BÀI GIẢNG KHÍ TƯỢNG LÝ THUYẾT CHƯƠNG 13

Chapter Thirteen CHAPTER THIRTEEN Fronts and Frontal Depressions Before proceeding with this chapter it is essential that the previous chapters have been read and understood AIR MASS BOUNDARIES When two air masses of differing characteristics meet they not mix freely but remain separated by a boundary called the frontal surface Some mixing of the air masses does take place but only along this boundary which is really a narrow zone of transition often referred to as the mixing zone Such a boundary is represented on the weather chart by a line called a FRONT When two air streams with different temperatures converge and meet the warmer air tends to override the colder denser heavier air whilst the colder air tends to undercut the lighter warm air (See Figures 13.1 (a), (b) and (c)) THE MAIN FRONTAL ZONES The positions of frontal zones marking the boundaries between the principal air masses fluctuate constantly whilst their mean positions move north and south with the seasons See Figure 13.1 (d) and compare the mean positions of the frontal zones in January with those for July, but bear in mind that the day-to-day positions of these zones can vary considerably from those shown in the figure especially in the temperate latitudes The Arctic front The Arctic front marks the transition between Arctic air and Polar maritime air There is a similar front in the North Pacific The Polar front The polar front marks the boundary between polar and tropical air masses in the Atlantic and Pacific Oceans In the North Atlantic its mean position in summer is from Newfoundland to Scotland In winter it moves southwards and extends from Florida to southwest England The Mediterranean front The Mediterranean front exists only in winter and extends from west to east across the Mediterranean separating polar continental air originating from Europe and tropical continental air from North Africa 111 Chapter Thirteen 112 Chapter Thirteen 113 Chapter Thirteen The intertropical convergence zone (I.T C.Z.) The intertropical convergence zone lies within the tropics and is a broad zone of separation between the N.E and S.E Trades which flow equatorwards from opposite hemispheres It was formerly known as the Intertropical Front but this term has fallen into disuse, because the opposing air masses not differ greatly in their characteristics and it bears little resemblance to other fronts The I.T.C.Z crosses and recrosses the equator at several point's and moves well north in the summer Due to the very large land masses in the northern hemisphere [he greater part of its length (in its mean position) lies north of the equator Its range of movement is small over the oceans but may be very large over the continents Areas of horizontal convergence along this belt vary from day to day in both position and activity but are generally associated with cloudy showery weather See Figure 13.1 (d) FRONTAL THEORY OF FORMATION OF DEPRESSIONS Within the temperate zones cold air flowing from high latitudes encounters warm air moving from sub-tropical regions The two air masses are separated by a frontal surface which slopes upward over the colder denser air at a gradient which varies from in 40 to in 200 (See Figures 13.1 (a), (b) and (c)) The polar front tends to remain inactive so long as the warm and cold air masses flow parallel to one another but when they converge warm moist air is forced upwards over the cold frontal surface This can result in the formation of much cloud and precipitation and often starts the mechanism which leads to the formation and development of a frontal depression The sequence of events is described in the following paragraphs: Under suitable conditions a small wave forms on the polar front, so that at this point there is a bulge of warm air protruding into the cold See Figure 13.2 and note that the two air masses flow more or less parallel to one another along the polar front except at the bulge where the winds are convergent (See Convergence in Appendix 1) The wave continues to grow in size Development usually follows and pressure falls at the crest of the wave Enlargement of the bulge continues and this is accompanied by a further fall in pressure The isobars then assume the closed form of a depression and the wind circulates round the tip of the bulge As pressure continues to fall the 114 Chapter Thirteen gradient becomes steeper and the winds stronger (See Figure 13.3) Figure 13.3 shows the isobars and fronts of a well developed and active depression The line LX is a warm front, since warm air is replacing cold air along this line LY is a cold front since cold air is replacing warm air along this line The warm and cold fronts are represented on the weather chart by the symbols shown in Figure 13.3 On working charts warm and cold fronts are represented by red and blue lines respectively The area between LX and LY is known as the "warm