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Chapter 2 Meteorology and Oceanography

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Chapter 2 Meteorology and Oceanography PART II ACTIONS AND MATERIAL STRENGTH REQUIREMENTS, CHAPTER 2 METROLOGY AND OCEANOGRAPHY Chapter 2 Meteorology and Oceanography 1 Meteorology and Oceanography Items to be Considered for Performance Verification 1.1 General The following meteorology and oceanography items, shall be considered with regard to the performance verification of port facilities. ① Atmospheric pressure and its distribution are factors that generate winds. ② Winds generate waves and storm surge, and affect the wind pressure that acts upon port facilities and moored vessels, and become a factor to interfere with cargo handling and other port operations. See 2 Winds for details. ③ The tidal level affects soil pressure and water pressure, which act on port facilities, and becomes a factor to interfere with cargo handling and other port operations. Also, it has an effect on waves in areas of shallow water. See 3 Tidal level for details. ④ Waves exert wave force on port facilities, and become a factor to interfere with the functioning of port facilities. They also act on moored vessels, causing them to move and interfere with cargo handling and other port operations. They also can raise the mean water level, which has effects similar to the tidal level as mentioned above. See 4 Waves for details. ⑤ Tsunami exerts wave force and fluid force on port facilities, and becomes a factor to interfere with the functioning of port facilities. It also acts on moored ships, causing them to move. See 5 Tsunamis for details. ⑥ Water currents affect sediments on the sea bottom and become a factor to interfere with the functioning of port facilities. See 6 Water Currents etc. for details. – 57 – TECHNICAL STANDARDS AND COMMENTARIES FOR PORT AND HARBOUR FACILITIES IN JAPAN 2 Winds Public Notice Winds Article 6 Characteristics of winds shall be set by the methods provided in the subsequent items corresponding to the single action or combination of two or more actions to be considered in the performance criteria and the performance verification: (1) Ocean surface winds to be used in the estimation of waves and storm surge shall be appropriately defined in terms of wind velocity, wind direction and others based on the long-term wind observation or weather hindcasting. (2) Winds to be used in the calculation of wind pressures shall be appropriately defined in terms of the wind velocity and direction corresponding to the return period through the statistical analysis of the long-term data of observed or hindcasted winds or other methods. (3) Winds to be used in the calculation of wind energy shall be appropriately defined in terms of the joint frequency distribution of wind velocity and direction for a certain duration of time, based on the long- term data of observed or hindcasted winds. [Commentary] 1) Winds to be used in the Estimation of Waves and Storm Surge: Winds to be used in the estimation of waves and storm surge shall be observed or hindcasted values for 30 years or more as a standard. 2) Winds to be used in the Calculation of Wind Pressure: Winds to be used in the calculation of wind pressure shall be observed or hindcasted values for 30 years or more as a standard. [Technical Note] 2.1 General (1) Wind is one of the most distinctive meteorological phenomena, namely, the phenomenon that the air moves due to atmospheric pressure differences and heat. The conditions under which winds blow over the ocean are usually very different than for those over land. Wind velocities over the ocean are much higher than those over land near the shore.1) For performance verification of port facilities, the effects of winds must be appropriately evaluated. (2) Gradient Winds ① The velocity of the gradient wind can be expressed as a function of pressure gradient, radius of curvature of barometic isolines, latitude, and air density as in equation (2.1.1). (2.1.1) where Vg : velocity of gradient wind (m/s); in the case of an anticyclone, equation (2.1.1) gives a negative value and so the absolute value should be taken. ∂p/∂r : pressure gradient (taken to be positive for a cyclone, negative for an anticyclone) (kg/m2/s2) r : radius of curvature of barometic isolines (m) ω : angular velocity of Earth's rotation (1/s) ω =7.27×10-5/s φ : latitude (°) 3 ρa : density of air (kg/m ) Before performing the calculation, measurement units should first be converted into the MKS units listed above. Note that 1º of latitude corresponds to a distance of approximately 1.11 × 105 m, and an air pressure of 1.0 hPa is 100kg/m/s2. – 58 – PART II ACTIONS AND MATERIAL STRENGTH REQUIREMENTS, CHAPTER 2 METROLOGY AND OCEANOGRAPHY ② A gradient wind for which the barometic isolines are straight lines (i.e., their radius of curvature in equation (2.1.1) is infinite) is called the geostrophic wind. In this