CHAPTER 13 (Moran and Morgan, 1997) A thunderstorm is a mesoscale convective phenomenon in the atmosphere, produced in a large cumulonimbus cloud. To stimulate deep convection within the troposphere, several ingredients are needed which may ultimately produce a thunderstorm. Thunderstorms can be single-cell or multicellular. Thunderstorm cells typically undergo a recognizable life cycle (cumulus, mature and dissipating), identified by certain characteristics associated with the vertical motions within the thunderstorm cell. Special synoptic situations can help develop severe thunderstorms, identified by large hail and damaging winds. Attention is also focused upon some of the thunderstorm hazards to include lightning, downbursts, flash floods, and hail. STUDY NOTES CHAPTER 13 Figure 13.1 -- Inspect the two photographs of cumuliform clouds that were take approximately 15 minutes apart. You should note the formation of a flat-topped anvil in the lower panel (B). Often, the anvil develops a filmy appearance as a result of the ice crystals. This anvil is a visual cue that distinguishes a cumulus congestus cloud (the primary cloud in the top panel) from the cumulonimbus cloud that would be the cloud type in the lower panel. Figure 13.2 -- Study the three panels showing the vertical cross-section through a thunderstorm cell during the three recognized stages in its life cycle. Each of these panels is separated by approximately 15 minutes. Pay particular attention to the direction of the red arrows marking the predominant vertical motion within the cell. You should also note the dashed line, or the 0 degrees C (32 degrees F) isotherm, used to indicate what is commonly called the "freezing level". At altitudes below this line, air temperatures are above 0 degrees, where liquid droplets would form. Above this isotherm, air temperatures would be colder than 0 degrees C, and water would become a supercooled liquid and eventually freeze into ice. Also note the approximate vertical and horizontal dimensions, with a cell that may be no more than 15 km wide developing through essentially the entire troposphere (to 12 km altitude). You should distinguish among the three stages, based upon the predominant vertical motion. In panel (A), the updrafts predominate in what is called the cumulus stage. In the mature stage (B), both updrafts and downdrafts are present. Finally, downdrafts dominate the dissipating stage (C). Figure 13.5 -- The white blotches on the visible satellite image over Texas and Oklahoma are the anvil tops of thunderstorm cells. These clouds typically reach and at times penetrate the tropopause. Close inspection of the eastern edge of the cells reveals a dark strip, which is the cloud shadow cast by the towering cumulonimbus cloud upon the region east of the cell. Figure 13.6 -- Look at the map view of model showing the progression of individual thunderstorm cells at an angle to the general storm movement. Figure 13.7 -- Locate the Mesoscale Convective Complex (MCC) described in the legend. An enhanced infrared satellite image is used to detect subtle variations in cloud top temperatures. Typically, a very cold cloud top, as indicated by the mottled appearance of the cluster, would indicate severe weather because of several factors. Cold cumulonimbus cloud tops usually indicate greater vertical development, a feature of severe weather. Colder conditions aloft also tend to enhance convection. Figure 13.8 -- One of the ways to destabilize the lower atmosphere is by differential advection, or transporting warm air into the region at low levels, while cold air in at high altitude. The solid line represents the temperature profile before the start of the process, while the dashed line is a less stable atmospheric profile at a later time. Figure 13.9 -- Inspect this map of the thunderstorm climatology across the continental US, expressed in terms of thunderstorm days per year. Locate what section of the nation has the largest concentration of thunderstorm days per year and which region would have the fewest. The region with the largest frequency of thunderstorm days would be found in Florida, while the region with the lowest annual frequency of thunderstorms would be in the Pacific Northwest. Use the contours on this chart to estimate the number of thunderstorm days per year that you would experience in your hometown. Figure 13.10 -- Study the map view of the Florida peninsula sea-breeze convergence responsible for the large number of thunderstorms in that state. Compare this map with the thunderstorm climatology map appearing Figure 13.9. Figure 13.11 -- Inspect this vertical cross-section of a severe thunderstorm and compare with the mature stage of a non-severe thunderstorm appearing in panel (B) of Figure 13.2. The tilted updraft helps prolong the thunderstorm cell and permits it to develop to greater heights. Also locate the gust front, along the leading edge of very strong winds. Figure 13.13 -- Severe thunderstorm synoptic situation that frequently develops in the southern Plains. A low-level southerly jet brings warm, moist