situation when there is a closed circulation (a closed isobar generally ​locates the center of the depression) and the maximum sustained wind speed is below 34 kt (gusts may exceed 34 kt). Warnings are issued on depressions only when intensification is expected.
4. Tropical Storm
If a tropical cyclone has maximum sustained winds of 34 to 63 kt it is termed a tropical storm and is assigned a name by the FWC/JTWC, Guam. Not all tropical cyclones reach this stage.
Assuming that the cyclone intensifies, the center pressure falls rapidly below 1000 mb and the winds form a tight band around the center. Cloud and rain patterns develop into narrow organized bands, spiraling inward toward the center.
5. Typhoon
A tropical cyclone in which the maximum sustained winds are 64 kt or greater is called a typhoon. The tropical cyclone is considered immature as long as the barometric pressure falls and the speed of the gusty winds increases. In an immature stage the typhoon strength winds may exist within a relatively small area.
When the center surface pressure stops falling, and the sustained wind speed regime stabilizes, the mature stage is reached; however the size of the storm may still increase and do so over a number of days. The circulation of a mature typhoon may be over 1000 n mi in diameter.
The right side of a tropical cyclone in the Northern Hemisphere is known as the "dangerous semicircle," while the left side is known as the "navigable semicircle" (navigable does not imply that a ship can maneuver without difficulty). Figure 1 gives an overall view of the wind pattern around the eye of a Northern Hemisphere 150-kt typhoon. Note the relationship between the wind intensity and the direction of movement.
​Figure 2 depicts the general wind field about a 150-kt Northern Hemisphere typhoon. The angle of inflow varies considerably with each typhoon. Note the tightening of the pressure gradient in the right semicircle relative to the direction of motion.
6. Dissipation Stage
Dissipation occurs when the maximum wind speed declines steadily along with a rising central barometric pressure.
2.2MOVEMENT
Individual tropical cyclones (henceforth the term tropical cyclone refers to tropical depressions, tropical storms, and typhoons) may follow irregular and widely differing tracks. In general they begin in the tropics and usually move west-northwest. Some cyclones move Some cyclones move from the tropical latitudes to the temperate latitudes, their northwest movement becoming northward and finally northeastward. This change in direction through north to the northeast is termed "recurvature." In a study of 586 tropical storms and typhoons which occurred during the months of May-December 1945-1969, Burroughs and Brand (1972) found that 236 (40%) of them recurved. The majority of typhoons which are a matter of concern to Yokosuka are the recurvers. Appendix A shows the tracks of all typhoons for the June-October period for the 24 years 1946-1959. The above value of 40% is derived from Table 1. Note the change in the percent that recurve for each month.
Tropical cyclones typically move at 8-14 kt prior to recurvature. After recurvature, they can accelerate within a period of 48 hours to speeds 2-3 times that at the point of recurvature. This acceleration is mainly due to the system entering the belt of prevailing westerly winds.
| Period | Per Period | Total Tropical Storms & Typhoons Per Period | Percent That Recurve |
| May June July August September October November December | 14 14 17 39 53 52 34 13 | 24 40 88 113 123 94 70 34 | 58 35 20 35 43 55 48 36 |
| May–December | 235 | 586 | 40 |
As a tropical cyclone moves into temperate latitudes, it comes into contact with cooler surface waters and cooler air is drawn into its circulation. These factors hasten its dissipation.
2.3SEA STATE AROUND TROPICAL CYCLONES
Wind is the driving force which generates the waves and produces the state of the sea. Swell, which affects ship movement, extends well beyond the wind field associated with a tropical cyclone. A miscalculation or imprudent decision concerning sea conditions may lead to a destructive rendezvous with the storm. Though the tropical cyclone's wind pattern can be forecasted with fair accuracy for any direction and distance relative to the center, a forecast of the sea state is more difficult. The character of the sea state is primarily a function of storm size, duration and intensity.
​Figure 3 shows the combined sea height associated with 21 tropical storms and typhoons (based on 173 analyses for the year 1971) plotted as a function distance from the storm center and storm intensity (Brand, et al., 1973). There is an obvious relationship between the sea state and storm intensity. A tropical storm (wind category 35-64 kt) can produce 12-ft seas 210 n mi from the storm center, while an intense typhoon (wind category ≥100 kt) can produce 12-ft seas 450 n mi from the center. The distances given are mean distances since the isopleths of combined sea height are not symmetric about the storm center. Brand et al. (1973) found that the actual wave heights are at least partially dependent on the direction in which the storm is moving. For example, Figure 4 shows the average combined sea-height pattern for recurved storms moving on headings between 001° and 090° and is based on 24 sea-state analyses for tropical storms and typhoons that occurred during 1971. The mean speed of movement and mean wind speed for these analyses were 12.1 kt and 64.2 kt, respectively. Note that the greatest area of higher seas (9-15 ft range) exists in the right (dangerous) semicircle of the storm.