Chapter 7 Air Masses, Fronts, and the Extratropical Cyclone

Transcription

1 Chapter 7 Air Masses, Fronts, and the Extratropical Cyclone 7.1 Air Masses Most major weather changes consist of the advances and interactions of different air masses. There will also be found certain characteristic weather patterns that exist within the air mass itself. It is the movement of these different air masses that transfers temperatures, latent heat, and moisture from one region of the earth to another. An air mass is defined as an extensive portion of the atmosphere having characteristics of temperature and moisture that are relatively homogenous in the horizontal. For a large body of air to acquire these properties (1) the air must rest for a reasonable length of time on a source region, such as a large land mass or over a large body of water, and (2) there must also be large scale subsidence and divergence of air over the source region. Air that subsides over a homogenous source region, will itself become homogenous and will keep that homogeneity as it moves away from the source region. Therefore, air mass source regions will be found in the semipermanent high-pressure areas on the surface of the earth. Regions of low pressure are regions of convergence and rising air and are not conducive to forming homogeneous air masses because the general movement of wind is into these areas and will bring with them air of very different temperature and moisture properties. The air will be constantly changing and cannot form uniform properties. The only exception to this is along the equator in the Equatorial Low. Even though it is a low pressure area the air is relatively stagnant, it is even called the doldrums, and can produce relatively homogenous properties of temperature and humidity throughout the horizontal. The different air masses on the earth are designated with two notations. The first is an indication of the thermal characteristics of the air mass, and is seen in table 7-1. The second designation is an indication of the amount of moisture or Table 7-1 Thermal Designation of Air Masses. Air Mass Designation Name of Air Mass Temperature of Air Mass A Arctic Air Mass Very Cold P Polar Air Mass Cold T Tropical Air Mass Hot E Equatorial Air Mass Very Hot AA Antarctic Air Mass Very Cold humidity in the air mass. This second designation uses the prefix m to denote that the air mass is of maritime origin and hence it contains a good deal of moisture. The prefix c is used to denote that the air mass is formed over a continental source region and hence the air mass has very little moisture in it and is considered a dry

2 air mass. When these two notations are placed together we get the different air masses shown in table 7-2. The interactions of these air masses are the regions of Table 7-2 Different Air Masses on the Earth. Air Mass Designation Name of Air Mass Characteristics of Air Mass me Maritime Equatorial Very Hot and Very Moist Maritime Tropical Hot and Very Moist ct Continental Tropical Hot and Dry Continental Polar Cold and Dry mp Maritime Polar Cold and Moist ca Continental Arctic Very Cold and Very Dry the most weather. We should point out what appears to be an error in our notation, and that is that the Arctic air mass is found farther north than the Polar air mass. Originally the Polar air mass was considered to be the only cold air that came down from the north, but with time even colder air was observed coming down from even farther north and this was then called the Arctic air mass. The identification of an air mass is based upon (1) the history of change in the air since it left its source region, (2) it horizontal characteristics at certain levels in the upper air and (3) the vertical distribution of temperature, winds and humidity. The upper air observations are carried out by radiosondes. A radiosonde is a group of weather instruments carried aloft by a weather balloon that then transmits this information to earth by radio. Radiosonde observations are called Raobs. If the radiosonde is followed by radar, the technique is called rawinsonde. 7.2 Fronts When air masses having very different temperature and humidity properties come together, they do not mix readily but maintain a boundary surface of discontinuity for some time, the warmer lighter air being forced aloft over the colder air mass that stays below. The sloping boundary surface between the two different air masses is called a front. On a weather map a front is drawn as a line on the weather map that represents the intersection of the frontal surface with the ground, but the front is actually three-dimensional. That is, the front extends both vertically and horizontally and has thickness. The frontal boundaries vary from 2 or 3 kilometers thick to more than 50 km in width. In the zone of contact between the prevailing westerlies and the polar easterlies there is a more or less permanent, undulating frontal discontinuity known as the Polar Front. Along this polar front the extratropical cyclones are developed. They occur in much the same way as whirlpools are formed between adjacent currents of water moving at different speeds. That is, giant waves or whirls form along the polar front. 7-2

