| Home > Newsevents > Training > Rcourse_notes > GENERAL_CIRCULATION > GENERAL_CIRCULATION > |
The general circulation of the atmosphere
By S. Tibaldi* and R
Mureau
* Current address: University of Bologna,
Department of Physics, Via Imerio 46, 40126 Bologna, Italy
Table of contents >>
Next Section >>
Previous Section >>
2 The integral energy balance and the external thermal forcing
We shall start from describing the overall (integral) observational framework at the basis: the integral energy balance.
Many physical processes are involved in energy exchanges between the various components of the climate system. Fig. 1 gives some quantitative (annually averaged) estimates of the energy exchanges between these components, referred to in 100 units of incoming solar radiation (actually equal to 344 W m-2 if averaged over a long time and over the entire earth's surface, which is one quarter of the solar constant). We can see from Fig. 2 that 30% of this energy is immediately reflected back into space in the form of short waves. This is mostly due to the high reflectivity of clouds, although the air itself backscatters approximately 6% and the earth's surface (mainly the deserts and the oceans) reflects another 4%. The remaining 70% is absorbed by atmospheric components (water vapour, ozone, dust etc. : 16%), clouds (suspended liquid water: 3%) and by the surface of the earth (both oceans and land: 51%, by far the greatest amount). It is then mostly the surface which has to communicate this energy to the atmosphere, via sensible (7%) and latent (23%) heat fluxes and via long wave radiation, absorbed by water vapour and carbon dioxide (15%). The net energy input into the atmosphere is then (16+3+7+23+15) = 64% of the total solar input. This energy, if there has to be no long term heating or cooling of the atmosphere, has to be ultimately radiated back into space, and this is done mostly by water vapour and carbon dioxide (38%) and clouds (26%).
Figure 1 . The annual mean global energy balance for the earth-atmosphere system. Numbers are given as percentages of the globally averaged solar irradiance incident upon the top of the atmosphere.
Figure 2 . The radiation balance of the earth. The upper solid curve shows the average flux of solar energy reaching the outer atmosphere. The lower solid curve shows the average amount of solar energy absorbed. The dashed line shows the average amount of outgoing radiation. The lower curves are average values from satellite measurements between June 1974 and February 1978, and are taken from Volume 2 of Winston et al. (1979). Values are in W m-2. The horizontal scale is such that the spacing between latitudes is proportional to the area of the earth's surface between them, i.e. is linear in the sine of the latitude.
Figure 3 . Zonal mean profiles of the northward transports of energy in the atmosphere-ocean system (TA+ T0) based on radiation requirements, in the atmosphere (TA) obtained from rawinsonde data, and in the ocean (T0) inferred as a residual. All curves are for annual mean conditions in 1015 W. Positive values indicate northward transports.
The solar heating input, however, is strongly latitudinally inhomogeneous, being much larger in tropical than in polar regions (see Fig. 2 ). This would create very large low-level latitudinal temperature gradients (because most of the heat input comes from the lower boundary). These would immediately reflect in density (and therefore pressure) gradients which are incompatible with an atmospheric state of rest (or, better, of solid body rotation). Atmospheric large scale motions are therefore the consequence of such gradients (see Fig. 3 ). The air motions, however, take place in a rotating frame of reference and their dynamics must be understood taking the earth's rotation into account.
A simple and very useful conceptual laboratory model to illustrate these processes is the rotating annulus.
Training Course Notes Front Page >>
Table of contents >>
Next Section >>
Previous Section >>
 |
07.06.2002 |
 |
© ECMWF |
