Laboratory Characterization of Cryogenic Frost‐Point Hygrometers Using an Upper Air Simulator

ABSTRACT Cryogenic frost‐point hygrometers (CFHs) are essential tools for reference‐level water vapor measurements in the upper atmosphere. This study evaluates CFH's measurements on frost‐points, with a focus on the transition from supercooled dew‐point to frost‐point, the accuracy, and stability of measurements across varying frost‐point conditions, and the effect of airflow temperature. An upper air simulator (UAS) is used to generate frost‐points traceable to water vapor standards. The CFH measures these frost‐points inside a climate chamber that regulates the ambient temperature. As the frost‐point was lowered from −5°C, CFH measurements consistently tracked closer to the supercooled dew‐point than to the frost‐point until the dew‐point reached approximately −26.5°C, at which point the measurement transitioned to the frost‐point. A method based on the frequency components of optical signals is proposed to differentiate between supercooled water and frost. The standard deviations of the CFH measurements increased from 0.009°C to 0.031°C, 0.172°C, and 0.356°C as the frost point decreased from −5°C to −39.5°C, −66°C, and −75°C, respectively, indicating less effective operation of the proportional‐integral‐derivative (PID) controller at lower water vapor pressures. The mean differences between the UAS and CFH measurements ranged from −0.042°C to 0.125°C across the above frost points and remained within the combined uncertainty of the two systems, which ranged from 0.11°C to 0.15°C over the same range. Additionally, CFH measurements were slightly lower at higher airflow temperatures compared to lower temperatures for the same water vapor pressure, potentially due to the condensate's exposure to warmer airflow, which led to evaporation and slight discrepancies in mirror temperature. The systematic bias of the CFH in response to changes in ambient airflow temperature remains at (−0.83 ± 0.26) mK K −1 over the frost‐point range from −66°C to −23°C but increases sharply at −5°C, reaching −5.3 mK K −1 . By combining CFHs with a humidity generator traceable to water vapor standards, this study establishes a robust laboratory framework for quantitatively evaluating CFH performance under simulated upper‐atmospheric conditions. The findings provide essential insight into the interpretation of CFH measurements in atmospheric sounding observations.