The Earth’s Atmosphere Contents Part 7 doc

Severe Weather and Doppler Radar Most of our knowledge about what goes on inside a tor- nado-generating thunderstorm has been gathered through the use of Doppler radar. Remember from Chapter 5 that a radar transmitter sends out microwave pulses and that, when this energy strikes an object, a small fraction is scat- tered back to the antenna. Precipitation particles are large enough to bounce microwaves back to the antenna. As a consequence, the colorful area on the radar screen in Fig. 10.35 represents precipitation inside a severe thunder- storm, as viewed by the older, conventional-type radar. Notice that the pattern on the left side of the screen is in the shape of a hook. A hook echo such as this indicates the pos- sible presence of a tornado. When tornadoes form, they do so near the tip of the hook. However, there is a problem here in that many severe thunderstorms (as well as smaller ones) do not show a hook echo, but still spawn tornadoes. Sometimes, when the hook echo does appear, the tornado is already touching the ground. Therefore, a better tech- 282 Chapter 10 Thunderstorms and Tornadoes Rotating clouds at the base of a severe thunderstorm often indicate that the storm is about to give birth to a tornado. But how do the clouds develop rotation? Figure 3 illustrates how rotating vortices can develop near the surface. Notice that there is wind direction shear as surface winds are southeasterly; aloft they are westerly. There is also wind speed shear as the wind speed increases with increasing height. This wind shear causes the air near the surface to rotate about a horizontal axis, producing narrow tubes of spiraling air called vortex tubes. A strong updraft of a thunderstorm may then tilt a rotating tube and draw it into the storm as depicted in Fig. 4. This situation sets up two spinning vertical columns of air—one rotating clockwise and the other counter- clockwise. As air is drawn more quickly into the storm, the spiraling columns spin faster. If the thunderstorm has a more complicated structure (as most do), additional rotating air columns may form. This phenomenon normally induces the southern flank of the storm to rotate in one direction, usu- ally counterclockwise (when viewed from above) and the northern flank in the other direction, usually clockwise. Hence, the thunderstorm rotates. FIGURE 3 Spinning vortex tubes created by wind shear. FIGURE 4 The strong updraft in the thunderstorm carries the vortex tube into the thunderstorm, producing two rotating air columns that are oriented in the vertical plane. THUNDERSTORM ROTATION Focus on an Observation N E S W Southeasterly surface winds Strong westerly flow aloft N E S W Rotation clockwise Updraft Rotation counter- clockwise nique was needed in detecting tornado-producing storms. To address this need, Doppler radar was developed. Doppler radar is like a conventional radar in that it can detect areas of precipitation and measure rainfall intensity. But a Doppler radar can do more—it can actu- ally measure the speed at which precipitation is moving horizontally toward or away from the radar antenna. Because precipitation particles are carried by the wind, Doppler radar can peer into a severe storm and unveil its winds. Doppler radar works on the principle that, as pre- cipitation moves toward or away from the antenna, the returning radar pulse will change in frequency. A simi- lar change occurs when the high-pitched sound (high frequency) of an approaching noise source, such as a siren or train whistle, becomes lower in pitch (lower fre- quency) after it passes by the person hearing it. This change in frequency is called the Doppler shift and this, of course, is where the Doppler radar gets its name. A single Doppler radar cannot detect winds that blow parallel to the antenna. Consequently, two or more units probing the same thunderstorm are needed to give a complete three-dimensional picture of the winds within the storm. To help distinguish the storm’s air motions, wind velocities can be displayed in color. Color contour- ing the wind field gives a good picture of the storm. Even a single Doppler radar can uncover many of the features of a severe thunderstorm. For example, studies conducted in the 1970s revealed, for the first time, the existence of the swirling winds of the mesocyclone inside tornado-producing thunderstorms. Mesocyclones have a distinct image (signature) on the radar display. Studies show that about 30 percent of all mesocyclones produce tornadoes and about 95 percent produce severe weather. The time between mesocyclone identification and the tor- nado