Transcript Severe Weather

Severe
Weather
ATS 351
Lecture 10
November 9, 2009
Types of Severe Weather
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Thunderstorms
Hail
Lightning
Flood
Tornado
Severe Wind (Straight-Line Winds)
Thunderstorm Distribution
Favorable Conditions
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Instability
Fuel
Initial Lift
Shear
Capping Inversion
Instability
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Steep lapse rate
Means warm, moist air near the surface
Colder air above it
Needs to be calculated from a sounding
Fuel
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Just like any other weather phenomenon, a
storm needs fuel to sustain itself
The fuel for a storm is just a continued supply of
what started it
- Heat
- Moisture
- Lift
The storm needs to remain in areas of warm,
moist air. If storm moves into a colder region, it
will die
Sources of Lift
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Convective lifting
Boundaries
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Fronts
Drylines
Outflow
boundaries
Orographic
Convergence
Shear
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Because of the way a thunderstorm works, it
needs to be tilted to remain strong
Therefore, winds need to change with height
Two kinds of shear
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Speed Shear: Wind is faster as you go up
Directional Shear: Wind changes direction with
height
Capping Inversion
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If the atmosphere is unstable all the way up,
you get a constant updraft
It is more effective when the energy is held back
and released all at once
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This can happen by having a stable layer near the
surface that suppresses convection
As ground heats during the day, energy builds
up until it can “break the cap”
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Also referred to as a “capping inversion”
CIN
Back to the Skew-T
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Meteorologists have formulated various
numbers that can tell how favorable the weather
is for a storm. These quantities can describe
things such as:
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CAPE (Convective Available Potential Energy
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Instability
Shear
Or a combination of both
How unstable atmosphere is
LI (Lifted Index; normally at 500mb levle)
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LI = Tenvironment – Tparcel
Thunderstorm Development
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Stages
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Cumulus
Mature
Dissipation
Cumulus Stage
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Warm moist air rises,
condenses
Latent heat release
keeps air in cloud
warmer than
environment
Grows to a towering Cu
Cloud particles grow
larger, begin to fall
No precipitation at
surface
Mature Stage
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Marked by appearance of
downdraft
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Falling cloud drops evaporate,
cooling the air
Storm is most intense during this
stage
Cloud begins to form anvil
May have an overshooting top
Lightning and thunder may be
present
Gust front forms
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Downdraft reaches the surface
and spreads out in all directions
Gust front forces more warm,
humid air into the storm
Dissipation Stage
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Usually follows mature stage by
~15-30 min
Gust front moves out away
from the storm, and moist air is
no longer lifted into the storm.
Downdrafts become dominant
Low level cloud drops can
evaporate rapidly, leaving only
the anvil as evidence of the
storm’s existence
Types of Thunderstorms
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Thunderstorms come in
many varieties
Likelihood of severity
proportional to storm lifetime
NWS definition of severe
(one or more of the following
elements)
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¾” or larger diameter hail
50 kt (58 mph) or greater winds
tornadoes
Single Cell Thunderstorms
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Also referred to as ordinary,
pulse, or air mass thunderstorms
Typically do not produce severe
weather
Three stages
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Cumulus
Mature
Dissipating
Life span: ~45-60 min.
Multi-Cell Storms
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Cluster of storms moving as a single unit
Stronger wind shear than the ordinary
cell case
More organized multi-cells
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Bow Echoes
Squall Lines
New cells tend to form on the upwind
(W or SW) edge of the cluster, with mature cells located
at center and dissipating cells found along the
downwind (E or NE) portion of the cluster
Multiple cells compete for warm, moist low-level air so
not incredibly strong and have short life spans
Multi-cell Storms
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Cell 1 dissipates while cell 2
matures and becomes
dominant
Cell 2 drops heaviest
precipitation as cell 3
strengthens
Severe multicell storms
typically produce a brief
period of hail and/or
downbursts during and
immediately after the
strongest updraft stage
Updrafts and Downdrafts
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Degree of
instability and
moisture
determine the
strengths of
updrafts and
downdrafts
Vertical Wind Shear
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Change of wind speed
and/or direction with
height
Weak vertical wind
shear: short-lived
since rainy downdraft
quickly undercuts and
chokes off the updraft
Sheared
environments are
associated with
organized convection
Vertical Wind Shear
Gust fronts
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An area of high pressure
created at the surface by
cold heavy pool of air from
downdraft called a mesohigh
Gust front: leading edge of
cold air from downdraft
Passage noted by calm
winds followed by gusty
winds and a temperature
drop then precipitation
Convergence region
between cold outflow and
warm, moist inflow
Can generate new cells
Leads to multi-cell storms
Production of shelf and roll
clouds
Downbursts
Overshooting tops
Mesoscale Convective
Systems (MCS’s)
• Individual storms can grow and
organize into a large convective
system (weak upper level winds)
• Definition: 100km contiguous group of
t-storms
• Range of lifetimes
• New storms grow as older ones
dissipate (reinvigorates itself)
• Provide widespread precipitation
• Can spawn severe weather
• Hail, high winds, flash floods,
tornadoes
• Formation (in U.S.)
• Usually during summer when a cold
front stalls beneath an upper level
ridge of high pressure
• Surface heating and moisture can
generate thunderstorms on the cool
side of the front
Squall Lines
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Multicell storms can form as a
line of storms extending for
hundreds of km, called a
squall line
Squall lines often form along
or just ahead of a cold frontal
boundary (called pre-frontal
squall lines)
Supercells may be embedded
within prefrontal squall lines
Leading line of thunderstorms
may be followed by large
region of stratiform
precipitation where the anvil
cloud trails behind the main
storm.
