E4 Atmospheric Radiation

Topic

Atmospheric radiation governs the energy balance of Earth and its atmosphere, a balance that begins with solar radiation—shortwave radiation arriving from the Sun with wavelengths ranging from 0.2 to 4 micrometers. Before reaching the surface, this solar radiation encounters various obstacles: ozone partially absorbs the ultraviolet component, while water vapor partially absorbs the near-infrared component. Radiation that is not absorbed may instead be scattered: air molecules scatter solar radiation via Rayleigh scattering—a process highly dependent on wavelength (the shorter the wavelength, the more intense the scattering; this explains why the sky appears blue, as blue light has the shortest wavelength in the visible spectrum and is scattered most strongly by air molecules)—whereas aerosols scatter solar radiation via Mie scattering, which depends primarily on particle size. Any solar radiation that is neither absorbed nor allowed to reach the surface—but is instead reflected back into space—constitutes what is known as planetary albedo.

The other component of Earth's energy balance is longwave radiation—featuring much longer wavelengths, between 4 and 100 micrometers—emitted back into space by Earth and its atmosphere in accordance with the Stefan-Boltzmann law: the hotter a body is, the more longwave radiation it emits, with the rate of emission increasing far more rapidly than the temperature itself. Greenhouse gases—water vapor, carbon dioxide, ozone, methane, and nitrous oxide—absorb this longwave radiation and re-emit it in specific infrared bands; the more greenhouse gases present in the atmosphere, the greater the atmospheric opacity to this longwave radiation. This increased opacity is precisely what raises the surface temperature above what it would be on an identical planet lacking an atmosphere.

The radiative forcing of any of these greenhouse gases measures exactly how much the net radiation flux at the top of the atmosphere changes when the gas's concentration changes: if the concentration of a greenhouse gas increases, it absorbs and re-emits more longwave radiation, and this change manifests as radiative forcing that alters the Earth's energy balance—the very balance with which this description began.

Both shortwave solar radiation and the longwave radiation emitted by Earth fundamentally follow the same physical description along any ray passing through the atmosphere: the radiative transfer equation. According to this equation, the intensity of radiation traveling along a ray is attenuated by absorption and scattering that divert energy away from the ray, while simultaneously being increased by the atmosphere's own emission and by scattering that redirects energy from other rays into it; the balance among these four processes, integrated along the entire path of the ray, determines the amount of radiation that ultimately reaches any given point.

Solving this radiative transfer equation exactly is too computationally expensive to repeat at every point in an atmospheric model; consequently, approximate numerical solutions form the core of the radiation models used in both numerical weather prediction and climate modeling. Thus, from incoming solar radiation to the longwave radiation Earth emits back into space—and including the radiative forcing imposed on this exchange by greenhouse gases—atmospheric radiation remains the ultimate control over the energy balance of Earth and its atmosphere.

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