Energy Cascade

Atmospheric waves may develop in layers of strong vertical wind shear. Like ocean waves, these too amplify and break generating smaller scale waves and eddies that become unstable (wave breaking). Through this progression of instabilities, kinetic energy may be extracted from the large-scale wind field, giving rise to a variety of small-scale motions that extend down the molecular scale—a process that inspired Richardson’s rhyme and resembles that of an energy cascade.

Energy cascades transfer energy from large (or small) scales of motion to the small (or large) scales. As indicated by the “drop in the bucket,” kinetic energy in the reservoir is transferred to smaller and smaller scales until it becomes indistinguishable from random molecular motions, becoming incorporated into the atmosphere’s reservoir of internal energy.

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].

Physical Principles: The Movement of Rossby Waves

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

Physical Principles: Maintaining General Circulation

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