actually touching the ground is about 20 minutes. Tornadoes also have a distinct signature, known as the tornado vortex signature (TVS), which shows up as a region of rapidly changing wind directions within the mesocyclone (see Fig. 10.36). Unfortunately, the resolu- tion of the Doppler radar is not high enough to measure actual wind speeds of most tornadoes, whose diameters Severe Weather and Doppler Radar 283 FIGURE 10.35 A tornado-spawning thunderstorm shows a hook echo in its rainfall pattern on a conventional radar screen. FIGURE 10.36 Doppler radar display showing a large supercell thunderstorm that is spawning an F4 tornado (circled area) near Lula, Oklahoma. The close packing of the winds indicates strong cyclonic rotation and the signature of a tornado. (Red and orange indicate winds blowing away from the radar. Green and blue indicate winds blowing toward the radar.) are only a few hundred meters or less. However, a new and experimental Doppler system—called Doppler lidar—uses a light beam (instead of microwaves) to measure the change in frequency of falling precipita- tion, cloud particles, and dust. Because it uses a shorter wavelength of radiation, it has a narrower beam and a higher resolution than does Doppler radar. In an attempt to obtain tornado wind information at fairly close range (less than 10 km), smaller portable Doppler radar units (Doppler on wheels) are peering into tor- nado-generating storms (see Fig. 10.37). The new network of 135 Doppler radar units de- ployed at selected weather stations within the continental United States is referred to as NEXRAD (an acronym for NEXt Generation Weather RADar). The NEXRAD system consists of the WSR-88D* Doppler radar and a set of computers that perform a variety of functions. The computers take in data, display it on a moni- tor, and run computer programs called algorithms, which, in conjunction with other meteorological data, detect severe weather phenomena, such as storm cells, hail, mesocyclones, and tornadoes. Algorithms provide a great deal of information to the forecasters that allows them to make better decisions as to which thunder- storms are most likely to produce severe weather and possible flash flooding. In addition, they give advanced and improved warning of an approaching tornado. More reliable warnings, of course, will cut down on the number of false alarms. Because the Doppler radar shows air motions within a storm, it can help to identify the magnitude of other severe weather phenomena, such as gust fronts, microbursts, and wind shears that are dangerous to air- craft. Certainly, as more and more information from Doppler radar becomes available, our understanding of the processes that generate severe thunderstorms and tornadoes will be enhanced, and hopefully there will be an even better tornado and severe storm warning sys- tem, resulting in fewer deaths and injuries. Waterspouts A waterspout is a rotating column of air over a large body of water. The waterspout may be a tornado that formed over land and then traveled over water. In such a case, the waterspout is sometimes referred to as a tornadic waterspout. Waterspouts that form over water, especially above the warm, shallow coastal waters of the Florida Keys, where almost 100 occur each month during the summer, are referred to as “fair weather” waterspouts.† These waterspouts are generally much smaller than an average tornado, as they have diameters usually between 3 and 100 meters. Fair weather waterspouts are also less intense, as their rotating winds are typically less than 45 knots. In addition, they tend to move more slowly 284 Chapter 10 Thunderstorms and Tornadoes FIGURE 10.37 Graduate students from the University of Oklahoma use a portable Doppler radar to probe a tornado near Hodges, Oklahoma. †“Fair weather” waterspouts may form over any large body of warm water. Hence, they occur frequently over the Great Lakes in summer. Storm-chasing scientists on May 3, 1999, using two Doppler radars mounted on separate vehicles, measured a record wind speed of 318 mi/hr inside a violent tor- nado that ultimately ravaged sections of Oklahoma City. *The name WSR-88D stands for Weather Surveillance Radar, 1988 Doppler. than tornadoes and they only last for about 10 to 15 min- utes, although some have existed for up to one hour. Fair weather waterspouts tend to form when the air is conditionally unstable and clouds are developing. Unlike the tornado, they do not need a thunderstorm to generate them. Some form with small thunderstorms, but most form with developing cumulus congestus clouds whose tops are frequently no higher than 3600 m (12,000 