1. Thunderstorms and Volcanic clouds—analogies Ashfall 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 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
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6. Thunderstorm Life Cycle
Mature Stage – maximum intensity
Stage typically lasts about 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
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7. Thunderstorm Life Cycle
When the upward billowing cumulonimbus cloud reaches the tropopause, it spreads out forming a flat anvil top. Cloud tops during the mature stage can build to altitudes in excess of 18,000 m (about 60,000 ft).
Thunderstorm cells can develop along the gust front ahead of the main thunderstorm. The radar image shows up as an arc-shaped band.
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8. Thunderstorm Life Cycle
In this visible satellite image, clusters of intense thunderstorm cells appear as bright white blotches over portions of Texas, Oklahoma, and Missouri
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9. 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
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10. Thunderstorm Classification
NOAA classification of thunderstorms, and the likelihood of severe weather.
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11. 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
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12. 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.

13. 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 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

14. 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

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

16. The Geography of Thunderstorms

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

19. 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.
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20. Severe Thunderstorms American Meteorological Society Education Program
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.
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21. 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
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22. Severe Thunderstorms American Meteorological Society Education Program
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.
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23. Severe Thunderstorms American Meteorological Society Education Program
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.
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24. 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
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25. 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
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26. Ice plays a vital role in lightning generation

27. 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
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28. Thunderstorm Hazards - Lightning
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29. Thunderstorm Hazards American Meteorological Society Education Program
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)
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30. 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
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31. Hail
Formation
Largest? Coffeyville, KS, (1.75 lb, 14 cm diameter)

32. 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
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33. Accretionary Lapilli are perhaps best explained by a process which involves vertically developed clouds and ice, especially hail and graupel.

34. Likely microphysical processes and particles in volcanic cloud.
C Textor et al., 2006, JVGR 150:

35. 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
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36. 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 May 2004
More than 180 tornadoes were reported

37. 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.

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

39. USA: Real-time Lightning

40. Overshooting Top

41. 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
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43. 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

44. Diagram of a Supercell

45. A Look from the SE

46. Umbrella Cloud
The volcanic cloud had a distinct morphology, with a column feeding an umbrella cloud. There was very high winds which drove the cloud eastward and it spread over eastern Washington in a few hours.
High winds at km height made the volcanic cloud spread like a mushroom

47. Recent work by Baines and Sparks calls attention to the fact that large ignimbrite eruptions will lead to giant volcanic clouds because of the large size of ignimbrite outflow sheets and the large amounts of bouyant fine ash. This probably helps to explain why the ashfall deposits are so extensive--note the comparison between MSH 1980 and the Bishop and Yellowstone deposits.
GRL 32 no L24808
GRL 32 L

48. 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?

49. 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)
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50. Mammatus

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

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

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

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

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

56. Schultz et al., 2006, J Atmos Sci 63: 2409-2435
Cloud polarization lidar of cirrus mammatus UTC 10 Sept 1994, Salt Lake City
Schultz et al., 2006, J Atmos Sci 63:

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

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

59. Fully glaciated volcanic cloud with abundant CCN
Above Ephrata on 18 May
Photo by Douglas Miller
Density of cloud increases as a whole
Latent heat effects significant in rise and possibly in fall
Bright band effects during descent (thawing)
Overall sublimation/evaporation
Quite different from a thunderstorm

60. Meteorological Cloud Volcanic Cloud
Durant et al., 2008, JGR 113
Few IN
Bergeron
Large Ice HM
Precipitation
Many IN
Small ice HM
Little Precip
Sublimation
Meteorological Cloud Volcanic Cloud

61. Virga – NOAA photo

62. Virga -- Australia Severe Weather

63. Conceptual Model: Distal Fallout Durant et al., 2009, JGR