Flux Emissivity Tables for Water Vapor, Carbon Dioxide and Ozone

Abstract

Flux emissivities for the H₂O bands and window, and for CO₂ and O₃, were evaluated from Elsasser's wavelength-dependent absorption coefficients and flux tranmissivities. The flux emissivities for water vapor differ significantly from previously published emissivities based on Elsasser's incorrect flux densities. A flux emissivity correction for the overlap of H₂O and CO₂, is defined and evaluated for combinations of optical depths. The largest partial emissivity is associated with the H₂O rotation band, and it increases markedly with decreasing temperature, while the partial emissivities associated with the 6.3 μ band and the window are smaller and decrease with decreasing temperature. Emissivity in the window increases very rapidly with optical depths ≳1 gm cm⁻². The total H₂O flux emissivity is remarkably independent of temperature, especially in the range from −40 to 20C, as a result of near cancellation of the temperature dependences in three regions of the spectrum. The total flux emissivity increases rapidly with optical depth beyond ∼1 gm cm⁻² as a result of emission in the window. An H₂O column with a temperature of 20C is 98% black for optical depths >50 gm cm⁻². The flux emissivity of CO₂ increases slightly with temperature at all but the very smallest optical depths. This result traces physically to the increase of absorption with temperature at the edges of the 15 μ band. The correction to flux emissivity resulting from the overlap of H₂O and CO₂ radiation is negative and increases markedly with the optical depths of both. It is of the order of −0.05 to −0.10 for typical total atmospheric optical depths. This emissivity correction for overlap shows a large percentage increase with temperature, principally as a result of increasing absorption with temperature in the wings of both the rotation band of H₂O and the 15 μ CO₃ band. The flux emissivity of O₃ undergoes a large percentage decrease with temperature. For typical total O₃ depths, the emissivity for −70C is less than half its value at 20C.