Skew T Log P Diagram AOS 330 LAB 9

Outline

• • • Adiabatic Processes (Review) Static Stability Equivalent Potential Temperature Θ e

Adiabatic Process

An adiabatic process is one in which no heat or mass is exchanged with the environment -- in other words, a parcel stays a parcel. In the atmosphere, a parcel expanding / being compressed adiabatically cools / warms at a constant rate with respect to height. The rate at which this happens is known as the dry adiabatic lapse rate, which we’ll denote as gamma: Γ d = g / c p ~ 10K / km (Note: Γ d is positive, because we define the lapse rate as a rate of cooling for expanding parcels. Don’t let this confuse you!)

Moist Adiabatic Process

Now, we know that as a parcel cools, it may reach saturation. When this occurs, the dry adiabatic lapse rate is no longer valid, since further cooling is mitigated by heat released during condensation. The process is now moist adiabatic and the lapse rate is: Γ m ~ 6K / km

Parcel Method

• A blob of air (parcel) starts moving adiabatically up or down relative to its environment from a hypothetical level in the atmosphere.

Assume: 1) No mixing between parcel and environment 2) Vertical movement of parcel does not cause a change in the overall characteristic of the environment. (no compensating downward motion) Neither assumptions fully meet real world convection requirements.

Parcel Path

Static Stability

Let Γ = - dT / dz represent the lapse rate in a layer of the atmosphere, we can then define the layer as: 1. Absolutely Stable if Γ < Γ m 2. Conditionally Unstable if Γ d > Γ > Γ m 3. Absolutely Unstable if Γ > Γ d Additionally, we define a special case of absolute stability if Γ < 0 (i.e. temperature actually increases with height.) What do we call this?

Static Stability

Let’s consider an air parcel that has the same temperature as the environment and is at rest. Now, give it a slight push upward. As it moves upward, it adiabatically expands and cools.

At the new level higher up, it could : 1) be denser than its surrounding, sink back down to its original position. (STABLE) 2) be less dense than its surrounding, continue to rise. (UNSTABLE) 3) have the same density as its surrounding, will continue to rise until it encounters enough friction to stop it. (neutral)

Absolutely Stable

Parcel is unsaturated, it cools following dry adiabatic lapse rate.

Γ d Γ Γ d As parcel rises(sinks), it is cooler(warmer) than its environment, it would want to move back to its original position.

Absolutely Unstable

Parcel is unsaturated, it cools following dry adiabatic lapse rate.

Γ Unstable for both saturated / unsaturated parcels.

d Γ Γ d Γ m As parcel rises(sinks), it is warmer(cooler) than its environment, it would want to continue to rise(sinks).

Conditionally Unstable

Parcel is unsaturated, it cools following dry adiabatic lapse rate.

Γ m Γ d Parcel is saturated, it cools following moist/pseudoadiabatic lapse rate.

Γ m Γ Γ d Stable when parcel is unsaturated. Unstable when parcel is saturated.

Γ

Neutral

Saturated neutral Γ m Dry neutral Γ d Γ

Stability Quiz

Equivalent Potential Temperature Θ

e • • Θ e - Maximum potential temperature that a parcel could have through condensation of all of its water vapor content. Conserved in both dry and moist adiabatic processes 

e

  exp 

Lw s



c p T

  

T d LCL T Θ Θ e

References

• • Petty, G (2008). A First Course in Atmospheric Thermodynamics, Sundog Publishing.

Potter and Coleman, 2003a: Handbook of Weather, Climate

and Water: Dynamics, Climate, Physical Meteorology,

Weather Systems and Measurements, Wiley, 2003