Development of a new cryogenically cooled water vapor radiometer for the 22 GHz line – quasi-optical design and preliminary laboratory receiver tests

Abstract. This paper reports on the instrumental design of a new cryogenically cooled middle-atmosphere water vapor radiometer developed by the University of Bern at the Institute of Applied Physics (IAP). Here, we present the instrument design for the breadboard stage. The key innovation of this new instrument is its cryogenically cooled front-end, which is designed to keep its size compact, reducing the required cooling power compared to existing cryogenically cooled radiometers. The advantage compared to uncooled instruments is the reduced receiver noise temperature and the possibility to extend the altitude coverage of the retrieval of water vapor profiles to even higher altitudes with better temporal resolution. The new radiometer is part of the Swiss H2O Hub and is supposed to replace the existing 22 GHz radiometer, MIAWARA, which has been in operation at the University of Bern for over 20 years at the Zimmerwald observatory. The calibration of the new instrument includes tipping curve calibration to determine tropospheric opacity, using the sky as a cold target. An ambient load serves as the hot target for the Hot-Cold calibration, and we also explore the possibility of using frequency-switch calibration to reduce the impact of non-linearities in the receiver chain, allowing for a higher integration time of the line observation compared to other calibration techniques. The combination of a cryogenic front-end and frequency switch microwave radiometers at 22 GHz has not been previously implemented in a single instrument. In addition to detailing the instrumental design and calibration techniques, we present preliminary results of atmospheric spectra obtained with the breadboard setup.