Numerical study of radiative transfer processes in the atmosphere

Abstract

Radiative transfer constitutes an important physical process in the atmosphere. A detailed knowledge of the radiative balance of the Earth and the processes pertaining to the transfer of solar and terrestrial radiation is essential in the study of climate and the mechanisms of climate change. In complex climate models, simplifications and parameterizations have been developed for the treatment of radiative processes. Many of these schemes, some of which are in use in operational models worldwide, have been found inadequate in the study of atmospheric radiation problems, wherein analyses are confined only to a few major parameters.

In the present study, comprehensive radiative schemes have been developed to analyze the transfer of solar and terrestrial radiation in the atmosphere. They make use of standard or user-specified atmospheric models and compute the flux and radiative heating/cooling rates in the atmosphere. Different aspects of the radiative properties of aerosols and gases have been treated accurately and completely. The temporal and spatial variability of aerosols and the absorption and scattering interactions between gases and aerosols have been accounted for in a physically consistent manner. Transmittances for absorption by CO?, O?, and O? are evaluated by employing empirical band models. Absorption due to water vapor, an optically major constituent influencing heating in the troposphere in the shortwave, is dealt with by means of modified k-distribution functions which employ a superior scaling approximation to account for the variation of pressure and temperature along atmospheric paths.

Computational schemes in the shortwave are based on a choice of different radiative transfer approximations, invoked on a selective basis. Two simplified methods have been devised for treating radiative transfer through multiple inhomogeneous layers of the atmosphere. These methods, while retaining the simplicity of two-stream approximations, have been found comparable in accuracy to the exact multi-stream adding method.

Extensive computations parallel to those of another highly elaborate and sophisticated numerical technique of Brasslau and Daue (1975) are performed to determine the flux, albedo, absorption, and heating rate profiles for several model atmospheres. Comparison of results with the latter model yields close agreement in many instances with similar conclusions. Computational procedures used by the latter have been found unsatisfactory for spectral regions associated with strong absorption by water vapor. Sensitivity studies involving aerosol concentration, their location, ground albedo, solar zenith angle, etc., reveal the significant impact of different types of aerosols on various aspects of the transfer of solar energy in the atmosphere.

Longwave radiative calculations are based on the integral form of the solution for the equation of radiative transfer in the atmosphere. Selective absorption by all trace gases and continuum absorption by water vapor throughout the longwave spectrum have been adequately accounted for. Aerosol scattering in the longwave is included by employing the delta-Eddington scaling of optical parameters.

Results obtained with the present model tally well with those of two other detailed narrowband models. A series of numerical experiments have been conducted to determine the sensitivity of boundary fluxes and cooling rate curves to various parameters. They suggest the importance of including the effect of water vapor continuum outside the 8–13 ?m atmospheric window (a relatively transparent spectral region characterized by little absorption) and aerosol scattering in longwave calculations.