Tornado
Workshop
Contents
Langen, Germany, 25 February 2005
• 1. Basics
– Parcel Theory
– Perturbation pressure field
– Updraft Rotation
• 2. Thunderstorm Classes
– The use of avoiding classifying thunderstorm
structures
– Single-, multi- and supercell as special cases of a
rather generic concept
• 3. Forecasting Thunderstorm Classes
Tornado
Workshop
1.1 Basic Parcel Theory
Langen, Germany, 25 February 2005
Boussinesq approximation:
Dw
1 p

B
Dt
 z
where

B  g

Non-hydrostatic pressure gradient force neglected,
and only the „Archimedian buoyancy force“ considered

Bdz  R 
B 0
T ' 0
T d (ln p)  CAPE
CAPE: Convective Available
Potential Energy
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Workshop
1.2 Skew T-log p diagram
Langen, Germany, 25 February 2005
• The green area is proportional
to CAPE
• Convective initiation in
Parcel Theory:
A convective cell is initiated if
a moist parcel is lifted above
its LFC.
Source: NWS Norman
Tornado
Workshop
1.3.1 Updraft Rotation
Langen, Germany, 25 February 2005
Linearized vorticity equation
D '
 ωh   h w'
Dt
The tilting of ambient vorticity:
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Workshop
1.3.2 Updraft Rotation
Langen, Germany, 25 February 2005
v
SRH  k   v inf ( z )  dz
z
zo
z
Storm-Relative Helicity
Tornado
Workshop
1.4 Pressure Perturbations
Langen, Germany, 25 February 2005
Dw
1 p

B
Dt
 z
... retaining the perturbation-pressure
terms in the vertical momentum
equation ...
The perturbation pressure field p‘ can be found by solving this equation:
1 2 B
  p'  e  ω 

2
z
1
Forcing related to:
2
Deformation
(Splat)
2
Rotation
(Spin)
Gradient of
Buoyancy
Tornado
Workshop
2.1.1 Classification
Langen, Germany, 25 February 2005
• Nature is continuous, any classification scheme naturally
arbitrary
• There are many structures that do not readily fit into a tight
classification scheme
• Too strong mental adherance to the archetypal structures
may limit one‘s ability to deal with a given („nonarchetypal“) situation
• Goal should thus be:
Use physical concept which describes all convective
structures, and consider certain classes merely as „special
cases“ in the continuous spectrum
Tornado
Workshop
2.1.2 Classification
Langen, Germany, 25 February 2005
Possible approach to avoid classification
• Convective cells develop where moist parcels are
lifted to their LFCs
• Strength of the (mainly) vertical accelerations
governed by the thermal buoyancy and perturbation
pressure gradient forces
• Rotational characteristics of the cells are determined
by the nature of the vorticity ingested by the updraft
• All contributions interact with each other!
Tornado
Workshop
2.2 Archetypes
Langen, Germany, 25 February 2005
• Single-cell thunderstorm
• Isolated supercell thunderstorm
• Multicell thunderstorm
– Weakly organized clusters
– Long-lived, well organized
(e.g. squall lines containing supercells and
bow echoes)
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Workshop
2.3 Single Cell Thunderstorm
Langen, Germany, 25 February 2005
• Localized low-level forcing (in terms of space/time)
• Weak/no wind shear (minimal dynamical contribution
to p‘)
• Weak/no vorticity in the thunderstorm inflow
Source:
Skywarn
Tornado
Workshop
2.4.1 Isolated Supercell
Langen, Germany, 25 February 2005
• Low-level forcing: Localized in space (moving
with the storm), persistent in time
• Large contributions to dynamical p‘ (windshear/updraft interaction, rotation)
• Large helicity in the inflow (updraft rotation)
A supercell is characterized by the presence of a deep,
persistent mesocyclone.
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Workshop
2.4.2 Isolated Supercell
Langen, Germany, 25 February 2005
(c) C.A.D. 3.0
Tornado
Workshop
2.5.1 Multicell Thunderstorm
Langen, Germany, 25 February 2005
• Low-level forcing: Spatially extensive,
persistent
• Strong contributions to p‘
(vertical wind shear, cold pool)
• Vorticity in the inflow (likely generated
along the cold pool), book-end vortices
Tornado
Workshop
2.5.2 Multicell Thunderstorm
Langen, Germany, 25 February 2005
(c) R. Houze, 1993, taken from www.mcwar.com
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Workshop
2.6 Upshear Tilt of an MCS
Langen, Germany, 25 February 2005
• Perturbation pressure gradient forces
cause convective updrafts to tilt upshear
Tornado
Workshop
Langen, Germany, 25 February 2005
Source: BAMEX
2.7 Bow Echo
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Workshop
3. Forecasting Thunderstorm Types
Langen, Germany, 25 February 2005
• Results based on idealized (numerical) models
concerning detailed shape of the
shear/thermodynamic profiles can seldom be
„literally translated“ into the real world
– Shear (and thermodynamic) profiles appear to vary
substantially in space and time, available data
unlikely to be representative for the environment of
a given storm
– Often, several storm structures occur at a time, or
storms morph from one type into another during
the their life time
Tornado
Workshop
3.2.1 „Classical Environments“
Langen, Germany, 25 February 2005
Subjective SFC analysis – Berliner Wetterkarte 12Z
De Bilt
June 18th, 2002
Tornado
Workshop
3.2.2 „Classical Environments“
Langen, Germany, 25 February 2005
500 hPa analysis – Berliner Wetterkarte 00Z
June 18th, 2002
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Workshop
3.2.3 „Classical Environments“
Langen, Germany, 25 February 2005
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Workshop
3.2.4 „Classical Environments“
Langen, Germany, 25 February 2005
(c) D. Kiese, M. Hubrig, S. Lueke
Tornado
Workshop
3.3.1 „Classical Environments“
Langen, Germany, 25 February 2005
Tornado
Workshop
3.3.2 „Classical Environments“
Langen, Germany, 25 February 2005
(c) W. Stieglmair
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Workshop
3.3.3 „Classical Environments“
Langen, Germany, 25 February 2005
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Workshop
4. Conclusions
Langen, Germany, 25 February 2005
• Thunderstorm structure is determinded by
– Morphology of the low-level forcing
– Thermodynamic profiles
– Kinematic profiles
• While severe thunderstorm threat can be forecasted reasonably
well, exact forecasts of dominant cell structure often very difficult
• Supercells may occur whenever convection develops in strongly
sheared environment (model-/sounding-derived SRH not
necessarily high)
• A slight chance of mesocyclones exists any time convection is
underway (i.e., one should never be surprised to see one on
radar, even though all available data might not have suggested
that supercells would be possible)
Tornado
Workshop
Langen, Germany, 25 February 2005
Thank you for your attention!
Questions?
Dahl@estofex.org