Chapter 15: Thunderstorms and Tornadoes

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Chapter 15 Thunderstorms and Tornadoes
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Weather Trivia October 26

• 2001 The seasons first major storm packing
  sustained south-westerly winds of 75-90 km/hour
  depressed already low water levels on the St.
  Clair and Detroit rivers and the western end of
  Lake Erie. Over 50 ships dropped anchor or
  remained tied up at the docks. Water levels
  dropped 1.5 m making it unsafe for loaded ships
  to sail and draining local marinas. Upbound
  traffic on the Welland Canal was halted due to
  the high winds.

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What is a Thunderstorm

• A storm in which thunder is heard. Since thunder
  is only generated by lightning thunderstorms are
  storms in which lightning is produced
• A thunderstorm is a deep convective storm that
  forms in a conditionally unstable moist air mass
• Severe thunderstorms may produce high winds
  flash floods damaging hail and the may spawn
  tornadoes
• Thunderstorms may be isolated short-lived (less
  than 1 hr) and small (a few km across). Such
  storms are referred to as air-mass storms single
  cell storms or ordinary thunderstorms.

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Life cycle of an Ordinary thunderstorm

• Cumulus stage
• Successive parcels of air rising from the
  surface gradually penetrate to greater and
  greater heights.
• Above the condensation level cloud droplets form
  and grow by condensation and collisions and
  coalescence.
• There are updrafts throughout the cloud
• At higher levels above the freezing level ice
  crystals may form
• There has not been time for precipitation to form

Cumulus stage
Figure 15.1a
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b) Mature Stage Precipitation starts to form. The
fall speed of the precipitation is greater than
the updraft and the precipitation starts to fall
towards the surface. The falling rain slows down
the updraft. At the same time cooler drier air
from the edges of the cloud is drawn into
(entrained into) the cloud. Some of the rain
evaporates in the drier air cooling the air
(latent heat is used). The cooled air is denser
than the surrounding air and so it descends
rapidly as a downdraft.
Mature Stage
Figure 15.1b
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• The mature stage is characterised by the presence
  of both updrafts and downdrafts.
• This is the most intense stage of the
  thunderstorm.
• The thunderstorm rises up to a level where the
  air is stable which could be as high as the
  tropopause.
• Updrafts and downdrafts reach their maximum
  values in the middle of the storm.
• Lightening and thunder are present.
• Heavy rain and possibly small hail reach the
  surface.
• At the surface there may be a strong cold winds
  spreading out from the region of the downdraft.
• The boundary of this cooler air is the gust
  front.
• In dry air where the cloud base is high the rain
  may evaporate before it reaches the surface
  (virga).

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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.
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Dissipating Stage
c) Dissipating stage If the updraft weakens there
is no supply of water vapour. This may be due to
the gust front moving too far ahead of the storm
or the downdraft overwhelming the updraft. The
latent heat from the condensation of the water
vapour supplies the energy for the storm. If the
updrafts are cut-off the storm dissipates.
Figure 15.1c
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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
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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.
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Thunderstorm Movement
Upper winds Lower winds
Upper winds Lower winds
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.
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Severe Thunderstorms
Storms producing a minimum of a) 3/4 inch hail
and/or b) wind gusts of 50 knots and/or c)
tornado winds classify as severe. (Hail may be
as large as grapefruits.) 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. (Updrafts
may exceed 40 m/sec)
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Severe Thunderstorms
Figure 15.5
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An organized mass or collection of
thunderstorms that extends across a large region
is called a mesoscale convective complex
(MCC). With weak upper level winds such MCCs
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.
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Mesoscale Convective Complex
Figure 15.15
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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 74000 were evacuated.
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Tornadoes

• A rapidly rotating column of air that blows
  around a small intense area of low pressure with
  a circulation that reaches the ground.
• A funnel cloud is a tornado whose circulation has
  not descended down to the ground.
• Diameters are typically between 100 and 600 m.
• Most last for a few minutes and have a path
  length of a few kilometers but some may last for
  an hour and travel more than 100 km.
• A single supercell thunderstorm may spawn several
  tornadoes.

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Tornado Breeding Supercell Storm
Figure 15.38
Supercell thunderstorms may have many of the
features illustrated here including a
mesocyclone of rotating winds.
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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
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Tornado Occurrence
Number per state in 25 yrs Number per 10000 sq.
miles per year
Figure 15.30
Tornadoes from all 50 states of the U.S. add up
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.
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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
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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
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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. The Fujita scale is used to
classify tornadoes according to their rotational
speed based on damage done by the storm. F0 has
40 72 mph winds F5 has 261 - 318 mph winds
(see Table 15.2).
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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
Hook Echo
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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
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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
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Chapter 16 Hurricanes
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Definition

• An intense storm of tropical origin with
  sustained winds exceeding 64 knots ( 120 km/hr)
• Has different names depending on where it forms
• Hurricane - N. Atlantic
• - Eastern N. Pacific
• Typhoon - Western N. Pacific
• Cyclone - India
• Tropical cyclone - Australia
• Typical diameter is about 500 km

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Hurricane Elena (1985) from space shuttle
Discovery
Figure 16.2
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Characteristic Features of a Hurricane

• Central Eye Clear region or broken clouds light
  winds. Typically 40 km diameter. Very
  low surface pressure.
• Eye Wall Ring of intense thunderstorms that
  rotate counterclockwise (NH) around the
  eye. Region of heaviest precipitation and
  strongest winds.
• Spiral Rain bands
• Spiral bands of storms and cloud that
  spiral counterclockwise in towards the eye
  wall. Winds increase in speed towards the eye
  wall.

