general circulation of the atmosphere lisa goddard 19 september 2006
TRANSCRIPT
General Circulation of the General Circulation of the AtmosphereAtmosphere
Lisa Goddard19 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
Outline1. 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”
•The primary horizontal balance of forces in atmospheric motion is the geostrophic balance:
Geostrophic Balance: the Coriolis force
•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.
Geostrophic Flow with Friction
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
•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.
Friction, Mass Continuity Convergence/Divergence
Geopotential heightSwitching between z & p coordinates
units: m
:
,
1
1
z p
z p
z p
p p z
x z x
p p z
x z x
p zg
x x
Geopotential gz
p
x xSo
p
x x
p
y y
Standard coordinatetransformation
Minus sign includedbecause p variesoppositely to z
Now invoke hydrostatic eqn.
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).
-
-
MidlatitudesBaroclinic instability
Eddy fluxes of momentum force an indirect meridional
circulation: the Ferrel cell
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.
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 energygravitational 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.