Hans J. Liebe

Abstract: Accurate laboratory measurement of attenuation rates α_x over a range from 0.1 to 10 dB/km at 138 GHz for water vapor (H₂O) and its mixtures with air, nitrogen (N₂), oxygen (O₂), and Argon (Ar) have been performed over a temperatures range from 8 to 43°C, relative humidities up to 95% RH, and total pressures reaching 1.5 atm. A computer-controlled resonance spectrometer was employed. The results are interpreted in terms of underlying absorption mechanisms. Broadening efficiencies m for mixtures H₂O + N₂, H₂O + O₂, + Ar agree among themselves with those measured within cores of the 22 and 183 GHz H₂O absorption lines. The m-factors are applied to predict what share αl of the total α_x results from the complete pressure-broadened H₂O spectrum. The results indicate that a substantial amount of the self-broadening term proportional to the square of vapor pressure is left unaccounted. The negative temperature coefficient of the excess absorption is consistent with a dimer (H₂O)₂ model. A spectroscopic data base limied to 30 local H₂O lines (centered below 1 THz) contributes about 1/3 to α_l at 138 GHz. An empirical formulation of the experimental findings is incorporated into the parametric propagation model MPM that utilizes a local (30x H₂O, 48 x x[sic] O₂) line base to address frequencies up to 1000 GHz. Predictions of moist air attenuation and delay rates by means of the revised MPM program generally compare favorably with reported (10 – 430 GHz) data from both field and laboratory experiments, except for subfreezing transmission data in the 190 to 260 GHz range.

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