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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 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.com upload.wikimedia.org www.unca.edu Air Parcel Stability Stability Rock on a Hill Rock in the Ditch Unstable Stable Tp > Te Tp < Te 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 – 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 collisioncondensation 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
