Thunderstorms and Volcanic clouds—analogies Ashfall Grad Class Fall 2009 lecture #11

1. Thunderstorms and Volcanic clouds—analogiesAshfall Grad Class Fall 2009 lecture #11

2. Thunderstorms • Definition: a storm containing lightning and thunder. • Associated with midlatitude cyclones, localized convection, orographic lifting and tropical cyclones.

3. Thunderstorm Formation • Ingredients • warm, moist air (often mT) • unstable (or conditionally unstable if lifting mech.) • encouraged by diverging air aloft

4. Thunderstorm Life Cycle

5. Thunderstorm Life Cycle • Towering Cumulus Stage • Cumulus clouds build vertically and laterally, and surge upward to altitudes of 8,000-10,000 m (26,000-33,000 ft) over a period of 10-15 minutes • Produced by convection within the atmosphere • Free convection – triggered by intense solar heating of Earth’s surface • Generally not powerful enough to produce thunderstorms • Forced convection – orographic uplift or converging winds strengthen convection • This is generally the cause of thunderstorms • Latent heat released during condensation adds to buoyancy • During the cumulus stage, the updraft is strong enough to keep water droplets and ice crystals suspended • As a result, precipitation does not occur in the cumulus stage American Meteorological Society Education Program

6. Thunderstorm Life Cycle • Mature Stage – maximum intensity • Stage typically lasts about 10-20 minutes • Begins when precipitation reaches Earth’s surface • Features heaviest rain, frequent lightning, strong surface winds, and possible tornadoes • Weight of droplets and ice crystals overcome the updraft • Downdraft created when precipitation descending through the cloud drags the adjacent air downward • Entrained dry air at the edge of the cloud leads to evaporative cooling, which weakens the buoyant uplift and strengthens the downdraft • At the surface, the leading edge of downdraft air resembles a miniature cold front and is called a gust front • Ominous-appearing low clouds associated with a gust front include a roll cloud and a shelf cloud American Meteorological Society Education Program

7. Thunderstorm Life Cycle • Dissipating Stage • Precipitation and the downdraft spread throughout the thunderstorm cell, heralding the cell’s demise • Subsiding air replaces the updraft and cuts off the supply of moisture • Adiabatic compression warms the subsiding air and the clouds gradually vaporize American Meteorological Society Education Program

8. Thunderstorm Classification NOAA classification of thunderstorms, and the likelihood of severe weather. American Meteorological Society Education Program

9. Thunderstorm Classification • Thunderstorms are meso-scale convective systems (MCS) and are classified based on the number, organization, and intensity of their constituent cells • Single-cell thunderstorms • Usually a relatively a weak system forming along a boundary within an air mass (i.e., gust front) • Typically completes its life cycle in 30 minutes or less • Multicellular thunderstorms • Characterizes most thunderstorms. Each cell may be at a different stage in its life cycle, and a succession of cells is responsible for a prolonged period of thunderstorm weather. • Two types: • Squall line • Mesoscale convective complex • Either can produce severe weather American Meteorological Society Education Program

10. Thunderstorm Classification A thunderstorm may track at some angle to the path of its constituent cells, complicating the weather system motion. In the above idealized situation, the component cells of a multicellular thunderstorm travel at about 20 degrees to the eastward moving thunderstorm. As they travel toward the northeast, the individual cells progress through their life cycle.

11. Thunderstorm Classification • Multicellular thunderstorm types • Squall line – elongated cluster of thunderstorm cells that is accompanied by a continuous gust front at the line’s leading edge • Most likely to develop in the warm southeast sector of a mature extra-tropical cyclone, ahead of and parallel to the cold front • Mesocyclone convective complex (MCC) • A nearly circular cluster of many interacting thunderstorm cells with a lifetime of at least 6 hrs, and often 12-24 hrs • Thousands of times larger than a single cell • Primarily warm season phenomena (March – September) • Usually develop at night over the eastern 2/3 of the U.S. • Is not associated with a front • Usually develops during weak synoptic-scale flow, often develops near an upper-level ridge of high pressure, and on the cool side of a stationary front • A low level jet feeds warm humid air into the system • Supercell thunderstorms are long-lived single cell storms • Exceptionally strong updraft, with rotational circulation that may evolve into a tornado

12. Thunderstorm Classification Radar image of a squall line stretching from Texas to Illinois Infrared satellite image showing meso-scale convective complexes over western Kansas and most of Arkansas

13. The Geography of Thunderstorms Frequency decreases with distance from equator. None above 60o Most occur during summer’s warm temperatures.

