| 21 Dec 2025
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.
General comments
The paper describes the development of a new cryogenically cooled water vapor radiometer observing the 22 GHz line from ground. Since the 22 GHz line is rather weak, especially for observing the upper mesosphere, where high spectral resolution is required to resolve the pressure broadened line shapes, cooling is beneficial. The goals of the development include to achieve optimal performance in terms of sensitivity (receiver temperature) at moderate power consumption, to get a compact design for several purposes and to introduce a novel calibration technique (frequency switch) for ground-based 22 GHz water vapor radiometers. The described novel instrument concept and the reported results show a clear progress in the development for these kinds of instruments and fits well within the scope of AMT. The overall presentation is well structured and clear. I recommend publication after minor revision
Specific comments
Introduction, lines 21-22 … accompanied by …: the sentence looks a bit ambiguous. Please shortly describe the mechanism of freeze drying on production of the hydroxyl radical (so that the reader does not have to read the reference).
Line 26: there are earlier papers showing the effect of SSWs on water transport as measured with 22 GHz radiometers (e.g. Seele & Hartogh GRL, 2000). Please add the reference
Line 37. Note that Kiruna is in Sweden. The cited instrument operated both in Norway (Andenes) and Sweden (Kiruna).
Instrumental design:
Lines 114-115: … better mechanical properties… Please list the properties
Line 118: Why sqrt(sqrt)? Please provide a short explanation.
Lines 118 ff and Figure 4 caption: it does not get clear how much the ARC improves the measurements. Please quantify. What is the maximum acceptable window deformation and how thick does the window have to be (for the given diameter) that the deformation stays below this tolerance? Is the thickness of one wavelength sufficient?
Line 134: Is there a list of instrument requirements that were defined before the instrument was developed? What defines the requirement for a sidelobe level of -35 dB?
Line 139: Why is the spillover unit dBi and not dB? What defines the -30 dB?
Line 142: please specify the mechanical mirror parameters a bit more precisely: what is the material thickness and mass of the mirror?
Line 149: It is a bit difficult to decipher all details in Figure 7. Please add a zoomed-in version with an x-scale of +- 5 degrees.
Line 164 ff: Please mention here that the Hallgren et al system also contains temperature-tunable internal hot and cold loads. This seems to be the main reason why the system is larger than the CRYOWARA system. A short discussion would fit here what are the pros and cons of internal loads compared to the ambient hot load you are using and the “atmospheric” cold load.
Line 170: please specify power consumption and service interval length
Figure 9 shall contain dimensions
Line 195: it does not get clear whether you are using MLI, etc. or not. Isn’t it beneficial to take advantage of it? How much would the thermal load be reduced?
Comparison with MIAWARA
Line 203: …far-field gain of 35.6 dB… ff: here it shall be dBi I guess (or is it related to a dipole antenna?)
Lines 209 ff: discussion about the performance comparison. I would like to see a bit more quantified discussion here. The advantage of the new system is the better noise temperature and at the same time the dual polarization measurements (reducing the measurement time for the same SNR by a factor of 2). On top of the receiver temperatures the observing conditions have to be taken into account. I propose to add number for typical seasonal conditions at the Zimmerwald location (you have a statistic from MIAWARA) e.g. in winter and summer and provide an integration time required for a retrieval up to 80 km with MIAWARA and CRYOWARA. This would demonstrate the big advantage (especially of the dual-polarization approach) of the new system.
Table 2: please explain why MIARAWA with its single-polarization approach also needs two HEMT amplifiers. Please check dB/dBis.
About the CRYOWARA spectrometer mentioned in Table 2: I checked the specification on the website of the manufacturer and found a maximum bandwidth of 160 MHz, but you mention 200 MHz here. Please clarify.
Calibration
Line 229: …channels is then calibrated… If you mean a total power calibration please mention it here.
Preliminary results
Figure 18b: please mention the integration time as in 18a. What is the little bump at about -50 MHz? Why is it shifted between the two polarizations? Please provide an additional plot 18c integrating the two polarizations. The spectral noise should go down by sqrt(2) in case the two signals are not correlated. Please prove that this is the case.
Typos etc:
Line 79: Table ??
General: check use of dB and dBi and correct it accordingly.