Transcript Slide 1

Lecture 17: Atmospheric circulation & pressure distrib’ns (Ch 8)
In the context of map discussions, already we have touched on a few of
the concepts of Ch8. Today we’ll cover:
• 3-cell model of dominant planetary scale wind motions, or “general
circulation”
• observed general circulation
• jet streams
• Rossby waves
We spoke earlier (slide 2, Lec. 9) of the vast and continuous range of scales of
motion in the atmosphere…
“The largest-scale patterns, called the general circulation, can be considered the
background against which unusual events occur, such as” (droughts etc.)… p 213
Hadley’s (1735) Single-cell Model
• ocean-covered planet
• sub-solar point perpetually
over equator
• equatorial heating produces lift and
symmetric poleward motion
aloft; sink at poles
• return equator-ward motion at
surface
• reasoned earth’s rotation must
deflect the wind
Fig. 8-2a
Single-cell Model** is
• consistent with the observed (and important)
surface equatorial “trade winds”, “the most
persistent on earth”
• though polar surface easterlies emerge only as a
long term average, not a prevailing feature
• BUT: surface easterlies everywhere would slow
down rotation rate of earth!
Air aloft “diverges” out of the air
column, air at surface
“converges” into column
** Hadley is honoured by a “Hadley
Centre for Climate Prediction & Research”
in UK
Fig. 8-2a
Ferrel’s (1865) Three-cell Model
System still conceived as an ocean-covered earth
with the subsolar point on the equator
• the low lat. “Hadley cell”is thermally
driven by powerful ITCZ convection
• the mid lat. Ferrel cell is indirect
(forced by the other two)
• the high lat. Polar cell is thermally
driven by sink at the poles,
but, we do not get vigorous
overturning
• modern view is that only the Hadley
cell is considered “real”
Fig. 8-2b
Three-cell Model
Fig. 8-2b
Equatorial lows + ITCZ
Convective cloud in the ITCZ
(“the doldrums”). The ITCZ migrates
N-S seasonally with changing solar
declination
Fig. 8-3
Three-cell model – winds aloft in the Hadley cell
Air rising at equator feeds into poleward upper streams, which are deflected
by the Coriolis force (to right in N. hemisphere) to produce a zonal
component of motion… result: westerly upper currents in both hemispheres
About 30o N
Rising parcel
equator
“meridional” component
“zonal” component
(of wind velocity)
Alternative view (Sec. 8-1) each parcel with fixed mass m
tends to conserve its “angular momentum” m v r . Constancy
of m v r demands an increase of the zonal component (v) of
the parcels velocity as it moves north, where the circumference
of a latitude line about the earth (2  r ) is shorter. (Friction
intervenes so conservation not quite perfect.)
Arrows denote
rate of movement
of the ground
surface towards
the east…
angular mtm of a
stationary parcel
is largest at the
equator
Three-cell Model
This poleward moving upper
current cools, and around 2030o latitude (ie. in the “horse
latitudes”) it sinks, with
consequent adiabatic warming
mitigating against cloud at that
latitude
Fig. 8-2b
Three-cell Model
polar surface easterlies do
emerge as a long term average
Surface westerlies (realistic)
+ upper easterlies (unrealistic)
realistic
Fig. 8-2b
Land/ocean mix + topography make true
climate rather different from 3-cell model
What’s observed: semipermanent surface pressure cells & winds
Migrates W and weakens in summer
Gone in summer
Fig. 8-4a
• note strong influence of continents!
(Sea-level isobars averaged over 30 Januaries)
Fig. 8-4b
What’s observed: mid-tropospheric winds
• heights largest over tropics
• lowest heights h and strongest gradient h / x (thus, strongest winds) in winter
• zonal component generally dominant
Fig. 8-6
Polar front jet
• Recall P falls more slowly with increasing height in warm air, where density lower
• so wherever there are strong horiz temperature gradients (fronts) there are strong
height gradients
• strong height gradient implies strong wind perpendicular to the height
gradient (see Geostrophic law)… strongest in winter (stronger T-grdnt)
• jet here shown as a
climatological feature
• as weather feature,
irregular - meander &
branch
Fig. 8-8
Waves in mid-latitude mid/upper troposphere causing
convergence and divergence aloft...
Long (Planetary/Rossby) Waves —
• wavelength 1000’s kilometers
• typically 3 - 7 around globe (fewer, longer, stronger in winter)
• not always unambiguously identifiable - but can be shown mathematically to exist
in ideal atmos. due to N-S variation of Coriolis force
• (usually) move slowly eastward
L
con
div
con
500 mb
div
Example of high amplitude Rossby wave over N. America
Edmonton low +4oC, high +20oC
Winnipeg low
-2oC,
high
+11oC
Fig. 8-13
Sept. 22, 1995
Rossby wave moving eastward
Fig. 8-12
Rossby wave moving eastward
Notice the “short wave” here; covered
later – are associated with storms
Rossby wave moving eastward
Rossby wave moving eastward