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. EL LFC LCL Tparcel Tenv SATURATED Td T 12 10 8 HEIGHT (km) 6 4 2 -40
-30
-20
-10 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. CAPE EL LFC SATURATED 12 10 8 HEIGHT (km) 6 4 2 -40 -30 -20 -10 10 2010 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. Tp=14oC LFC Tp=20oC LCL Tp=30oC, Td=22oC
FREELY CONVECT
2 km: Te=14oC
Tp=14oC
LFC
SATURATED
LIFTING MECHANISM
1 km: Te=22oC
Tp=20oC
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. CAPE EL LFC LCL Td T 12 10 8 HEIGHT (km) 6 4 2 -40 -30 -20 -10 10 20
TEMPERATURE 30oC
DEWPOINT 14oC 8
CAPE
HEIGHT (km) 6
LFC 4 2
LCL Td T
-40
-30
-20
-10 10 20 30
TEMPERATURE (oC)

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