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Stability and Thunderstorms

Stability and Thunderstorms. ESS 111 – Climate and Global Change. Atmospheric Stability & Clouds. • Stability – Parcel Theory • Parcel of air expands and contracts freely • Remains as a single unit • No heat exchange with the outside air (Adiabatic Process).

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Stability and Thunderstorms

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  1. Stability and Thunderstorms ESS 111 – Climate and Global Change

  2. Atmospheric Stability & Clouds • Stability – Parcel Theory • Parcel of air expands and contracts freely • Remains as a single unit • No heat exchange with the outside air (Adiabatic Process)

  3. Atmospheric Stability & Clouds • Stability – Adiabatic Process • Parcel expands when lifted due to lessening pressure • Molecular action slows = cools at a steady rate • Compresses when forced down = Warms at a steady rate

  4. Atmospheric Stability & Clouds • Lapse Rate: Rate of cooling (or warming) of the atmosphere: • Environmental Laps Rate = actual temperature as measured by a radiosonde (weather balloon) • Dry Adiabatic Lapse Rate (DALR) – Unsaturated parcel of air = 10° °C per km • Moist Adiabatic Lapse Rate (MALR) – Saturated parcel of air = 6° °C per km.

  5. Cloud & Storm Updraft Development – Mechanisms • Surface Heating – Free Convection • Topography • Convergence of Surface Air • Fronts – Free Convection: Ground heated by radiation • Thermal Forms (Convective Updraft) – Air in contact with warmest ground is warmer and therefore less dense than surrounding air – Air rises…..it cools as it rises….it expands due to lessening pressure acting on it (Adiabatic Processes) – If the rising (and cooling) parcel that started at the surface continues to be warmer than the surrounding air, then the parcel is unstable and will continue to rise. – Numerous unstable parcels over time (tens of minutes) can eventually form a thunderstorm updraft.

  6. Cloud Development Triggers

  7. Convection & Clouds Pockets of warm air rise as thermals with invisible water vapor, and at the dew point temperature condensation creates the cloud base. Rising air from below is replaced by sinking air from above, creating areas of blue sky.

  8. Thunderstorms A cumulonimbus cloud that produces lightning (and hence thunder) Very common (globally) Transport heat and moisture vertically in the atmosphere Most are not severe Severe thunderstorm: a) Hail > 1 inch diameter (¾ inch prior) b) Wind gust >50 kt (58 mph) c) Tornado Ingredients for thunderstorm initiation: a) Water vapor b) Instability c) Lifting mechanism For a severe thunderstorm, a 4th ingredient is: d) Strong vertical wind shear 8 (change in wind speed / direction)

  9. Thunderstorm Facts & Climatology • At any given time there are 2000 thunderstorms in progress • About 45,000 thunderstorms take place each day • Annually, The U.S. experiences about 100,000 thunderstorms. • About 16 million thunderstorms occur annually around the world! • The lightning from these storms strikes Earth about 100 times each second

  10. Thunderstorm Frequency Map

  11. Global Lightning Frequency

  12. Upscale development of thunderstorms Individual thunderstorms • Single cell thunderstorm • Multicell thunderstorm • Supercell thunderstorm – will examine this first Larger-scale systems with embedded showers and/or thunderstorms: • Mesoscale Convective System (MCS) • Squall line (quasi-linear convective system, QLCS) • Hurricane 12

  13. Single-Cell Thunderstorm

  14. Single-Cell Thunderstorm • Life span of about 20 to 30 minutes • Usually not strong enough to produce severe weather • Single Cell Pulse Storms often produce severe weather – Usually strong winds as it collapses – Sometimes small hail

  15. Life cycle of a single cell (air mass) thunderstorm 3 stages of development 1 2 3 Fig. 18.3 Vertical shear is weak (as in the SE during summer) Microbursts (strong downdrafts and their outflows) may form during first part of the mature stage. Form along weak boundaries or convergence zones. Small size: <10 km diameter 15 Short lifetime: <1 h

  16. Towering Cumulus (Cumulus Congestus)

  17. Is the Cumulus Cloud still growing? Wispy soft top = Weaker updraft & dying storm Cauliflower-type hard top = Strong updraft & growing storm

  18. Life Cycle – Mature Stage

  19. Life Cycle – Dissipating (Orphan Anvil)

  20. Air Mass Thunderstorms Summary • Usually weak (but can produce heavy rain in a short period of time). • Usually not severe • Usually move slowly (weak winds aloft) • Often develop and dissipate in less than one hour • Form in a weakly sheared environment and thus have a BUILT-IN SELF-DESTRUCT MECHANISM that guarantees a short lifetime (downdraft collapsing down on the updraft).

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