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The Distinction between Large-Scale and Mesoscale Contribution to Severe Convection

This case study explores the dynamic and thermodynamic features of a tornadic severe thunderstorm in Topeka, Kansas in 1983. It analyzes the roles of large-scale and mesoscale processes in convective initiation and highlights the need for forecasting mesoscale processes.

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The Distinction between Large-Scale and Mesoscale Contribution to Severe Convection

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  1. The Distinction between Large-Scale and Mesoscale Contribution to Severe Convection: A Case Study Example A summary of Charles A. Doswell III’s: By Matt Gough

  2. OVERVIEW • Why did Doswell choose this event? • Large-Scale setting • “subsynoptic” setting • Define large-scale and mesoscale processes • Dynamic/Thermodynamic features • Conclusions

  3. Late in the day of May 6, 1983 a tornadic severe thunderstorm struck Topeka, Kansas. • F3 • 1 fatality • 25 injuries

  4. So, why did Doswell choose this storm? • No widespread intense convection as in the case of previous case studies. • Here he can separate out the synoptic scale from the mesoscale. • Mesoscale processes hard to predict. “… one never proves hypothesis…” - Doswell

  5. Large scale: cyclogenesis 12Z 850mb map

  6. 12Z 500mb

  7. Surface analysis: • Dryline • Developing cold front

  8. Two major obstacles to convective development: • Capping inversion • Modest low-level moisture • Note CAPE

  9. Sub-synoptic features • By 2130Z strong convection over Nebraska and isolated thunderstorms over western Kansas. • Cold front intensifies and approaches the dryline. • By 000Z front intersects dryline and a squall line developes (includes the Topeka tornado)

  10. Doswell believed that it was not coincidental that deep convection began with the arrival of the cold front. • How intense was the lifting associated with the front? • Is this lifting enough to initiate convection despite the capping inversion?

  11. Defining the roles of large-scale and mesoscale processes • 3 ingredients of Deep Moist Convection: • Moisture, Conditional instability, lift • “If large scale lifting were the main initiating mechanism, deep convection would be expected to begin as relatively extensive cloudiness before breaking down into individual convective elements” • “one can conclude that the lift needed to start deep convection is generally a product of MESOSCALE processes”.

  12. So, how do you separate mesoscale from large-scale???? • large scale = quasi-geostrophic • mesoscale – defined as “between” scales. Must consider both large-scale and microscale parameters.

  13. Q-G forcing for vertical motion • Forcing in Iowa and Missouri decrease w/ht • Forcing in CO, WY, Mont., increase w/ht. This tilting supports cyclogenesis (Holton 9.2.1)

  14. Barotropic = no temp gradient wrt. Pressure • For upward forcing, must have convergence of Q: negative Horizontal variation in geostrophic wind Horizontal temp/thck gradient wrt. pressure

  15. But, (Hoskins and Pedder, 1980) the equation above shows no large-scale frontogenesis in west Kansas at both 00z and 12z where the cold front formed. Can’t explain frontogenesis completely with Q-G.

  16. “we must look elsewhere for the lifting”

  17. Summary and Discussion • “Large scale flows create a favorable environment while mesoscale processes provide the lift needed for convective initiation.” • Mesoscale processes must be coupled with large scale analysis by forecasters. • How to forecast meso-scale processes remains unanswered.

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