Transcript 5 - Atmos Circulation

General Circulation of the Atmosphere
Lisa Goddard
19 September 2006
Main Points
• * Atmosphere wants to attain balance
- Circulation results from the atmosphere adjusting
to remove imposed imbalances
• * Know which way the wind blows
- and how that establishes the local climate &
environment
• * Know why the circulation should behave
as it does
- Geostrophic balance
- Hydrostatic balance
- Ideal Gas Law
- Momentum & energy conservation
Outline
1. Review: The Forces : pressure gradient
force, Coriolis force, friction.
2. and the Balances: Geostrophic balance,
Hydrostatic balance
• 3. What does the atmosphere look like?
(time averaged view)
• 4. Why does it look that way? Zonally
averaged thermally-driven circulation
• 5. Longitudinal asymmetries: oceans,
mountains
• 6. Poleward energy transport
Vertical pressure
gradient force
Due to random molecular
motions, momentum is
continually imparted to the
walls of the volume
element by the
surrounding air.
The momentum transfer
per unit time, per unit
area, is the pressure
In the absence of
atmospheric motions the
gravity force must be
exactly balanced by the
vertical component of the
pressure gradient force.
“Hydrostatic Balance”
Sea-level pressure
Horizontal pressure
gradient force
... eastward pressure-gradient force per unit mass
Coriolis Force: Geostrophic balance
A large scale dynamical balance
Geostrophic Balance: the Coriolis force
•The primary horizontal balance of forces in atmospheric motion
is the geostrophic balance:
Geostrophic Flow with Friction
•Friction slows down the wind, causing a weakening in the Coriolis force. A new
balance is achieved between the resultant of the Coriolis force (CF) and friction on
one hand and the pressure gradient force (PGF) on the other hand.
What does the General Circulation look like?
Why does it look that way?
It is driven by
differential
heating
Equator to Pole
Climate Zones according to Koeppen
•
For more information about this map see:
•
http://www.blueplanetbiomes.org/climate.htm
Sea-level pressure
units: hPa
Surface winds
units: m/s
Schematic of
“Hadley Circulation”
Friction, Mass Continuity
Convergence/Divergence
•Friction leads to the convergence of air into the centers of low pressure and divergence out of
the centers of high pressure. Mass continuity (or mass balance) implies that there is rising
motion in a low pressure system and sinking motion in a high, leading to a reversal of the
convergence/divergence patterns aloft.
•The tendency of air to rise over a low pressure system creates favorable conditions for the
formation of rain clouds.
•In high pressure systems the sinking motion leads to clear and dry conditions.
Precipitation
units: mm/day
Schematic of
“Hadley Circulation”
500 hPa Geopotential height (Z)
units: m
Geopotential height
Switching between z & p coordinates
 p   z  Standard coordinate
 p 

  x 
transformation
 z   x  p
z
Minus sign included
 p 
 p   z  because p varies


  
 x 
oppositely to z
z   x  p

z
Now invoke
 p 
 z 
 x       g   x  hydrostatic eqn.
z
p
units: m
Geopotential :   gz
p


x
x
So,
1 p 

 x x
1 p 

 y y
Latitude-height crosssection of Zonal wind
units: m/s
500 hPa (= mb) Geopotential
Height (m)
N. Hem.
S. Hem.
units: m
Thermal Wind:
the vertical shear of the Geostrophic
Wind
Horizontal temperature
gradients
- a “baroclinic” atmosphere imply a vertical shear in the
geostrophic wind, through
hydrostatic balance
-
-
Warmer temperatures on the right
raise the 980 mb surface, creating
a vertical shear in the geostrophic
wind.
PGF is right to left, geostrophic flow is into the
page (in the northern hemisphere).
Planetary-scale
Thermal wind
-
-
units: m/s
Midlatitudes
Baroclinic instability
Eddy fluxes of momentum force an
indirect meridional circulation: the
Ferrel cell
Baroclinic Instability
(Temperature)
Baroclinic Instability
(Winds)
Zonal Asymmetries
Land-sea contrasts. Highly seasonally dependent.
Effects of Mountains on Local Climate
•Moist convection can explain the local climate effect of mountains, namely the tendency
for large mountain ranges to have excess precipitation on the upwind side and a desert or
rain shadow on the downwind side. The former is due to the lifting of the incoming air by
the mountain. The latter is due to the warming of the rising air due to latent heat release.
Surface winds
units: m/s
300 hPa January geopotential height
Zonal asymmetries
The Walker Circulation
Atmospheric energy
transport
Poleward transport of moist static
energy occurs mainly through the
Hadley circulation in the tropics,
and by baroclinic waves (”eddy
heat transports”) at higher
latitudes.
internal energy
gravitational PE
latent heat content
(of an air parcel)
The Zonally Averaged Mass
Circulation
The annually-averaged atmospheric mass circulation in the latitude
pressure plane (the meridional plan). The arrows depict the direction of
air movement in the meridional plane. The contour interval is 2x10 10
Kg/sec - this is the amount of mass that is circulating between every two
contours. The total amount of mass circulating around each "cell" is
given by the largest value in that cell. Data based on the NCEP-NCAR
reanalysis project 1958-1998.
Summary
• General circulation driven by horizontal gradients in diabatic
heating
•
•
Meridional: equator - pole heating differential
Zonal asymmetries: land - sea contrasts; mountains
• Tropical circulations characterized by thermally-direct cells
(Hadley Cell, Walker Cell)
• The flow is largely zonal and geostrophic, but meridional flow
across isobars maintains a thermal wind balance between
the geostrophic wind and the temperature field
• Extratropical circulation is dominated by baroclinic instability
• Poleward transport of moist static energy occurs mainly through
the Hadley cell in the tropics, and by baroclinic waves at higher
latitudes.