A brief synopsis of Johnson and Mapes: Mesoscale Processes

and Severe Convective Weather

From Severe Convective Storms sections 3.3b, 3.3c.1, 3.4

ByMatt Gough

overview

• Advective Processes and examples that lead to severe weather

• Dynamical Processes and examples

• Convective triggering mechanisms

Advective Processes

Moisture Advection• Increases CAPE and lowers LFC

Synoptic scale Differential Advection• Destabilizes atmosphere• Provides vertical wind shear• establishes capping inversions

Convergence Lines• Preconditioning and triggering convection

Examples of mesoscale Differential Advection

• Low-level jets with high θ(e) overrunning fronts and cold-pools (bow echoes).

• Flow of moist boundary layer air out from beneath an inversion

• Transport of clouds and moisture aloft downstream of mountain barriers (dry microbursts)

• Short periods of jet streak circulations and boundary layer heating

Examples of Convergence Lines• Warm and cold fronts – associated with

Mesoscale cross-front circulations.• Precipitation driven downdrafts (gust fronts)• Drylines – usually needs to be coupled with

other mechanisms.• Land and Sea breezes – convergence of

offshore or onshore flow.• Mountain/Valley breezes – upslope flow in

western US providing convergence and lifting.

Off the east coast of Florida in the morning.

Dynamical processes

• Upper level jets

• Lower level jets

• Coupling of upper and lower level jets

Dynamical processesUpper level jets

Dynamical processesLower level Jets• Enhancement of moisture and temperature

advection.• Increased low level convergence• Increase in vertical wind shear

Coupling of upper and lower level jets• Upper level conv. forces low level jet Coupling between indirect circulation and

cold cold front.

Triggering of convection

Mechanisms can operate in isolation

• Storms along a gust front

or in combination

• Gust fronts colliding with each other

Triggering of convection

Local Processes

Buoyancy driven circulation

• Thermals, horizontal convective rolls, overshooting buoyant eddies.

Terrain Forcing

• Lee-side convergence, thermally-induced lifting, flow deflection, and surface inhomogeneities.

Lee side convergence

Triggering of convection

Advective Processes

• Convection along convergence

• Cold/warm front lifting

• Gust fronts, sea breezes, and drylines

Usually the intersection of these features cause severe convection.

Cold front lifting

Triggering of convection

Dynamical Processes:

• Horizontal Convective Rolls (operate in conjunction with other processes)

• Gravity waves (in tropics – small CIN)

• Horizontal shear

Triggering of convection

Combined Lifting Processes:• Boundary layer rolls intersecting with

convergence lines • HCRs intersecting with sea breezes.• Dryline convection: mesoscale lows, HCRs,

fronts, and gravity waves.• Gust fronts with terrain, gravity waves, etc.

Denver, CO, 17 Jul 1987