Slide 15 of 26
Read Horel and Geisler Chapter 2
The ERL (Emitted Radiation Level), is that level in the atmosphere above which there is sufficiently little greenhouse gas (Water and CO2) that infra red radiation emitted upwards is just able to reach outer space without being absorbed. The greater the concentration of water and CO2, the higher in the atmosphere is the ERL. If there were no greenhouse gases or clouds, the ERL would be at the Earth surface. This simplified model treats the tropopause as effectively the top of the atmosphere, and lumps all wave lengths together. In practice each infra red wavelength has a different ERL.
The intensity of emitted radiation increases with the temperature. Because the temperature decreases upwards, the higher the ERL the less intense is the infrared radiation leaving to space. Below the ERL heat is transferred from the surface primarily by convection, which maintains a fixed lapse rate of temperature with height.
On average, the radiation leaving to space must equal that coming in from the sun (342 W/m2). Assuming a fixed fraction (about 30.7 % or 105 W/m2) of the incoming solar is reflected from the surface or from clouds, the remaining 69.3% or 337 W/m2 must leave as infra red. We idealize by assuming all the infra red leaves from the ERL, with intensity set by the temperature at that level.