Thunderstorms

1
Thunderstorms Tornadoes

• This chapter discusses
• The different types and development stages of
  thunderstorms
• The life-threatening storm components of
  lightning and tornadoes

2
Ordinary Thunderstorms Stage I
Three stages have been identified in ordinary
thunderstorms An unstable atmosphere and
vertical updrafts keep precipitation suspended
Figure 15.1A
3
Ordinary Thunderstorms Stage II
Entrainment of dry air that causes cooler air
from evaporation, triggering downdrafts and
falling precipitation and gust fronts
Figure 15.1B
4
Ordinary Thunderstorms Stage III
Weakening updrafts and loss of the fuel source
after 15 to 30 minutes.
Figure 15.1C
5
Mature Stage Thunderstorm
Figure 15.2
During the mature stage, updrafts may stop at the
troposphere where the cloud ice crystals are
pushed horizontally by winds and form an anvil
top, or they may overshoot further into the
tropopause.
6
Dissipating Stage of Thunderstorm
Once downdrafts dominate updrafts, the storm ends
as precipitation leaves the cloud faster than it
is replenished by rising, condensing air.
Often, lower level cloud particles will evaporate
leaving an isolate cirrus anvil top section.
Figure 15.3
7
Multicell Storms
Figure 15.4
Cool downdrafts leaving a mature and dissipating
storm may offer relief from summer heat, but they
may also force surrounding, low-level moist air
upward. Hence, dying storms often trigger new
storms, and the successive stages may be viewed
in the sky.
8
Severe Thunderstorms
Figure 15.5
9
Severe storms classified as producing a minimum
of a) 3/4 inch hail and/or b) wind gusts of 50
knots and/or c) tornado winds. In ordinary
storms, the downdraft and falling precipitation
cut off the updraft. In severe storms, winds
aloft push the rain ahead and the updraft is not
weakened and the storm can continue maturing.
The single supercell storm shown here maintained
its structure for hours.
10
Gust Front Microburst
Turbulent air forms along the leading edge of the
gust front, which can generate tumbling dust
clouds. Such gust fronts and associated cold
dense air often feel like a passing cold front,
and may cause a 1 to 3 mb local rise in pressure,
called a mesohigh.
Figure 15.6
11
Gust Front Shelf Cloud
When unstable air is prevalent near the base of
the thunderstorm, the warm rising air along the
forward edge of the gust front is likely to
generate a shelf, or arcus, cloud.
Figure 15.6
12
Gust Front Roll Clouds
Figure 15.7
Turbulence in the fast moving gust front will
spawn eddies and possibly roll clouds beneath the
shelf cloud. These clouds spin about a
horizontal axis near the ground.
13
Microbursts from Dense Air
Figure 15.9
Dry air entrained into the thunderstorm will
evaporate and cool the falling mix of
precipitation and air, which may create dry, and
in humid areas wet, microbursts of strong winds.
14
Flying into a Microburst
Figure 15.10
A pilot flying into a microburst must anticipate
sudden and strong changes in wind direction and
speed. Initially a headwind is encountered that
lifts the plane, followed by a strong downdraft,
and when leaving the storm a tailwind causes a
loss of altitude.
15
Storm Radar Bow Echo
Derecho, or straight-line winds, may form ahead
of a several hundred kilometer cluster of storms,
known as a squall line or mesoscale convective
system, often formed a few hundred kilometers
ahead of a cold front. This image shows a
squall line within the bow shaped radar echo.
Figure 15.11
16
Pre-Frontal Squall Line Storms
Pre-frontal squall lines identify major storms
triggered by a cold front that may contain
several severe thunderstorms, some possibly
supercells, extending for more than 1000
kilometers. This 1989 storm spawned 25
tornadoes, the worst killing 25 people.
Figure 15.12
17
Gravity Waves
Figure 15.13
Pre-frontal squall line formation is not fully
understood. One theory suggests that a surging
cold front may initiate "gravity waves" aloft,
where the rising motion of the wave causes
cumulus cloud development.
18
Trailing Stratified Clouds
Figure 15.14
An extensive region of stratified clouds may
follow behind a squall line. This figure shows
a loop of rising and falling air that supplies
the moisture to the stratiform clouds and
associated light precipitation.
19
Mesoscale Convective Complex
An organized mass of thunderstorms extending
across a large region is called MCC. With weak
upper level winds, such MCC's can regenerate new
storms and last for upwards of 12 hours and may
bring hail, tornadoes, and flash floods. They
often form beneath a ridge of high pressure.
