METEOROLOGY

1. METEOROLOGY GEL-1370

2. Chapter Ten Thunderstorms andTornadoes

3. Goal for this Chapter We are going to learn answers to the following questions: • What atmospheric conditions produce thunderstorms? • How severe thunderstorms are produced? • Why severe thunderstorms are not common in polar latitude? • How lightning are produced? • How thunders are produced? • What are tornadoes and how they are produced? • What is Fujita scale? • Major characteristics of a tornadoe? • Why highest frequency of thunderstorms occur in US?

4. Thunderstorms • Thunderstorm: A storm that contains lightning and thunder • Birth occurs when warm humid air rises in a conditionally unstable environment • What can trigger the birth of thunderstorm – unequal heating of the surface, terrain, lifting of warm air along a frontal zone • Ordinary thunderstorms (or air-mass thunderstorms): Develop in warm, humid air masses away from weather fronts; usually short-lived and rarely produce strong winds or large hail • Severe thunderstorms may produce high winds, flash floods, changing hail & tornadoes

5. Thunderstorms • Stages of ordinary thunderstorms: • Cumulus Stage: Humid air rises, cools, & condenses in to cumulus clouds • Transformation of water-vapor into liquid or solid cloud particles releases large quantities of latent heat; this keeps the air inside the cloud warmer than the surrounding air • During cumulus stage, insufficient time for precipitation to form, and the updrafts keep water droplets and ice crystals suspended within the cloud; no lightning or thunder during this stage • As the cloud builds well above the freezing level, cloud particles grow larger and heavier; drops begin to fall; drier air around the cloud is being drawn into it; entrainment of drier air leads to evaporation of raindrops; air becomes colder & heavier; air begins to descend as a downdraft

6. Thunderstorms – contd. • Appearance of the downdraft marks the beginning of the mature thunderstorms; downdraft & updraft within the mature thunderstorm constitute a ‘cell’ • In most storms, there are several cells, each of which may last for an hour or so • Updrafts & downdrafts reach their greatest strength in the middle of the cloud, creating severe turbulence • Overshooting: Intrusion of the updraft above the cloud top in to the stable atmosphere • Dissipating stage: When updrafts weaken & downdrafts tend to dominate throughout much of the cloud • Three stages: Cumulus stage, maturing thunderstorm stage, & dissipating stage

7. Thunderstorms – contd. • A single ordinary thunderstorm may go through its three stages in an hour or less • The cold downdraft may force warm, moist surface air upward; this air may condense and can gradually build into a new thunderstorm – multicell thunderstorms • Most ordinary thunderstorms are multicell storms • Severe Thunderstorms: Capable of producing large hail, strong, gusty surface winds, flash floods, and tornadoes • Can form from moist air when it is forced to rise into a conditionally unstable atmosphere; severe thunderstorms also form in areas with a strong vertical wind sheer

8. Air motions associated with thunderstorms; severity depends on the intensity of the storm’s circulation pattern

9. Ordinary thunderstorm in its mature stage

10. A multicell thunderstorm; in the middle is in its mature stage; to its right of the cell, a thunderstorm is its cumulus stage

11. A simplified model describing air motions & other features associated with a severe thunderstorm; severity depends on the intensity of the storm’s circulation pattern

12. Severe Thunderstorms – contd. • The storm in the previous figure, moves from left to right & the upper-level winds cause the system to tilt so that the updrafts move up and over the downdrafts • The updrafts in a severe thunderstorm may be so strong that the cloud top is able to intrude well into the stable atmosphere; top of the cloud may even extend to more than 18 km above the surface • Gust Front: The boundary separating the cold downdraft from the warm surface air • Along the leading edge of the gust front, the air is turbulent; strong winds here can pick-up loose dust and soil and lift them into a huge tumbling cloud

13. Gust Front & Microburst • Downbursts: A severe localized downdraft that can be experienced that fall slowly and reduce visibility more than light rain • Microburst: A downburst with winds extending only 4-kms or less • Supercell and Squall-line thunderstorms: • Supercell Storm: An enormous severe thunderstorm whose updrafts (can exceed 90 knots) and downdrafts are nearly in balance, allowing it to maintain itself for several hours. It can produce large tornadoes & hail (> grapefruit size); most supercell storms move to the right of the steering winds aloft • Squall-line storms form as a line of thunderstorms along a cold front or out ahead of it

14. The lower half of a severe squall-line type thunderstorms and some of the features associated with it

15. Dust clouds rising in response to the outburst winds of a microburst north of Denver, CO

16. Doppler radar display showing a line of thunderstorms bent in the shape of a bow (Red, orange, and yellow)

17. Supercell near Spearman, TX has a tornado extending downward from its base

18. Some of the features of a classic supercell thunderstorm, viewed from southeast

19. Diagram of the thunderstorm from above, looking down on the storm; shaded red: updraft; shaded gray: downdraft

20. Severe Thunderstorms – contd. • Dry Line (dew-point fronts): A zone of instability along which thunderstorms form; dew point temp may drop along this boundary by as much as 9°C/km • Mesoscale Convective Complexes: A large organized convective weather system comprised of a number of individual thunderstorms; size of an MCC ~ 1000 times larger than individual thunderstorm

