Contents

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Contents

• 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

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1.1 Basic Parcel Theory

• Boussinesq approximation

where
Non-hydrostatic pressure gradient force
neglected, and only the Archimedian buoyancy
force considered

CAPE Convective Available Potential
Energy
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1.2 Skew T-log p diagram

• 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
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1.3.1 Updraft Rotation
Linearized vorticity equation
The tilting of ambient vorticity
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1.3.2 Updraft Rotation
Storm-Relative Helicity
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1.4 Pressure Perturbations
... retaining the perturbation-pressure terms in
the vertical momentum equation ...
The perturbation pressure field p can be found
by solving this equation
Rotation (Spin)
Gradient of Buoyancy
Deformation (Splat)
Forcing related to
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2.1.1 Classification

• 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 ones ability to deal with a
  given (non-archetypal) 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

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2.1.2 Classification

• 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!

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2.2 Archetypes

• 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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2.3 Single Cell Thunderstorm

• 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
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2.4.1 Isolated Supercell

• Low-level forcing Localized in space (moving
  with the storm), persistent in time
• Large contributions to dynamical p
  (wind-shear/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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2.4.2 Isolated Supercell
(c) C.A.D. 3.0
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2.5.1 Multicell Thunderstorm

• 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

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2.5.2 Multicell Thunderstorm
(c) R. Houze, 1993, taken from www.mcwar.com
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2.6 Upshear Tilt of an MCS

• Perturbation pressure gradient forces cause
  convective updrafts to tilt upshear

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2.7 Bow Echo
Source BAMEX
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3. Forecasting Thunderstorm Types

• 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

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3.2.1 Classical Environments
Subjective SFC analysis Berliner Wetterkarte 12Z
De Bilt
June 18th, 2002
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3.2.2 Classical Environments
500 hPa analysis Berliner Wetterkarte 00Z
June 18th, 2002
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3.2.3 Classical Environments
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3.2.4 Classical Environments
(c) D. Kiese, M. Hubrig, S. Lueke
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3.3.1 Classical Environments
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3.3.2 Classical Environments
(c) W. Stieglmair
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3.3.3 Classical Environments
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4. Conclusions

• 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)

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Thank you for your attention!
Questions? Dahl_at_estofex.org