Validating clear air absorption models using ground-based microwave radiometers and vice-versa

Microwave radiometer observations are compared with various radiative transfer model calculations based on simultaneous radiosondes. This analysis uses observations from Payerne, Switzerland, in cloud free conditions during the Temperature Humidity and Cloud (TUC) experiment in winter 2003/04. The results show a systematic bias in the brightness temperatures measured by the Radiometrics profiler at 55-59 GHz, which has since been corrected in the control software. Observations at lower frequencies (22-30 GHz) in these cold conditions do not support recent proposed changes to the width of the 22.235 GHz water vapour line, although this is subject to the assumption of no residual bias in the radiosonde humidity. At intermediate frequencies (51-54 GHz), the absorption models produce large differences, which may be due to differences in oxygen line coupling and highlight the need for further laboratory measurements at low temperatures.