sector" of the depression The centre of the depression will move along the polar front in a direction roughly parallel to the isobars in the warm sector and at a speed approximating to that of the air in the warm sector (See Figure f3.3) 115 Chapter Thirteen 116 Chapter Thirteen 117 Chapter Thirteen 118 Chapter Thirteen 119 Chapter Thirteen THE OCCLUDING OF A DEPRESSION The cold front advances faster than the warm front and gradually overtakes it, commencing at the tip of the warm sector and working down the length of the front until the occluding process has been completed and all the tropical air has been lifted off the ground Figures 13.6 and 13.7 show the stages in the occluding process Note that the symbol for an occlusion is a combination of those used for warm and cold fronts On working charts an occlusion is represented by a purple line An occlusion is classified as warm or cold according to whether the overtaking polar air is warmer or colder than the retreating polar air respectively In the former case the overtaking air will override the colder air and, in the latter case, will undercut the warmer air (See Figures 13.8 (a) and (b), 13.9 (a) and (b)) In Figures 13.8(a) and 13.9(a) the position of the upper front is indicated by a dotted line Compare these two figures and notice that the line marking a cold occlusion is continuous with the line of the cold front whereas the warm occlusion is shown as a continuation of the warm front MATURITY AND DISSOLUTION OF A DEPRESSION The energy to develop and sustain an active frontal depression is derived mainly from the supply of air in the warm sector Thus a depression having a wide warm sector will usually continue to deepen and grow in size whilst unoccluded, and often during the early stages of occlusion Later this development ceases and the speed and direction of movement of the centre is no longer related to that of the air in the warm sector, instead it becomes dependent on the general flow of air over a wide area Movement tends to become slow at this stage and the depression may 120 Chapter Thirteen take several days to fill up, but the process of weakening is likely to be more rapid over a surface which is relatively cold The arrival of a new, more vigorous system can destroy the old circulation and cause it to fill up within 24 hours MOVEMENT OF DEPRESSIONS (a) Small active depressions move faster than large dying ones (b) Small depressions tend to follow the flow of isobars in the general pattern-Le to follow the main stream (c) All depressions move from areas of rising pressure tendency towards areas of falling pressure tendency-i.e from isallobaric high to isallobaric low (See pressure tendency and isallobar in Appendix 1) If barometric tendencies are the same all round the centre of a depression it will remain stationary (d) Depressions tend to follow the flow of air round the perimeter of large, well established warm anticyclones (e) An unoccluded depression moves in a direction parallel to the isobars in the warm sector and, at sea, at approximately the same speed as the surface wind in the warm sector (f) A partly occluded depression tends to slow down as the occluding process continues (g) A fully occluded depression becomes slow and sometimes erratic in movement, but generally moving in the direction of the average flow of air up to the tropopause It also tends to move off to the left of its original track in the northern hemisphere and to the right in the southern hemisphere Large completely occluded depressions are liable to become stationary or nearly so especially if there is little horizontal change in temperature within the area covered (h) A depression within a family (see Fig 13.10) follows the approximate path of its parent but tends to move • off" towards lower latitudes (i) Secondary depressions tend to move with the main circulation of air round the primary (j) A non-frontal depression tends to move in the same direction as the strongest winds circulating round it 121 Chapter Thirteen 122 Chapter Thirteen 123 Chapter Thirteen THE FUTURE MOVEMENT OF A DEPRESSION This can be estimated by "extrapolation" That is, movement can be assumed to continue as shown by a succession of synoptic charts: but other factors should be taken into consideration e.g (a) to (j) above A "FAMILY" OF DEPRESSIONS The speed of movement of a cold front is greatest where the winds are strongest, that is, near the depression's centre Near the perimeter of the system movement is less rapid and, as the whole system advances, the cold front tends to trail out well to the rear where it is continuous with the more or less inactive part of the polar front A new depression may be formed on this trailing cold front and as it matures and occludes the process is repeated and another depression is born 10 this way a family of three, four or five depressions may be formed, each new one on the trailing cold front of its parent (See Fig 13.10) The cold air circulating in the rear of each system pushes the polar front further towards the sub-tropics; thus the track of each depression in the family commences in a lower latitude than that of its parent Finally the cold air breaks through the polar front and flows equatorwards to feed the trade winds Meanwhile an anticyclone builds up in the polar air, a new family commences to form on its poleward side and the whole cycle may be repeated A family of depressions approaching the British Isles from the Atlantic will give a period of very unsettled weather The high pressure ridges between the lows will generally give short-lived periods of fine weather 124 Chapter Thirteen FORMATION OF SECONDARY DEPRESSIONS A secondary depression is one which is contained within the circulation of a larger depression called the primary The depression forming on the trailing cold front of a depression is an example of a secondary depression (See Fig 13.10) Occasionally depressions can also form at the tip of the warm sector or on the warm front of a partly occluded depression These secondaries form when the movement of the centre of the primary depression is blocked (See Fig 13.11 (a) and (b)) 125 Chapter Thirteen MOVEMENT OF SECONDARY DEPRESSIONS Secondaries have a tendency to move with the main flow of air round the primary centre Their speed of movement is governed by the strength of wind in the primary circulation As a secondary deepens it tends to approach the centre of the primary and eventually absorbs it completely When, however, a secondary develops to about the same size and depth as the primary the two centres (which together form a "dumb-bell" shaped depression) tend to rotate about one anotheranticlockwise in the northern hemisphere Secondaries which form at the occlusion point (Fig 13 11 (a)) move in the direction of the warm sector isobars and sometimes to the right of it QUESTIONS Name the three main frontal zones and the air masses they separate Describe the I.T.C.Z and its associated weather Draw simple diagrams, in vertical cross-section, illustrating the warm and cold air masses at: (a) A warm front (b) A cold front Indicate the movement of each air mass with arrows 4 Draw the international symbols used on synoptic weather charts for: (a) A warm front (b) A cold front (c) An occlusion How would you know, from these symbols in which direction the fronts are moving? Describe, with the aid of sketches of isobars and fronts, the formation, growth and development (up to the early stage of occlusion) of a typical polar front depression Tabulate the sequence of cloud, wind, weather and instrumental readings you would expect to encounter whilst heading westwards through an unoccluded frontal depression in the N Atlantic assume you will pass to the south of the centre, through the warm and cold fronts Show, by means of simple sketches in vertical cross-section, the positions of the warm and cold air masses at: (a) A cold occlusion (b) A warm occlusion Describe how the movement of a depression is related to barometric tendencies Sketch isobars and fronts in a "family" of depressions (Northern hemisphere) Describe briefly its formation and life history 10 Describe the formation, development and movement of secondary depressions 126 [...]... trailing cold front of a depression is an example of a secondary depression (See Fig 13. 10) Occasionally depressions can also form at the tip of the warm sector or on the warm front of a partly occluded depression These secondaries form when the movement of the centre of the primary depression is blocked (See Fig 13. 11 (a) and (b)) 125 Chapter Thirteen MOVEMENT OF SECONDARY DEPRESSIONS Secondaries... and occludes the process is repeated and another depression is born 10 this way a family of three, four or five depressions may be formed, each new one on the trailing cold front of its parent (See Fig 13. 10) The cold air circulating in the rear of each system pushes the polar front further towards the sub-tropics; thus the track of each depression in the family commences in a lower latitude than that... completely occluded depressions are liable to become stationary or nearly so especially if there is little horizontal change in temperature within the area covered (h) A depression within a family (see Fig 13. 10) follows the approximate path of its parent but tends to move • off" towards lower latitudes (i) Secondary depressions tend to move with the main circulation of air round the primary (j) A non-frontal... primary the two centres (which together form a "dumb-bell" shaped depression) tend to rotate about one anotheranticlockwise in the northern hemisphere Secondaries which form at the occlusion point (Fig 13 11 (a)) move in the direction of the warm sector isobars and sometimes to the right of it QUESTIONS 1 Name the three main frontal zones and the air masses they separate 2 Describe the I.T.C.Z and its