CHAPTER 14 (Moran and Morgan, 1997) Tornadoes are one of the hazards associated with a severe thunderstorm. A tornado is a mesoscale phenomenon produced within a supercell thunderstorm that begins to rotate forming a mesocyclone. Approximately three-quarters of all tornadoes in the world occur in North America, particularly in Tornado Alley. The characteristics of a tornado are examined. One of the tools that a meteorologist employs to detect tornadoes is the Doppler radar system. STUDY NOTES CHAPTER 14 Figure 14.1 -- Look at the map of the region affected by devastating tornadoes on what T. Fujita called the Super Outbreak of 3-4 April 1974. Note that many of these tornadoes followed parallel tracks, moving from southwest to northeast, indicative of the strong, southwesterly steering winds over the region in the mid-troposphere at about the 500 mb level. Figures 14.2 and 14.3 -- Figures are self-explanatory. Figure 14.4 -- Study the map of tornado climatology, expressed as the average annual number of tornado occurrences per unit area. The 91 km (56.5 mi.) distance noted in the figure legend was selected because it corresponds to the radius of a circle with an area of approximately 10,000 square miles. You should locate the region of maximum tornado intensity in what is called "Tornado Alley" in Oklahoma and Kansas, where during an average year slightly more than 10 tornadoes would be expected per 10,000 square miles. You should also find the region of the lowest tornado frequency to the west of the Rocky Mountains. Check to see the tornado frequency for your home community. Compare this map with the thunderstorm climatology map (Figure 13.9). Note that the greatest density of tornadoes is in the southern Plains, while the greatest frequency of thunderstorms is found in the Florida peninsula. Table 14.1 -- Review the recommended personal safety rules designed to protect you from tornado hazards. Figure 14.5 -- Look at the schematic of the multiple suction vortices, considering the extremely rapid changes in wind speed over short distances. Use the accompanying notation to identify the three relative speeds. Note that as the individual vortices spin around a common point that moves across the terrain, the additive effects of the winds would increase the wind speeds along the right-side boundary (when looking downstream to the right). On the left-side boundary the opposing wind direction reduces the net wind speeds. Table 14.2 -- Skim through this tabular listing of the Fujita Tornado Intensity Scale. Note that the wind speeds in the most "violent" F-5 tornadoes ever recorded have had wind speeds that have been estimated at speeds of 420 km per hour (260 mph or higher). Figure 14.6 -- Self-explanatory. Figure 14.7 -- Map of Wichita-Andover tornado. Figure 14.8 -- The study vertical cross-section of a typically supercell that may spawn a tornado. The tornado (not identified with a label) would appear under the parent cumulonimbus cloud base near the collar and wall clouds (directly above the word "West"). Note the various accessory clouds appearing around the parent cumulonimbus cloud. Figures 14.9 and 14.10 -- Look at the pictures of cumulonimbus clouds that formed mesocyclones. Figure 14.12 -- Study the schematic that shows how a radar unit works. Figure 14.13 -- Note that the radar dome encloses a radar antenna for the WSR-88D radar that is approximately 10 meters (33 ft) wide. Figures 14.14 through 14.17 are various radar images where the reflectivity is monitored for specific information, such as to obtain the rainfall rates or the total cumulative rainfall as in Figure 14.15. Figures 14.18 and 14.19 -- Study the schematics depicting the Doppler radar principles. Figures 14.30 and 14.21 -- Spend a moment to look at the displays using the velocity or Doppler mode of a Doppler radar. Read Weather Fact (Tornado Oddities) on page 341. Read the Special Topic (Tornado Look-Alikes) on page 348. Skim the Special Topic (Wind Profilers) on page 352. CHAPTER 14 (Moran and Morgan, 1997) TORNADOES This chapter covers tornadoes, small but violent weather systems. We describe their characteristics, geographical and temporal distribution, and associated hazards. The special combination of atmospheric conditions that favors tornado development occurs most frequently in spring in the central United States. Based on rotational wind speeds reconstructed from property damage, tornadoes are rated from 1 to 5 on the Fujita-scale. Most North American tornadoes are spawned by supercell thunderstorms that develop along or ahead of well-defined cold fronts. Tornadoes also spin off gust fronts and are associated with land-failing hurricanes. In the final section of this chapter, we describe weather radar, a valuable tool for tracking areas of precipitation and weather systems. Modern weather radars can operate in either the reflectivity mode (to detect precipitation) or the Doppler mode (to determine the circulation within a small-scale weather system). CHAPTER OBJECTIVES After reading this chapter, the student should be able to: distinguish between a tornado and a funnel cloud. identify the principal force operating within a tornado. explain why tornadoes are most frequent in tornado alley. explain why tornadoes are most frequent in spring. describe the hazards of tornadoes. explain the basis for the F-scale of tornado intensity. describe the linkage between severe thunderstorm cells and tornadoes. identify some weather phenomena that resemble funnel clouds. compare and contrast the two modes of operation of weather radar (reflectivity, Doppler). describe the role of radar in monitoring small-scale circulation systems such as tornadic thunderstorms. 14 Tornadoes 334 Tornado Characteristics 335 Distribution of Tornadoes 337 Hazards of Tornadoes 339 The F-Scale 340 The Tornado-Thunderstorm Connection 341 Weather Radar 344 Conclusions 351 Weather Fact: Tornado Oddities 341 Special Topic: Tornado Look-Alikes 348 Special Topic: Wind Profilers 352 Key Terms 351 Summary Statements 351 Review Questions 353 Questions for Critical Thinking 353 Selected Readings 353