THE DISTINCTION BETWEEN LARGESCALE 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 Setting
850 mb Analysis
• Large scale cyclogenesis indicated by negative tilting
trough
• Strong low-level jet
Large Scale Setting
500 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