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Hurricane Formation

• Over tropical oceans with surface temperature
  greater than 26.5oC over a vast area.
• Low friction abundant supply of water vapour.
  Water vapour condenses to release latent heat.
• Winds must be light.
• Humidity must be high through a considerable
  depth of the troposphere.
• The Coriolis force must not be too small.
  Therefore hurricanes dont form within 5o north
  or south of the equator.
• Require a trigger to initiate their formation.

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Tropical Weather Waves
A wave known as an Easterly Wave may form on
the ITCZ. Convection is common on the ITCZ. The
Easterly wave may organize the convection and act
as the trigger that allows a storm to grow into a
tropical depression a tropical storm or
possibly a hurricane.
Figure 16.1
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• Hurricanes derive their energy from the release
  of latent heat resulting from the condensation of
  water vapour that has evaporated from the warm
  ocean. The heat energy is converted to kinetic
  energy (energy associated with wind motion)
  inside the deep convective storms.
• Hurricanes grow stronger as long as the air aloft
  moves outward more quickly than air flows in
  towards the centre at low levels.

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• Hurricanes move in the general direction of the
  tropospheric winds. Thus Atlantic hurricanes
  typically move westward in the NE Trades and then
  towards the north and northwest around the
  Bermuda High.
• Hurricanes dissipate rapidly when they move over
  cold water or over a large landmass.

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Hurricane Stages of Development
The initial components of a hurricane may form as
a tropical disturbance grow into a tropical
depression when winds exceed 20 knots become a
tropical storm when winds exceed 35 knots and
finally then qualify as a hurricane when winds
exceed 64 knots.
Figure 16.6
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Hurricane Movement
Global patterns of tropical cyclone formation and
movement have been recorded on this figure which
notes regional names for these systems. Travel
speeds for the hurricane my range from 10 to 50
knots but they may also stall over a region and
cause destructive flooding.
Figure 16.7
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Erratic Paths of Hurricanes
Figure 16.8
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Historical charts of hurricane location may
reveal erratic and hard to predict patterns of
movement. As this figure shows hurricanes may
occasionally double back. Further when removed
from the ocean and without a moisture source to
supply energy they may still continue an inland
journey. In the North Atlantic on average 3
storms per year move inland and bring damaging
winds and rain.
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North Atlantic Hurricanes
Composite infrared imagery of Hurricane Georges
reveals the pattern of a seasonal threat for
Central and North America coastlines. Tropical
cyclones at the same latitude survive longer in
the Atlantic than Pacific Ocean because of warmer
Atlantic Ocean waters.
Figure 16.9
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Hurricane Damage Warning
Figure 16.11
Figure 16.10
Hurricanes have their highest wind speeds on the
side where storm pushing winds amplify cyclonic
or counterclockwise rotational winds. In coastal
areas flooding is aggravated by the hurricane
low pressure triggering higher tides and Ekman
transport piling up water.
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Hurricane Watch Warning
Figure 16.5A
The National Hurricane Center in Florida issues a
hurricane watch 24 to 48 hours before a
threatening storm arrives and if it appears that
the storm will strike within 24 hours a
hurricane warning is issued. While some consider
the warning area too large causing unneeded
evacuation such evacuations have saved many
lives. Hurricane Hugo (1989) with peak winds
near 174 knots caused tremendous damage.
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Hurricane Saffir-Simpson Winds
Figure 16.14
Figure 16.13A
In 1989 Hugo caused nearly 7 billion in damages
in the U.S. killing 49 in the Caribbean and
United States. Current classification of
hurricanes is based on their wind speed however
and not on human or property damage. Hurricanes
range from category 1 to 5 with winds of 64 to
more than 135 knots.
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Hurricane Names and Cost
Category 5 Hurricane Andrew (1992) was the
costliest US storm but it ranks as less intense
than 1935 and 1969 hurricanes. Hurricane names
are chosen from an alphabetical list of male and
female names for the Atlantic and Pacific some
of which are retired if the storm was especially
damaging.
Figure 16.14
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Hurricane Andrew Devastation in Homestead
Florida August 24 1992
Figure 16.15
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Likelihood for Landfall
Between 1900 and 1999 only two category 5
hurricanes (1935 Camille in 1969) have made
landfall along the Gulf or Atlantic. Numerous
category 1 and less damaging storms that do
make landfall may not cause much damage but
bring needed rainfall.
Figure 16.16
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Hurricanes and Mid-Latitude Storms

• Hurricanes
• Energy from latent heat
• Warm core cyclone weakens with height
• Central clear eye sinking air
• Strongest winds at surface
• Circular isobars stronger pressure gradients no
  fronts

• Mid-Latitude Storms
• Energy from temperature contrast
• Cold core low strengthens with height
• Centres of rising cloudy air
• Winds strongest aloft
• Isobars have kinks weaker pressure gradients
  fronts