Thunderstorms & Tornadoes

1. Thunderstorms & Tornadoes • This chapter discusses: • The different types and development stages of thunderstorms • The life-threatening storm components of lightning and tornadoes

2. Ordinary Thunderstorms • Three stages have been identified in ordinary thunderstorms: • an unstable atmosphere and vertical updrafts keep precipitation suspended • entrainment of dry air that causes cooler air from evaporation, triggering downdrafts and falling precipitation and gust fronts • weakening updrafts and loss of the fuel source after 15 to 30 minutes. Figure 14.2A

3. Mature Stage Thunderstorm Figure 15.2 During the mature stage, updrafts may stop at the troposphere where the cloud ice crystals are pushed horizontally by winds and form an anvil top, or they may overshoot further into the tropopause.

4. Dissipating Stage of Thunderstorm Once downdrafts dominate updrafts, the storm ends as precipitation leaves the cloud faster than it is replenished by rising, condensing air. Often, lower level cloud particles will evaporate leaving an isolate cirrus anvil top section. Figure 15.3

5. Multicell Storms Figure 15.4 Cool downdrafts leaving a mature and dissipating storm may offer relief from summer heat, but they may also force surrounding, low-level moist air upward. Hence, dying storms often trigger new storms, and the successive stages may be viewed in the sky.

6. Severe Thunderstorms Figure 15.5

7. Storms producing a minimum of a) 3/4 inch hail and/or b) wind gusts of 50 knots and/or c) tornado winds, classify as severe. In ordinary storms, the downdraft and falling precipitation cut off the updraft. In severe storms, winds aloft push the rain ahead and the updraft is not weakened and the storm can continue maturing. The single supercell storm shown here maintained its structure for hours.

8. Gust Front & Microburst Turbulent air forms along the leading edge of the gust front, which can generate tumbling dust clouds. Such gust fronts and associated cold dense air often feel like a passing cold front, and may cause a 1 to 3 mb local rise in pressure, called a mesohigh. Figure 15.6

9. Gust Front Shelf Cloud When unstable air is prevalent near the base of the thunderstorm, the warm rising air along the forward edge of the gust front is likely to generate a shelf, or arcus, cloud. Figure 15.6

10. Gust Front Roll Clouds Figure 15.7 Turbulence in the fast moving gust front will spawn eddies and possibly roll clouds beneath the shelf cloud. These clouds spin about a horizontal axis near the ground.

11. Microbursts from Dense Air Figure 15.9 Dry air entrained into the thunderstorm will evaporate and cool the falling mix of precipitation and air, which may create dry, and in humid areas wet, microbursts of strong winds.

12. Flying into a Microburst Figure 15.10 A pilot flying into a microburst must anticipate sudden and strong changes in wind direction and speed. Initially a headwind is encountered that lifts the plane, followed by a strong downdraft, and when leaving the storm a tailwind causes a loss of altitude.

13. Storm Radar Bow Echo Derecho, or straight-line winds, may form ahead of a several hundred kilometer cluster of storms, known as a squall line or mesoscale convective system, often formed a few hundred kilometers ahead of a cold front. This image shows a squall line within the bow shaped radar echo. Figure 15.11

14. Pre-Frontal Squall Line Storms Pre-frontal squall lines identify major storms triggered by a cold front that may contain several severe thunderstorms, some possibly supercells, extending for more than 1000 kilometers. This 1989 storm spawned 25 tornadoes, the worst killing 25 people. Figure 15.12

15. Gravity Waves Figure 15.13 Pre-frontal squall line formation is not fully understood. One theory suggests that a surging cold front may initiate "gravity waves" aloft, where the rising motion of the wave causes cumulus cloud development.

16. Trailing Stratified Clouds Figure 15.14 An extensive region of stratified clouds may follow behind a squall line. This figure shows a loop of rising and falling air that supplies the moisture to the stratiform clouds and associated light precipitation.

