7.1: Prelude to Applications of Atmospheric Radiation Principles

For climate, we need to understand the global energy budget comprised of solar radiation coming into the Earth’s atmosphere and infrared radiation leaving the atmosphere to go into space. We will see that, when averaged over the Earth and over sufficient time, the energy associated with infrared radiation emitted to space by the Earth’s surface and atmosphere essentially always balances the energy associated with solar radiation absorbed by the Earth’s surface and atmosphere. By increasing atmospheric concentrations of CO₂ and other greenhouse gases during the industrial era we have slightly perturbed this balance such that less infrared radiation is currently leaving the Earth system as compared to solar radiation being absorbed by it. This leads to additional energy being deposited into the Earth system that has been exhibited, in part, as a rise in surface air temperatures. At Earth’s surface the energy budgets of both downwelling solar and downwelling longwave radiation at short (second to minute to hour) timescales depends strongly on the presence of gases that absorb, emit, and scatter radiation in the atmosphere. Thus, Earth’s local surface temperature is exquisitely sensitive to the amounts and radiative properties of those gases and particles. We will do some simplified radiation calculations to show you how the Earth’s atmosphere affects the surface temperature.

For weather, we make predictions using models that consist of the equations of thermodynamics, motion, and microphysics. We initialize the models with observations and then let the model calculate the air motions going into the model future, thus giving weather forecasts. The models are good, but not so good that they can run for many days and continue to make accurate forecasts. So periodically, the models are adjusted by adding more observations, a process called data assimilation, in order to correct them and keep the forecasts accurate. Increasingly, satellite observations are being assimilated into the models to improve weather forecasts.

Satellite instruments observe atmospheric radiation: both visible sunlight scattered by Earth’s surface, clouds, and aerosols; and infrared radiation emitted by Earth’s surface and many of its atmospheric constituents. What the satellites measure depends on the wavelengths at which they collect radiation coming up to them. Typically, satellites observe in different wavelength bands, some of which cover wavelengths at which water vapor absorption is much stronger than for others. Taken together, the radiation in these different bands tells us much about the atmosphere’s temperature and moisture structure, which is just the kind of information that the models need to assimilate. You will learn how to interpret satellite observations of atmospheric radiation in support of applications such as vertically resolved temperature and moisture retrievals.