Download Thunderstorms and Tornadoes

Thunderstorms and
Tornadoes
Chapter 14
Thunderstorms
• A storm containing lightning and thunder
– convective storms
• Severe thunderstorms (NWS def) one of following:
– large hail - ¾ in dia
– Surface wind gusts greater than or equal to 50kts(58mph)
– or produces a tornado
• Ordinary Cell Thunderstorms, usually simple, ―pop –up‖
– Air-mass thunderstorms: limited wind sheer
– May form at sea breeze fronts, topographic irregularities, outflow
boundaries
– Stages: cumulus, mature, dissipating
Stages of Development
Simplified model depicting the life cycle of an ordinary cell thunderstorm that is nearly stationary
as it forms in a region of low wind shear.
Mature Stage - As cloud droplets get heavier, they fall trough drier air, causing it to cool, and
become denser downdraft…process is called entriainment, and is enhanced by falling
precipitation.
Dissipating Stage – updrafts have weakened and downdrafts dominate…no longer ―fueled‖
• Multi-cell Thunderstorms
– Thunderstorms that contain a number of convection cells, each in a different
stage of development, moderate to strong wind shear
– Form when there is moderate to severe vertical wind shear
– Causes the convection cell to tilt, with updraft riding over downdraft
– Precip does not fall into updraft, so, fuel not cut off, and it lasts much longer
– Gust Front forms out ahead, with possible a shelf cloud
Figure 14.7
A dramatic example of a shelf cloud (or arcus cloud) associated with an intense
thunderstorm. The photograph was taken in the Philippines as the thunderstorm
approached from the northwest.
Figure 14.8
A roll cloud forming behind a gust front.
Figure 14.9
Radar image of an outflow boundary. As cool (more-dense) air from inside the severe
thunderstorms (red and orange colors) spreads outward, away from the storms, it comes
in contact with the surrounding warm, humid (less-dense) air, forming a density
boundary (blue line) called an outflow boundary between cool air and warm air. Along
the outflow boundary, new thunderstorms often form.
 Micro-bursts: localized downdraft(downburst) that hits
the ground and spreads horizontally in a radial burst of
wind 4km or less outward
 generate wind shear - rapid change in wind speed and
direction
Figure 14.11
Flying into a
microburst. At position
(a), the pilot
encounters a
headwind; at position
(b), a strong downdraft;
and at position
(c), a tailwind that
reduces lift and causes
the aircraft to lose
altitude.
– Squall-line thunderstorms
• line of multi-cell thunderstorms
• pre-frontal squall-line, form out ahead of advancing
cold front
Pre-frontal squall-line thunderstorms may form ahead of an advancing cold
front as the upper-air flow develops gravity waves downwind from the cold front.
Figure 14.14
A model describing air motions and precipitation associated with a squall line that has
a trailing stratiform cloud layer.
Figure 14.15
A side view of the lower half of a squall-line thunderstorm with the rear-inflow jet
carrying strong winds from high altitudes down to the surface. These strong winds
push forward along the surface, causing damaging straight-line winds that may reach
100 knots. If the high winds extend horizontally for a considerable distance, the wind
storm is called a derecho.
Figure 14.16
The red and orange on this Doppler radar image show an intense squall line moving
south southeastward into Kentucky. The thunderstorms are producing strong straightline winds called a derecho. Notice that the line of storms is in the shape of a bow. Such
bow echos are an indicator of strong, damaging surface winds near the center of the
bow. Sometimes the left (usually northern) side of the bow will develop cyclonic rotation
and produce a tornado.
 Meso-scale Convective Complex( MCC)
 a number of individual multi-cell thunderstorms grow in size and organize
into a large circular convective weather system
 Tend to form in summertime, with weak upper level winds
 Low level jet brings in moisture, and is at max late at night, early morning
 May cover entire states - 100,000km2
Figure 14.17
An enhanced infrared
satellite image showing
the cold cloud tops (dark
red and orange colors) of
a Mesoscale Convective
Complex extending from
central Kansas across
western Missouri. This
organized mass of
multicell thunderstorms
brought hail, heavy rain,
and flooding to this area.
