The greenhouse effect and the spectral decomposition of the clear-sky terrestrial radiation

In this paper the clear-sky infrared radiation field of the Earth-atmosphere system is characterized by the spectral decomposition of the simulated upward and downward flux density components into three distinct wave number regions. The relative contributions of the far infrared, middle infrared, and windows spectral regions to the total longwave flux densities have been established. The approximate qualitative description of the meridional distributions of the zonal averages gave us a detailed insight into the role of the less explored far infrared spectral region. We demonstrate that on a global scale, the far infrared contribution to the clear-sky normalized greenhouse factor is significantly increasing toward the polar regions. Accurate computation of the transmitted and re-emitted part of the outgoing longwave radiation showed that in the far infrared the normalized upward atmospheric emittance increases poleward. This phenomenon is the direct consequence of the downward shift of the peak of the weighting functions in the strongly absorbing opaque spectral regions. The clear-sky total longwave terrestrial flux transmittance seems to be well correlated with the far infrared flux transmittance, which implies the possibility of inferring total longwave flux densities solely from far infrared observations. The zonal averages of the total normalized atmospheric upward emittances are almost independent of the water vapor column amount, they have no meridional variation, and they are constantly about fifty percent of the surface upward flux density, an indication, that the gray atmosphere in the IR is in radiative equilibrium. The meridional distribution of the greenhouse temperature change and its dependence on the atmospheric water vapor content were also evaluated. Solving the Schwarzschild-Milne equations for the bounded atmosphere the infrared atmospheric transfer and greenhouse functions were derived. The theoretically predicted greenhouse effect in the clear atmosphere is in perfect agreement with simulation results and measurements. Key-words: greenhouse effect, radiative equilibrium, far infrared. * Corresponding author. Tel: 757-827-4627; Fax: 757-825-8659