21. Surface conditions that can produce a dryline with severe thunderstorms; A developing mid-latitude cyclone with a cold front, warm front, and three distinct air masses (cP, cT & mT)

22. IR image showing a Mesoscale Convective Complex extending from central Kansas across western Missouri

23. Floods & Flash Floods • Flash floods: Floods that rise rapidly with little or advance warning; results when thunderstorms stall or move slowly, causing heavy rainfall over a relatively small area • Causes for Flash Floods: • Thunderstorms stall or move slowly • Thunderstorms move very quickly but keep passing over the same area (phenomenon called ‘training’) • Heavy rain and melting of snow taking place in spring • Torrential rains from tropical storms Summer of 1993 rain in the upper Midwest caused the worst flood 6.5 billion $ crop lost; 43 human lives; 45,000 homes were lost; evacuation of 74,000 people

24. Distribution of Thunderstorms • >40,000 thunderstorms/day (14 millions/yr) in the world • 14 million/year • Conducive conditions for thunderstorm formation: Combination of warmth and moisture • Where thunderstorms are prevalent: i) Southeastern states along the Gulf Coast with a maximum in Florida (mainly during summer); ii) Central Rockies; iii) Over water along the intertropical convergence zone where the low-level convergence of air helps to initiate uplift • Where thunderstorms are rare: i) Dry regions such as polar regions and the desert areas of the subtropical highs; ii) Pacific coastal and interior valleys

25. Average number of days each year on which thunderstorms are observed in US; mountainous west has sparse data

26. Average number of days each year hail observed

27. Thunderstorms and Lightning • Lightning: A giant spark discharging electricity that occurs in mature thunderstorms; can take place within a cloud, from one cloud to another, cloud to surrounding air or cloud to ground (~20%); 80% within the clouds • Lightning stroke can heat the air surrounding it to 30,000°C which in turn causes the air to expand, thus initiating a shock wave that becomes a booming sound wave-thunder • Light travels faster than sound (345 m/s @25 °C) • Time difference between the light and sound can be utilized to determine how far away the stroke took place

28. Lightning & Thunder – contd. • Close distance lightning: Clap sound or crack followed immediately by a loud bang • Farther away: rumbling sound due to sound emanating from different areas of the stroke • Lightning, but no thunder: Thunder waves were refracted and the sound waves got attenuated, making the thunder inaudible • Sonic boom: Produced when an aircraft exceeds the speed of sound at the altitude at which it is flying • Condition for lightning to occur: Separate regions containing opposite electrical charges must exist within the cumulonimbus cloud

29. Electrification of clouds • Several theories to explain the formation of lightning • When hail fall through supercooled droplets, the droplets freeze and release latent heat; this heat warms the hailstone; contact of warmer hailstone and colder ice crystal leads to a net transfer of positive ions from the warmer object to the colder object --- hailstone is negatively charged and ice crystals +ively charged • Positively charged ice particles carried to the upper part of the cloud by updrafts & larger haldstones with –ive charge fall toward the bottom of the cloud • Cold, upper part becomes +ively charged & middle of the cloud becomes –ively charged

30. Electrification of the Clouds – contd. • Another school of thought: Regions of separate charge exist within tiny cloud droplets and larger precipitation particles during the formation of precipitation --- Negative charge in the upper part of these particles & +ive charge in the lower part of the particles --- when falling precipitation collides with smaller particles, larger precipitation particles become negatively charged and the smaller particles positively charged --- updraft sweeps smaller sized particles leading to net +ive charge

31. Generalized charge distribution in a mature thunderstorm

32. The Lightning Source • Negative charge at the bottom of the cloud causes a region of the ground beneath to become +ively charged; as the thunderstorms move, the positive charge moves along with it; the positive charge is most dense on protruding objects; charge separation causes electric field existence; electric potential difference between cloud and the ground --- when electric potential builds up, current flow results and lightning occurs • Cloud-to-ground lightning begins when the localized electric potential gradient >3 million volt/m --- leads to the discharge of electrons toward the cloud base and then to the ground

33. Lightning source – contd. • Stepped Leader: An initial discharge of electrons that proceeds intermittently toward the ground in a series of steps in a cloud-to-ground lightning stroke • Return Stroke: The luminous lightning stroke that propagates upward from the earth to the base of a cloud • Dart Leader: Discharge of electrons that proceeds intermittently toward the ground along the same ionized channel taken by the initial lightning stroke • Different types of Lightning: Forked lightning (crooked or forked in shape), ribbon lightning (ribbon hanging in the cloud), bead lightning (series of beads tied to a string), ball lightning (sphere appears to float in the air) & sheet lightning (cloud appears like a white sheet)

