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

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

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?

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

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

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

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

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

Large Scale Setting850 mb Analysis• Large scale cyclogenesis indicated by negative tilting

trough• Strong low-level jet

Large Scale Setting500 mb Analysis

Surface Analysis at 1200 UTC

Convective Inhibition• Strong capping inversion in place

• Modest low-level moisture• Dewpoints at or below 50⁰F

• Can these negative factors be overcome?

12 and 00 UTC Topeka, KS Soundings

LFM Analysis

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

Visible Satellite Image 2130 UTC

Surface Analysis at 2100 UTC

Cold Front Strengthening

Vis. Satellite Image at 0000 UTC

Quasi-Geostrophic Frontogenesis

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

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

Questions?

Quasi-Geostrophic forcing for vertical motion

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

Surface Analysis at 1200 UTC

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