Study Guide (lecture # 12)

Book pages: 120-121, 128-134, skim: 116-119, 122-127

Review:

- potential energy considerations:

- introduction to APE
- conceptual models of APE
- Hadley cell as a Carnot cycle

Today's topics:

- "Global" definitions of APE
- limited area APE -- Interpretation of:

- efficiency factor
- diabatic processes

Jargon:

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"Global" APE:

How is APE defined mathematically? (“global” definition now, limited area next) (Chap. 4.3)

- the reference state -- measures the minimum PE
- APE proportional to departures from the mean: P on theta surfaces; theta on P surfaces, or T on P surfaces
- APE is created by heating the air that is already hot and/or cooling the cold air
<< Note: interpretation of the 4 formulae is important, NOT the derivation. >>

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Limited-volume APE derivation:

- why formulate limited area equations?
- reference pressure
- << Note: interpretation of the formulae: 4.30, 4.31, 4.34 is important, NOT the derivation >>
- efficiency factor

- weighting based on deviation from reference state

Interpretation of limited domain APE and KE:

- five groups of terms change APE_j : only first 3 listed are important to discuss

- diabatic processes (includes 5 sources/sinks)
- 2nd term: similar term found in the KE_j and APE_j equations
- divergence of heat flux -- conversion between APE forms: internal to the domain and external.
- divergence of mass in domain (sort of)
- changes in composition that alter the reference pressure in the domain

- distribution of the efficiency factor —

- tropical max; high latitude minimums,
- largest magnitudes mainly in middle troposphere

diabatic processes: the epislon*q integrand in (4.30)

Solar Radiation: q>0. So:

- generates APE in winter since tropical q >> polar q.
- albedo causes weak generation in summer too

Terrestrial Radiation: q<0. (slightly greater in tropics, so:

- might expect this to destroy APE, but
- generates APE because emission in high latitudes is from cloud tops where epsilon<0. (subtropial latitudes have emission where epsilon is much smaller)

NOTE: [epsilon] is nearly zero in midlatitudes, so the sign of epsilon probably varies as follows:

- epsilon >0 for relatively warm areas (e.g. oceans in winter, or warm sector of a frontal cyclone)
- epsilon <0 for relatively cold areas (e.g. continents in winter or cold air sector (behind a cold front). )

Sensible Heat Surface Flux: (SHF)

- SHF is largest near WBC's
- ambiguous as to APE tendency since:

- heating input where air is hot thus reducing T departure,
- But, heating reduces atmospheric static stability, which encourages cyclogenesis

Latent Heat Surface Flux: (LHF)

- while most evaporation is near WBCs, the heat is released elsewhere
- when the latent heating increases warm front precipitation (which it often does) eddy APE is generated

Friction:

- only important for KE
- always destroys KE