17. Mesoscale Convective Complex Figure 15.15

18. An organized mass, or collection, of thunderstorms that extends across a large region is called a mesoscale convective complex (MCC). With weak upper level winds, such MCC's can regenerate new storms and last for upwards of 12 hours and may bring hail, tornadoes, and flash floods. They often form beneath a ridge of high pressure.

19. Dryline Thunderstorms Figure 15.16

20. Abrupt geographic changes from moist to dry dew-point temperature, called drylines, form in western TX, OK, and KS in the spring and summer. The diagram illustrates how cool cP air pushes hot and dry cT air, at the height of the central plains, over the warm moist mT air. Such mixing causes large scale instabilities and the birth of many supercell storms.

21. Thunderstorm Movement Figure 15.17 Middle troposphere winds often direct individual cells of a thunderstorm movement, but due to dying storm downdrafts spawning new storms, the storm system tends to be right-moving relative to the upper level winds. In this figure, upper level winds move storms to the northeast, but downdrafts generate new cells to the south, which eventually cuts off moisture to the old cell.

22. Flash & Great Floods Figure 15.18 Figure 15.19 Thunderstorms frequently generate severe local flooding, but in the summer of 1993 a stationary front beneath the unusually southerly polar jet triggered several days of thunderstorms and rain. The jet caused weak surface waves and provided uplift of warm, moist Gulf air for thunderstorm growth throughout the northern Mississippi region. Floods took 45 human lives and 74,000 were evacuated.

23. Average Thunderstorm & Hail Days Figure 15.21 Figure 15.20 Observed frequency in the pattern and occurrence of thunderstorms does not overlap with hail frequency, possibly because hail falling into the thick layer of warm Gulf air will melt before reaching the ground.

24. Lightning & Thunder Charge differences between the thunderstorm and ground can cause lightning strokes of 30,000°C, and this rapid heating of air will creates an explosive shock wave called thunder, which requires approximately 3 seconds to travel 1 kilometer. Figure 15.22

25. Lightning Stroke Development Charge layers in the cloud are formed by the transfer of positive ions from warmer hailstones to colder ice crystals. When the negative charge near the bottom of the cloud is large enough to overcome the air's resistance, a stepped leader forms. A region of positive ions move from the ground toward this charge, which then forms a return stroke into the cloud. Figure 15.23A

26. Types of Lightning Figure 15.24

27. Nearly 90% of lightning is the negative cloud-to-ground type described earlier, but positive cloud-to-ground lightning can generate more current and more damage. Several names, such as forked, bead, ball, and sheet lightning describe forms of the flash. Distant, lightning with unheard thunder is often called heat lightning.

28. Lightning Rods & Fulgurite Figure 15.26 Figure 15.25 Metal rods that are grounded by wires provide a low resistance path for lightning into the earth, which is a poor conductor. The fusion of sand particles into root like tubes, called fulgurite, may result.

29. Lightning Detection & Suppression Figure 15.27 Figure 15.28 When lightning is nearby, trees are not safe because they may generate a return stroke, but a car may provide protection by transferring the charge through its body to the tires. Lightning is more often the cause for forest fires, triggering nearly 10,000 yearly in the U.S. A National Lightning Detection Network helps monitor this storm activity.

30. Tornado A rapidly rotating column of air often evolve through a series of stages, from dust-whirl, to organizing and mature stages, and ending with the shrinking and decay stages. Winds in this southern Illinois twister exceeded 150 knots. Figure 15.29

31. Tornado Occurrence Figure 15.30 Tornadoes from all 50 states of the U.S. add up to more than 1000 tornadoes annually, but the highest frequency is observed in tornado alley of the Central Plains. Nearly 75% of tornadoes form from March to July, and are more likely when warm humid air is overlain by cooler dryer air to cause strong vertical lift.

