ATOC 4720 class 29 1 How the energy

ATOC 4720 class 29 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=1380 W/m 2 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 In: 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 thermosphere (most above 90 km, N 2, O gives rise to E- and F-layer): UV & X-ray. Due to this absorption, T in thermosphere (Z>80 km) 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 100 km, although it is highly Reactive. [mean free path is long at high level] At lower levels: (20 -60 km, say), although O is trace constituent, it is important in forming ozone layer. [Mean Free path is lower at lower level] M: 3 rd 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 O 2, 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.