Emission of the Atmosphere

Abstract

Emission of the atmosphere as a thin air layer located over the Earth’s surface results from radiation of water and carbon dioxide molecules, as well as due to water microdroplets which constitute the clouds. Since the emission spectrum of the atmosphere consists mostly of individual broaden spectral lines due to vibration–rotation and rotation states of molecules, the infrared atmospheric spectrum consists of thousands broaden peaks. Therefore, in evaluation of the radiative flux from the atmosphere, we are based on the “line-by-line” method combined with thermodynamic equilibrium between the radiation field and air molecules, as well as the model of standard atmosphere for molecules with accounting for a weak altitude dependence for the atmospheric temperature. Assuming clouds as an opaque matter to be located above a certain altitude \(h_{cl}\) and to have a sharp boundary, one can describe the radiative flux created by atmospheric molecules at a given frequency \(\omega \) by two atmospheric parameters, namely an optical thickness \(u_\omega \) and an opaque factor \(g(u_\omega )\) for atmospheric layer below clouds. It is of principle for these evaluations the HITRAN database that contains parameters radiative transitions in greenhouse molecules located in atmospheric air. The altitude \(h_{cl}\) of the cloud boundary (or the radiative temperature \(T_{cl}\) for clouds) follows from comparison of the total radiative flux of the atmosphere to the Earth’s surface according to the energetic balance of the Earth, as well as the calculated radiative flux created by atmospheric molecules. As a result, the partial radiative fluxes for each component are determined at a given frequency, as well as the total radiative fluxes to the Earth due to each component. In particular, the contribution to the total radiative flux toward the Earth is approximately \(64\%\) due to \(H_2O\) molecules, \(18\%\) due to clouds, \(17\%\) due to \(CO_2\) molecules, and about \(1\%\) due to \(CH_4\) and \(N_2O\) trace molecules. The greenhouse instability is represented that is realized if one water molecule during its residence in the atmosphere emits such radiative flux that causes evaporation more than one water molecule from the Earth’s surface. The threshold of this instability occurs if the contemporary global temperature increases by 7K.