CHAPTER 11 (Moran and Morgan, 1997) In this unit attention is focused upon the macro-scale weather features (also known as synoptic-scale) of mid-latitudes that typically appear on the national surface weather maps. These features include air masses, fronts, and the migratory low pressure (cyclones) and high pressure (anticyclones) systems. Attention is focused upon the midlatitudes, not only because many of us live in this region, but also because the mid-latitudes represent the region having the greatest north-south temperature contrast. Recall that the largest meridional transport of heat energy by the atmosphere and oceans is found here. Typically, meteorologists focus their attention upon the migratory cyclones and the associated fronts because most of the inclement and severe weather accompanies these features. Closer inspection of the frontal structure reveals that clouds and precipitation associated with a warm front differ from those of a cold front. In addition, midlatitude low pressure systems have a characteristic pattern of clouds, temperature structure. These systems undergo a recognizable life cycle called the occlusion process. In midlatitudes, these lows are typically move from west to east, carried along by the mid- and upper- tropospheric winds. Using a model cyclone, we can determine the various weather sequences that we could expect to experience as a low pressure system approaches us. Migratory anticyclones represent another weather feature of the midlatitudes. These fair-weather systems have characteristics. STUDY NOTES CHAPTER 11 Figure 11.1 -- Study this figure, locating the primary source regions for the various air masses that affect the North American continent. Find the air masses influence the weather in your hometown, and determine their source regions. Relate this diagram to your own experience. In other words, if you live along the Gulf Coast, you would probably realize that the warm, humid air that often frequents your community has originated from over the Gulf. On occasion, a polar or even arctic air mass from northwest Canada would pass across your area. Table 11.1 -- Study the thermal, humidity and stability characteristics of the listed air masses, emphasizing the thermal and humidity characteristics on the right-hand side of the table. Figures 11.2, 11.3 and 11.5 -- Inspect the series of block diagrams that show the three-dimensional view of stationary, warm and cold frontal surfaces, respectively. Orient yourself as to the location of the warmest and coldest air in each diagram. Also, note the direction that the front may move by looking at the standard frontal symbols. The fronts normally appearing on the surface weather maps would appear where this surface intersects the earth's surface (the floor of each of these diagrams). The three-dimensional frontal surface (not shown) would extend outward from the page. In Figure 11.2, the stationary front would show little movement. In Figure 11.3, the warm front would be advancing from right to left. In Figure 11.5, the cold front would be advancing from left to right. Note that in Figure 11.2 and 11.3, stratiform (layer) clouds are the predominant clouds, indicating a gradual uplift, while in Figure 11.5 the clouds were mainly cumuliform indicative of the more rapid, convective uplift. Figure 11.6 -- Occasionally, a cold front moves in a manner that is not typical of what we would expect of a midlatitude storm system. The example shown here is the backdoor cold front that occasionally moves south or even southwestward over the Northeast. Figures 11.7 and 11.8 -- The vertical cross-section shows the structure of two different types of occlusions. The major point to be drawn from these panels is that the warm air in the warm sector has been forced aloft, leaving only the occluded front on the surface weather map. Figure 11.9 -- Study the series of diagrams depicting the progression of a midlatitude cyclone through its life cycle. These diagrams portray the link between surface weather features (the low center and accompanying fronts) and upper airflow (appearing as dashed lines) in the mid- to upper-troposphere. In Panel (A) an open wave cyclone develops with a trough-like feature in the winds aloft. This upper-level trough flows around the southern edge of a cold pool of air. Over time, the surface low undergoes its typical life cycle, while the upper-level trough "deepens", with more intense winds and a larger meridional (north-south) excursion of the wind regime. Note that the original position of the surface low usually appears below the east limb of the upper level trough, but by the time the surface low reaches the occluded stage, the upper-level trough often is positioned over the surface low. Figures 11.10 and 11.11 -- Take a look at the series of surface weather maps, visible satellite images and 500 mb upper air charts for three consecutive days. Compare where the clouds appear on the satellite images with respect to the positions of the surface lows and fronts on the surface weather maps. Also trace how