II. Synoptic Atmospheric Destabilization Processes

• Elevated Mixed Layer (EML)

• Synoptic Lifting

• Dynamic Destabilization

• Differential Advection

Static Stability and Instability

• Static Stability: push an air parcel upward (downward) and it will fall (rise) back to its original position transforming the work exerted to accelerate the parcel into the kinetic energy of the parcel’s return movement.

• Static Neutrality: push an air parcel upward (downward) and it will continue its motion without further acceleration.

• Static Instability: push an air parcel upward (downward) and it will continue to accelerate its motion upward (downward)

Potential Instability

• If layer is lifted, it will become (conditionally) unstable

• Characterized by Equivalent potential temperature decreasing with height, or having a minimum value

• Typical of tropical and spring/summer season soundings

Destabilization• Increase Environmental Lapse Rate at middle levels (850-300 hPa)

– Mix air vertically; neutralize layer stability• Warm from below and thermals mix warmth upward through a deper layer• Vertical shear (of horizontal wind) generates turbulence that will mix a layer

– Differential Advection• Warming below

– Differential advection– Diabatic (radiation) or moist adiabatic

• Cool above– Differential advection– Diabatic (radiation, precipitation evaporation)

– Layer subsidence or Lifting

• Warming and Moistening below– Surface fluxes increase low level moisture and temperature

• Solar heating, evaporation• Evapotranspiration• Warm or moist surface, warm body of water, warm soil surface, etc

– Strong low-level thermal/moisture advection, such as Sawyer Elliasen Surge

• Maintain an inversion (cap) – separate low level moistening (some heating) from upper destabilization processes – Prevent deep moist convection from erupting and restabilizing

Layer Lifting

Differential Advection

• Layer aloft moving at a different velocity than one below

• Cold air advection Aloft

• Warm or moist air advection below

Differential AdvectionCaused by Topography

Elevated Mixed Layers• Surface-Based formation of a deep mixed layer

– Vigorous vertical (mostly cloud-free) mixing of a dry daytime boundary layer driven by solar heating at high Bown ratio (sensible/latent)

– Potential temperature near constant with height (very steep lapse rate)

– Usually is desert climates, where moisture is very low

– Often occurring on during warm season mountain plateaus that are isolated from moisture sources (lee side)

• Mexican Plateau

• Great Basin

• Front Range of Rockies

• Elevation of Mixed Layer– Movement off high terrain downwind to middle levels over low elevations

• Advection off high terrain

• Mountain-plains thermal solenoid

– Movement off warm desert over relatively cool surface boundary layer• Advection by low level surge produced by circulation of baroclinic system, I.e. Sawyer-

Eliassen Circulation

Characteristics of Orographically Generated North American EML

• Deep, warm, dry well mixed mountain boundary layer advected over plains featuring a relatively cool (potential temperature-wise) boundary layer layer topped by a capping inversion– Slope flow inversion cap

– Frontal cap

• Plains boundary layer moistened and warmed below EML creating conditional instability:

– Daily heating and evapo-transpiration

– Advection from a moist region

• Point at which EML leaves surface and becomes “elevated” is the “Great Plains Dryline”

EML Moving From Rockies

Dry line

Sahara Generated EML

• Deep dry, dusty, well-mixed Desert Boundary Layer (DBL) builds over Sahara desert of North Africa

• DBL advected northward over Mediterranean Marine Boundary Layer (MBL) which is relatively cool (potential temperature-wise) and moist– Usually in advance of an approaching upper level trough, i.e.

Sawyer-Eliassen Circulation

• MBL further moistened and warmed (or possibly cooled) creating conditional or potential instability:– WISHE (wind induced surface heat exchange)

EML Moving over Mediterranean from Sahara

Dynamic Destabilization

• Ageostrophic circulation in the right exit region of the jet streak forces subsidence and a capping subsidence inversion– Spreads isentropes (lines of constant potential temperature) vertically

creating lower stability (measured by the vertical gradient of potential temperature)

• Brunt-Vasallai Frequency is a measure of static stability;

•

• Sawyer-Eliassen Circulation advects warm air under dry destabilized layer completing the formation of “loaded gun” sounding

• Progression into the right entrance region initiates rising motion that “pulls trigger”

N 2

g

z

Load the Gun

Pull the Trigger!

High Elevation Sounding

Loaded Gun Sounding