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Thunderstorms

Thunderstorms. ASTR /GEOL 1070. Physics of Thunderstorms. Two fundamental ideas: Convection Latent heat of vaporization/condensation. Energy Source. Energy difference between Warm, moist surface air Cool, dry upper air. Humid Surface Air. Some energy is “latent” in humidity

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Thunderstorms

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  1. Thunderstorms ASTR /GEOL 1070

  2. Physics of Thunderstorms • Two fundamental ideas: • Convection • Latent heat of vaporization/condensation

  3. Energy Source • Energy difference between • Warm, moist surface air • Cool, dry upper air

  4. Humid Surface Air • Some energy is “latent” in humidity • Lower temperature than if dry • Will not immediately rise • Unstable when condensation starts

  5. Lifting a Surface Parcel • Air rises, expands, and cools at the dry adiabatic lapse rate (fast T drop) • Until it reaches its dew point • Then rises and cools at the moist adiabatic lapse rate (slower T drop) • Warmer than the surrounding air

  6. Lifting Mechanism • “Kick” to bring air to LCL • Uplift over mountains • Advancing cold front • Late afternoon heating

  7. End Result • When moist air finally begins to condense, it becomes very buoyant • Continues rising • If there is enough warm, moist air, it rises all the way to the tropopause

  8. Anvil • Air in the stratosphere becomes warmer with altitude • Cloud stops rising • Piles up at neutral buoyancy • May have overshooting top if energetic

  9. Potential for Convection Lifted Index

  10. Lifted Index • Compares theoretical lifted temperature of surface air to actual temperature of upper air • If (lifted temperature) > (upper-air temperature), parcel unstably rises

  11. Lifted Index • LI = (upper-air temp)−(lifted air temp) • Meanings > 0: stable air; no thunderstorms 0 to −2: possible thunderstorms with lifting mechanism −2 to −6: thunderstorms likely, possibly severe < −6: severe thunderstorms likely

  12. Determine a L.I.! Educational! Easy! • “Lift” parcel, cool at dry lapse rate until saturated • Continue to “lift,” but at saturated lapse rate (less T drop) • Compare to actual air temp at 500 mbar Fun!

  13. Making a Stüve Plot Potential for convective storms

  14. Thermodynamic Plot • Solid slanted lines (dry adiabats) show “lapse rate:” temperature drop with elevation gain • If you know surface T,p you know it for the rising parcel

  15. Thermodynamic Plot • Rising moisture-saturated air condenses • Releases heat • Temperature drop is inhibited • Slanted dashed curves: saturated adiabats

  16. Thermodynamic Plot • Mixing ratio: (mass of water vapor)/(mass of air) (g/kg) • Dotted lines: saturation mixing ratios • Dew point T, p at that mixing ratio

  17. Lifting a Surface Parcel • Air rises, expands, and cools along a dry adiabat… • Until it reaches its dew point • Then rises and cools along a saturated adiabat

  18. Finding the Dew Point • Lift along mixing ratio until it meets the dry adiabat • That is when the moisture begins to coondense

  19. Lifting a Surface Parcel • Lift along the dry adiabat and the saturation mixing ratio until they meet • Then lift along the saturated adiabat

  20. Lifted Index • Lift surface air along dry adiabat until saturation • Then lift along saturated adiabat to 500 mb • Lifted index = (air temp at 500 mb) − (lifted parcel temp at 500 mb) • Best chance of severe thunderstorms when L.I. < −6 °C

  21. Task • Plot the temperatures and dew points • Lift the surface parcel to 500 mb pressure • Determine the lifted index

  22. Thunderstorm Varieties

  23. Single-Cell Storm • Begins as a simple cumulus cloud (Cumulus stage)

  24. Single-Cell Storm • Grows into a towering cumulus cloud • Falling rain creates a downdraft • Mature stage

  25. Single-Cell Storm • Cool air sinks into updraft • Cuts off storm’s energy source (dissipating stage) • Storm dies in a few hours

  26. Multicell Storm: Squall Line • Cold front initiates lifting WARM, MOIST COOL • Storms appear in a line L

  27. Multicell Storm: MSC • Wind shear displaces downdraft • Downdraft from one storm spawns another

  28. Supercell • Requires unstable atmosphere and strong vertical wind shear • “Capping inversion” prevents gradual energy release • Entire storm rotates • Updraft and downdraft in different positions

  29. Thunderstorm Effects

  30. U.S. Weather-Related Deaths Average deaths per year Source: Ackerman and Knox, Meteorology: Understanding the Atmosphere

  31. Lightning and Hail • Wind shear (rising and falling air) causes static charges → lightning • Rain caught in updrafts can freeze—sometimes repeatedly → hail

  32. Hail • Destroy 1% of world agricultural production annually • “Hail Alley:” Denver basin

  33. Angular Momentum • Moving toward a rotational axis causes spin to speed up!

  34. Tornadoes • Usually arise in supercells • Horizontal wind shear causes horizontal-axis rotation • Updraft re-orients vortex

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