3 7.3 The Life Cycle of the Extratropical Cyclone The extratropical cyclone is the name given to a low pressure area that forms in the mid latitude regions. (We should note that the expression extratropical means that this is a cyclone that is formed outside of the tropics. Cyclones that form in the tropics are called hurricanes and will be discussed in the next chapter.) Sometimes the extratropical cyclone is simply referred to as the mid latitude cyclone. The steps in the life cycle of the Extratropical Cyclone are: (a) The Initial Stage (b) The Wave Develops (c) The Warm Sector is well defined (d) The Cold Front starts to overtake the warm front (e) The Cold Front Occlusion (f) The Dissipation Stage (a) The Initial Stage The extratropical cyclone begins as indicated in the Initial Stage shown in figure 7.1. A stationary front separates the continental polar () air mass to the north from the maritime tropical () air mass to the south. The continental polar air mass is a dry and cold air mass, while the maritime tropical air mass is warm and moist. The front is called a stationary front and is designated by the alternately blue and red thick line, while the thinner lines are isobars that are essentially N cold air B S Figure 7.1 The Initial Stage of the Extratropical Cyclone parallel to the front. The small triangles and semicircles located on the front, circles above -- triangles below, is also the symbol for a stationary front. Notice that south of the front, the winds are flowing from the west toward the east (called westerly winds) and are parallel to the isobars and parallel to the front. Also notice that north of the front, the winds are blowing from the east toward the west (called easterly winds) and are also parallel to both the isobars and the front. It is because the winds are all parallel to the front that the front is a stationary front (there are A 7-3

4 no winds perpendicular to the front to cause it to move). If you were located at the position A in figure 7.1, the wind would be from the west and the air would be hot and humid. There would not be any significant weather at this location. If you were located at the position B in figure 7.1, the winds would be from the east, and the air would be cold and dry. There might be some few clouds very close to the front but in a relatively short distance away from the front there would not be any significant weather. (b) The Wave Develops The winds flowing easterly to the north and westerly to the south, in figure 7.1, will cause a cyclonic rotation to begin and a wave will start to develop on this polar front which can be seen in figure 7.2. As an example, to demonstrate this effect, place a pencil between your two hands, perpendicular to the fingers. Move your top hand to the left representing the easterly wind, and your bottom hand to the right representing the westerly wind, notice how the pencil rotates in your hand in a counterclockwise or cyclonic motion. The two motions in opposite directions cause a rotational motion, and this is exactly what happens to the air. The air to cold air NW L B SE cool air C A SW Figure 7.2 The Wave Develops the south will now have a southerly component to it, while the air to the north will now have a northerly component to it, as seen in figure 7.2, to produce the cyclonic motion. The start of the cyclonic rotation causes the pressure to drop, a small low pressure area now appears, and the extratropical cyclone is born. The cyclonic rotation causes a wave to develop in the polar front. The winds in the cold area ( air mass region) are now moving from the northwest and as they push against the front they cause this part of the polar front to advance. The boundary of this advancing cold air is called the cold front, and is shown as the blue line with small triangles attached to it, showing the direction that the front is moving. On the other side of the developing wave, a warm front is formed. In a warm front the cool air north of the front recedes to the north. As can be seen in figure 7-4

5 7.2, there is a southeasterly wind blowing north of the warm front. The warm air south of the front is now moving from the southwest and as this pushes against the cooler air, because it is less dense than the cooler air, it rises above the cool air. The rising air will expand adiabatically, and cool to the saturation point. Condensation then occurs, and clouds will now start to form just north of the warm front from this rising air. The extratropical cyclone is developing. If you were located at the point A in the figure, you would be in the warm humid maritime tropical air mass. Because it is a maritime tropical air mass, the relative humidity would be quite high. If on the other hand, you were located at the point B in the figure, you would be in the cool continental polar air mass (). We call this part of the air mass cool, rather than cold, because it is now retreating northward, with a southeasterly wind component. There would now be some layered clouds, such as stratus or alto stratus moving into your area. If you are located at the point C in the figure, you are north of the cold front, and in the cold continental polar air mass. The winds are from the northwest and much colder, drier air from the north is now moving into your location. The skies are usually clearer behind the cold front. (c) The Warm Sector is well defined As the wave develops, the warm sector becomes very well defined, as seen in figure 7.3. The storm is deepening, the wave is getting larger, and the weather associated with the system is becoming more pronounced. If you were located at the point A in cold air C NW L B SE cool air SW A Figure 7.3 The Warm Sector is well defined the figure, you would be in the warm humid maritime tropical air mass (). Because it is a maritime tropical air mass, the relatively humidity would be quite high, and this, in general would make you somewhat uncomfortable. The winds would be blowing up from the southwest. Because the winds are from the southwest 7-5