the surface low moves. You may recall the discussion associated with the barograph trace for Green Bay, WI appearing in Figure 4.5. Finally, note that the surface low is carried along by the mid-tropospheric wind flow appearing on the sequence of 500 mb charts. Figure 11.12 -- Take a moment to consider the visible satellite image of an occluded midlatitude storm system. Note the comma shaped cloud pattern. Visualize the counterclockwise flow around the low pressure center. The text also refers to the color enhanced infrared image on page 252, at the beginning of the chapter. This figure provides a slightly different perspective of the same storm. Figure 11.13 -- Compare the various weather elements in the three panels for an ideal midlatitude cyclone located in the Midwest. The position of the cold and warm front would indicate that this low would be at the mature stage in its life cycle. The low center and surface isobars are shown in each panel. Starting with the Panel (A), note how the winds spiral in a counterclockwise manner around the low and how the warm and cold fronts are nearly perpendicular to the flow. As we discovered previously, the surface winds show a slight cross-isobar orientation toward low pressure because of the effects of friction. Panel (B) has a distinctive temperature pattern. A "warm sector" is found in the southeast quadrant of the surface low (the region appearing with red shading) and a "cold sector" located to the northwest of the surface low. The surface fronts represent the narrow transition zone between the warm and cold air. Finally in Panel (C), the typical comma shaped cloud shield is depicted by the purple shaded region, together with the standard weather symbols. Along the cold front extending southward from the surface low, rainshowers and thunderstorms would indicate convective type precipitation. However, just to the north of the warm front, the presence of fog, drizzle and continuous rain suggests more stable or non-convective precipitation types. Steady snow is noted to the northwest of the surface low. Convective snowshowers may appear to the west as cold northwesterly winds move over previously warm ground. Figure 11.14 -- Spend a moment to orient yourself with the three-dimensional view of the conveyor belt model of the extratropical cyclone. In this figure, the surface low is located off the East Coast, and the upper-level depicts a trough in the flow pattern displaced slightly to the west of the surface low. To the east of the system, a red arrow identified as a "warm conveyor belt" represents the southerly winds that bring warm and moist air northward in the warm sector. This conveyor then ascends along the warm front surface over a "cold conveyor belt" located northeast of the surface low. Rising motion of air in the warm conveyor belt produces the clouds and precipitation shield found in the northeast sector of the surface low. An upper-level dry airstream associated with the upper tropospheric jet stream is also depicted with a set of yellow arrows. Figure 11.15 -- Use this figure in conjunction with Table 11.2 to study the weather sequences associated with a passage of a migratory mid-latitude storm system. Since these storm systems have a warm and a cold side, different weather regimes should be expected depending upon where the storm tracks. For this example, suppose that you were situated in Chicago. Take time to visualize the sequence of weather events that you would expect to see as a low pressure system passes to the north of Chicago (with Track A so that the warm side of the low passes to you). Repeat the process as the low tracks to the south of Chicago (Track B so that the cold side of the low passes). We will consider changes in each weather element separately. First, make sure that you understand how the low would pass by you if you were located in Chicago for each situation. Next you should try to visualize how the near-surface winds would change with time as the low approaches and then moves away. Remember the rule that states that if you stand with your back to the wind, the low will be to your left in the Northern Hemisphere. Then study how the fronts would be associated with only the case where the low would remain to your north (Track A). Next, consider the sequence of warm air or cold air advection. Finally consider the air pressure tendency, or how the air pressure changes over time. Note that as the low approaches, the pressure "falls" with time, then after the low passes close to your position, the pressure "rises" with time. You should also consider the sequence of clouds that are associated with the system. After you have understood the progression of the individual elements, you should attempt to visualize the succession of multiple weather events that you would experience if a low were to pass to the south of your community or, on a different occasion, pass to your north. Figure 11.16 -- Study the map containing the typical wintertime tracks of midlatitude low pressure systems that move across the nation. Note the