34. Development of stepped leader: when –ive charge near the bottom of the cloud becomes large enough to overcome air’s resistance, flow of electrons rushes to the earth

35. As the electrons approach the ground, a region of + charge moves up into the air through any conducting object, such as trees, buildings

36. When the downward flow of electrons meets the upward surge of +ive charge, a strong electric current – a bright return stroke – carries +ve charge upward into the cloud

37. Lightning rod extends above the building: when lightning strikes, it follows an insulated conducting wire into the ground

38. Four marks on the road surface represent areas where lightning, after striking a car entered the roadway; 3 tires were flattened

39. Lightning Detection & Suppression • Heat Lightning: Distant lightning from thunderstorms that is seen, but not heard • As the electric potential near the ground increases, a current of +ive charge moves up pointed objects, such as antennas • Lightning rods (made of metal with a pointed tip) are placed that extend well beyond the height of the structure • Lightning Direction-finder: It detects the direction of lightning by measuring the radiowaves produced by lightning

40. Damages by lightning in US • 10,000 fires/yr in US are started by lightning • 50 million $ worth of timber is destroyed per yr • Can we reduce the cloud-to-ground lightning?? Seeding cumulonimbus clouds with hair-thin pieces of Al wire (10-cm long) --- metal will produce many tiny sparks and prevent the electrical potential in the cloud from building to a point where lightning occurs

41. Tornadoes • Tornadoes: A product of thunderstorms; rapidly rotating winds that blow around a small area of intense low pressure • Tornadoe’s circulation is present on the ground either as a funnel-shaped cloud or as a swirling cloud of dust & debris; majority rotate counterclockwise • Other shapes: • Twisting ropelike funnels • Cylindrical-shaped funnels • Massive black funnels • Funnels that resemble an elephant’s trunk hanging from a large cumulonimbus cloud

42. Tornadoes – Features and stages • Diameter (most): 100-600 m (few meters – 1,600 m rare) • Most last only a few minutes & average path length of ~7 km (largest one: 470 km; lasted for 7 hrs) in Illinois and Indiana in 1917 • Stages of a Tornadoes (most common): • Dust-Whirl stage: Dust swirling upward from the surface – damage is light • Organizing Stage: Tornado increases in intensity with an overall downward extent of the funnel • Mature Stage: funnel reaches its greatest width & is almost vertical; damage is most severe • Shrinking stage: Overall decrease in the funnel’s width & increase in the funnel’s tilt; still capable of intense &

43. Tornadoes – Features and stages • Sometimes violent damage • Decay Stage: The final stage, usually finds the tornado stretched into the shape of a rope Minor tornadoes may evolve only through certain stages Damages: ~ 100 people/year killed (11/10/02 – 37 people died on a single day) 45% mortalities in mobile homes March 18, 1925 tornadoes: 695 people died, 7 tornadoes traveled a total of 703 km across portions of Missouri, Illinois and Indiana

44. Tornado outbreak • Tornado Outbreak: A series of tornadoes that forms within a particular region, often associated with widespread damage and destruction; a region may include several states • April 3, 1974: 16 hour period, 148 tornadoes cut through parts of 13 states, 307 people killed, >3700 people injured, damage >600 million $ • Occurrence: Most numbers in US; average: >1,000/yr; 1,424 during 1998 • Tornado alley: Tornado belt, Central Plains, stretches from central Texas to Nebraska

45. A mature tornado with winds >150 knots rips through southern illinois

46. Tornado incidence by State; upper:number by each state (25 yrs); lower: average annual number/100,000 square miles; darker: greater frequency

47. Tornadoes and their impact • Lifting railroad coach with 117 passengers and dumping it 25 m away • Schoolhouse was demolished and 85 students inside were carried over 100 m without one of them being killed • Most tornadoes have winds of less than 125 knots • Pressure in the center of a tornado may be more than 100 mb lower than the surrounding & there is a momentary drop in outside pressure when tornado is above a structure • When confronted with a tornado, take shelter immediately (basement, stay away from windows, small bathroom, closet, interior hallway)

48. Fujita scale for damaging wind • .

49. Fujita Scale – contd. • Fujita Scale: Theodore Fujita in late 1960s --- classifying tornadoes according to their rotational wind speed based on the damage done by the storm • Majority of tornadoes are F0 and F1 (weak ones) and only a few % are above the F3(violent) with ~ 1 F5/yr • Tornado Formation: Tornadoes tend to form with intense thunderstorms and a conditionally unstable atmosphere is essential for their development • Most strong and violent tornadoes develop near the right rear sector of a severe thunderstorm • In order for a tornado to spawn a tornado, the updraft must rotate

50. Total wind speed of a tornado is greater on one side than on the other