6 the tendency is for the temperature of the air to be getting even warmer, because the hotter air south of you is moving north up to you. There might be some scattered cumulus clouds in your area. Except for the heat and the humidity, the weather should be nice. If on the other hand, you were located at the point B in the figure, you would be in the cool continental polar air mass (). We call this part of the air mass cool, rather than cold, because it is now retreating northward, with a southeasterly wind component. Even though it was originally cold air, the fact that it was south of you allowed it to warm up a bit from the increased radiation from the sun. Hence, air that was originally south of you, and hence warmer, is now moving north toward you. Since the air south of you should be warmer than the air at your location, the air at your location is now becoming warmer than it was. This is why it is now called a cool air mass, even though it is part of the polar air mass. Because it is a continental air mass the humidity is much lower than in the air mass at point A. The warm front has a very gentle slope and the less dense air rides up above the cooler and denser air mass. The rising air expands and cools adiabatically to saturation, then condensation occurs and clouds occur. Because of the gentle slope, the clouds extend back over a relatively large area, as seen by the gray color in the figure. Hence, if you are located at the point B you would find that the winds are from the southeast, the air is cool and the humidity is reasonably low, and there would be layered altostratus or altocumulus clouds at your location. At this point you may not be getting rain yet but as the storm deepens you can expect it shortly. If you are located at the point C in the figure, the cold front has already passed you and you are now in the cold continental polar air mass. The winds are from the northwest and much colder air from the north is now moving into your location. Since it is a continental air mass, the humidity is usually quite low. If this cold front is passing you in the summer time you will certainly notice the lower relative humidity and hence more comfortable air. If the cold front has passed you in the winter time you will probably notice the drop in the temperature more than the drop in the humidity. In either case though, you will notice that the wind changes from a southwesterly wind to a northwesterly wind as the cold front passes your location. At this stage in the development of the storm, there is only a very narrow band of clouds associated with the cold front, usually of the cumulus family, and as the front passes the sky usually clears very quickly thereafter. As the storm is moving in an easterly direction, because of the prevailing mid-latitude westerlies, the area of lowest pressure is moving away from you. Hence the pressure is starting to increase in your area. As we mentioned in chapter 6, the winds are vertically downward in a high pressure area, hence on the backside of the low pressure area, air pressure is increasing, and the vertical motion downward causes whatever clouds that might have been present, to essentially evaporate into the air, leaving clear skies. So if you are located at the position C you will be experiencing cold, dry northwesterly winds and the sky will be clear or clearing. 7-6

7 (d) The Cold Front starts to overtake the warm front As the storm deepens, the cold front starts to overtake the warm front. This means that the warm sector between the two fronts starts to get smaller, as seen in figure 7.4. Essentially everything we said about the system in section c still applies, but cold air C NW L B cool air SE A SW Figure 7.4 The cold front starts to overtake the warm front. the weather is becoming more pronounced. The cloudy region north of the warm front is increasing in size, and there probably is precipitation taking place in this region. If it is winter time, the air will be relatively cold and there is a good chance that you are having snow. If it is summer time, the air will be relatively warm and the precipitation will be rain. The cold air behind the cold front is more dense, and hence heavier, than the warmer moist air out in advance of the cold front. As the cold front moves forward it pushes this warmer moist air ahead of it up into the atmosphere. The rising air will expand and cool, saturation and condensation occur, and clouds will form. These clouds usually form into a relatively narrow band of cumulus type clouds along the cold front, as shown in the figure. The precipitation from these clouds is usually the showery type, and do not last for long periods of time. In the spring time there might even be a band of thunderstorm clouds out in advance of the cold front, called a squall line. We will discuss this in the next chapter. (e) The Cold Front Occlusion As the cold front keeps moving, it eventually overtakes the warm front. The cold air behind the cold front has been pushing the lighter moist air mass aloft as the cold front moves forward. When it actually catches up with the warm front all the air mass has been pushed aloft. There is no more warm moist () air still on the ground at that point. The cold air mass now pushes into the cool air mass. At this point the cold front occlusion has started to be formed. This is shown in figure 7.5, as the purple line drawn on the map. Notice that both triangles from the 7-7