popular names that are associated with each track, usually based upon where the storm originated. You should note that many of these storms develop in one of two regions of preferred cyclogenesis. One such region is found near the coastline. The East Gulf storms represent several of these storms that develop off the coast where a strong horizontal temperature gradient has developed between the cold continent and warmer ocean. Another region where storms develop is downwind from large mountain barriers. Note that several other storm tracks, to include the Panhandle, Colorado and Alberta storms appear to originate on the lee side of the Rockies. You should also note that the storms tend to move with the upper airflow. Since a trough is typically located over the cold continent in winter, a southwesterly wind flow guides the storms along a storm track that often is from southwest toward the northeast. Figures 11.17, 11.18 and 11.19 -- Study these vertical cross-sections through the centers of several types of low pressure systems. In Figure 11.17, a vertically oriented cold core low, you should realize that the circulation regime remains throughout the troposphere. Figure 11.18 depicts the more typical situation in a midlatitude wave cyclone where the axis of the cold core low tends to bend with height. In other words, the upper-level low is not directly over the surface low as appearing in Figure 11.17, but tends to be displaced to a position over the colder air. In Figure 11.19, a vertical cross-section is shown through a warm core low, typical of either a low that may appear over the U.S. Southwest or as a tropical cyclone (such as a hurricane or tropical storm). This type of low differs from the cold core low example in Figure 11.17 in that the warm core low is relative shallow with a relative high pressure system found in the upper troposphere. As a result, the cyclonic circulation regime weakens with height and becomes an anticyclonic circulation regime aloft. Figure 11.20 -- Example of an arctic outbreak. Snow is on the ground. Read the Weather Fact (Coastal Impact of Nor'easters) on page 269. Read the Special Topic (The Case of the Missing Storm) on page 275. Spend a moment inspecting Figure 1, visualizing how a storm system, as represented by the spinning column would change its shape and spin rate as it encounters a mountain barrier. Once on the downwind (to the right) the stretching causes an increased spin. Read the Special Topic (Storm of the Century) on page 277 for its informational content. Skim the Mathematical Note (Vorticity) on pages 280 to 282. CHAPTER 11 (Moran and Morgan, 1997) AIR MASSES, FRONTS, CYCLONES, AND ANTICYCLONES This chapter concerns synoptic-scale weather systems that appear on surface weather maps: air masses, fronts, cyclones, and anticyclones. An air mass is a huge volume of air that is relatively uniform horizontally in temperature and humidity. As air masses travel away from source regions (where initial characteristics are acquired), they modify. Where contrasting air masses meet, fronts and frontal weather develop. Air that is forced to ascend along a front undergoes expansional cooling that may lead to clouds and precipitation. Our description of the life cycle of a wave cyclone shows how air masses, fronts, and pressure systems are interrelated. With upper-air support (horizontal divergence), cyclones develop and are steered by the westerlies. From our description of the weather pattern associated with a cyclone, it is evident that the specific track of a cyclone determines the weather that is experienced at any locality. Anticyclones follow cyclones and these fair-weather systems are accompanied by characteristic patterns of air mass advection. CHAPTER OBJECTIVES After reading this chapter, the student should be able to: describe the characteristics of the various air masses that regularly form over or invade North America. describe how and why air masses modify as they travel away from their source regions. identify and distinguish among the various types of fronts and frontal weather. distinguish between warm frontal weather and cold frontal weather. explain why fronts are associated with cyclones but not with anticyclones. sketch the life cycle of a midlatitude synoptic-scale cyclone. describe the components of the conveyor-belt model. explain the linkage between a surface cyclone and the westerly flow aloft. identify the principal storm tracks across North America. distinguish between cold and warm cyclones and between cold and warm anticyclones. describe the air mass advection associated with an anticyclone. 11 Air Masses, Fronts, Cyclones, and Anticyclones by Patricia Pauley 252 Air Masses 253 Frontal Weather 255 Midlatitude Cyclones 261 Anticyclones 277 Conclusions 283 Weather Fact: Coastal Impact of Nor'easters 269 Special Topic: The Case of the Missing Storm 275 Special Topic: Storm of the Century 277 Mathematical Note: Vorticity 280 Key Terms 283 / Summary Statements 283 Review Questions 284 Questions for Critical Thinking 284 Selected Readings 284