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

11. 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?

12. 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”.

13. 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.

14. 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)

15. 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

16. 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.

17. “we must look elsewhere for the lifting”

18. 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.