ft) and do not extend to the freezing level. Apparently, the warm, humid air near the water helps to create atmospheric instability, and the updraft beneath the resulting cloud helps initiate uplift of the surface air. Studies even suggest that gust fronts and converging sea breezes may play a role in the formation of some of the waterspouts that form over the Florida Keys. The waterspout funnel is similar to the tornado funnel in that both are clouds of condensed water vapor with converging winds that rise about a central core. Contrary to popular belief, the waterspout does not draw water up into its core; however, swirling spray may be lifted several meters when the waterspout fun- nel touches the water. Apparently, the most destructive waterspouts are those that begin as tornadoes over land, then move over water. A photograph of a particu- larly well-developed and intense waterspout is shown in Fig. 10.38. Summary In this chapter, we examined thunderstorms and the atmospheric conditions that produce them. The ingredi- ents for an isolated ordinary (air-mass) thunderstorm are humid surface air, plenty of sunlight to heat the ground, and a conditionally unstable atmosphere. When these conditions prevail, small cumulus clouds may grow into towering clouds and thunderstorms within 20 minutes. When conditions are ripe for thunderstorm devel- opment and a strong vertical wind shear exists, the stage is set for the generation of severe thunderstorms. Super- cell thunderstorms may exist for many hours, as their updrafts and downdrafts are nearly in balance. Thun- derstorms that form in a line, along or ahead of an advancing cold front, are called a squall line. Lightning is a discharge of electricity that occurs in mature thunderstorms. The lightning stroke momentar- ily heats the air to an incredibly high temperature. The rapidly expanding air produces a sound called thunder. Along with lightning and thunder, severe thunderstorms produce violent weather, such as destructive hail, strong downdrafts, and the most feared of all atmospheric storms—the tornado. Tornadoes are rapidly rotating columns of air that extend downward from the base of a thunderstorm. Most tornadoes are less than a few hundred meters wide with wind speeds less than 100 knots, although violent tornadoes may have wind speeds that exceed 250 knots. A violent tornado may actually have smaller whirls (suction vortices) rotating within it. With the aid of Doppler radar, scientists are probing tornado- spawning thunderstorms, hoping to better predict tor- nadoes and to better understand where, when, and how they form. A normally small and less destructive cousin of the tornado is the “fair weather” waterspout that com- monly forms above the warm waters of the Florida Keys and the Great Lakes in summer. Summary 285 FIGURE 10.38 A powerful waterspout moves across Lake Tahoe, California. Key Terms The following terms are listed in the order they appear in the text. Define each. Doing so will aid you in reviewing the material covered in this chapter. Questions for Review 1. What is a thunderstorm? 2. Describe the stages of development of an ordinary (air-mass) thunderstorm. 3. How do downdrafts form in thunderstorms? 4. Why do ordinary thunderstorms most frequently form in the afternoon? 5. What atmospheric conditions are necessary for the development of an ordinary thunderstorm? 6. (a) What are gust fronts and how do they form? (b) If a gust front passes, what kind of weather will you experience? 7. (a) Describe how a microburst forms. (b) Why is the term wind shear often used in conjunction with a microburst? 8. Why are severe thunderstorms not very common in polar latitudes? 9. Give a possible explanation for the generation of pre- frontal squall-line thunderstorms. 10. What do thunderstorms tend to do when they pro- duce devastating flash floods? 11. What is a Mesoscale Convective Complex (MCC)? 12. Where does the highest frequency of thunderstorms occur in the United States? Why there? 13. Why is large hail more common in Kansas than in Florida? 14. Explain how a cloud-to-ground lightning stroke de- velops. 15. How is thunder produced? 16. If you see lightning and ten seconds later you hear thunder, how far away is the lightning stroke? 17. Why is it unwise to seek shelter under a tree during a thunderstorm? 18. What is a tornado? 19. List the major characteristics of tornadoes, including their size, wind speed, and direction of movement. 20. How does a tornado watch differ from a tornado warning? 21. Why is it suggested that one not open windows when a tornado is approaching? 