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. 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.! “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 Educational! Easy! 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 Until it reaches its dew point Then rises and cools along a saturated adiabat Air rises, expands, and cools along a dry 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 Then lift along the saturated adiabat Lift along the dry adiabat and the saturation mixing ratio until they meet

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. Morning Inversion Morning air is often “inverted:” ground cools faster than air overnight

23. Thunderstorm Varieties

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

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

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

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

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

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

30. Thunderstorm Effects

31. U.S. Weather-Related Deaths Average deaths per year Flood136 Lightning85 Tornado73 Hurricane25 Hail1 Source: Ackerman and Knox, Meteorology: Understanding the Atmosphere

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

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

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

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