\Courses\atm240\s-guide.wpd

Study Guide (lecture #1)

Topics:

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:

Introductory questions: What is, What causes the general circulation?

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."

Balances & Physical Reasoning

1. 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
2. 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
3. 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
4. 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)
5. surface torques
   • must have easterly torque applied to earth (surface easterlies) to balance surface westerlies

Anticipating the circulation from 8 constraints: (some physical, two empirical)

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:

1. must transport heat poleward
2. must conserve mass
3. have westerlies, especially in middle latitudes
4. have westerly vertical shear, especially in middle latitudes
5. have meridional motions generate westerly accelerations (ang.momentum cons.)
6. have equal (torques from) surface easterlies and westerlies
7. incorporate baroclinic eddies if the vertical shear is "strong enough"
8. 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)

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