32. Tornado Wind Speed As the tornado moves along a path, the circular tornado winds blowing opposite the path of movement will have less speed. For example, if the storm rotational speed is 100 knots, and its path is 50 knots, it will have a maximum wind of 150 knots on its forward rotation side. Figure 15.31

33. Suction Vortices & Damage A system of tornadoes with smaller whirls, or suction vortices, contained within the tornado is called a multi-vortex tornado. Damage from tornadoes may include its low pressure centers causing buildings to explode out and the lifting of structures. Human protection may be greatest in internal and basement rooms of a house. Figure 15.32

34. Fujita Tornado Scale Figure 15.33 Tornado watches are issued when tornadoes are likely, while a warning is issued when a tornado has been spotted. Once the storm is observed, or has passed, the Fujita scale is used to classify tornadoes according to their rotational speed based on damage done by the storm.

35. Atmospheric Conditions for Tornadoes A specific pattern of events often coincide during the formation of tornadoes and severe thunderstorms. This may include when an open-wave mid-latitude cyclone mixes together cold dry air with warm moist air at the surface, and 850 mb warm moist and 700 mb cold dry air aloft flow north and north east, as shown in this figure. Further, at the 500 mb level a trough of low pressure pressure forms to the west of the surface low, and the 300 mb polar jet swings over the region. Figure 15.34

36. Thunderstorm Sounding Figure 15.35 Temperature and dew point have typical vertical profile in the warm sector before a tornado occurs, including the shallow inversion at 800 mb that acts like a cap on the moist air below. The cold dry air above warm humid air produces convective instability and lifting.

37. Vorticity from Horizontal to Vertical Figure 15.36 Figure 15.37 Spinning horizontal vortex tubes created by surface wind shear may be tilted and forced in a vertical path by updrafts. This rising, spinning, and often stretching rotating air may then turn into a tornado.

38. Tornado Breeding Supercell Storm Figure 15.38 Supercell thunderstorms may have many of the features illustrated here, including a mesocyclone of rotating winds formed when horizontal vorticity was tilted upwards.

39. Radar Image of Supercell The area of precipitation and winds in the mesocyclone is known as the bounded weak echo region (BWER) which the radar is unable to detect and displays as a black core to this storm. The cyclonic flow of precipitation on the radar screen is often shaped like a hook echo. Figure 15.39

40. Rear Flank Downdraft Supercell thunderstorm development may create an area where the updraft and counterclockwise swirl of upper winds converge into a rear flank downdraft. This downdraft can then interact with lower level inflow winds and spawn a tornado. Figure 15.40

41. Rotating Clouds as Tornado Signal The first sign that a supercell may form a tornado is the sight of rotating clouds at the base of the storm, which may lower and form a wall cloud, shown in this picture. Figure 15.41

42. NonSupercell Tornadoes If a pre-existing wall cloud was not present, than any tornado formed is not from a supercell storm, and is often called a funnel cloud, or may be a gustnado if the form along a gust front. Figure 15.42

43. Landspout Formation Figure 15.43A

44. Landspouts, which form over land but look like waterspouts, form when surface winds converge along a boundary where opposite blowing wind creates a horizontal rotational spin. If a storm passes above, its updraft may lift and stretch the horizontal spinning air, causing it to narrow and increase in rotational speed due to the conservation of angular momentum.

45. Doppler Radar Analysis A single Doppler radar unit can uncover many features of thunderstorm rotation and movement, but cannot detect winds parallel to the antenna. As such, data from two or more units might be combined to provide a complete view of the storm. Dopplar lidar (light beam rather than microwave beam) provides more details on the storm features, and will help measure wind speeds in smaller tornadoes. Figure 15.44

46. NEXRAD Wind Analysis NEXt Generation Weather RADar (NEXRAD) is operated by the National Weather Service and uses Doppler measurement to detect winds moving toward (green) and away (blue) from the antenna, which indicates areas of rotation and strong shear. Figure 15.45

47. Portable Radar Units Thunderstorm chasers may carry portable radar to image finer details of a storm as it moves along the flat lands of Tornado Alley. Figure 15.46

48. Waterspout Funnel Warm, shallow coastal water is often home to waterspouts, which are much smaller than an average tornado, but similar in shape and appearance. The waterspout does not draw water into its core, but is a condensed cloud of vapor. A waterspout may, however, lift swirling spray from the water as it touches the water surface. Figure 15.47