Download Ordinary Thunderstorms

Thunderstorms
Tornadoes
Huricanes
Ordinary Thunderstorms
Three stages have been
identified in ordinary
thunderstorms:
a)
an unstable atmosphere
and vertical updrafts keep
precipitation suspended
b)
entrainment of dry air that
causes cooler air from
evaporation, triggering
downdrafts and falling
precipitation and gust
fronts
c)
weakening updrafts and
loss of the fuel source
after 15 to 30 minutes.
Mature Stage Thunderstorm
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.
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.
Multicell Storms
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.
Severe Thunderstorms
Storms producing a minimum of
• 3/4 inch hail and/or
• wind gusts of 50 knots and/or
• tornado winds, classify as severe.
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.
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.
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.
Microbursts from Dense Air
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.
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.
Trailing Stratified Clouds
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.
Mesoscale Convective Complex
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 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.
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.
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.
Lightning & Thunder
Charge differences between the
thunderstorm and ground can cause
lightning strokes of 30,000°C, and
this rapid heating of air will creates
an explosive shock wave called
thunder, which requires
approximately 3 seconds to travel 1
kilometer.
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.
A region of positive ions move from the
ground toward this charge, which then
forms a return stroke into the cloud.
Types of Lightning
Nearly 90% of lightning is the negative cloud-to-ground type, 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, unseen lightning is often called heat lightning.
Lightning Rods & Fulgurite
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.
Tornado
A rapidly rotating column of
air often evolve through a
series of stages, from dustwhirl, to organizing and
mature stages, and ending
with the shrinking and decay
stages.
Winds in this southern
Illinois twister exceeded 150
knots.
Tornado Occurrence
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.
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.
Suction Vortices & Damage
A system of tornadoes with smaller
whirls, or suction vortices, contained
within the tornado is called a multivortex 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.
Fujita Tornado Scale
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 of F0 – F5 is used to
classify tornadoes according to their rotational speed based on damage done by the
storm.
Atmospheric Conditions
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
Thunderstorm Sounding
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.
Vorticity from Horizontal to Vertical
Spinning horizontal vortex tubes created by surface wind shear may be tilted and forced in a
vertical path by updrafts. This rising, spinning, and often stretching rotating air may then turn
into a tornado.
Tornado Breeding Supercell Storm
Supercell thunderstorms may have many of the features illustrated here,
including a mesocyclone of rotating winds formed when horizontal vorticity
was tilted upwards.
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.
Rotating Clouds
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.
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.
Landspouts, which form over land
but look like waterspouts.
Landspouts
They 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.
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.
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.
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.
Things Often Mistaken For
Tornadoes





Heavy Precipitation
Downbursts
Dust Devils
Cold Funnels
If There’s No Evidence of Rotation, It’s Not a
Tornado
Virga
Downburst, May 1994
Downburst Damage, Ontario
Dust Devil
Cold Funnels
Cold Funnels
Hurricanes
Hurricane: Atlantic and East Pacific
 Typhoon: West Pacific
 Cyclone: Indian Ocean

Intense Low-Pressure Systems
 Need 60 m (200 feet) of ocean water at
26.5 C or warmer to form

World Hurricane Tracks 19952003
Hurricane Forming Regions
Hurricane-Free Regions
No Coriolis effect at equator, hence no
hurricanes within 5 degrees of equator
 No warm sea water in South Atlantic,
hence no South Atlantic Hurricanes
 No warm sea water in Southeast Pacific,
hence no Southeast Pacific Hurricanes
 Apart from Caribbean coast, no hurricanes
in South America

March 2004: Brazil’s First Hurricane?
Coriolis Effect at Equator
Coriolis Effect at Equator
Coriolis Effect at Equator
Westbound: Deflected away from Equator
 Eastbound: Directed along Equator
 Unlikely for winds but does happen in
oceans (Equatorial Countercurrent)
 Weather systems can’t spin

Saffir-Simpson Scale
Defined by instruments
1. 74-95 mph
1-2m storm surge
2. 96-110 mph
2-3 m
3. 111-130 mph
3-4 m
4. 131-155 mph
4-6 m
5. >155 mph
>6m
Naming Hurricanes
No naming system until 1953
 Women’s names 1953-79
 Regional Name Lists
 Lists maintained by World
Meteorological Organization
 Names can be retired after especially
significant storms

Dangers of Hurricanes
Wind Pressure
 Flying Debris
 Storm Surge
 Flash Flooding
 Tornadoes

Eye of Hurricanes
100 km or less in diameter
 30 minutes or so calm weather
 Definitely not the end of the storm!
 Post-eye storm is stronger
 “Centrifugal” force counteracts inward
air flow
 In strongest storms, air flow can get so
congested a second eyewall forms
(Andrew)

Trailing Side is Most
Dangerous
Decay of Hurricanes
Need warm water for energy
 Decay rapidly over land
 Lose strength over cold water
 Can still cause destructive flooding long
after cyclonic structure is gone
 Degenerate into low pressure systems

Cold Water Trail
Extratropical “Hurricanes”
Two-Ocean Hurricanes
Winter Storms