RADIATIVE-TRANSFER IN STRATIFIED ATMOSPHERES - DEVELOPMENT AND VERIFICATION OF A UNIFIED MODEL

A reliable and efficient discrete method is verified for multiple scattering, radiative-transfer calculations in vertically inhomogeneous, non-isothermal atmospheres in local thermodynamic equilibrium. The linear-in-optical-depth approximation to the Planck function is used to obtain accurate solutions for thermal radiation. We show that this approximation significantly improves computing efficiency while still maintaining adequate accuracy. Using this multiple-scattering scheme, we have constructed and validated a radiation model for stratified atmospheres, taking into account molecular (Rayleigh) scattering and absorption/emission. The exponential sum fitting of transmissions technique is utilized to parameterize gaseous absorption, thereby achieving a unified treatment of shortwave and longwave radiative transfer. To validate the model, we present computed fluxes and heating/cooling rate profiles for the five McClatchey atmospheres. The results are compared with other models having different spectral resolution (Δs) and gaseous scattering and absorption/ emission structure. Specifically, we compare broad-band (Δs > 100 cm-1), narrow-band (Δs < 100 cm-1) and line-by-line computations and find that at the expense of accuracy by a few W-m-2 for flux or a few tenth °C/day for heating/cooling rate computations, the broad-band models are very fast and may be suitable for many applications. We also find that in spite of good agreement between fluxes (at the top and bottom boundaries) computed by different methods, the heating/cooling rate profiles may differ substantially due to compensating errors. © 1990.