case, the wind velocity is as equation (2.1.2). (2.1.2) ③ The actual sea surface wind velocity is generally lower than the value obtained from the gradient wind equation. Moreover, although the direction of a gradient wind is parallel to the barometic isolines in theory, the sea surface wind blows at a certain angle α to the barometic isolines in reality as illustrated in Fig. 2.1.2. In the northern hemisphere, the winds around a cyclone blow in a counterclockwise direction and inwards, whereas the winds around an anticyclone blow in a clockwise direction and outwards. It is known that the relationship between the velocity of gradient winds and that of the actual sea surface wind varies with the latitude. This relationship under the average conditions is summarized as in Table 2.1.1.3) α α Low High (a) Cyclone (b) Anticyclone Fig. 2.1.2 Wind Direction for a Cyclone (Low) and an Anticyclone (High) Table 2.1.1 Relationship between Sea Surface Wind Speed and Gradient Wind Speed Latitude (°) 10 20 30 40 50 Angle α (°) 24 20 18 17 15 Velocity ratio Vs /Vg 0.51 0.60 0.64 0.67 0.70 (3) Typhoon Winds In calculations concerning the generation of storm surge or waves due to a typhoon, it is common to assume that the air pressure distribution follows either Fujita’s equation (2.1.3) 4) or Myers’ equation (2.1.4) 4) ; the constants in the chosen equation are determined based on actual air pressure measurements in the region of typhoons. Fujita’s formula (2.1.3) Myers’ formula (2.1.4) where p : air pressure at a distance r from the center of typhoon (hPa) r : distance from the center of typhoon (km) pc : air pressure at the center of typhoon (hPa) r0 : estimated distance from the center of typhoon to the point where the wind velocity is maximum (km) ∆p : air pressure drop at the center of typhoon (hPa) ∆p=p∞-pc p∞ : air pressure at r =∞ (hPa); p∞=pc+∆p The size of a typhoon varies with time, and so r0 and ∆p must be determined as the functions of time (4) Meteorological GPV Organizations such as the Japan Meteorological Agency, the European Center for Medium-Range Weather – 59 – TECHNICAL STANDARDS AND COMMENTARIES FOR PORT AND HARBOUR FACILITIES IN JAPAN Forecasts (ECMWF), and America’s National Center for Environmental Protection (NCEP), calculate the values of items such as air pressure, wind velocity, wind direction, and water vapor flux, based on calculation models for meteorological values that use a three-dimensional calculation grid, and the values at the grid points (GPV: grid point values) are saved. These GPV’s may be used instead of wind hindcastings based on equation (2.1.1) through equation (2.1.4). However, when a grid with large spacing is used for meteorological calculations the atmospheric pressure and winds may not be satisfactorily reproduced at places where meteorological conditions change drastically with position, such as near the centers of typhoons. Therefore, when GPV’s are used, it is preferable to use observational values to verify the precision. (5) Wind Energy If winds are considered as the movement of the air then the wind energy that crosses a unit cross-sectional area in unit time is given by equation (2.1.5).1) Winds for estimating the wind energy shall be appropriately specified with joint statistic distributions for velocity and direction for a fixed time (usually, one year), based on long-term (usually, three years or more) observed or hindcasted data. (2.1.5) where P : wind force energy per unit cross-sectional area (W/m2) 3 ρa : air density (kg/m ) V : wind velocity (m/s) In other words, the wind force energy is proportional to the cube of the wind velocity, so a small difference in wind velocity can mean a big difference in energy (power generation). Therefore, during performance verification of facilities that use wind force energy, it is important to accurately understand how the conditions change with regard to time and space. In the coastal zone the wind conditions varies drastically between land and sea. Also, wind velocity shows great variation on land due to altitude, but over the sea the changes in wind velocity with altitude are gradual, so it is possible to obtain highly stabilized winds that are appropriate for power generation at relatively low altitudes. For example, the results of measurements in the vicinity of the Kansai International Airport, show that the wind energy over the course of a year at a measurement tower (MT station) placed at a height of 15 meters over the ocean were roughly the same as at a land station (C station) with an altitude of 100 meters, and about five times greater than at a land station with an altitude of 10 meters.5) 2.2 Characteristic Values of Wind Velocity (1) Determination of Wind Characteristics The elements of winds are direction and velocity, where the wind direction is expressed as one of sixteen directions and the wind velocity is the mean velocity over 10 minutes.
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