Chapter 9 - Radiation

Abstract

Radiation imbalances provide the energy that drives large-scale atmospheric circulation and smaller-scale processes, which affect our weather. Radiation in weather models is considered in two parts: shortwave radiation from the Sun, and longwave radiation, or thermal radiation, from the Earth. Absorbed shortwave radiation is the dominant source of energy input to the Earth's atmosphere and surface. Along with reflected shortwave radiation and emitted longwave radiation, from the surface and atmosphere, it determines the local radiative heating rates. Uncertainties in local radiative heating rates primarily arise from external factors such as clouds and aerosols. For instance, if clouds are missing from the input data to the radiation scheme, the shortwave, longwave, and total radiative heating rates will be incorrect. The cloud microphysics as characterized by the cloud phase, the effective liquid cloud drop radius and the cloud ice equivalent radius are also important external factors. Top-of-atmosphere solar irradiance and surface radiative properties determine boundary conditions for the radiative transfer calculations and are also a source of uncertainty. Internal computations in radiation schemes also cause uncertainties, but to a much lesser extent. These include parameterizations of optical properties, radiative transfer, and surface-radiation interactions. In addition, subgrid-scale assumptions can cause considerable uncertainties. These include assumptions about how multilevel clouds overlap within a grid box, and 3D radiative effects of clouds and complex surface topographies such as mountains. Finally, the discretization of the equations and input in space and time add to the uncertainties.