BÀI GIẢNG KHÍ TƯỢNG THỰC HÀNH CHƯƠNG 8 VÀ 9

CHAPTER Observation of Sea Surface Temperature CHAPTER Observation of Sea Surface Temperature Sea surface temperature plays an important role in understanding the interactive relation between the atmosphere and the ocean The difference between the air temperature and the sea surface temperature gives a basic measure of the vertical stability of the atmosphere and the heat exchange between the atmosphere and the ocean The sea surface temperature should therefore be very carefully observed The temperature to be observed is that of the sea water at the depth of 1-2 m in the nearsurface mixing layer underlying the ocean skin and is representative of general oceanic conditions The scale of your sea-water thermometer should be read to the order of O.l°e The sea surface temperature is normally observed by one of the following ways: 1) Measuring the temperature of the condenser intake water; 2)Exposing an electrical thermometer to the sea-water either directly or through the hull; 3) Measuring the temperature of a sample of the sea-water with a sea-bucket The methods 3) and 1) have been used for many years Recently, as the speed and dimension of ships have increased, method 2) has been more widely used 8.1 Intake method The intake sea-water temperature is measured by a thermometer provided within the intake pipe when the ship is built When a direct-reading thermometer is installed in cramped conditions the observer should be warned of the possibility of error in his readings due to parallax A distant reading system with the display elsewhere (e.g in the engine room or on the bridge) overcomes this problem The observer should also be aware that for ships of deep draught, or when a marked temperature gradient exists within the sea surface layer, intake temperature readings usually differ considerably from those close to the sea surface The intake temperature should not be taken when the ship is stationary, for the cooling water may not be circulating then In addition to the above, the temperature of the intake pipe may change due to the heat of the engine room Thus this method is not necessarily appropriate for the observation of the seasurface temperature However, it is adopted to some extent in consideration of its safe and convenient ways 8.2 Hull-attached thermometer method The sensor in this method is mounted either externally in direct contact with the sea using a "through-the-hull" connection, or internally (the "limpet" type) attached to the inside of the hull Such hull-attached thermometer provides a very convenient and accurate means of measuring sea-surface temperature Its distant-reading device is installed in a cabin for observation The sensor has to be located forward of all discharges at a depth of - m below the waterline When large changes of draught can occur, more than one sensor may be needed 59 CHAPTER Observation of Sea Surface Temperature 8.3 Sea-bucket method In this method, a preparatory process to haul sea-water by a sea-bucket (especially a simple bucket or a canvass bucket) lowered over the side of the ship and then to pour it away should be repeated several times to make the temperature of the bucket attain the same value with that of the sea-water before a sample of a full bucket of sea-water is hauled on board to obtain its temperature by a thermometer The sample should be taken near the bow, from the leeward side of the ship during navigation, from the windward side while at anchor, and well forward of all outlets The thermometer should be read as soon as possible after it has attained the temperature of the water sample When not in use the bucket should be in a shady place to drain 60 CHAPTER Observation of Ocean Waves CHAPTER Observation of Ocean Waves Navigation of a ship is most profoundly affected by winds and waves Needless to say, correct information on ocean waves is essential for disaster prevention and economical navigation Recent development in the observation by satellites not the least diminishes the important role of the observation of ocean waves by ships The data of ocean waves observed by ships contribute to the analysis and forecasting of ocean waves and are also utilized for statistical studies in the climatology of ocean waves 9.1 Classification of ocean waves Due to various reasons, thousands of waves with different wave heights and wave periods give rise on the ocean The sea surface is incessantly moving up and down Among such waves, those caused by winds over the sea surface are called ocean waves and ocean waves have periods between around second and about 30 seconds Ocean waves are generally classified into wind waves (or sea) and swell Wind waves and/ or swell change into surfs in approaching a coast or into tidal races in colliding with tidal or ocean currents (1) Wind waves (or Sea) The system of waves raised by the local wind blowing at the time of observation is usually referred to as "wind waves" or "sea" Development of wind waves depends on three characteristics of wind: wind speed, fetch (the distance along which a wind is blowing straight) and duration (the time during which a wind with an almost fixed wind direction and speed is blowing along a fetch) (2) Swell Waves not raised by the local wind blowing at the time of observation, but due either to winds blowing at a distance or to winds that have ceased to blow, are referred to as "swell" Fig.9.1 shows a schematic view of wind waves (sea) with sharp crests and swell with gentle ones 9.2 Characteristics of ocean waves Major characteristics of ocean wave are as follows: 1) Wave direction: the direction from which the waves come 2) Wave period: the time between the passage of two successive wave crests past a fixed point It is equal to the wave length divided by the wave speed 3) Wave height: the vertical distance between trough and crest (see Fig 9.2) 4) Wave length: the horizontal distance between successive crests or troughs It is equal to the wave period multiplied by the wave speed (see Fig 9.2) 5) Wave speed: the distance traveled by a wave in a unit of time It is equal to the wave length divided by the wave period Among these characteristics, the observations should include the measurement or estimation of 1),2), and 3) of the above in respect of each distinguishable system of waves, i.e wind waves (sea) and swell 9.3 Non-instrumental observation 62 CHAPTER Observation of Ocean Waves Bearing in mind the distinction between wind waves (sea) and swell, the observer should differentiate between the recognizable wave systems, on the basis of the direction, the appearance and the period of the waves (1) Direction The direction (one of the 36 directions) from which the waves are coming is most easily found by sighting along the wave crests and then turning 90° to face the approaching waves The observer is then facing the direction from which the waves are coming (2) Period Measurement of wave period is made in the unit of second by a stopwatch The observer notes some object floating on the water at some distance from the ship: if nothing better is available, a distinctive patch of foam can usually be found which remains identifiable for the few minutes required for the observation He starts his watch when the object appears at the crest of the wave As the crest passes on, the object disappears into the trough, then reappears on the next crest, etc The time at which the object appears to be at the top of each crest is noted The observations are continued for as long as possible; they will usually terminate when the object becomes too distant to identify, on account of the ship's motion Obviously the longest period of observation will be attained by choosing an object initially on the bow as far off as it can be clearly seen Another method is to observe two or more distinct consecutive periods from an individual group while the watch is running continuously; with the passage of the last distinct crest of a group or the anticipated disappearance of the object, the watch is stopped, then restarted with the passage of the first distinct crest of a new group The observer keeps count of the total number of periods until he reaches 15 or 20 at least With observation of a period less than five seconds and low wind velocity, the above observation may not be easily made, but such waves are less interesting than those with longer periods (3) Height With some experience fairly reliable estimates can be made For estimating the height of waves having wave lengths much shorter than the ship, the observer should take up a position as low down in the ship as possible, preferably amidships where the pitching is least, and on the side of the ship from which the waves are coming Use should be made of the intervals which occur every now and then, when the rolling of the ship temporarily ceases In case of waves longer than the ship, the preceding method fails because the ship as a whole rises over the wave Under these circumstances the best results are obtained when the observer moves up or down in the ship until, when the ship is in the wave trough and vertical, the oncoming waves appear just level with the horizon (see Fig.9.3) The wave height is then equal to the height of the observer above the level of the water beneath him (a) If the ship is rolling, care should be taken to ensure that the approaching wave is in line with the horizon at the instant when the ship is vertical, otherwise the estimate of height will be too large or too small (b) 63 CHAPTER Observation of Ocean Waves By far the most difficult case is that in which the wave length exceeds the length of the ship but the wave height is small The best estimate of height can be obtained by going as close to the water as possible, but even then the observation can only be rough