Bow Echoes
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Bow Echo – a bowed convective
line (25 – 150 km long) with a
cyclonic circulation at the northern
end and an anticyclonic circulation
at the southern end
Strong jet in from behind
Can produce long swaths of
damaging winds
Form in conditions of large
instability and strong low level
shear
Observed both as isolated
convective systems or as
substructures within much larger
convective systems (such as a
squall line)
May contain strong winds or
tornadoes
Supercells
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Characterized by rotating
updrafts (called a mesocyclone)
Differ from multicell cluster
because of rotation and that
updraft elements merge into a
main rotated updraft rather than
developing separate and
competing cells
Can persist for 12 hours and
travel hundreds of miles
Forms in environments of
strong winds aloft
Winds veer with height from the
surface
Can be classified as either High
Precipitation (HP) or Low
Precipitation (LP)
Hail
• Storms contain updraft and
downdraft
– Not same strength everywhere
• Hail that swept upwards in a region
of lesser updraft
• Begins to fall, can fall into stronger
updraft
• Cycling may occur
• Important contributors to creating
charged regions in clouds
Lightning
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Inside a cloud, updrafts and turbulence toss ice
particles around
Each collision creates a small amount of electric
charge
After a few million of those, the charge is too
much to be held back by the air
Discharges all at once in a flash of lightning
Lightning
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The temperature of lightning is roughly 30,000
degrees C
 The surface of the sun is only about 5700
degrees C
One bolt of lightning carries enough electricity to
power the entire United States for 0.1 seconds
Lightning has been known to strike up to 15
miles from the actual storm
Lightning Misconceptions
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Lightning comes down from the clouds
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Entire process happens in under 0.001 seconds
Lightning always hits the tallest object
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It actually comes down AND goes up.
As a bolt begins the trip down, a “streamer” from the ground
shoots upward toward the oppositely charged cloud.
The flash happens when they meet in the middle.
Not true. It may seem that way, but lightning simply takes the
“path of least resistance”.
If you conduct electricity better than the 30 ft. tall tree next to
you, you will get hit
Lightning never hits the same place twice
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That’s just wrong.
There are many documented cases of lightning hitting twice in
the same spot
Sometimes only a few seconds apart!
Lightning Fatalities
Thunder
If air is heated from 75 to 90 degrees, it will
expand
 If air is heated from 75 to 50,000 degrees, it
will expand quickly
• Thunder is a compression wave due to this
rapid heating
 The thunder you hear is not lightning “hitting
the ground” but actually a sonic boom
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Tornadoes
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Formation
Life Cycle
Definition
Types
Damage
EF-scale
Wall clouds
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Lowering of cloud base
Visible manifestation of the mesocyclone at low levels
(contains significant rotation)
Develop when rain-cooled air is pulled upward, along
with more buoyant air
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Rain-cooled air usually very humid so upon being lifted, will
quickly saturate to form the lowered cloud base
Tornado often forms from within
wall cloud
Formation
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Tornadogenesis is the formation of tornadoes
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We know relatively little about this process
Basic formation steps are known
Details are missing, but they are very crucial
details
• Vertical wind shear crucial
Rotation tilting
• After horizontal rotation is established, the storm’s
updraft works to tilt it upright
• Now the storm has a vertically rotating component
Mesocyclone
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The new rotating storm is called a mesocyclone
Characterized by rotating updraft
At this point, the rotation can be picked up on Doppler
radar if it is strong enough
Supercell Tornado Formation
Funnel Cloud
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Area of rotation that does not touch the ground
Often mistaken for a tornado
Ground Contact - Tornado
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Once the rotation reaches the ground, the
downward moving air will spread out
Some will go back toward the center of the
funnel, converging and forcing it back up
The upward motion will begin to kick up debris
Suction Vortices
• Many violent tornadoes
contain smaller whirls that
rotate inside them
• Rotate faster, and do a
great deal of damage
• How these form is still not
completely understood
Damage
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The highest (strongest) winds on Earth are
found inside tornadoes
The strongest tornado ever recorded had winds
over double that of the strongest hurricane
Damage can be devastating
Fujita Scale
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In 1973, Ted Fujita of the Univ. of Chicago devised a
scale for rating the intensity of a tornado
Subjective damage scale that classified a tornado on a
scale from F0 to F5
Assessed by going to damage sites and using a
checklist
Enhanced Fujita Scale
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Proposed in early 2005, adopted in 2007
Replaces Fujita Scale
Uses more criteria to assess damage
Has 28 “damage indicators” that surveyors look at
FUJITA SCALE
DERIVED EF SCALE
OPERATIONAL EF
SCALE
F
Numbe
r
Fastest 1/4mile (mph)
3 Second
Gust (mph)
EF
Number
3 Second
Gust
(mph)
EF
Number
3 Second
Gust (mph)
0
40-72
45-78
0
65-85
0
65-85
1
73-112
79-117
1
86-109
1
86-110
2
113-157
118-161
2
110-137
2
111-135
3
158-207
162-209
3
138-167
3
136-165
4
208-260
210-261
4
168-199
4
166-200
5
261-318
262-317
5
200-234
5
Over 200
http://www.spc.noaa.gov/efscale/ef-scale.html
EF0 - “Light damage”
EF1 - “Moderate damage”
EF2 - “Considerable damage”
EF4 - “Devastating damage”
EF3 - “Severe damage”
EF5 - “Incredible damage”