• Supercell thunderstorms
– Large, long-lasting thunderstorm with a single
violent rotating updraft
– Strong vertical wind shear
– Outflow never undercuts updraft, updrafts may
exceed 90kts and can cause large sized hail
– 3 types:
 Classic,
 high precipitation
 low precipitation
FIGURE 14.19 Some of the features associated with a classic tornado-breeding supercell thunderstorm as viewed from the
southeast.
The storm is moving to the northeast.
A wall cloud photographed southwest of Norman, Oklahoma.
Conditions leading to the
formation of severe
thunderstorms, and especially
supercells.
The area in yellow shows where
supercell thunderstorms are
likely to form. Why?
- position of cold air above warm
creates conditionally unstable
atmosphere
- Strong vertical wind shear
induces rotation
Creates a rotating updraft and
sets stage for a tornado
Figure 14.22
A typical sounding of air temperature and dew point that frequently precedes the
development of supercell thunderstorms.
cap on instability at 800mb
cold dry air above means – convective instability
• Thunderstorms and the Dryline
– Sharp, horizontal change in moisture
– Thunderstorms form just east of dryline
cP
 cT
mT
Intense thunderstorms often can create flash flood
conditions especially if storms are ―training‖
Big Thompson Canyon
– July 31, 1976, 12 inches of rain in 4
hours created a flash flood associated
with $35.5million in damage and 135
deaths
• Distribution of Thunderstorms
– Most frequent Florida, Gulf Coast,
Central Plains
– Fewest Pacific coast and Interior valleys
– Most frequent hail Central Plains
Figure 14.26
The average number of days each year on which thunderstorms are observed
throughout the United States. (Due to the scarcity of data, the number of
thunderstorms is underestimated in the mountainous far west.)
Figure 14.27
The average number of days each year on which hail is observed throughout the
United States.
• Lightning and Thunder
– Causes of electrification of clouds
– graupel and hail fail into region of
supercooled water , water freezes,
releasing latent heat and keeping the
hailstone warmer than surrounding ice
crystal nuclei
• Net transfer ….+ ions from warmer to colder,
this leaves larger hail stones negatively
charged and smaller ice crystals positively
charged
Updrafts carry the tiny positively charged ice crystal into the upper reaches of the cloud,
while the heavier hailstone falls through the updraft toward the lower region of the cloud.
Figure 14.28
The lightning stroke can travel in a number of directions. It can occur within a cloud, from
one cloud to another cloud, from a cloud to the air, or from a cloud to the ground. Notice
that the cloud-to-ground lightning can travel out away from the cloud, then turn
downward, striking the ground many miles from the thunderstorm. When lightning
behaves in this manner, it is often described as a “bolt from the blue.”
Figure 14.30
The generalized charge
distribution in a mature
thunderstorm.
• The Lightning Stroke
– A discharge of static electricity
– Positive charge on ground, cloud to ground
lightning
• Thunder
–
–
–
–
Lightning heats air to 54,000deg F – hotter than Sun’s surface
Explosive expansion of air - shock wave
Sound travels at 330m/s or 1100 ft/s, so delay… about 5 sec per mile
Sound is refracted upward in unstable atm and we do not hear
lightning at approximately 15km away … Heat Lightning
The development of a lightning stroke.
(a) When the negative charge near the bottom of the cloud becomes large enough to
overcome the air’s resistance, a flow of electrons—the stepped leader —rushes toward
the earth.
(b) As the electrons approach the ground, a region of positive charge moves up into the
air through any conducting object, such as trees, buildings, and even humans.
(c) When the downward flow of electrons meets the upward surge of positive charge, a
strong electric current— a bright return stroke —carries positive charge upward into the
cloud.
Figure 14.33
The lightning rod extends above the building, increasing the likelihood that lightning will
strike the rod rather than some other part of the structure. After lightning strikes the
metal rod, it follows an insulated conducting wire harmlessly into the ground.
Figure 14.35
The four marks on the road surface represent areas where lightning, after striking a car
traveling along south Florida’s Sunshine State Parkway, entered the roadway through
the tires. Lightning flattened three of the car’s tires and slightly damaged the radio
antenna. The driver and a six-year-old passenger were taken to a nearby hospital,
treated for shock, and released.
Figure 14.36
Cloud-to-ground lightning strikes in the vicinity of Chicago, Illinois, as detected by the
National Lightning Detection Network
Tornadoes
• Rapidly rotating column of air that blows
around a small area of intense low pressure
with a circulation that reaches the ground.