22. Explain why the central part of the United States is more susceptible to tornadoes than any other region of the world. 23. Describe the atmospheric conditions at the surface and aloft that are necessary for the development of the majority of tornado-spawning thunderstorms. 24. Describe how Doppler radar measures the winds inside a severe thunderstorm. 25. Explain both how and why there is a shift in tornado activity from winter to summer within the continen- tal United States. 26. What atmospheric conditions lead to the formation of “fair weather” waterspouts? Questions for Thought and Exploration 1. Why does the bottom half of a dissipating thunder- storm usually “disappear” before the top? 2. Sinking air warms, yet thunderstorm downdrafts are cold. Why? 3. If you are confronted by a large tornado in an open field and there is no way that you can outrun it, your only recourse might be to run and lie down in a depression. If given the choice, when facing the tornado, would you run toward your left or toward your right as the tor- nado approaches? Explain your reasoning. 286 Chapter 10 Thunderstorms and Tornadoes ordinary (air-mass) thunderstorms cumulus stage mature thunderstorm dissipating stage multicell storms severe thunderstorm gust front shelf cloud roll cloud downburst microburst derecho supercell storm squall line dryline Mesoscale Convective Complexes (MCCs) flash flood lightning thunder sonic boom stepped leader return stroke dart leader heat lightning St. Elmo’s Fire tornadoes funnel cloud tornado outbreak suction vortice Fujita scale mesocyclone gustnado wall cloud tornado watch tornado warning Doppler radar NEXRAD waterspout 4. Use the Severe Weather/Lightning section of the Blue Skies CD-ROM to examine the anatomy of a light- ning stroke. 5. Using the Severe Weather/Microburst section of the Blue Skies CD-ROM, try to land a plane while flying through a microburst. 6. Lightning, sprites, and jets (http://www.sprite.lanl .gov/) Compare photographs of lightning, red sprites, and blue jets. What similarities can you observe among these three electrical phenomena? In your own words, describe the physical mechanism behind sprites and jets. For additional readings, go to InfoTrac College Edition, your online library, at: http://www.infotrac-college.com Questions for Thought and Exploration 287 Tropical Weather Anatomy of a Hurricane Hurricane Formation and Dissipation Hurricane Stages of Development Hurricane Movement Focus on a Special Topic: How Do Hurricanes Compare with Middle-Latitude Storms? Destruction and Warning Focus on a Special Topic: Modifying Hurricanes Naming Hurricanes Summary Key Terms Questions for Review Questions for Thought and Exploration Contents O n September 18, 1926, as a hurricane approached Miami, Florida, everyone braced themselves for the devastating high winds and storm surge. Just before dawn the hurricane struck with full force—torrential rains, flooding, and easterly winds that gusted to over 100 miles per hour. Then, all of a sudden, it grew calm and a beautiful sunrise appeared. People wandered outside to inspect their property for damage. Some headed for work, and scores of adventurous young people crossed the long causeway to Miami Beach for the thrill of swimming in the huge surf. But the lull lasted for less than an hour. And from the south, ominous black clouds quickly moved overhead. In what seemed like an instant, hurricane force winds from the west were pounding the area and pushing water from Biscayne Bay over the causeway. Many astonished bathers, unable to swim against the great surge of water, were swept to their deaths. Hundreds more drowned as Miami Beach virtually disappeared under the rising wind-driven tide. Hurricanes 289 B orn over warm tropical waters and nurtured by a rich supply of water vapor, the hurricane can indeed grow into a ferocious storm that generates enor- mous waves, heavy rains, and winds that may exceed 150 knots. What exactly are hurricanes? How do they form? And why do they strike the east coast of the United States more frequently than the west coast? These are some of the questions we will consider in this chapter. Tropical Weather In the broad belt around the earth known as the trop- ics—the region 23 1 ⁄ 2 ° north and south of the equator— the weather is much different from that of the middle latitudes. In the tropics, the noon sun is always high in the sky, and so diurnal and seasonal changes in tem- perature are small. The daily heating of the surface and high humidity favor the development of cumulus clouds and afternoon thunderstorms. Most of these are individual thunderstorms that are not severe. Some- times, however, the storms will