New measurements of atmospheric continuum for refinement of millimeter wave propagation models

Results of extensive experimental study of the continuum absorption in a mixture of water vapor and nitrogen in millimeter wave band are presented. A new approach based on variation of the spectrometer optical path-length was employed to exclude systematic uncertainty caused by water adsorption on the cavity elements of the resonator spectrometer. It allowed investigation of the water-related continuum in a wide temperature range from -12 to +55°C including a first-time laboratory study of the continuum at temperatures below freezing. Detailed investigation of the continuum absorption dependences on radiation frequency and gas meteorological parameters is fulfilled. Coefficients of the common empirical parameterization of the continuum are derived and compared with results of the most known previous experimental and theoretical studies demonstrating very good qualitative and in some cases quantitative agreement. Dominating types of intermolecular interactions leading to the observed continuum are discussed. Parameterization of the total measured absorption (including contributions of the continuum and local lines) is made to provide the possibility of derivation of the continuum using any appropriate model of the resonance absorption. Additional analysis of the results is performed to demonstrate possible contribution of the systematic error due to water adsorption on the cavity elements of the resonator spectrometer.