1. Thermodynamic Diagrams and Severe Weather

2. What is Severe Weather? Tornado Hail > or = 3/4inch
Wind > 50 knots

3. Convective Available Potential Energy (CAPE)
Remember: Area on a thermodynamic diagram is proportional to energy.
CAPE is also called buoyant energy.
CAPE on a thermodynamic diagram is the area between the parcel and the environment temperatures from the LFC to the EL
CAPE is a measure of instability

4. CAPE

5. CAPE
Note in the figure the CAPE is given in hundreds. So the scale goes from at the lower end to greater than 4500 Jkg-1 at the upper end.

6. Maximum Updraft Speed
If we convert the potential energy of CAPE to a kinetic energy, we can get the maximum speed of any updraft that may develop.

7. Convective Inhibition (CIN)
CIN is NOT negative CAPE!!!!!!
CAPE integrates from the LFC to the EL, CIN integrates from the surface to the LFC
Is a measure of stability
Reported as an absolute value

8. CIN

9. Overcoming Convective Inhibition
A convective outbreak rarely occurs from surface heating alone!
Triggering Mechanisms for T-Storms
fronts
dry lines
sea-breeze fronts
gust fronts from other thunderstorms
atmospheric bouyancy waves
mountains

10. Cap Strength Very important for severe weather to develop
Too little or no cap: happy little cumulus everywhere
Too strong of a cap: nothing happens
Just the right amount of a cap: Severe Weather

12. At the inversion* look at the temperature difference between the parcel and the environment.

13. Shear vs. CAPE
Need a balance between Shear and CAPE for supercell development
Without shear: single, ordinary, airmass thunderstorm which lasts minutes
If shear is too strong: multicellular t-storms
(gust front moves too fast)

14. CAPE and Shear
This figure is the same as the last except that it includes all 242 cases, regardless of what season they occurred in.

15. Shear Just Right 2-D equilibrium: squall line develops
3-D equilibrium: right moving and left moving supercells A B A B L
Left Mover L
Right Mover

16. Bulk Richardson Number (BRN)
BRN= CAPE
1/2Uz2
(where Uz is a measure of the vertical
wind shear)
This is out of the same 242 strong/violent cases.

17. Hodographs South East West North V
Draw wind vectors in direction they are going
This is opposite of how the wind barbs are drawn U
East
West
Wind speed
North

18. Example

19. Straight Line Shear Storm Splitting:
500
Storm Splitting:
R and L storm cells move with mean wind but drift outward
700
850
900
1000

20. Curved Hodograph Emphasizes one of the supercells
Veering (clockwise curve):
right moving supercells
warm air advection in northern hemisphere
Backing (counter clockwise curve):
left moving supercells
warm air advection in southern hemisphere
700
500
300
850
900
1000

21. Straight Line Hodograph
Curved hodograph

22. Helicity
Can be thought of as a measure of the “corkscrew” nature of the winds.
H = velocity dotted with vorticity
= V • ζ
= u (dyw - dzv) - v (dxw - dzu) + w (dxv - dyu)
Higher helicity values relate to a curved hodograph.
large positive values--> emphasize right cell
large negative values--> emphasize left cells
Values near zero relate to a straight line hodograph.

23. CAPE and Helicity Plainfield, IL tornado: CAPE=7000 Helicity=165
Energy Helicity:
This figure shows the inverse relationship between CAPE and helicity. Weak CAPE with strong helicity can produce strong/violent tornados. Strong CAPE with weak helicity can also produce strong/violent tornados. The Plainfield, IL tornado had a CAPE of 7000, but the helicity was only Strong/violent tornados can also develop from moderate CAPE with moderate helicity. In the 242 cases plotted of strong and violent tornados (strong is F2-F3, violent F4-F5) none of them had both a strong CAPE and a strong helicity.

24. Supercell Index
Weights various parameters which are indicative of possible supercell development