align into a narrow band called a nonsquall cluster. On other occasions, the thun- derstorms will align into a row of vigorous convective cells or squall line. The passage of a squall line is usually noted by a sudden wind gust followed immediately by a heavy downpour. This deluge is then followed by sev- eral hours of relatively steady rainfall. Many of these tropical squall lines are similar to the middle-latitude squall lines described in Chapter 10. As it is warm all year long in the tropics, the weather is not characterized by four seasons which, for the most part, are determined by temperature variations. Rather, most of the tropics are marked by seasonal differences in precipitation. The greatest cloudiness and precipitation occur during the high-sun period, when the intertropical convergence zone moves into the region. Even during the dry season, precipitation can be irregular, as periods of heavy rain, lasting for several days, may follow an extremely dry spell. The winds in the tropics generally blow from the east, northeast, or southeast—the trade winds. Because the variation of sea-level pressure is normally quite small, drawing isobars on a weather map provides little useful information. Instead of isobars, streamlines that depict wind flow are drawn. Streamlines are useful because they show where surface air converges and diverges. Occasionally, the streamlines will be disturbed by a weak trough of low pressure called a tropical wave, or easterly wave (see Fig. 11.1). Tropical waves have wavelengths on the order of 2500 km (1550 mi) and travel from east to west at speeds between 10 and 20 knots. Look at Fig. 11.1 and observe that, on the western side of the trough (heavy dashed line), where easterly and northeasterly surface winds diverge, sinking air produces generally fair weather. On its eastern side, where the southeasterly winds converge, rising air generates showers and thun- derstorms. Consequently, the main area of showers forms behind the trough. Occasionally, a tropical wave will intensify and grow into a hurricane. Anatomy of a Hurricane A hurricane is an intense storm of tropical origin, with sustained winds exceeding 64 knots (74 mi/hr), which forms over the warm northern Atlantic and eastern North Pacific oceans. This same type of storm is given different names in different regions of the world. In the western North Pacific, it is called a typhoon, in the Philippines a baguio (or a typhoon), and in India and Australia a cyclone. By international agreement, tropical 290 Chapter 11 Hurricanes 10° 20° 30° ° N D iv e r g e n c e C o n v e r g e n c e Latitude, °N FIGURE 11.1 A tropical wave (also called an easterly wave) as shown by the bending of streamlines—lines that show wind flow patterns. (The heavy dashed line is the axis of the trough.) The wave moves slowly westward, bringing fair weather on its western side and showers on its eastern side. The word hurricane derives from the Taino language of Central America. The literal translation of the Taino word hurucan is “god of evil.” The word typhoon comes from the Chinese word taifung, meaning “big wind.” cyclone is the general term for all hurricane-type storms that originate over tropical waters. For simplicity, we will refer to all of these storms as hurricanes. Figure 11.2 is a photo of Hurricane Elena situated over the Gulf of Mexico. The storm is approximately 500 km (310 mi) in diameter, which is about average for hur- ricanes. The area of broken clouds at the center is its eye. Elena’s eye is almost 40 km (25 mi) wide. Within the eye, winds are light and clouds are mainly broken. The sur- face air pressure is very low, nearly 955 mb (28.20 in.).* Notice that the clouds align themselves into spiraling bands (called spiral rain bands) that swirl in toward the storm’s center, where they wrap themselves around the eye. Surface winds increase in speed as they blow coun- terclockwise and inward toward this center. (In the Anatomy of a Hurricane 291 Rain free area Eye Eye wall Spiral rain band FIGURE 11.2 Hurricane Elena over the Gulf of Mexico, about 130 km (80 mi) southwest of Apalachicola, Florida, as photographed from the space shuttle Discovery during September, 1985. Because this storm is situated north of the equator, surface winds are blowing counterclockwise about its center (eye). The central pressure of the storm is 955 mb, with sustained winds of 105 knots near its eye. *An extreme low pressure of 870 mb (25.70 in.) was recorded in Typhoon Tip during October, 1979, and Hurricane Gilbert had a pressure reading of 888 mb (26.22 in.) during September, 1988. [...]