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”