mT air northward across Texas, while in the upper troposphere a westerly jet brings colder and drier air in aloft. Differential advection where warm moist air is brought in near the surface, and cold dry air aloft, helps destabilize the atmosphere. The wind shear, or marked change from southerly to westerly winds, could also enhance rotation. A "dry line" along the western boundary of the tongue of moist air can serve as a mechanism for lifting and initiating severe weather. The dry line is similar to a cold front because a marked contrast in dewpoint also produces a distinct density boundary between air masses. Recall that at the same temperature, humid air is less dense than dry air. As a result, the dry air to the west of the advancing dry line lifts the less dense humid air to the east, triggering potentially vigorous convection. Figure 13.14 -- Development of a capping inversion, which may ultimately aid in the development of a severe thunderstorm cell. Figure 13.15 -- Notice the mammatus clouds under the anvil of a cumulonimbus cloud. You should recognize these clouds and should be alert to the possibility for severe weather, even though the presence of mammatus does not always signify severe weather. Figure 13.16 -- A mesoscale display of a lightning detection network. Note that the crosses indicate the location of recent cloud-to-ground lightning strikes. The lightning strikes tend to cluster into bands as thunderstorm cells move across a particular region. Figure 13.17 -- Check the charge distribution of a thunderstorm cell. Table 13.1 -- Look at the various types of lightning. Figure 13.19 -- Look at this self-explanatory figure that shows some of the hazards to aviation associated with a microburst, especially when the aircraft lands. Figures 13.20 and 13.22 -- Self-explanatory. Table 13.2 -- Review the personal safety rules designed for protecting you from flash flooding, one of the thunderstorm hazards. Figure 13.23 -- Look at the sequence of vertical cross-sections through a thunderstorm that show a model of the development of a hailstreak. The individual figures are at five separate times, with the initial time on the left. Figure 13.24 -- Acquaint yourself with the national map of hail frequency, locating the region experiencing the greatest average number of days with hail per year. You should notice that the greatest hail frequency occurs in northeast Colorado, to the east of the Rockies. You should also remember that while thunderstorms are more frequent in Florida, as based upon Figure 13.9, the warmer conditions in the atmosphere over Florida usually are sufficient to diminish the size of hail. Read through the Special Topic entitled Lightning Safety (pg. 324-325) Consider the recommended personal safety rules to protect yourself from lightning, one of the thunderstorm's major hazards. Read the Weather Fact (The Rumble of Thunder) on page 328. Read the Special Topic (Hail Suppression) on page 330 and 331. CHAPTER 13 (Moran and Morgan, 1997) THUNDERSTORMS This chapter covers the genesis, properties, and hazards of thunderstorms, mesoscale convective systems. Here we apply what we learned earlier regarding conditions favorable for deep convection within the troposphere. We describe the three stages in the life cycle of a thunderstorm cell (cumulus, mature, and dissipating) and distinguish among single cell and multicellular thunderstorms. The latter include squall lines and mesoscale convective complexes (MCCs). We also describe the special synoptic situation that is favorable for the development of severe thunderstorms. The final section of this chapter is concerned with thunderstorm hazards: lightning, downbursts, flash floods, and hail. This discussion sets the stage for our consideration of tornadoes in the next chapter. CHAPTER OBJECTIVES After reading this chapter, the student should be able to: describe the characteristics of each stage in a thunderstorm life cycle. distinguish among single cell and multicellular thunderstorms. describe the role of atmospheric stability in thunderstorm development. explain why thunderstorm frequency varies across North America. explain why some thunderstorms become severe whereas others do not. identify the characteristics of a severe thunderstorm. sketch the synoptic weather pattern that favors development of a severe thunderstorm. describe the atmospheric conditions that lead to a lightning discharge. explain why lightning is dangerous. distinguish between a microburst and a macroburst. explain why a microburst can be hazardous to aviation. explain why urban areas are particularly vulnerable to flash flooding. describe the origin and characteristics of hail. 13 Thunderstorms 306 Thunderstorm Life Cycle 307 thunderstorm Classification 311 Geographical and Temporal Distribution 313 Severe Thunderstorms 316 Thunderstorm Hazards 319 Conclusions 329 Special Topic: Lightning Safety 324 Special Topic: Hail Suppression 330 Weather Fact: The Rumble of Thunder 328 Key Terms 329 Summary Statements 331 Review Questions 332 Questions for Critical Thinking 332 Selected Readings 332