severe weather ats 351 spring 2010 lecture 11. outline prerequisites for severe weather how...
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Severe Weather
ATS 351Spring 2010Lecture 11
Outline Prerequisites for severe
weather How conditions are
assessed with respect to severe weather
Types of severe weather Single/multi-cell
thunderstorms Squall lines/MCS’s Supercells
Lightning Tornadoes
Types of Severe Weather
Thunderstorms Hail Lightning Flood Tornado Severe Wind
(Straight-Line Winds)
Thunderstorm Distribution
Favorable Conditions
Instability Shear Initial Lift Fuel Restricting
Cap
Instability
Steep lapse rate
Means warm, moist air near the surface
Colder air above it
Needs to be calculated from a sounding
Updrafts and Downdrafts
Degree of instability and moisture determine the strengths of updrafts and downdrafts
Sources of Lift
Convective lifting Boundaries
Fronts Drylines Outflow
boundaries Orographic Convergence
Shear
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
Speed Shear: Wind is faster as you go up Directional Shear: Wind changes direction with
height
Vertical Wind Shear 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
Fuel
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
The storm needs to remain in areas of warm, moist air. If storm moves into a colder region, it will die
Restricting Cap
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 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” Also referred to as a “capping inversion” Remember CIN?
Back to the Skew-T Meteorologists have formulated various
numbers that can tell how favorable the weather is for a storm. These quantities can describe things such as: Instability Shear Or a combination of both
CAPE (Convective Available Potential Energy How unstable atmosphere is
LI (Lifted Index; normally at 500mb level) LI = Tenvironment – Tparcel
Types of Thunderstorms Thunderstorms come in
many varieties Likelihood of severity
proportional to storm lifetime NWS definition of severe
(one or more of the following elements)
¾” or larger diameter hail 50 kt (58 mph) or greater winds tornadoes
Single Cell Thunderstorms
Also referred to as ordinary,
pulse, or air mass thunderstorms Typically do not produce severe
weather Three stages
Cumulus Mature Dissipating
Life span: ~45-60 min.
Cumulus Stage
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 Marked by appearance of
downdraft 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
Downdraft reaches the surface and spreads out in all directions
Gust front forces more warm, humid air into the storm
Dissipation Stage Usually follows mature stage
by ~15-30 min Gust front moves out away
from the storm, and most 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 storms existence
Gust fronts 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
Multi-Cell Storms
Cluster of storms moving as a single unit Stronger wind shear than the ordinary
cell case More organized multi-cells
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
Updraft competition for warm, moist low-level air so not incredibly strong and have short life spans
Multi-cell Storms
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
Mesoscale Convective Systems (MCS’s)
• Individual storms can grow and organize into a large convective system (weak upper level winds)• Nominal 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
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.
Squall Lines
Bow Echoes
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 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 up to 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)
Supercells
Wall clouds 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 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
Wall clouds
Lightning 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 Misconceptions
Lightning comes down from the clouds 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.
Entire process happens in under 0.001 seconds Lightning always hits the tallest object
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 There are many documented cases of lightning hitting twice in
the same spot Sometimes only a few seconds apart!
Lightning Facts 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
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
Lightning Fatalities
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
Tornadoes
Formation Life Cycle Definition Types Damage EF-scale
Formation Tornadogenesis is the
formation of tornadoes 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 Vorticity Tilting
• After horizontal rotation is established, the storm’s updraft works to tilt it upright
• Now the storm has a vertically rotating component
Mesocyclone 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
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
Supercell Tornado Formation
Funnel Cloud Area of rotation that does not touch the ground Often mistaken for a tornado
Ground Contact - Tornado 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
Damage
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 beyond devastation
Damage
Fujita Scale 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 There are none higher (no F6’s).
Assessed by going to damage sites and using a checklist
Enhanced Fujita Scale• 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 SCALEOPERATIONAL EF
SCALE
F Number
Fastest 1/4-mile (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”
EF3 - “Severe damage”
EF4 - “Devastating damage”
EF5 - “Incredible damage”