Tornado Workshop Langen, Germany, 25 February 2005 Contents 1. Basics –Parcel Theory –Perturbation pressure field –Updraft Rotation 2. Thunderstorm Classes

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Langen, Germany, 25 February 2005

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:

Bz

p

Dt

Dw

1

gB

CAPEpdTRBdzTB

0'0)(ln

Non-hydrostatic pressure gradient force neglected, and only the „Archimedian

buoyancy force“ considered

CAPE: Convective Available Potential Energy

where

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1.2 Skew T-log p diagram

Source: NWS Norman

• 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.

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1.3.1 Updraft Rotation

''

wDt

Dhh ω

Linearized vorticity equation

The tilting of ambient vorticity:

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1.3.2 Updraft Rotation

z

zo

dzz

zSRHv

vk )(infStorm-Relative Helicity

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1.4 Pressure Perturbations

Bz

p

Dt

Dw

1

z

Bep

222

2

1'

1ω

Deformation(Splat)

Rotation(Spin)

Gradient ofBuoyancy

... retaining the perturbation-pressure terms in the vertical momentum equation ...

The perturbation pressure field p‘ can be found by solving this equation:

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 one‘s 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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500 hPa analysis – Berliner Wetterkarte 00Z

June 18th, 2002

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(c) D. Kiese, M. Hubrig, S. Lueke

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(c) W. Stieglmair

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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?

[email protected]

Documents

Tornado Workshop Langen, Germany, 25 February 2005 Contents 1. Basics –Parcel Theory –Perturbation pressure field –Updraft Rotation 2. Thunderstorm Classes