Chapter 11Chapter 11

Thunderstorms and Tornadoes

Thunderstorms and Tornadoes

Figure CO: Chapter 11, Thunderstorms and Tornadoes

© Dobresum/ShutterStock, Inc.

What is a thunderstorm?• A cloud or a cluster of clouds that produces

thunder, lightning, heavy rain, and sometimes hail and tornadoes

• Tall cumulonimbus clouds– Form when air rises or is lifted from the surface or

near the surface– Energy is released when saturated air condenses

moisture– Air rises rapidly and to great heights—the anvil cloud

• Overshooting tops briefly overshoot the tropopause• Mammatus cloud may form beneath the anvil

Figure 01: Photo of thunderstorm

Courtesy of Dr. John R. Mecikalski

Figure 02A: Visible satellite image of thunderstorms.

Courtesy of CIMSS/SSEC/University of Wisconsin-Madison

Figure 02B: Infrared image of thunderstorms.

Courtesy of CIMSS/SSEC/University of Wisconsin-Madison

Figure 03: World thunderstorm climatology.

Adapted from WMO (World Meteorological Organization), 1956: World Distribution of Thunderstorm Days. WMO Publ. No. 21, TP. 21.

Figure 04: U.S. thunderstorm climatology

Courtesy of Oklahoma Climatological Survey

Conditions for Thunderstorm Formation

• A warm moist air mass at or just above the surface – High dew-point temperature– Maritime tropical air mass

• A deep layer of conditional instability– Saturated air parcels rise freely– In the tropics this is the average condition– Stability of an air mass can change

Conditions for thunderstorm formation (continued)

• How stability changes (becomes less stable or more unstable)– Warm air advection at low levels– Cold air advection at upper levels– Lifting of a stable layer that is humid at its base

and dry at its top (convective or potential instability)

• Also described a blowing a capping inversion

– Surface heating

Conditions for thunderstorm formation (continued)

• Lifting mechanisms (more than one may be active) to get air parcels saturated– Free convection from buoyancy due to surface

heating• Common in warm season• Thermals, then cumulus clouds

– Forced lifting from topography• Many thunderstorms occur on the upwind side of

mountains

Conditions for thunderstorm formation (continued)

• Frontal lifting– Especially cold fronts– Also drylines

• Convergence– Sea breezes converging in central Florida– Low pressure centers in the tropics– Hurricane is the largest collection of thunderstorms

• Nocturnal low-level jet (just above the surface)– An important ingredient in severe weather– Supplies moisture and energy at low levels

Lifted Index• Is a way to describe stability with one number• Is not a perfect measure of stability• Is useful for forecasting, but not the only criterion

forecasters use• Is a difference in temperature at 500mb• The parcel temperature is determined by raising

a surface parcel to saturation at the DALR, and to 500mb at the SALR

• LI = T(environment) – T(air parcel)• Negative values are unstable

Figure T01: Lifted Index, Stability, and Possible Weather

Figure 05: Satellite image of lifted index and severe weather reports

Courtesy of CIMSS/SSEC/University of Wisconsin-Madison

Conditions for thunderstorm formation (continued)

• For the more severe thunderstorms, vertical shear of the horizontal wind– The westerly (west to east) part of the wind

increasing as height increases– Clockwise turning of the direction from which the

wind blows as height increases• For example, southeasterlies at the surface, southerlies

at 850mb, southwesterlies at 700mb, and westerlies at 500mb

Figure 06: Severe weather schematic

Adapted from Athrens, C.D. Meteorology Today, Ninth edition. Brooks Cole, 2009

Figure 07: LANDSAT image of tornado destruction

Courtesy of USGS EROS Data Center with processing by Environmental Remote/Landsat-7

Thunderstorm Cells

• A cell is a compact region of a cloud that has a strong vertical updraft– Ordinary cells are a few km in diameter, last less

than an hour– Supercells are larger and can last several hours

• Account for the vast majority of severe thunderstorm weather

• Multicell thunderstorms are composed of lines or clusters of thunderstorm cells, ordinary, supercell, or both

Figure T02: The three thunderstorm types

Ordinary Single-Cell Thunderstorm

• Life cycle has three distinct stages– Cumulus—parcels ascend in the updraft and get

saturated at the lifting condensation level or LCL, which marks cloud base

• Mixing with environment air is called entrainment– Mature—begins when precipitation starts to fall

• Time of most lightning, rain, small hail• A downdraft develops with cooling due to evaporating

precipitation– Dissipating—updraft weakens, downdraft dominates

• Also known as the air mass thunderstorm

Figure 08: Thunderstorm life cycle

Figure 09: Photo of ordinary thunderstorm

Courtesy of Tim Webster

Multicell Thunderstorms

• Composed of several individual single-cell storms, each one at a different stage of development– Can last several hours– A moderate amount of vertical wind shear

• Updraft and downdraft can coexist• Updraft and downdraft meet at the gust front