... into work, then ejected at a low temperature In a hurricane, small swirling eddies transfer sensible and latent heat from the ocean surface into the overlying air The warmer the water and the greater the wind speed, the greater the transfer of sensible and latent heat As the air sweeps in toward the center of the storm, the rate of heat transfer increases because the wind speed increases toward the eye... form where the upper-level winds are diverging (the air is spreading out) and, at the same time, the air aloft is leaving a vertical column of air more quickly than the air at the surface is entering The energy for a hurricane comes from the direct transfer of sensible heat from the warm water into the atmosphere and from the transfer of latent heat from the ocean surface One idea (known as the organized... radiational cooling The maximum strength a hurricane can achieve is determined by the difference in temperature between the ocean surface and the top of its clouds As a consequence, the warmer the ocean surface, the lower the minimum pressure of the storm, and the higher its winds Presently, there is much debate whether hurricanes are driven by the organized convection process, by the heat engine process,... that exceed 64 knots (74 mi/hr) and blow counterclockwise about their centers in the Northern Hemisphere A hurricane consists of a mass of organized thunderstorms that spiral in toward the extreme low pressure of the storm’s eye The most intense thunderstorms, the heaviest rain, and the highest winds occur outside the eye, in the region known as the eye wall In the eye itself, the air is warm, winds... counterpart at the same latitude over the eastern Pacific The reason is, of course, that the surface water of the Atlantic is much warmer DESTRUCTION AND WARNING When a hurricane is approaching from the east, its highest winds are usually on its north (poleward) side The reason for this phenomenon is that the winds that push the storm along add to the winds on the north side and subtract from the winds... Similarly, the higher wind speeds cause greater evaporation rates, and the overlying air becomes nearly saturated 295 Near the eye wall, turbulent eddies transfer the warm moist air upward, where the water vapor condenses to form clouds The release of latent heat inside the clouds causes the air temperature in the region of the eye wall to be much higher than the air temperature at the same altitude further... can strengthen again 4 Use the Hurricanes/Virtual Hurricane activity on the Blue Skies CD-ROM to answer the following questions (a) Calculate the pressure gradient at a distance of 200 km from the center of the hurricane Is the pressure gradient different depending on whether you’re north, south, east, or west of the eye? What effects do these differences, if present, have on the winds? (b) Do the same... the ITCZ during the Southern Hemisphere’s warm season discourages their development Hurricanes that form over the North Pacific and North Atlantic are steered by easterly winds and move west or northwestward at about 10 knots for a week or so Gradually, they swing poleward around the subtropical high, and when they move far enough north, they become caught in the westerly flow, which curves them to the. .. surface winds increase, the depression becomes a tropical storm At this point, the storm is given a name Some tropical storms continue to deepen into full-fledged hurricanes, as long as the outflow of air at the top of the storm is greater than the convergence of air at the surface In the organized convection theory, the energy source that drives the hurricane comes primarily from the release of latent... then gradually swing northwestward around the subtropical high to the north If the storm moves into middle latitudes, the prevailing westerlies steer it northeastward Because hurricanes derive their energy from the warm surface water and from the latent heat of condensation, they tend to dissipate rapidly when they move over cold water or over a large mass of land, where surface fric- tion causes their . the broad belt around the earth known as the trop- ics the region 23 1 ⁄ 2 ° north and south of the equator— the weather is much different from that of the middle latitudes. In the tropics, the. Gradually, they swing poleward around the subtrop- ical high, and when they move far enough north, they become caught in the westerly flow, which curves them to the north or northeast. In the middle. spell. The winds in the tropics generally blow from the east, northeast, or southeast the trade winds. Because the variation of sea-level pressure is normally quite small, drawing isobars on a weather