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 • Review: The Forces : pressure gradient force, Coriolis force, friction. • 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
In the absence of atmospheric motions the gravity force must be exactly balanced by the vertical component of the pressure gradient force. “Hydrostatic Balance” 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
... eastward pressure-gradient force per unit mass Horizontal pressure gradient force
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
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
500 hPa Geopotential height (Z) units: m
Geopotential heightSwitching between z & p coordinates Standard coordinatetransformation Minus sign includedbecause p variesoppositely to z Now invoke hydrostatic eqn. units: m
Latitude-height cross-section 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
MidlatitudesBaroclinic instability Eddy fluxes of momentum force an indirect meridional circulation: the Ferrel cell
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
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.