Figure 15.15
20
Dryline Thunderstorms
Abrupt geographic changes from moist to dry
dew-point temperature, called drylines, form in
western TX, OK, and KS in the spring and summer.
The diagram illustrates how cool cP air pushes
hot and dry cT air, at the height of the central
plains, over the warm moist mT air. Such mixing
causes large scale instabilities and the birth of
many supercell storms.
Figure 15.16
21
Thunderstorm Movement
Figure 15.17
Middle troposphere winds often direct individual
cells of a thunderstorm movement, but due to
dying storm downdrafts spawning new storms, the
storm system tends to be right-moving relative to
the upper level winds. In this figure, upper
level winds move storms to the northeast, but
downdrafts generate new cells to the south, which
eventually cuts off moisture to the old cell.
22
Flash Great Floods
Figure 15.18
Figure 15.19
Thunderstorms frequently generate severe local
flooding, but in the summer of 1993 a stationary
front beneath the unusually southerly polar jet
triggered several days of thunderstorms and
rain. The jet caused weak surface waves and
provided uplift of warm, moist Gulf air for
thunderstorm growth throughout the northern
Mississippi region. Floods took 45 human lives
and 74,000 were evacuated.
23
Average Thunderstorm Hail Days
Figure 15.21
Figure 15.20
Observed frequency in the pattern and occurrence
of thunderstorms does not overlap with hail
frequency, possibly because hail falling into the
thick layer of warm Gulf air will melt before
reaching the ground.
24
Lightning Thunder
Charge differences between the thunderstorm and
ground can cause lightning strokes of 30,000C,
and this rapid heating of air will creates an
explosive shock wave called thunder, which
requires approximately 3 seconds to travel 1
kilometer.
Figure 15.22
25
Lightning Stroke Development
Charge layers in the cloud are formed by the
transfer of positive ions from warmer hailstones
to colder ice crystals. When the negative
charge near the bottom of the cloud is large
enough to overcome the air's resistance, a
stepped leader forms and approaches earth.
Figure 15.23A
26
Lightning Stroke Development
A region of positive ions move from the ground
toward this charge,through any conducting object,
such as trees.
Figure 15.23A
27
Lightning Stroke Development
When the downward flow of electrons meets the
upward surge of positive charge, a strong
electric current or bright return stroke,
carries positive charge upward into the cloud.
Figure 15.23A
28
Types of Lightning
Nearly 90 of lightning is the negative
cloud-to-ground type described earlier, but
positive cloud-to-ground lightning can generate
more current and more damage. Several names,
such as forked, bead, ball, and sheet lightning
describe forms of the flash. Distant lightning,
where the thunder is unheard, is often called
heat lightning.
Figure 15.24
29
Lightning Rods Fulgurite
Figure 15.26
Figure 15.25
Metal rods that are grounded by wires provide a
low resistance path for lightning into the earth,
which is a poor conductor. The fusion of sand
particles into root like tubes, called fulgurite,
may result.
30
Lightning Detection Suppression
Figure 15.27
Figure 15.28
When lightning is nearby, trees are not safe
because they may generate a return stroke, but a
car may provide protection by transferring the
charge through its body to the tires. Lightning
is more often the cause for forest fires,
triggering nearly 10,000 yearly in the US. A
National Lightning Detection Network helps
monitor this storm activity.
31
Tornado
A rapidly rotating column of air often evolve
through a series of stages, from dust-whirl, to
organizing and mature stages, and ending with the
shrinking and decay stages. Winds in this
southern Illinois twister exceeded 150 knots.
Figure 15.29
32
Tornado Occurrence
Figure 15.30
Tornadoes from all 50 states of the US sum to
more than 1000 tornadoes annually, but the
highest frequency is observed in tornado alley of
the Central Plains. Nearly 75 of tornadoes
form from March to July, and are more likely when
warm humid air is overlain by cooler dryer air to
cause strong vertical lift.
33
Tornado Wind Speed
As the tornado moves along a path, the circular
tornado winds blowing opposite the path of
movement will have less speed. For example, if
the storm rotational speed is 100 knots, and its
path is 50 knots, it will have a maximum wind of
150 knots on its forward rotation side.