14. The Geography of Thunderstorms

16. Thunderstorm Frequency • Probably 1500 to 2000 thunderstorms active around the world at any given time.

17. Severe Thunderstorms • A severe thunderstorm is accompanied by locally damaging surface winds, frequent lightning, or large hail • Surface winds stronger than 50 kts (58 mph) and/or hailstones 0.75 in. (1.9 cm) or larger in diameter • May also produce flash floods or tornadoes • What causes some thunderstorms to be severe? • Key is vertical wind shear, the change in horizontal wind speed and direction with increasing altitude • Weak vertical wind shear favors short-lived updrafts, low cloud tops, and weak thunderstorms • Strong vertical wind shear favors vigorous updrafts, great vertical cloud development, and severe thunderstorms • With increasing vertical wind shear, the inflow of warm humid air is sustained for a longer period because the gust front cannot advance as far from the cell. Also, most precipitation falls alongside the titled updraft, sustaining the updraft. American Meteorological Society Education Program

18. Severe Thunderstorms A synoptic weather pattern that favors development of severe thunderstorms. A dryline is the western boundary of the mT air mass and brings about uplift in a manner similar to a cold front. American Meteorological Society Education Program

19. Severe Thunderstorms • The polar front jet stream produces strong vertical wind shear • This maintains a vigorous updraft • This supports great vertical development of thunderstorms • The jet contributes to stratification of air that increases the potential instability of the troposphere • A jet streak induces both horizontal divergence and convergence of air in the upper troposphere • Convergence occurs in the right front quadrant of a jet streak, causing weak subsidence of air • Sinking air is compressionally warmed and forms an inversion (capping inversion) over the mT air mass • The underlying air mass becomes more humid • Contrast between air layers mounts • All that is needed is a lifting mechanism for severe weather to occur American Meteorological Society Education Program

20. Severe Thunderstorms A temperature sounding that favors the development of severe thunderstorm cells. A capping inversion separates subsiding dry air aloft from warm, humid air near the surface. American Meteorological Society Education Program

21. Severe Thunderstorms Mammatus clouds occur on the underside of a thunderstorm anvil and sometimes indicate a severe storm system. Their appearance is caused by blobs of cold, cloudy air that descend from the anvil into the clear air beneath the anvil. American Meteorological Society Education Program

22. Thunderstorm Hazards • Lightning • A brilliant flash of light caused by an electrical discharge within a cumulonimbus cloud or between the cloud and Earth’s surface • Direct hazard to human life • Ignites forest and brush fires • Very costly to electrical utilities • Lightning detection network provides real-time information American Meteorological Society Education Program

23. Thunderstorm Hazards • Lightning, continued • What causes lightning? • Large differences in electrical charge develop within a cloud, between clouds, or between a cloud and the ground • Upper portion and much smaller region of the cumulonimbus cloud become positively charged, with a disk-shaped zone of negative charge in between. A positive charge is induced on the ground directly under the cloud • Lightning may forge a path between oppositely charged regions • Charge separation within a cloud may be due to collisions between descending graupel striking smaller ice crystals in their path. At temperatures < -15 °C (5 °F) graupel become negatively charged while ice crystals become positively charged. Vigorous updrafts carry ice crystals to upper portions of the cloud. • Positive charge near cloud base also due to graupel-ice crystal collision, but temps > -15 °C (5 °F) induce positive charge to graupel and negative charge to ice crystals American Meteorological Society Education Program

24. Ice plays a vital role in lightning generation

25. Thunderstorm Hazards • Lightning, continued • A cloud-to-ground lightning flash involves a regular sequence of events • Stepped ladders: streams of electrons surge from the cloud base to the ground in discrete steps • Return stroke: forms as an ascending electric current when the positive and negative charges recombine; often emanates from tall, pointed structures • Dart leaders, subsequent surges of electrons from the cloud, follow the same conducting path • Sequence takes place in < two-tenths of a second • Lightning causes intense heating of air so rapidly that air density cannot initially respond • Shock wave is generated and propagates outward, producing sound waves heard as thunder • Flash-to-bang method: Thunder takes about 3 seconds to travel 1 km (or 5 seconds to travel 1 mi) • If you must wait 9 seconds between lightning flash and thunderclap, the lightning is about 3 km (1.8 mi) away American Meteorological Society Education Program

