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.