Figure 15.31
34
Suction Vortices Damage
A system of tornadoes with smaller whirls, or
suction vortices, contained within the tornado is
called a multi-vortex tornado. Damage from
tornadoes may include its low pressure centers
causing buildings to explode out and the lifting
of structures. Human protection may be greatest
in internal and basement rooms of a house.
Figure 15.32
35
Tornado Numbers and Deaths by Decade
36
Fujita Tornado Scale
Figure 15.33
Tornado watches are issued when tornadoes are
likely, while a warning is issued when a tornado
has been spotted. Once the storm is observed,
or has passed, the Fujita scale is used to
classify tornadoes according to their rotational
speed based on damage done by the storm.
37
Fujita Scale for Damaging Wind
38
Atmospheric Conditions for Tornadoes
A specific pattern of events often coincide
during the formation of tornadoes and severe
thunderstorms. This may include when an
open-wave mid-latitude cyclone mixes together
cold dry air with warm moist air at the surface,
and 850 mb warm moist and 700 mb cold dry air
aloft flow north and north east, as shown in this
figure. Further, at the 500 mb level a trough
of low pressure pressure forms to the west of the
surface low, and the 300 mb polar jet swings over
the region.
Figure 15.34
39
Thunderstorm Sounding
Figure 15.35
Temperature and dew point have typical vertical
profile in the warm sector before a tornado
occurs, including the shallow inversion at 800 mb
that acts like a cap on the moist air below.
The cold dry air above warm humid air produces
convective instability and lifting.
40
Vorticity from Horizontal to Vertical
Figure 15.36
Figure 15.37
Wind shear is created when winds pass in
different directions aloft and the surface, and
also created by more rapid speeds aloft, causing
wind to form vortex tubes rotating at the
surface. A strong updraft in the storm tilts the
horizontal vortex tube into a vertical tube,
creating a tornado precursor.
41
Tornado Breeding Supercell Storm
Figure 15.38
Supercell thunderstorms may have many of the
features illustrated here, including a
mesocyclone of rotating winds formed when
horizontal vorticity was tilted upwards.
42
Radar Image of Supercell
The area of precipitation and winds in the
mesocyclone is known as the bounded weak echo
region (BWER) which the radar is unable to detect
and displays as a black core to this storm. The
cyclonic flow of precipitation on the radar
screen is often shaped like a hook echo.
Figure 15.39
43
Rear Flank Downdraft
Supercell thunderstorm development may create an
area where the updraft and counterclockwise swirl
of upper winds converge into a rear flank
downdraft. This downdraft can then interact
with lower level inflow winds and spawn a tornado.
Figure 15.40
44
Rotating Clouds as Tornado Signal
The first sign that a supercell may form a
tornado is the sight of rotating clouds at the
base of the storm, which may lower and form a
wall cloud, shown in this picture.
Figure 15.41
45
NonSupercell Tornadoes
If a pre-existing wall cloud was not present,
than any tornado formed is not from a supercell
storm, and is often called a funnel cloud, or may
be a gustnado if the form along a gust front.
Figure 15.42
46
Landspout Formation
Landspouts, which form over land but look like
waterspouts, form when surface winds converge
along a boundary where opposite blowing wind
creates a horizontal rotational spin. If a
storm passes above, its updraft may lift and
stretch the horizontal spinning air, causing it
to narrow and increase in rotational speed due to
the conservation of angular momentum.
Figure 15.43AB
47
Doppler Radar Analysis
A single Doppler radar unit can uncover many
features of thunderstorm rotation and movement,
but cannot detect winds parallel to the
antenna. As such, data from two or more units
might be combined to provide a complete view of
the storm. Dopplar lidar (light beam rather
than microwave beam) provides more details on the
storm features, and will help measure wind speeds
in smaller tornadoes.
Figure 15.44
48
NEXRAD Wind Analysis
NEXt Generation Weather RADar (NEXRAD) is
operated by the National Weather Service and uses
Doppler measurement to detect winds moving toward
(green) and away (blue) from the antenna, which
indicates areas of rotation and strong shear.
Figure 15.45
49
Portable Radar Units
Thunderstorm chasers may carry portable radar to
image finer details of a storm as it moves along
the flat lands of Tornado Alley.
Figure 15.46
50
Waterspout Funnel
Warm, shallow coastal water is often home to
waterspouts, which are much smaller than an
average tornado, but similar in shape and
appearance. The waterspout does not draw water
into its core, but is a condensed cloud of
vapor. A waterspout may, however, lift swirling
spray from the water as it touches the water
surface.
Figure 15.47