# Stability and Severe Storms AOS 101 Discussion Sections 302 and 303.

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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 larger And/or  Wind gusts 58 mph or greater  A tornado  Warnings typically last one hour  What is required to create thunderstorms?

Stability Rock on a HillRock in the Ditch Unstable Stable

An Air Parcel  An invisible, imaginary, and infinitely elastic container  Usually a cubic meter in size (1 m 3 )  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.com upload.wikimedia.org www.unca.edu

Air Parcel Stability

Stability Rock on a HillRock in the Ditch T p > T e T p < T e Unstable Stable If T p = T e 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 – Green Temp – Red Parcel – Yellow Dew Point Trace – Blue LCL – Lifting Condensation Level LFC – Level of Free Convection EL – 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 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