– Funnel cloud – tornado not on ground
• Tornado life cycle
– Dust whirl, organizing, mature, shrinking, decay stage
• Tornado outbreaks
– families of T. usually due to a long lived
supercell
– outbreak , usually 6 or more
 Tornado Occurrence
 US experiences most tornadoes – all 50 states
 Tornado Alley (warm, humid surface; cold dry air
aloft)
 Highest spring, lowest winter
 Tornado winds
 Measurement based upon damage after storm or
Doppler radar
 For southwest approaching storms, winds
strongest in the northeast of the storm, 220 kts
maximum - most less than 125 kts
Figure 14.38
Tornado incidence by state. The upper figure shows the average annual number of
tornadoes observed in each state from 1953–2004. The lower figure is the average
annual number of tornadoes per 10,000 square miles in each state during the same
period. The darker the shading, the greater the frequency of tornadoes. (NOAA)
Figure 14.40
The total wind speed of a
tornado is greater on one
side than on the other.
When facing an onrushing
tornado, the strongest
winds will be on your left
side.
Example forward motion –
50kts
Rotational speed – 100
kts
Figure 14.41
A powerful multi-vortex
tornado with three suction
vortices.
• Dr. T. Theodore Fujita
• The Fujita scale was revised in 2007 as the EF-scale
(Enhanced F-Scale)
– The EF-scale is based on
rotational wind speeds
estimated from property
damage
– Ranges from EF0 to EF5
• EF5 tornadoes are rare
• About 77% of tornadoes in the U.S. are considered weak (EF0 to EF1)
and 95% are below EF3
• Seeking shelter
– Basement or small, interior room on
ground floor
– Indoor vs. outdoor pressure
Tornadic Formation
• Basic requirements are an intense
thunderstorm, conditional instability,
and strong vertical wind shear

Supercell Tornadoes
 Wind sheer causes spinning vortex tube that is
pulled into thunderstorm by the updraft
Figure 14.44
(a) A spinning vortex tube created by wind shear.
(b) The strong updraft in the developing thunderstorm carries the vortex tube into
the thunderstorm producing a rotating air column that is oriented in the vertical
plane.
Figure 14.45
A tornado-spawning supercell thunderstorm over Oklahoma City on May 3, 1999,
shows a hook echo in its rainfall pattern on a Doppler radar screen. The colors
red and orange represent the heaviest precipitation.
Stepped Art
Fig. 14-46, p. 402
• Nonsupercell Tornadoes
– Gustnadoes – form along a gust front
– Land spout - form out of cumulus congestus clouds and
similar look to water spout
(a) Along the boundary of converging winds, the air rises and condenses into a cumulus
congestus cloud. At the surface the converging winds along the boundary create a
region of counterclockwise spin.
(b) As the cloud moves over the area of rotation, the updraft draws the spinning air up into
the cloud producing a nonsupercell tornado, or landspout.
Figure 14.47
A well-developed landspout moves
over eastern Colorado.
Waterspouts
• Rotating column of air that is
connected to a cumuliform
cloud over a large body of
water
• Tornadic waterspout – one
that formed over land
• ―fair weather‖ waterspouts
from under cumulus
congestus clouds ,45kts
• Does not pull up water more
than a few meters…what you
see is a cloud of condensation
Severe Weather and Doppler Radar
• Doppler radar measures the speed of
precipitation toward and away radar unit
• Two Doppler radars can provide a 3D view
Figure 14.49
Doppler radar display of winds
associated with the supercell storm
that moved through parts of
Oklahoma City during the
afternoon of May 3, 1999. The
close packing of the horizontal
winds blowing toward the radar
(green and blue shades), and
those blowing away from the radar
(yellow and red shades), indicate
strong cyclonic rotation and the
presence of a tornado.
• Doppler Lidar
• 150 NEXRAD units - WSR-88D and
computer system
– Also portable
Figure 14.50
Graduate students from the University of
Oklahoma use a portable Doppler radar to
probe a tornado near Hodges, Oklahoma.
Monitoring Tornadic Thunderstorms
The annual number of reports of tornadoes in the U.S.
1950-2008