Geography Reference
In-Depth Information
effects of the annual cycle, there would be no meridional or vertical circulation
and no stratosphere-troposphere exchange for such a hypothetical state.
A cross section of the radiatively determined temperature during a Northern
Hemisphere winter is shown in Fig. 12.4. It should be compared to the observed
temperature profile for the same season that was shown in Fig. 12.2. Although
the rather uniform increase of temperature from winter pole to summer pole in
the region of 30- to 60-km altitude is qualitatively consistent with the radiatively
determined distribution, the actual temperature difference between the two poles is
much smaller than in the radiatively determined state. Above 60 km even the sign of
the gradient is opposite to that in the radiative solution; the observed temperatures
near the summer polar mesopause are much colder than those near the winter polar
mesopause.
Departures from this radiatively determined state must be maintained by eddy
transports. Thus, rather than
causing
the mean circulation, the radiative heating
Fig. 12.4
Radiatively determined middle atmosphere temperature distribution (K) for Northern winter
soltice from a radiative model that is time marched through an annual cycle. Realistic
tropospheric temperatures and cloudiness are used to determine the upward radiative flux
at the tropopause. (Based on Shine, 1987.)
