Lapse Rates

Lapse rates are progression at which air temperature changes with increasing / decreasing height in the atmosphere.  The rate is considered positive when temperature decreases with elevation, zero when temperature is constant with elevation, and negative when temperature is increasing with elevation.  While there are two different class of lapse rates, normal and adiabatic. The difference between normal and adiabatic lapse rates determine the vertical stability, or instability, of the atmosphere. That is, an air parcel’s tendency to embrace or prohibit vertical motion.

Environmental Lapse Rate—non-rising air that is affected by radiation, convection, and/or condensation. It averages about 6.5°C/km.

Dry Adiabatic Lapse Rate—rate of cooling with increasing altitude. It is constant at about 10°C/km.

Moist Adiabatic Lapse Rate—air, saturated with water vapor, is not constant but is determined by the combined effects of expansion cooling and latent heating (LH) because saturated air cools slower than dry due to the heating produced by condensing water vapor. Is always less than the dry adiabatic lapse rate.

Due to the fact that density differences are affected by the differences between the adiabatic lapse rates and the environmental lapse rate, one may notice that absolute instability occurs when the environmental lapse rate (Г_E) exceeds the dry adiabatic lapse rate (Г_D) [i.e. Г_E > Г_D]. Whereas, absolute stability occurs when the environmental lapse rate (Г_E) is less than the wet adiabatic lapse rate (Г_W) [i.e. Г_E < Г_W]. However, when the environmental lapse rate (Г_E) falls between the wet adiabatic lapse rate (Г_W) and the dry adiabatic lapse rate (Г_D) [i.e. Г_W < Г_E < Г_D] the atmosphere is considered conditionally unstable, as you can see from the picture below.

Atmospheric Stability and Instability

            Instability is a race to get cold between the parcel and the environment, and we want to environment to win. We could help the environment win by making the environment cool more slowly and / or make the parcel cool at a slower rate. The parcel method, for example, talks about the parcel being a hypothetical box that does not allow any transfer of heat in or out but, allows only adiabatic temperature changes.

The stability of the parcel is dependent on the parcel’s motion after a forced displacement. As the parcel undergoes adiabatic change, its temperature is compared to the surrounding environment to relate differences in density. If the parcel returns to its original position it is considered stable, whereas if the parcel continues moving away from its original position it is considered unstable. Moreover, if a parcel is displaced but remains at its new position it is considered neutral.

            Due to the fact that density differences are affected by the differences between the adiabatic lapse rates and the environmental lapse rate, one may notice that absolute instability occurs when the environmental lapse rate (Г_E) exceeds the dry adiabatic lapse rate (Г_D) [i.e. Г_E > Г_D]. Whereas, absolute stability occurs when the environmental lapse rate (Г_E) is less than the wet adiabatic lapse rate (Г_W) [i.e. Г_E < Г_W]. However when the environmental lapse rate (Г_E) falls between the wet adiabatic lapse rate (Г_W) and the dry adiabatic lapse rate (Г_D) [i.e. Г_W < Г_E < Г_D] the atmosphere is considered conditionally unstable, as you can see from the picture below.

            On the other hand, especially with regard to the potential for severe storm development, another type of stability becomes important: potential instability. While, static stability (discussed above) considers what happens to a small parcel (box) of air when lifted or lowered while the surrounding air is kept in place, potential instability contemplates what happens when an entire layers of air are displaced upward [i.e. a mass of warm air displaced upward by the movement of a cold front].

Forecasting: Determining Stability from Thermodynamic Diagrams

Understanding Weather and Climate (7th Edition) (MasteringMeteorology Series) by Edward Aguado, James E. Burt

Atmospheric Stability & Instability

Stability & Instability

·         Instability - Possessing the ability to move away from the original position; allows convection and enhances vertical motions.

·         Stability - Possessing the ability to return to its original position; suppresses convection.

·         Inversion - Temperatures increasing with increased altitude, or height. A negative lapse rate.

·         Lapse rate (Γ) - The change of temperature with a change in height. Rate of decrease in temperature. Getting colder with height = Positive lapse rate. dT / dZ = temperature changing with height.

 

·         Adiabatic – not heat

·         Isotherm – A line of constant temperature.

·         Isothermometer – A line of constant dew point temperature.

