1. Tornado Workshop 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 –

2. Tornado Workshop Langen, Germany, 25 February 2005 1.1 Basic Parcel Theory Boussinesq approximation: Non-hydrostatic pressure gradient force neglected, and only the Archimedian buoyancy force considered CAPE: Convective Available Potential Energy where

3. Tornado Workshop Langen, Germany, 25 February 2005 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.

4. Tornado Workshop Langen, Germany, 25 February 2005 1.3.1 Updraft Rotation Linearized vorticity equation The tilting of ambient vorticity:

5. Tornado Workshop Langen, Germany, 25 February 2005 1.3.2 Updraft Rotation Storm-Relative Helicity

6. Tornado Workshop Langen, Germany, 25 February 2005 1.4 Pressure Perturbations Deformation (Splat) Rotation (Spin) Gradient of Buoyancy... 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:

7. Tornado Workshop Langen, Germany, 25 February 2005 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

8. Tornado Workshop Langen, Germany, 25 February 2005 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!

9. Tornado Workshop Langen, Germany, 25 February 2005 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)

10. Tornado Workshop Langen, Germany, 25 February 2005 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

11. Tornado Workshop Langen, Germany, 25 February 2005 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.

12. Tornado Workshop Langen, Germany, 25 February 2005 2.4.2 Isolated Supercell (c) C.A.D. 3.0

13. Tornado Workshop Langen, Germany, 25 February 2005 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

14. Tornado Workshop Langen, Germany, 25 February 2005 2.5.2 Multicell Thunderstorm (c) R. Houze, 1993, taken from www.mcwar.com

15. Tornado Workshop Langen, Germany, 25 February 2005 2.6 Upshear Tilt of an MCS Perturbation pressure gradient forces cause convective updrafts to tilt upshear

16. Tornado Workshop Langen, Germany, 25 February 2005 2.7 Bow Echo Source: BAMEX

17. Tornado Workshop Langen, Germany, 25 February 2005 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

18. Tornado Workshop Langen, Germany, 25 February 2005 3.2.1 Classical Environments Subjective SFC analysis – Berliner Wetterkarte 12Z De Bilt June 18th, 2002

19. Tornado Workshop Langen, Germany, 25 February 2005 3.2.2 Classical Environments 500 hPa analysis – Berliner Wetterkarte 00Z June 18th, 2002

20. Tornado Workshop Langen, Germany, 25 February 2005 3.2.3 Classical Environments

21. Tornado Workshop Langen, Germany, 25 February 2005 3.2.4 Classical Environments (c) D. Kiese, M. Hubrig, S. Lueke

22. Tornado Workshop Langen, Germany, 25 February 2005 3.3.1 Classical Environments

23. Tornado Workshop Langen, Germany, 25 February 2005 3.3.2 Classical Environments (c) W. Stieglmair

24. Tornado Workshop Langen, Germany, 25 February 2005 3.3.3 Classical Environments

25. Tornado Workshop Langen, Germany, 25 February 2005 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)

26. Tornado Workshop Langen, Germany, 25 February 2005 Thank you for your attention! Questions? Dahl@estofex.org