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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
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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
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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?
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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
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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
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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
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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
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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
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Large Scale Setting850 mb Analysis• Large scale cyclogenesis indicated by negative tilting
trough• Strong low-level jet
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Large Scale Setting500 mb Analysis
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Surface Analysis at 1200 UTC
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Convective Inhibition• Strong capping inversion in place
• Modest low-level moisture• Dewpoints at or below 50⁰F
• Can these negative factors be overcome?
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12 and 00 UTC Topeka, KS Soundings
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LFM Analysis
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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
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Visible Satellite Image 2130 UTC
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Surface Analysis at 2100 UTC
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Cold Front Strengthening
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Vis. Satellite Image at 0000 UTC
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Quasi-Geostrophic Frontogenesis
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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
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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
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Questions?
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Quasi-Geostrophic forcing for vertical motion
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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
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Surface Analysis at 1200 UTC
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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