Severe Weather ATS 351 Lecture 10 November 9, 2009.

Severe Weather

ATS 351Lecture 10

November 9, 2009

Types of Severe Weather

ThunderstormsHailLightningFloodTornadoSevere Wind (Straight-Line Winds)

Thunderstorm Distribution

Favorable Conditions

InstabilityFuelInitial LiftShear Capping Inversion

Instability

Steep lapse rateMeans warm, moist air near the surfaceColder air above itNeeds to be calculated from a sounding

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- 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

Convective liftingBoundaries

Fronts Drylines Outflow

boundariesOrographicConvergence

Shear

Because of the way a thunderstorm works, it needs to be tilted to remain strong

Therefore, winds need to change with heightTwo kinds of shear

Speed Shear: Wind is faster as you go up Directional Shear: Wind changes direction with

height

Capping Inversion

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 convectionAs ground heats during the day, energy builds

up until it can “break the cap” Also referred to as a “capping inversion” CIN

Back to the Skew-TMeteorologists 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 levle) LI = Tenvironment – Tparcel

Thunderstorm Development

Stages Cumulus Mature Dissipation

Cumulus Stage

Warm moist air rises, condenses

Latent heat release keeps air in cloud warmer than environment

Grows to a towering CuCloud particles grow

larger, begin to fallNo precipitation at

surface

Mature StageMarked by appearance of

downdraft Falling cloud drops evaporate,

cooling the airStorm is most intense during this

stageCloud begins to form anvilMay have an overshooting topLightning and thunder may be

presentGust 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 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

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.

Multi-Cell Storms

Cluster of storms moving as a single unitStronger wind shear than the ordinary

cell caseMore 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

Multiple cells compete 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

Updrafts and Downdrafts

Degree of instability and moisture determine the strengths of updrafts and downdrafts

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

Gust frontsAn 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 cellsLeads to multi-cell stormsProduction 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

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 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

LightningInside a cloud, updrafts and turbulence toss ice

particles aroundEach collision creates a small amount of electric

chargeAfter a few million of those, the charge is too

much to be held back by the airDischarges all at once in a flash of lightning

Lightning

The temperature of lightning is roughly 30,000 degrees CThe surface of the sun is only about 5700

degrees COne 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

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 secondsLightning 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 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

Tornadoes

FormationLife CycleDefinitionTypesDamageEF-scale

Wall cloudsLowering of cloud baseVisible 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

Formation

Tornadogenesis is the formation of tornadoes We know relatively little about this process

Basic formation steps are knownDetails 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

MesocycloneThe new rotating storm is called a mesocycloneCharacterized by rotating updraftAt this point, the rotation can be picked up on Doppler

radar if it is strong enough

Supercell Tornado Formation

Funnel Cloud

Area of rotation that does not touch the groundOften mistaken for a tornado

Ground Contact - TornadoOnce the rotation reaches the ground, the

downward moving air will spread outSome will go back toward the center of the

funnel, converging and forcing it back upThe 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

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 ScaleIn 1973, Ted Fujita of the Univ. of Chicago devised a

scale for rating the intensity of a tornadoSubjective damage scale that classified a tornado on a

scale from F0 to F5Assessed 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 Numbe

r

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”