1. Paper by Charles A. Doswell III Powerpoint by Christopher J. Stumpf
The Distinction between Large-Scale and Mesoscale Contribution to Severe Convection: A Case Study Example
Paper by Charles A. Doswell III
Powerpoint by Christopher J. Stumpf

2. The May 6th 1983 Topeka, KS Tornado
A Squall line moved through on the evening of the 6th
Embedded in the squall a tornadic storm formed leaving 1 fatality and injuring 25 people, the tornado was rated an F3
To determine what caused this storm to develop me must understand both the large-scale and mesoscale processes on this day
The Event was not part of a widespread convective outbreak, which are the more typically studied events.

3. Differences between Large-Scale and Mesoscale definitions and terms
Large-Scale Processes
Synoptic and sub-synoptic
Mesoscale Processes
Thermodynamic environment
Usually separated on an order of magnitude basis
How do we distinguish between where large scale end and mesoscale begins?

4. Distinguishing between large-scale and mesoscale
Large-scale processes can be restricted to:
Adiabatic
Hydrostatic
Mass continuity must be satisfied
Advection is dominated by the geostrophic wind
Variation of Coriolis parameter is insignificant
Quasi-geostrophic forcing
Omega equation
Height tendency

5. Mesoscale Processes Mesoscale stands in between large and small scales
Defined as processes which cannot be understood without considering the large scale and microscale processes

6. Defining the Roles of Large-Scale and Mesoscale Processes
Deep Moist Convection can be broken down into three ingredients
Moisture
Conditional Instability
Source of lift
Moisture and Instability can be combined in CAPE.
However, lift needs to be addressed separately
Remove any one of these processes and you no longer have DMC. However, you may still get significant weather just not DMC. Other factors such as VWP can influence the type of conv.

7. Lift Rarely is the environment completely unstable
Significant lift is required to overcome the negative buoyancy before a rising air parcel can reach its LFC
Large-Scale vertical motions (cm/s) are simply too small to accomplish the needed lift in a reasonable time
Large-scale processes however setup the environments necessary for convection to occur but do not initiate convection
It is instead the mesoscale processes which initiate lift.

8. Dynamic and Thermodynamic Factors
A. Large Scale Setting
850 and 500 mb analysis
Surface low pressure
Topeka, KS Sounding
Limited-Area, Fine-Mesh Model (LFM) analysis
B. Sub-synoptic Features
Thunderstorms near NE and KS border
Dryline in Western KS, OK and TX
Sfc. Pressure Rises

9. Large Scale Setting 850 mb Analysis
Large scale cyclogenesis indicated by negative tilting trough
Strong low-level jet

10. Large Scale Setting 500 mb Analysis
Notice negatively tilted trough/how the lows are stacked. Cyclone is in the development stage.

11. Surface Analysis at 1200 UTC
Complex set of surface features with fairly strong southerly flow, dryline extending through Western KS and OK and into W-Central TX. Cold front extending into CO. Pressure is solid contours….Isotherms are the dashed contours.
Note the sfc. Pressure trough this mesoscale feature could create localized convergence and create lift that is non-existent on the synoptic scale.

12. Large lapse rates means…the environment is potentially more unstable.
However, two obstacles to convection are present on this day…1. A strong capping inversion and 2. modest low-level moisture (dewpoints below 50 deg. F)

13. Convective Inhibition
Strong capping inversion in place
Modest low-level moisture
Dewpoints at or below 50⁰F
Can these negative factors be overcome?
A few possible means to diminish these negative factors? Forecasters might consider these factors when making their forecast for convection
Strong low-level jet could import more moisture thus increasing CAPE and decreasing the LCL and LFC
Vertical motion associated with the cyclone coupled with strong daytime heating could weaken capping inversion

14. 12 and 00 UTC Topeka, KS Soundings
Note strong capping inversion at 12Z and no capping inversion at 00Z
The darker lines are for the 00Z sounding. This was released just ahead of the approaching squall line.

15. LFM Analysis
Limited-area Fine Mesh model is consistent with morning observations about the evolution of the cyclone. However, the model has problems addressing a few factors specifically the strength and movement of the surface cold front.

17. Dynamic and Thermodynamic Factors
A. Large Scale Setting
850 and 500 mb analysis
Surface low pressure
Topeka, KS Sounding
Limited-Area, Fine-Mesh Model (LFM) analysis
B. Sub-synoptic Features
Thunderstorms near NE and KS border
Dryline in Western KS, OK and TX
Sfc. Pressure Rises

18. Visible Satellite Image 2130 UTC
Note convection over NE already occurring at this time.

19. Surface Analysis at 2100 UTC
Sfc. Cold front is catching up to the dryline and will soon overtake it after 2100Z. Sfc. Trough near Topeka, KS this feature could have provided enough lift to enhance the convection as it began to develop. Wind speeds have increased and sfc. Trough seems to have strengthened compared to earlier.

20. Cold Front Strengthening
2-h pressure rises (solid lines) and falls (dashed lines). The track of the sfc low is plotted, we can see strong pressure rises behind cold front indicating the strengthening of the front.

21. Vis. Satellite Image at 0000 UTC
Notice the convection/squall line forming/developed over KS. Doswell states that its not coincidental that deep convection began with the arrival of the front.

22. Quasi-Geostrophic Frontogenesis
Little to no frontogenesis located in C & W KS. The LFM indicates the presence of a front however the diagnostics indicate that this front cannot be explained by large scale processes. Is there a disconnect between large scale processes and mesoscale processes?

23. Summary of Event
The large-scale processes on this day established the environment needed for deep moist convection
However the vertical motion was not sufficient enough to initiate the convection solely by large-scale forcing
Mesoscale processes were needed to initiate the deep convection
Dryline combined with advancing cold front created enough vertical motion to overcome the negative buoyancy
The convection/clouds over NE and clear skies in KS allowed for differential heating to take place
The large scale cyclone is responsible for the cold air behind the dryline
May have weakened the capping inversion
Not enough to overcome the CIN

24. Summary of Event
The boundary/front was established by mesoscale processes
After its development the subsequent march across KS was done by large-scale processes/cyclogenesis
Large-scale dynamic processes are not the trigger to convection
The triggers are the mesoscale processes
Large-scale processes can be modified by mesoscale processes and vise versa
CAPE for instance could be diminished by a decrease in moisture

25. Questions?

26. Quasi-Geostrophic forcing for vertical motion
Q-Vector is the combination of differential vorticity and thickness advection into one term. Negative values (convergence of the Q-Vector) imply forcing which favors upward motion. Net result is that quasi-geostrophic forcing tilts westward with height which is supportive of cyclogenesis.

27. Connection between Dynamics and Thermodynamics
Dynamics: large-scale, quasi-geostrophic forcing
Result in increased lapse rates
Its easier to lift air when the lapse rates are large
Thermodynamics: combination of moisture and lapse rate distributions which makes deep moist convection possible

28. Surface Analysis at 1200 UTC
Complex set of surface features with fairly strong southerly flow, dryline extending through Western KS and OK and into W-Central TX. Cold front extending into CO. Pressure is solid contours….Isotherms are the dashed contours.
Note the sfc. Pressure trough this mesoscale feature could create localized convergence and create lift that is non-existent on the synoptic scale.

30. Yes a few equations
Omega equation can be rewritten in terms of Q-Vector Divergence
This combines differential vorticity and thickness advection into one term
This says that if there is forcing for vertical motion that there has to be a horizontal variation in the geostrophic wind (height gradient cannot be uniform)