Stability and Severe Storms

AOS 101 Discussion Sections 302 and 303

What is a Severe Thunderstorm?

NWS Severe Thunderstorm Definition Hail of 1 inch diameter or

largerAnd/or Wind gusts 58 mph or

greater A tornado

Warnings typically last one hour

What is required to create thunderstorms?

Stability

Rock on a Hill Rock in the Ditch

Unstable Stable

An Air Parcel An invisible, imaginary, and infinitely elastic

container Usually a cubic meter in size (1 m3)

It is used to “test” the atmosphere It does not actually occur in nature Can be used to determine how a portion of the

atmosphere evolves Can be roughly shown with weather balloons Shows atmospheric instability

Extremely useful when studying heat and energy exchange in the atmosphere

voices.washingtonpost.comupload.wikimedia.org www.unca.ed

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Air Parcel Stability

Stability

Rock on a Hill Rock in the Ditch

Tp > Te Tp < Te

Unstable Stable

If Tp = Te then the parcel is neutral

Absolutely Stable

Absolutely Stable – Temperature Inversion

Absolute Instability

Conditionally Unstable

How to “Test” the Atmosphere Lift the parcel along the

dry adiabat Once the parcel reaches

dew point it becomes saturated and rises along moist adiabat

Parcel Reaches Saturation – Lifting Condensation Level (LCL)

Level of Free Convection

Equilibrium Level (EL)

Example Skew-T

Dew Point – GreenTemp – RedParcel – YellowDew Point Trace – BlueLCL – Lifting Condensation LevelLFC – Level of FreeConvectionEL – Equilibrium Level

When and Why Do Thunderstorms Occur? During the spring and

summer, certain atmospheric conditions can be set up which drive severe convective storms: Conditionally unstable

atmosphere Moisture Upward vertical motion

(“Lifting”) Wind shear

Moisture Since a conditionally unstable atmosphere

is only unstable with respect to an saturated air parcel, some moisture source is required to create severe weather

Vertical Lifting An unstable

atmosphere will only generate severe weather when it is given a “push”

Unstable air parcels can be lifted by the following mechanisms: Convection Convergence Frontal forcing Topography

Convection

Convergence

Frontal Forcing: Cold Fronts

Frontal Forcing: Warm Fronts

Topography

Wind Shear

Wind shear is a term which describes how the speed and direction of the wind change with height Critically important for the

formation of severe weather Wind shear will keep upward

motion and downward motion in the storm separated, allowing the storm to survive

Directional Wind Shear Wind changes direction with height Wind is backing if the wind direction rotates

counterclockwise with height Wind is veering if the wind direction rotates

clockwise with height Generally, severe weather will only be found if

the wind is veering with height

Life Cycle of a Storm Building block for any thunderstorm is a

thunderstorm cell Typical thunderstorm lasts approx. 30 minutes Three stages

Developing stage Mature Cumulus stage Dissipating Stage

Developing Stage Warm, humid air rises and

develops an updraft Air parcels saturate and

form a towering cumulus cloud

Little or no rainfall Lasts near 10 minutes No severe weather yet

Mature Cumulus Phase Precipitation begins to fall,

creating downward motion (downdraft)

When downdraft hits the ground, it spreads out and creates a “gust front”

Storm develops overshooting (“anvil”) top

Lasts an average of 10-20 minutes.

Most likely time for severe weather (large hail, gusty winds, tornadoes)

Dissipating Phase Downdraft dominates

and shuts off the updraft Gust front moves out

ahead of the storm and cuts off inflow of warm, moist air

Severe weather threat diminished

Lightning still a threat Other storms may

develop along outflow boundary

Hail Hail forms from a large updraft and collision-

condensation

Lightning Lightning is a result of cloud ionization

discharging with the ground Thunder comes from the extreme heating

from a lightning strike creating a sonic boom VIDEO

Types of Thunderstorms Ordinary Cell Multi-cell Cluster Multi-cell Line (aka Squall line) Supercell thunderstorm

Supercell Thunderstorm

Tornadoes A tornado is defined as “a violently rotating

column of air descending from a thunderstorm and IN CONTACT with the ground.” -NWS

Can sometimes last for more than an hour and travel several miles

Most violent tornadoes are capable of tremendous destruction with wind speeds of 250 mph or more

Tornadoes in the winter and early spring are often associated with strong frontal systems that form in the Central US and move east

How are tornadoes formed? Before thunderstorms develop, a change in

wind shear creates an invisible, horizontal spinning effect near the surface.

How are tornadoes formed? Rising air within the thunderstorm updraft tilts

the rotating air from horizontal to vertical

How are tornadoes formed? An area of rotation, 2-6 miles wide, now

extends through much of the storm. Most strong and violent tornadoes form within

this area of strong rotation

Tornado Video Tornado Time Lapse Video