Understanding Severe Weather Dynamics: Ingredients for Thunderstorms

1. AOS 101 Severe Weather April 1/3

2. Lifting Condensation Level (LCL) • Level at which a parcel lifted from the surface would reach saturation (i.e. level where temperature = dewpoint) • Last week: DALR →T decreases at 10oC/km • In addition, dewpoint (Td) decreases at 2oC/km • LCL will coincide with how high above the ground the cloud bottom is.

3. Level of Free Convection (LFC) • Level at which a parcel raised from the surface would become warmer than the environment. • Above this level air is able to freely convect, or ascend unabated to the tropopause. Equilibrium Level (EL) • Level at which parcel is no longer warmer than the environment • Usually near the tropopause where environmental lapse rate is near 0oC/km • Coincides with height of cloud top.

4. 12 EL 10 Tparcel 8 Tenv SATURATED HEIGHT (km) 6 LFC 4 2 LCL Td T -40 -30 -20 -10 0 10 20 30 TEMPERATURE (oC)

5. Ingredients for Thunderstorms • Instability • Lifting mechanism • Shear • Moisture

6. 1. INSTABILITY • In an unstable atmosphere, lifted parcels of air will be warmer than its surroundings. • In this situation, a buoyancy force acts on the object, accelerating it upward.

7. Buoyancy Force • The larger the difference in Tparcel and Tenv ,the larger the force and acceleration (F = ma) • More buoyancy leads to stronger updrafts, up to 50 m s-1

8. CAPE (Convective Available Potential Energy) • A measure of how buoyant parcels will be as they ascend in a thunderstorm cloud. • Higher CAPE means stronger updrafts and more intense thunderstorms. • Is equal to the area between parcel path and environmental temperature curve when the parcel is warmer (between the LFC and EL).

9. 12 EL 10 8 CAPE SATURATED HEIGHT (km) 6 LFC 4 2 -40 -30 -20 -10 0 10 20 30 TEMPERATURE (oC)

10. 2. LIFTING MECHANISM • On the typical summer day, the atmosphere will be conditionally unstable. • Stable for unsaturated parcels • Unstable for saturated parcels • Surface (unsaturated) parcels will not be able to rise on their own. • Some mechanism must raise the parcel until it reaches saturation (LCL) and then past a level at which it is warmer than its surroundings (LFC).

11. FREELY CONVECT Tp=14oC LFC 2 km: Te=14oC SATURATED LIFTING MECHANISM Tp=20oC 1 km: Te=22oC LCL UNSATURATED 0 km: Te=30oC Tp=30oC, Td=22oC

12. Examples • Convergence of winds • Orography (upslope) • Intense surface heating • Outflow boundary from storm in the vicinity

13. 3. SHEAR • Difference in winds with height • 2 types: • Directional shear (wind changes direction with height). • Speed shear (same direction of winds, but speed increases with height). 2 km 1 km 3 km sfc DIRECTIONAL 3 km 2 km 1 km sfc SPEED

14. Air mass thunderstorm CUMULUS MATURE DISSIPATING COOL DOWNDRAFT WARM MOIST UPDRAFT LIGHT RAIN HEAVY RAIN

15. Why shear is needed… • With no shear, downdrafts cut off source of moisture (updraft) • Vertical shear displaces downdraft from updraft, allowing t’storm to continually replenish moisture • Storms last longer and become more intense.

16. 4. MOISTURE • Thunderstorms need plentiful source of moisture to drive circulation • High moisture →High dewpoint →Low dewpoint depression →Low LCL →Low LFC →Less lifting needed for free convection (also more CAPE).

17. 12 EL 10 TEMPERATURE 30oC DEWPOINT 14oC 8 CAPE HEIGHT (km) 6 LFC 4 2 LCL Td T -40 -30 -20 -10 0 10 20 30 TEMPERATURE (oC)

18. EL 12 10 TEMPERATURE 30oC DEWPOINT 22oC 8 CAPE HEIGHT (km) 6 4 LFC 2 LCL Td -40 -30 -20 -10 0 10 20 30 TEMPERATURE (oC)

19. Severe Weather Criteria • Wind in excess of 58 mph • Hail larger than .75 inches • Tornado

20. WIND • 58 MPH wind gust or higher • Strong winds are created by downdrafts which strike the ground and spread out • Downburst, Microburst • Can exceed 120 mph (weak tornado strength)

21. HAIL • .75 inch hail (nickel size) or greater • Strong updrafts keep ice chucks aloft so that more water freezes to them • Record hailstone: 7.0 inches (volleyball size) • Aurora, Neb. • June 22, 2003

22. TORNADO • Can have winds up to 318 mph • Moore, OK – May 3, 1999 • Stay on the ground for an hour or more • Record: 1924 Tri-State Tornado (3.5 hours) • Track for tens of miles • Record: Tri-state, 219 miles • Up to a mile wide • Record: 2.5 miles – Hallam, NE – May 22, 2004

23. Ranked using the Fajita Scale • Assessors look at damage, than match to wind speeds • F0 = weakest, F5 = strongest • Only 1% are F4 or F5 • Last F5: Greenville, KS 2007 (first since 1999)

24. Types of Thuderstorms • Airmass (usually not severe) • Multi-cell • Squall line (bow echo, derecho) • Supercell

25. Multicell • Groups of cells moving in a line • Outflow of one storm provides lifting mechanism for the next cell in the line • Can “train” over one area for hours • Flash Flooding

26. Squall Line (Bow Echo, Derecho) • Continuous line of storms moving quickly • Typical if speed shear but no directional • Outflow in front of storm lifts air upwards • Can persist for over a day and travel 1000 km or more • Very strong winds (120+ mph)

27. Supercell • Most severe of all storms. • Produce most strong tornadoes (F2+) and large hail (2”+) • Need directional shear, winds turning clockwise with height. • Hook echo (TVS) • Mesocyclone • Overshooting top

29. Mesocyclone

30. Overshooting Top