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

IR rad latent 21% 23% 7% sen

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:

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

How the balance is achieved

The energy balance at the earth’s surface 21% 23% 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)

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% )

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.

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:

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).

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

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.