8 cold air C D NW L SE B cool air A SW Figure 7.5 The cold front occlusion begins. cold front, and semicircles from the warm front are drawn together on the purple occluded front. The entire storm is intensifying everywhere. The pressure is much cold air C D NW L A B cool air SE SW Figure 7.6 The cold front occlusion is well developed. lower, and there are more clouds and more precipitation present. As time goes by, the cold front keeps on moving, pushing even more of the warm moist air mass aloft. The size of the occluded front increases, as shown in figure 7.6 as the much 7-8

9 larger purple occluded front. This is the most intense portion of the entire storm. The area of clouds and precipitation is at its greatest. (f) The Dissipation Stage What is a front, but a boundary between two different air masses. However, as the occlusion reaches its maturity, the air mass is on both sides of the occluded front, and there is no longer two different air masses at the surface to have a front at all. Hence, at this point the storm starts to die out. This is shown as the dashed purple line, in figure 7.7, indicating what remains of the occluded front. Without D L NW C B SE E SW Figure 7.7 The cold front occlusion dissipates. A the difference in temperature between the two different air masses, there is much less energy to support the low pressure area. The low pressure area starts to fill. Without the vertical motion upward characterized by a low pressure area, the clouds aloft start to dissipate. The entire storm starts to break up. We have reached the end of the life cycle, the end of this particular storm. But look closely at the easterly end of the cold front in figure 7.7. Notice that north of the cold front there is a air mass with easterly winds. Also notice that south of that cold front there is an air mass with westerly winds. However, if we go back to figure 7.1 we see that this is exactly the conditions that started the wave that developed into the extratropical cyclone in the first place. Hence another new cycle will begin and another new extratropical cyclone is going to be born. We therefore see that there is always a family of extratropical cyclones that travel around the earth. As one dies out, a new one is formed, goes through its life cycle, and then it dies. The process continues around the earth. This is also one of the reasons why it is sometimes 7-9

10 difficult to make long range forecasts of the weather. It is difficult to forecast the behavior of a storm that hasn t even been born yet. Some general comments on the Extratropical Cyclone. Since air is forced to rise along the imbedded fronts of the extratropical cyclone, they are usually accompanied by appreciable cloudiness and precipitation. Conditions along the fronts are spoken of as frontal weather whereas air mass weather prevails in the areas away from the fronts. These cyclones vary in diameter from about 100 miles to as much as 2000 miles. Most, however, are in the range of 500 miles to 1000 miles. The form of the extratropical cyclone is roughly circular, or elongated and oval. Occasionally they may be broad, shallow weak depressions. Usually they travel in groups or families whose movements follow waves in the pressure patterns aloft. The general direction of their motion is from west to east in the mid-latitude westerlies. Over north America they tend to curve toward the southeast into the Mississippi Valley and then toward the Northeast. The speed of an extratropical cyclone is on the average between 20 to 30 mph, or from about 500 miles per day to about 700 miles per day. The rate of motion is usually greater in the winter time than in the summer time. As long as the discontinuity in temperature and moisture conditions is maintained along its front the cyclone is likely to persist, but a cyclonic pattern of pressure and winds is also essential. 7.4 The Cold Front The Cold Front is the leading edge of the cold dry continental air mass that is advancing into an area that was originally occupied by the warm moist tropical air mass. Because the cold air is denser, and hence heavier, it pushes under the lighter warmer air like a wedge, forcing the lighter to rise up into the atmosphere, as seen in figure 7.8. Because there is a frictional force between the moving cold air and the ground, the moving air at the surface slows down, while the air above it moves at a greater speed. The net result of this frictional drag, causes the slope of the cold front to be very steep, as seen in the figure. This steep slope causes most of the clouds that will be formed along the cold front to be clouds of vertical development, which are usually accompanied by heavy showery precipitation. Since the Polar air mass is colder than the Tropical air mass, when the cold front passes by you in winter, there is usually a sharp drop in the temperature of the air. Also since the continental Polar air () is less moist than the maritime Tropical air (), the humidity of the air will also decrease when the cold front passes. This drop in the relative humidity sometimes helps the meteorologist to define the exact position of the front. 7-10

