\Courses\atm240\s-guide.wpd

1. course admin matters

2. what is the gen circ? (some defs)

3. what properties must it have? (balances & physical constraints)

4. imagining what circs might work

Jargon: geostrophic balance, hypsometric equation, zonal mean, eddies;
stationary pattern, transient, Hadley cell, Ferrel cell, baroclinic
instability

Chapter 1:

*Note: a better definition than in the book would be:*

"All those
atmospheric processes sufficient to provide an explanation for observed
large scale ( >1000 km) circulations that persist for a season or
longer."

- radiation & temperature (T)
- (essentially) no net heating (over a year)
- local variations (e.g. tropics absorb more solar radiation than polar latitudes) lead to T gradients
- T gradient implies meridional heat transport (if so, then atmosphere generally not in pure radiative balance)
- need some T gradient to drive motions

- pressure & geopotential (Z)
- from hypsometric eqn: thickness greater in tropics than poles
- 1000 mb geopotential has less variation than 500 mb (say)
- these imply meridional Z gradient that increases with elevation

- winds
- from geostrophy: Z gradient implies westerly winds
- Z gradient increasing with elevation means westerlies do too (thermal wind balance)
- meridional winds are needed for heat transport
- but meridional winds generate westerly acceleration/deceleration if conserving (even approximately) angular momentum

- mass balance
- meridional motions must have equal mass transport N as well as S
- no net mass build up means (annual average, say) surface P is constant
- mass balance for water is separate issue (it can be added & removed from atmosphere)

- surface torques
- must have easterly torque applied to earth (surface easterlies) to balance surface westerlies

Can we anticipate what the general circulation might be?

- Doing so guides interpretation of data to be seen in this course.
- From the discussion above and some additional information we conclude that the circulation should satisfy several (8) constraints

Hypothetical "Candidate" circulations:

- must transport heat poleward
- must conserve mass
- have westerlies, especially in middle latitudes
- have westerly vertical shear, especially in middle latitudes
- have meridional motions generate westerly accelerations (ang.momentum cons.)
- have equal (torques from) surface easterlies and westerlies
- incorporate baroclinic eddies if the vertical shear is "strong enough"
- have tropical and midlatitude maxima in precipitation (vertical motion)

Various Examples can be proposed.

- - most simple circulations can be excluded;
- - not just "any" flow would work
- - Hadley type circulation (with modifications) can work in tropics (at other latitudes u becomes too fast)
- - Eddies work in midlatitudes (matching these to the Hadley cell is challenging but revealing)

-------------------------------------------------------------------------------------------------------------------