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Mesoscale Processes and Severe Convective Weather Richard H. Johnson and Brian E. Mapes Presentation by Chris Medjber. Severe Convective Storms, Meteorological Monographs , 28, no. 50, American Meteorological Society, pg 71-122. Main Topics. Mesoscale Mechanisms for Environment

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Mesoscale Processes and Severe Convective WeatherRichard H. Johnson and Brian E. MapesPresentation by Chris Medjber

Severe Convective Storms, Meteorological Monographs, 28, no. 50, American Meteorological Society, pg 71-122

main topics

Main Topics

Mesoscale Mechanisms for Environment


Convection Triggers

mesoscale mechanisms for environment preconditioning

Mesoscale Mechanisms for Environment Preconditioning


For severe storms to develop, synoptic and/or mesoscale processes must act to provide adequate moisture and instability for convection to initiate.

Once initiation has begun, the interaction of convection with the shear environment produces a pattern of storm evolution that can lead to severe weather.

mesoscale preconditioning processes for severe weather

Differential Advection

Creation of capping inversion


Formation of deep, dry PBL

(leading to microbursts)

Convergence Lines



Sea/Land/Lake breezes

Mountain/Valley breezes

Moisture Advection

Increase CAPE, lower LFC

Local cumulus moistening


Secondary Circulations

Geostrophic adjustment


Gravity Currents, Waves

Cold pool lifting

Localized reduction of CIN

Modification of vertical shear

Mesoscale Instabilities

Boundary Layer Processes

Horizontal convective rolls

Inertial oscillation (low-level jets)

Mesoscale Preconditioning Processes for Severe Weather
moisture advection
Moisture Advection


  • Low-level jets


  • Increased CAPE
  • Lowered LFC
  • Promotes new cloud growth
  • MCCs
differential advection
Differential Advection


  • Low-level jets
  • Ageostrophic circulations about a mesoscale jet-streak
  • Transport of clouds and moisture aloft downstream of mountain barriers
  • Jet-streak circulations and boundary layer heating changing rapidly over short periods of time
differential advection cont d
Differential Advection (Cont’d)


  • Long-lived bow echoes and MCCs or mesoscale vorticity centers
  • Dry microbursts
  • Convective outbreaks
  • On the synoptic scale, differential advection can cause destabilization, vertical wind shear, or establish capping inversions
converging lines
Converging Lines


  • Cross-front circulations along cold, warm, stationary, or quasi-stationary fronts
  • Precipitation-driven convective downdrafts (“gust fronts”)
  • Drylines
  • Sea and land breezes
  • Mountain/Valley breezes
converging lines cont d
Converging Lines (Cont’d)


  • Destabilization of the environment
  • Reduces CIN to the point where severe can occur even in the absence of CAPE
  • Derecho and bow echo development
  • Nocturnal MCC development
  • Waterspout formation
secondary circulations
Secondary Circulations

Upper-level wind maxima (jet streaks)

  • Transverse ageostrophic circulations about the jet axis are argued to initiate convection with clouds and precipitation being most prevalent in the right entrance and left exit region of the jet streak

Low-level jet

  • Convection is favorable through the enhancement of moisture and temperature advection, increased low-level convergence, and an increase in vertical wind shear associated with this jet
triggering of convection

Triggering of Convection


Isolated convective-triggered mechanisms (storms along a gust front, drylines, terrain features, etc.)

Combined convective-triggered mechanisms (gust fronts colliding or intersecting other low-level perturbations such as other gust fronts, drylines, cold fronts, terrain features, etc.)

local processes
Local Processes
  • Buoyancy-driven circulations in the convective boundary layer (CBL)

* forced and active cumulus

  • Terrain forcing

* cloud initiation from leeside convergence and upslope flow development

* “Caprock”, Ozark Mountains, and Wichita Mountains

  • Surface inhomogeneities from soil moisture or vegetation type
advective processes1
Advective Processes
  • Boundary layer convergence lines (convective scale)
  • Cold front lifting
  • Collision or intersection of advective phenomena (gust fronts, sea/lake breezes, drylines, etc.)

* e.g. intersections of drylines and fronts, and gust

fronts and sea breezes

  • CBL thermals
dynamical processes1
Dynamical Processes
  • Horizontal convective rolls
  • Collision and intersection of gravity waves, and bores with other lifting mechanisms is the most common trigger for severe weather
combined lifting processes
Combined Lifting Processes
  • Genesis of severe weather most often occurs from the combination of local, advective, and dynamical processes
      • Boundary layer rolls with convergence lines
      • Dryline intersection with fronts, boundary layer rolls, and mesoscale low pressure areas
      • Gust fronts with terrain