11 Ci Cb cold air Ns Figure 7.8 Vertical cross section of the cold front. When the cold front passes you in summer, there is not always such a sharp drop in the temperature, but usually there is a significant drop in the humidity. In either case, however, there is always a wind shift at the passage of the cold front, and this is one of the most significant identifying features of the cold fronts passage. This can be seen clearly in figures 7.3 through 7.5, where the wind is blowing from the northwest behind the cold front, and blowing from the southwest in advance of the cold front. Also note from those diagrams, that the cold front sits in a pressure trough. Hence, there will be a drop in pressure in advance of the cold front, and a rise in pressure after the cold front passes. This drop then rise is sometimes helpful to the meteorologist in picking out the exact location of the cold front passage on a particular weather map. Since the zone of cloudiness associated with the cold front is a relatively narrow band of clouds, the zone of precipitation is also much narrower than the clouds associated with the warm front. A few hours after the frontal passage, clearing weather can normally be expected. Sometimes when the cold air moves over the warmer ground, the air near the ground gets warmer and an instability may develop in the lowest layers of the atmosphere. This can result in some cumulus clouds after the passage of the cold front, and with it some resulting scattered showers. In the winter when the precipitation is snow, the combination of low temperature, high winds and blowing snow along and following the cold front is called a blizzard in the United States. 7.5 The Warm Front The warm front is the boundary between the retreating cool dry continental air mass and the advancing warm moist tropical air mass. Because there is a frictional force between the retreating cool air and the ground, the retreating air at the surface slows down, while the retreating air above it moves at a greater speed. The net result of this frictional drag, causes the slope of the warm front to be very shallow, as seen in figure 7.9. The slope is in the ratio of about 1 mile high versus 200 miles in the horizontal. Because the cool continental air is denser, and hence 7-11

12 heavier than the warm maritime tropical air, it stays on the ground while the lighter warmer tropical air rises up into the atmosphere, as seen in figure 7.9. This Cirrus stratus Cirrus Nimbostratus Altostratus Stratus cool air Precipitation Figure 7.9 Vertical cross section of the warm front. gradual slope causes most of the clouds that will be formed along the cold front to be layered, or stratiform, type clouds, which are usually accompanied by light to moderately heavy and continuous types of precipitation, extending over a large area. The great area expansion of rising air produces a vast cloud system with the highest clouds lying far in advance of the front. High wispy cirrus clouds (Ci) often in the form of mares tails appear first. As the front moves in, the clouds lower and thicken progressively to cirrostratus (Cs), altostratus (As), and nimbostratus (Nb). Fogginess and poor visibility are common in the precipitation area. Occasionally the air is unstable, and then the slight lifting action of the front is enough to initiate strong vertical development, and cumulonimbus type clouds will be formed producing heavy showery precipitation just ahead of the front. The warm front normally extends east and southeast from the center of the low pressure area. The wind shift on the passage of the warm front, is not as pronounced as it is with the cold front. Maximum wind speed, sometimes with gustiness, is experienced slightly in advance of the warm front. The barometric pressure decreases gradually until the front passes and then tends to level off. In the winter time, ice pellets or freezing rain may occur when rain falls from the mass to the colder air mass below. The sequence of precipitation in the winter is quite often snow, ice pellets, and then rain as the warm front approaches. The clouds lower and the temperature rises. When the warm front passes, warm weather follows as the warm moist tropical air mass moves into the area. The weather will stay warm until the arrival of the cold front. 7.6 The Occluded Front (a) The Cold Front Occlusion. The best way to visualize the cold front occlusion, is to look at the vertical cross section of the cold front and the vertical cross section of the warm front, as shown in figure 7.10(a). As the cold front starts to catch up to the warm front all the weather associated with each front starts to merge together, figure 7.10 (b). 7-12

13 (a) Vertical cross section showing the cold front approaching the warm front. Cumulus (b) Vertical cross section showing the beginning of the occlusion as the cold front just touches the warm front. Cumulus (c) Vertical cross section showing the cold front occlusion. Figure 7.10 Vertical cross section of the cold front occlusion. As the cold front advances, all the warm moist tropical air mass in advance of the cold front is pushed upward into the atmosphere. By the time the cold front catches up to the warm front, it has pushed all the warm moist air aloft. There is no longer any moist tropical air at the ground. As the cold continental air mass advances 7-13