– Groups of multicell thunderstorms are called mesoscale convective systems

Figure 10: Schematic of a multicell thunderstorm

Squall Lines

• A squall line is composted of individual intense thunderstorm cells arranged in a line or band– Occur along a boundary of unstable air– Have life spans of 6 to 12 hours or more– Extend over several states simultaneously– A shelf cloud is often observed above the gust front– Often observed ahead of a cold front– Divergence aloft and a broad, low-level inflow of

moist air favor development of squall lines

Figure 11A: Radar image of squall line

Courtesy of NWS/NOAA

Figure 12: Photo of shelf cloud

© Peter Wollinga/Dreamstime.com

Mesoscale Convective Complex (MCC)

• An MCC is a complex of individual storms that covers a large area in an infrared satellite image and lives more than 6 hours– Often form in late afternoon and evening– In satellite images give the appearance of a large

circular storm with cold cloud-top temperatures– Often form underneath a ridge of high pressure

• Because upper-level divergence can occur in a ridge– Do not require as much vertical shear as squall lines– Can be maintained by the low-level jet

Figure 13: Satellite image of MCC

Courtesy of SSEC and CIMSS, University of Wisconsin-Madison

Figure 14: Time-lapse MCC schematic

Supercell Thunderstorms

• The supercell thunderstorm is a large single-cell storm, sometimes 32 km or more across, that almost always produces dangerous weather– Strong wind gusts, large hail, dangerous lightning

and tornadoes– Require a very unstable atmosphere– Require both directional and speed shear

• Vertical wind shear causes supercell thunderstorms to rotate about a vertical axis

Figure 15: Hand and pencil explanation of supercell thunderstorm rotation

Figure 16: Supercell thunderstorm, from the side

Courtesy of NOAA

Cyclonic Right-Moving Supercell

• The spinning updraft is called a mesocyclone– Resembles an extratropical cyclone– 5 to 20 km across– Narrows and rotates more quickly when it stretches– Is too large and too slow in rotation to be a tornado– Has an overshooting cloud top– Has two downdrafts ahead of (forward flank) and

behind (rear flank) the center of the storm– Downdrafts caused by precipitation– Gust fronts ahead of the downdrafts

Figure 17: The surface conditions associated with a typical supercell thunderstorm

Courtesy of Oklahoma Climatological Survey

Microbursts

• Microbursts develop when rain falling from a thunderstorm evaporates underneath the cloud, cooling the air beneath– Cold heavy air plunges to the surface and splashes

against the ground– Air then rushes sideways and swirls upward as a

result of the pressure gradient between the cold air and the warm surroundings

• Microbursts can do as much damage as a small tornado

Figure 18ab: Microburst

Courtesy of Mike Smith, www.mikesmithenterprises.com

Figure 18cde: Microburst

Courtesy of Mike Smith, www.mikesmithenterprises.com

Figure 19: Flightpath of an airplane through a microburst

Figure 20: Ronald Reagan landing before a microburst

Adapted from Fujita, T. The Downburst. University of Chicago Press, 1985.

Tornadoes

• Tornadoes are rapidly rotating columns or funnels of high wind that spiral around very narrow regions of low pressure beneath a thunderstorm– Visible because of condensation, dust, and debris– Nearly always rotate cyclonically, often move to

the northeast– If the circulation does not reach the ground, called

a funnel cloud– Usually < 1.6 km across

Figure 21: Girl in front of tornado

Courtesy of Marilee Thomas

Figure B01: Don Lloyd photo of tornado

© Cailyn Lloyd

Tornado Formation

• Most form underneath supercell thunderstorms• The cloud base underneath the updraft on the

rear side of the thunderstorm may lower– Forming a rotating wall cloud

• A rapidly rotating column of air much smaller than the mesocyclone may protrude beneath the wall cloud

• As water vapor condenses in the air rushing up into this column, a funnel cloud may form and reach the ground, becoming a tornado

Figure 22: The ominous approach of a rotating wall cloud is a sign that a tornado may develop at any moment.

Courtesy of Nolan T. Atkins

Tornado Life Cycle

• Four stages in the life cycle– Organizing stage, funnel picks up debris, as it

reaches the surface and widens– Mature stage, tornado at peak intensity and width– Shrinking stage, the funnel narrows– Decaying or rope stage, when the funnel thins out

to a very narrow ropelike column after which it eventually dissipates

Figure 23: The life cycle of the Union City, Oklahoma, tornado on May 24, 1973

Modified from Golden, J. H., and D. Purcell, Mon. Wea. Rev., 106 [1978]: 3–11.