26. Thunderstorm Hazards - Lightning American Meteorological Society Education Program

27. Thunderstorm Hazards • Downbursts • Exceptionally strong downdrafts that occur with or without rain • Starburst pattern causes ground destruction • Also very dangerous to aircraft because they trigger wind shear • Aircraft have warning systems that use the same principle as Doppler radar • A macroburst cuts a swath of destruction > 4 km (2.5 mi) wide with surface winds that may top 210 km per hr (130 mph) • A microburst is smaller and shorter lived • Derecho: a family of straight-line downburst winds that may be hundreds of kilometers long; sustained winds in excess of 94 km per hr (58 mph) American Meteorological Society Education Program

28. Thunderstorm Hazards • Flash Floods • Short-term, localized, often unexpected rise in stream level usually in response to torrential rain falling over a relatively small geographical area • Caused by excessive rainfall in slow moving or stationary thunderstorm cells • Atmospheric conditions that favor flash floods: • More common at night and form in an atmosphere with weak vertical wind shear and abundant moisture through great depths • Precipitation efficient atmosphere has high values of precipitable water and relative humidity and a thunderstorm cloud base with temperatures above freezing American Meteorological Society Education Program

29. Hail • Formation • Largest? Coffeyville, KS, 1970 (1.75 lb, 14 cm diameter)

30. Thunderstorm Hazards • Hail • Frozen precipitation in the form of balls or lumps of ice > 5 mm (0.2 in.) in diameter, called hailstones • Almost always falls from cumulonimbus clouds that are characterized by a strong updraft, great vertical development, and an abundance of supercooled water • Develops when an ice pellet is transported vertically through portions of the cloud containing varying concentrations of supercooled water droplets • Composed of alternating layers of glaze and rime • Grows by accretion (addition) of freezing water droplets and falls out of cloud base when it becomes to large and heavy to be supported by updrafts American Meteorological Society Education Program

31. Accretionary Lapilli are perhaps best explained by a process which involves vertically developed clouds and ice, especially hail and graupel.

32. Likely microphysical processes and particles in volcanic cloud. C Textor et al., 2006, JVGR 150: 359-373.

33. Thunderstorm Hazards • Hail, continued • May accumulate on the ground in a long, narrow strip known as a hailstreak; typically 2 km (1.2 mi) wide and 10 km (6.2 mi) long • The figure below is a model of hailstreak development American Meteorological Society Education Program

34. Tornadoes • About 10% of the annual 10,000 U.S. severe thunderstorms produce tornadoes • A tornado is a violently rotating column of air in contact with the ground • Most are small and short-lived and often strike sparsely-populated regions • The most prolific tornado outbreak on record occurred over the Great Plains and Midwest on 29-30 May 2004 • More than 180 tornadoes were reported

35. Lightning • discharge of electricity that occurs in mature thunderstorms • Cause: charge separation in cloud sets up electrical potential • Role of lightning is to equalize these differences in electrical potential. • Important fixer of Nitrogen.

36. Stepped leader Upward leader Return stroke Electrons down Protons up Circuit complete Repeats every few microseconds with new leader.

37. USA: Real-time Lightning http://www.weather.com/

38. Overshooting Top

39. Overshooting Top • Overshooting top - characteristic of a strong updraft • The updraft goes higher than the rest of the clouds near it (in the anvil) • Overshoots the tropopause or equilibrium level btwn the troposphere & stratosphere • Updraft penetrates stratosphere and then is forced back down to equilibrium level American Meteorological Society Education Program

41. Supercell Thunderstorms • A supercell thunderstorm is a t.s. with a deep rotating updraft (mesocyclone) • Updraft elements usually merge into the main rotating updraft and then accelerate rapidly • Flanking updrafts "feed" the supercell updraft, rather than compete with it • Small percentage of all t.s.’s are supercells but they cause the majority of damage

42. Diagram of a Supercell

43. A Look from the SE

44. Umbrella Cloud High winds at 10-11 km height made the volcanic cloud spread like a mushroom

45. GRL 32 no L24808 GRL 32 L24808 2005

46. Features of Supercells • Mesocyclone (p.125) organizes updraft and downdraft and keeps them separate • Updraft is slanted downwind (aloft) so hail/rain doesn’t fall through it and kill it • Supercell can last for hours and travel a hundred plus miles • Often moves to the right of the mean flow - has to do with rotation (vorticity) and propagation • What does propagation mean?

47. How Supercells Move • Movement = Advection + Propagation • This little formula applies to pretty much everything in weather • advection = just the horizontal transport of the feature (like a supercell) along with the winds • propagation = development of the feature (usually happens towards inflow or flanking line in the case of a supercell) American Meteorological Society Education Program

48. Mammatus

49. Schultz et al., 2006, J Atmos Sci 63: 2409-2435

50. Schultz et al., 2006, J Atmos Sci 63: 2409-2435