Lesson 1 – Ingredients for
severe thunderstorms
B. Barrett – SO441
Synoptic Meteorology
A severe thunderstorm near Lusk, WY 18 May 2014
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
Lifting mechanisms in the atmosphere
• Convective
heating
• Convergence
along a density
gradient
• Motion up
topography
• Convergence into
surface low
pressure
Source: http://web.gccaz.edu/~lnewman/gph111/topic_units/moisture/moisture_stabil_prec/4_lifting.jpg
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
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: http://www.metoffice.gov.uk/media/image/o/1/
Lightning_Strikes_map_%28Credit_NASA%29.jpg
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, southsouthwest 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
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:


SRH    uenv  c   kˆ     uenv  dz
Storm-relative helicity
• Storm-relative helicity can be calculated as:
SRH    uenv  c    kˆ    uenv   dz
• It can be approximated as:
  u 
 v  
SRH    vsr 
  usr    z
 z  
  z 
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
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: http://tornado.sfsu.edu/geosciences/classes/m500/Shear_Helicity/Helicity.htm
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.