Atmospheric Destabilization Processes

• Upper Level Mixed Layer• 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)

Dry Adiabats

• Dry Adiabats (lines of constant theta on a thermodynamic diagram):

/1000 pR c

mbT

P

Moist Adiabats

• Moist Adiabats (lines of constant pseudo-equivalent potential temperature on thermodynamic diagram)

0.2854(1 0.28 )1000 3376

exp (1_ 0.81 ) 2.54vr

ep v vsat

mbT r r

P T

284055

3.5ln ln 4.805satT T e

0.622v

er

p e

Dry Adiabatic Lapse Rate

m

T

z

Moist Adiabatic Lapse Rate

Static Stability

absolutely unstable

conditionally unstable

absolutely stable

d e

d e m

m e

Destabilization• Increase Lapse Rate at middle levels

– Mix air vertically– Warm below

• Advection• diabatic

– Cool above• Advection• diabatic

• Increase low level moisture and temperature– Daytime heating– Warm moist advection, Sawyer-Elliasen surge

• Maintain inversion (cap) separating low level moisture and heat from increased upper level lapse rate

Layer Lifting

Differential Advection

• Layer aloft moving at a different velocity than one below

• Cold air advection Aloft• Warm or moist air advection below

Example of Differential Advection

Elevated Mixed Layer

• Deep dry mixing of a dry daytime boundary layer produces high lapse rate through deep layer– Deep dry adiabatic layer (steep lapse rate)– Very dry layer, nearly constant vapor mixing ratio– Best layers formed in regions of very dry air (too make

dry lapse rate) found in:• Deserts • High plateau or lee side of a mountain range

Elevated Mixed Layer

• Deep, warm, dry well mixed layer advected over the top of a relatively cool (potential temperature-wise) layer forming a capping inversion

• Boundary layer below inversion moistened and warmed creating conditional instability:– Daily heating and evapo-transpiration– Advection from a moist region

EML Moving From Rockies

Dynamic Destabilization

• Quasi-geostrophic sinking motion in the right exit region of the jet forces subsidence and a capping subsidence inversion

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

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

Load the Gun

Pull the Trigger!

High Elevation Sounding

Loaded Gun Sounding