Tornado Signatures on Radar

• Hook echo– The pattern of heavy rain inside a supercell forms a

kind of hook around the region most likely to produce a tornado

• Tornado vortex signature– A couplet of red and green colors on Doppler radar

images• Mesocyclone signature

– A couplet of red and green colors on Doppler radar images larger than the tornado vortex signature

Figure 24A: Radar reflectivity image of Enterprise, AL tornado

Courtesy of NCDC/NOAA

Figure 24B: Radar velocity image of Enterprise, AL tornado

Courtesy of NCDC/NOAA

Tornado Winds and the EF Scale

• The Enhanced Fujita (or EF) scale ranges from 0 to 5, with 5 the most damage

• The scale uses 28 damage indicators, like schools, barns and vegetation and the damage to each helps place the tornado on the scale

• The higher the scale number, the more severe are the tornado’s wind and damage

• Some of the most severe tornadoes are multiple vortex tornadoes

Figure T03: The Enhanced Fujita Scale for Tornadoes

Figure 25: Fujita scale pie charts

Courtesy of Tom Grazulis, tornadoproject.com

Figure 26: Multiple-vortex tornado

Modified from Tom Grazulis, The Tornado: Nature’s Ultimate Windstorm, Oklahoma University Press, 2000, p. 111.

Figure 27: U.S. tornado climatology

Courtesy of Oklahoma Climatological Survey

Figure 28: A climatology of the relative frequency of killer tornado events from 1950 to 2004.

Courtesy of Dr. Walker Ashley, Meteorology Program, Department of Geography, Northern Illinois University

Figure 29: Time of year of maximum tornado risk

Adapted from H. E. Brooks et al., Wea. Forecasting, 4 (2003).

Figure 30: Line graph of tornado deaths per million Americans

Courtesy of Dr. Charles A. Doswell III, www.flame.org/~cdoswell/Tornado_essay.html

Figure B03: Photo of Parsons Company damage

Courtesy of NOAA/Matt Dayhoff, Peoria Journal Star

Figure 31: Tornado paths from Superoutbreak

Source: NOAA

Figure 32: Greensburg, KS devastation

Courtesy of Greg Henshall/FEMA

Figure 33: Path of Atlanta tornado in EF scale

Courtesy of National Weather Service Forecast Office, Peachtree City, GA/NOAA

Figure 34: Atlanta skyscraper post-tornado

Courtesy of Bruce Bracey www.flickr.com/photos/broo2/2358025514

The Waterspout

• Waterspouts are narrow spinning funnels of rising air that form underneath clouds– Usually shorter cumulus clouds that are not

rotating– Low pressure at the center sets up a pressure

gradient that drives air inward– Air rising and cooling condenses, making the

funnel visible– Strongest waterspouts only as strong as the

weakest tornadoes, < 160 km/hr

Figure 35: Waterspout

Courtesy of Dr. Joseph Golden/NOAA

Lightning

• Lightning is a huge electrical discharge• Lightning caused by rising and sinking air

motions that occur in mature thunderstorms– Can travel from cloud to cloud, within the same

cloud, or from cloud to ground– In-cloud discharges by far most common

• A lightning bolt is actually a series of flashes

Figure 36: Lightning

© Harald Edens, www.weatherscapes.com

Lightning Strikes a Tree• First, charge separates in the cloud

– Collisions of ice crystal with graupel• Second, the ground becomes positively charged

– The base of the cloud is negatively charged, and like charges repel

– The voltage between cloud and ground builds up• Third, lightning formation begins

– A pilot leader, then dart leaders of negative charge move down from the cloud

• Fourth, a brilliant flash is observed– Current flows upward in the return stroke

Figure 37A: Charges collect in the base of the cloud

Figure 37B: Negative charges build up near the base of the cloud, the ground repels negative charges and changes from its usual negative to a

positive charge.

Figure 37C: The stepped leader connects the cloud to the ground

Figure 37D: The bright return stroke surges upward

Figure 38: Color graphic of lightning climatology

Adapted from Orville, R., and Huffines, G., Monthly Weather Review, May 2001.

Figure 39: Schematic of upper-atmosphere lightning: elves, sprites, etc.

Courtesy of National Severe Storms Laboratory/NOAA

Flash Floods and Flooding

• A flood is a substantial rise in water that covers areas not usually submerged– Water flows into a region faster than it can be

absorbed, stored, or removed into a drainage basin– Caused by high-intensity rainfall, prolonged rainfall, or

both– Great threats to human life

• A flash flood is a sudden local flood that has a great volume of water and a short duration– Key elements: rainfall intensity and duration

Figure 40: Six Flags under water

© Erik S. Lesser/Landov

Hail• Hail is precipitation in the form of large balls or

lumps of ice• Hailstones begin as small ice particle• Hailstones grow by accretion of supercooled

water droplets• Dry growth occurs when the drops freeze on

contact—little liquid water on the surface• Wet growth occurs when the droplet don’t freeze

quickly and spread across the surface of the hailstone—a film of liquid water on the surface

Figure 41: Largest hailstone ever

Courtesy of NOAA

Producing Hailstones

• Production of large hailstones requires a strong updraft that is tilted and an abundant supply of supercooled water– Hail occurs in regions near the strong updraft– Supercell thunderstorms often produce the

largest hail– The curtain of hailstones that falls below cloud

base is called the hailshaft– The hailswath is the section of the ground covered

with hail

Figure 42: Graphic of hail occurrence across U.S.

Hailstorms Across the Nation by S. Channgon, D. Channgnon, and S. Hilbert, Image courtesy of the Midwestern Regional Climate Center, Illinois State Water Survey