Air-Broadened Half-Widths of the 22- and 183-GHz Water-Vapor Lines

Air-broadened half-widths of the 22- and 183-GHz water-vapor lines and associated uncertainties have been determined using comparisons between ground-based radiometric measurements from Atmospheric Radiation Measurement sites in Oklahoma and Alaska, and MonoRTM, a radiative transfer model. Values of the widths obtained using the measurements are 0.0900 cm^-1/atm with 1.6% uncertainty for the 22-GHz line and 0.0992 cm^-1/atm with 2.4% uncertainty for the 183-GHz line. Also presented are spectroscopic parameters for these lines from new calculations performed using the complex implementation of the Robert-Bonamy theory (CRB). The CRB values of the air-broadened widths are 0.0913 cm^-1/atm with 3% uncertainty and a temperature exponent of 0.755 for the 22-GHz line and 0.0997 cm^-1/atm with 3% uncertainty and a temperature exponent of 0.769 for the 183-GHz line. The values for the air-broadened half-widths derived from the measurement/model comparisons show good agreement with the new CRB calculations. For future versions of MonoRTM, width values of 0.0900 and 0.0997 cm^-1/atm are to be adopted with temperature dependences of 0.76 and 0.77 for the 22- and 183-GHz lines, respectively.