Transcript Document

Chapter 11
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 singlecell 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