1. ATOC 4720 class29
1. How the energy balance is achieved in the earth-atmosphere system
2. The energy balance of the upper
atmosphere
3. The energy balance of the troposphere

2. IR rad
latent
21%
23%
7% sen

3. The mean planetary irradiance back into space
Assume the net energy flux through the earth’s surface is negligible, and energy stored in the atmosphere is not systematically changing,
Then at the top of the atmosphere, incoming energy should be balanced by outgoing energy:

4. S=1380W/m2
Incoming solar radiation
Outgoing planetary radiation
The earth’s radius
A---planetary albedo

5. How the balance is achieved

6. The energy balance at the earth’s surface
21%
23% 7%

7. Balance at the top of the atmosphere
100 units incident:
Out: 30% reflected
6% longwave radiation from earth
64% emission by atmosphere (38% by water and carbon
dioxide molecules; 26% by clouds)

8. Balance in the atmosphere
19% absorbed solar radiation;
15% absorbed the earth’s IR radiation;
30% latent & sensible heat fluxes;
Out: 64% IR emission (38%
& 26% )

9. 2. The energy balance of the upper atmosphere
Above the tropopause: absorption of solar radiation
is important.
Photoionization and photodissociation of various
Gaseous constituents of the upper atmosphere by
UV and X-ray.

10. Photoionization of the thermosphere (most above 90km, N2, O2, O gives rise to E- and F-layer): UV & X-ray Due to this absorption, T in the thermosphere (Z>80km) increases with height to maintain thermal equilibrium; random molecular
motion can conduct heat downward. (radiation is ineffective because of the thin air density).
T is strongly influenced by solar activity.
Photodissociation of oxygen:

11. O--major consituent above 100km, although it is highly
Reactive. [mean free path is long at high level]
At lower levels: (20-60km, say), although O is trace
constituent, it is important in forming ozone layer.
[Mean Free path is lower at lower level]
M: 3rd molecule to carry excess energy away
from the reaction (3 body collision).

12. The Ozone layer UV: Not much absorbed by photodissociation, so
O Quickly recombines with O2, maintaining ozone layer.

13. 3. The energy balance of the troposphere
Troposphere: structure and dynamics are profoundly
Influenced by the earth’s surface; Latent and sensible
Heat fluxes account for 30% of the total 51% absorbed.
Almost all solar radiation with
Is absorbed above tropopause;
Very little absorption in visible band (maximum solar
Emission)
Absorption of solar IR radiation mostly occurs in troposphere,
Where water vapor is located.