Ground-based radiometric observations of atmospheric emission and attenuation at 20.6, 31.65, and 90.0 GHz: a comparison of measurements and theory

During 1987 and 1988, ground-based zenith-viewing observations of atmospheric thermal emission were made at frequencies of 20.6, 31.65, and 90.0 GHz. At the locations of the experiments (San Nicolas Island, CA, and Denver, CO) radiosonde observations of temperature and humidity were also available. Both National Weather Service and CLASS radiosondes were used in the study. After conversion to attenuation by use of the mean radiating temperature approximation, the data were processed to derive attenuation statistics. Both clear and cloudy attenuation characteristics were examined and compared with results from recent theories. For the clear atmosphere, water-vapor models of Waters (1976) and of Liebe (1989) were compared. At 20.6, and 31.65 GHz, the model of Waters agrees better with measurements; at 90.0 GHz, the model of Liebe is far superior. A recent model of P.W. Rosenkranz (1988) was used for oxygen absorption. For the average mass absorption coefficients for liquid clouds, measurement and theory generally agreed to within 30%. The predictability and interdependence of the three separate channels were also examined. It was found that attenuation for any two channels can predict that of the third to within 25%.< >