1. Lesson 1 – Ingredients for severe thunderstorms
B. Barrett – SO441
Synoptic Meteorology
A severe thunderstorm near Lusk, WY 18 May 2014

2. Two basic ingredients for severe thunderstorms
Good buoyancy
Provides strong lift
Wind shear
Keeps warm, buoyant updrafts separate from cold, rainy downdrafts
If buoyancy and moisture are limited, often you simply get shallow convection
But if both are sufficient and in presence of a lifting mechanism, get deep convection

3. Lifting mechanisms in the atmosphere
Convective heating
Convergence along a density gradient
Motion up topography
Convergence into surface low pressure
Source:

4. More on buoyancy
Quantified by Convective Available Potential Energy (CAPE)
CAPE quantifies difference in temperatures from the LCL (lifting condensation level) to the EL (equilibrium level): parcel minus environment
CAPE depends on many factors:
Surface air temperature
Surface dew point temperature
Environmental temperature throughout the troposphere

5. Buoyancy climatology Examine global mean CAPE in November versus May
What similarities do you see? What differences?
Compare mean CAPE to annual lightning flash distribution
Similarities? Differences?
Source: Lightning_Strikes_map_%28Credit_NASA%29.jpg

6. More on wind shear
Wind shear: a change in wind speed and/or direction with height
Speed shear example:
10 kts at surface, 20 kts at 850 mb, 30 kts at 700 mb, 50 kts at 500 mb
Directional shear example:
Southeast at surface, south-southwest at 850 mb, southwest at 700 mb, west at 500 mb
Often wind profile contains both speed and directional shear
Sometimes messy though:
Speeds increase, then decrease, then increase again
Direction veers (like figure at the right), but then backs, then veers again
A wind profile favorable for supercellular thunderstorms

7. Storm-relative helicity
Storm-relative helicity (SRH) measures low-level vertical wind shear as “felt” by a thunderstorm
Storm motion is removed from the calculation
You already understand relative winds. Consider this example: you are jogging to the east at 5 mph and the wind is from the east at 5 mph. You feel a 10 mph “relative” wind. If you were jogging to the west at 5 mph, and the wind was also to the west at 5 mph, you would feel a 0 mph relative wind.
SRH can be calculated as:

8. Storm-relative helicity
Storm-relative helicity can be calculated as:
It can be approximated as:

9. Shear and storm-relative helicity
Assume storm motion is from 225 degrees (from the SW) at 12 m s-1. Calculate the following for this environment:
0-6 km deep-layer shear
0-3 km SRH
0-1 km SRH

10. Buoyancy and low-level shear acting together
In severe thunderstorms, buoyancy and helicity act together:
Low-level helicity gets tilted into the vertical by the thunderstorm updraft!
Source:

11. Tilting of vorticity
Another view of vorticity being tilted into the vertical
Once tilted, buoyancy acts to stretch it
Stretching of vorticity increases it
(Hang on – later in the semester, we will see the vorticity equation)
The greater the buoyancy, the greater the vertical motion and thus greater the stretching
Image source: Penn State Univ.