14 further, it, being colder and hence more dense and heavier than the less dense cooler continental polar air, now pushes under this less dense cooler continental polar air also, as seen in figure 7.10(c). Because the occlusion is the merger of the cold front weather with the warm front weather, the occlusion will be the region of the most intense weather of the entire system. Cold Front Occlusions form over continents and the east coast of continents. Living in the New York area, you will only see cold front occlusions. (b) The Warm Front Occlusion. The warm front occlusion occurs in winter along the west coast of continents. The advancing air mass is now coming off the ocean and hence is a maritime polar air mass, rather than the advancing continental polar air mass that was advancing over the continent in the cold front occlusion. The beginning phase of the warm front occlusion is very much like the cold front occlusion. The vertical cross section of the cold front and the vertical cross section of the warm front are approaching each other, as shown in figure 7.11(a). As the cold front starts to catch up to the warm front all the weather associated with each front starts to merge together as before, figure 7.11 (b). As the cold front advances, all the warm moist tropical air mass in advance of the cold front is pushed upward into the atmosphere. By the time the cold front catches up to the warm front, it has pushed all the warm moist tropical air aloft. There is no longer any moist tropical air at the ground. As the cool maritime air mass advances further, it pushes into the cold continental polar air mass. That is, the retreating air mass behind the warm front is a cold continental polar air mass, while the advancing air mass is a cool maritime air mass. The retreating continental polar air mass is colder than the advancing maritime polar air mass. When they collide, the retreating continental polar air mass, being colder and hence more dense and heavier than the less dense cooler continental maritime air stays on the ground while the less dense advancing cooler maritime polar air mass rises up over the more dense continental air mass, as seen in figure 7.11(c). Because the occlusion is the merger of the cold front weather with the warm front weather, the occlusion will be the region of the most intense weather of the system. Warm Front Occlusions occur in winter along the west coasts of continents. If you lived in California, Washington, or Oregon, you would only see warm front occlusions. As the occlusion process continues, the whole system tends to dissipate because there is a lack of sharp persistent differences in the air mass properties. 7-14

15 Cirrus Cumulonimbus Cirrus stratus Cirrus Nimbostratus Altostratus Nimbostratus mp Cool air Stratus Cold air Precipitation (a) Vertical cross section showing the cold front approaching the warm front. Cumulus mp Cool air Precipitation Cirrus Cumulonimbus Nimbostratus Nimbostratus Cirrus stratus Altostratus Cold air (b) Vertical cross section showing the beginning of the occlusion as the cold front just touches the warm front. Cirrus Cumulus mp cool air Cumulonimbus Nimbostratus Precipitation Cirrus Cirrus stratus Altostratus cold air Cirrus (c) Vertical cross section showing the warm front occlusion. Figure 7.11 Vertical cross section of the warm front occlusion. The Language of Meteorology Air mass An air mass is defined as an extensive portion of the atmosphere having characteristics of temperature and moisture that are relatively homogenous in the horizontal. For a large body of air to acquire these properties (1) the air must rest for a reasonable length of time on a source region, such as a large land mass or over a large body of water, and (2) there must also be large scale subsidence and 7-15

16 divergence of air over the source region. Therefore, air mass source regions will be found in the semipermanent high-pressure areas on the surface of the earth. Front The sloping boundary surface between the two different air masses is called a front. Polar Front A more or less permanent, undulating frontal discontinuity in the zone of contact between the prevailing westerlies and the polar easterlies. Extratropical cyclone The extratropical cyclone is the name given to a low pressure area that forms in the mid latitude regions. Cold Front The Cold Front is the leading edge of the cold dry continental air mass that is advancing into an area that was originally occupied by the warm moist tropical air mass. Warm Front The warm front is the boundary between the retreating cool dry continental air mass and the advancing warm moist tropical air mass. Occluded Front The occluded front is the most intense portion of the extratropical cyclone. It occurs when the cold front merges with the warm front. Questions for Chapter 7 To go to another chapter, return to the table of contents by clicking on this sentence. 7-16