·         Barometer - measures pressure

 

Air Parcel’s

·         “Blob” of air

·         Expands and contracts freely

·         Rises and sinks through the atmosphere

·         If it is warmer it is less dense then the air which makes it rise (compression)

·         If it is colder it is more dense then the air which makes it sink (expansion)

·         Temperature will change from pressure – does not exchange from inside and outside the box

·         Temperature is NOT a measure of heat

  

Types of Heat Transfer

1.    Conduction

a.    The transfer of sensible heat from a warm object to a cool object through contact

2.    Convection

a.    fluid/anything that is flowing

3.    Radiation

How Pressure changes with height?

a.    dp / dZ < 0

b.    Pressure decreases with height, which is a negative change, that is less than

1^st Law of Thermodynamics

̶        Heat In = Work + delta-T

a.    Adiabatic process:        

Warm air is less dense then cold air ONLY at the same pressure

Static Stability

If the environment is cooling slower than the wet adiabatic lapse rate is	Stable
If the parcel is cooling faster than the dry adiabatic lapse rate	Unstable
̶        Depends on if the parcel is cooling at the wet adiabatic lapse rate or the dry adiabatic lapse rate ̶         When ΓE falls between ΓW and ΓD.	Conditionally unstable
Depends on water vapor capacity	Saturation

 

̶        Race to get cold (lapse rate): Instability is in a race (atmosphere) between the environment and the parcel, we want the environment to win (by being colder) at a rate of 10 at the PBL (planetary boundary layer)

̶        PBL (Planetary Boundary Layer): The part of the atmosphere that cools at a rate of 10/km

“Since the density differences are affected by the differences between the adiabatic lapse rates and the environmental lapse rate, one may denote absolute instability occurring when the environmental lapse rate, ΓE , exceeds the dry adiabatic lapse rate, ΓD; absolute stability occurring when ΓE is less than the wet adiabatic lapse rate, ΓW ; and conditional instability when ΓE falls between ΓW and Γ . The atmosphere may be considered potentially unstable, (or synonymously convectively unstable) when referring to the atmosphere's potential for releasing instability, even when the atmosphere appears to be stable. A layer may be strongly stable (that is, it has a negative D lapse rate) and yet still considered to be potentially unstable. This is favored when the bottom of a specific layer is warm and moist while the top of the layer is substantially drier.”

Dynamically lifting a layer of the atmosphere

̶        Lapse rate decreases as it goes up

̶        Bottom layer saturates more quickly than the top

“The original layer is considered convectively unstable if at the point of total saturation, the layer has a lapse rate greater than the ΓW. This criterion can be represented by determining the change of the equivalent potential temperature with height. If dθe/ dZ<0, the atmosphere is considered convectively unstable.”

“If θ_e at the bottom is greater than θ_e at the top, as it is in this case, then the layer’s lapse rate is greater than the local wet adiabatic lapse rate and the layer is convectively unstable.”

̶        Turbulent Eddies mixes the atmosphere in the PBL (or CBL)

o   Mechanically induced T.E.

§ 
Result of roughness and wild speed

o   Thermally induced T.E

̶        Redistributes heat and water vapor (w)

̶       
Constant: Heat, w, potential temperature,

Sun give off radiation à ground absorbs radiation à warms up the ground à ground warms up the air à upward flux of sensible heat

Flux: flow through an area

Sensible heat: The heat through touch

Three ways to transfer heat

o   Convection

o   Conduction

o   Radiation

How does the density change with height?

̶        Decrease with height because pressure is decreasing with height because there is less weight of the air above you

̶        dp / dz > 0

̶        Heating the air makes it less dense and needs to cool at 34C /km

As I go down into the molecular boundary layer the decrease in density due to an increase in temperature has to more than compensate for the increase in density due to an increase in pressure.

What 2 things influence density?

̶        Temperature

̶        Pressure

OLR (Outgoing Long-wave Radiation): making the ground cold à air in contact with ground gets cold à atmosphere more dense à doesn’t want to overturn àT.E. makes it overturn anyway (calm night will be the coldest) àcauses frost because the temperature closer to the ground is colder

LLNJ (Low Level Nocturnal Jet)

CCL - Convective condensation level; the lowest level at which condensation will occur as a result of convection due to surface heating.

The CCL, or Convection Condensation Level, is the height to which a parcel of air, if heated sufficiently from below, will rise adiabatically until condensation begins. In the most common case, this is the height of the base of cumulus clouds which are produced by thermally-induced turbulent eddies (convection solely from surface heating).

The CAP will break when the PBE between the LFC and EL is greater than the NBE/CIN above the EL and the next LFC.

T and T_D affect θe.

The higher the θe the more likely I am to get thunderstorms.

The Anvil forms at the EL and the overshooting top is when the parcel bounces around the EL.

To break a CAP

̶        Increase temperature

̶        Increase Dewpoint

̶        Convective battering

                                                         

What is the effect of the air coming down into the boundary layer?

̶        The air is colder but as I bring it down it gets warmer

Convective battery

Mon Test: Part 2 of lab, soundings,

Tues: Multiple choice and short answer

Labeling the Graph

LIFTED INDEX LI = T₅₀₀ - Tp₅₀₀

If ( - ) then it is unstable

θ on the dry adiabat = constant

θ on the wet adiabat = increasing

θe = total heat, never changes

PBL (Planetary Boundary Layer): The part of the atmosphere that cools at a rate of 10/km

NBE/CIN/CAP