The Cryogenic Frost point Hygrometer (CFH): Updates, Data Evaluation, and Opportunities

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The Cryogenic Frost point Hygrometer (CFH): Updates, Data Evaluation, and Opportunities Telayna Gordon, Martin Stuefer, Douglas Sisterson, Donna Holdridge, Jenni Kyrouac, Chris Martin, James Martin, David Breedlove, Matthew Gibson, Mark Smith, John Schatz, Nicki Hickmon, Lori Borg The Cryogenic Frost point Hygrometer (CFH) has been deployed monthly at the ARM Southern Great Plains (SGP) site since September, 2014 to measure tropospheric and stratospheric water vapor with increased accuracy and sensitivity. The CFH, which uses a temperature-controlled chilled mirror to directly measure the ambient frost point, allows for the construction of water vapor mixing ratio profiles, and is particularly useful for studying the dry conditions of the low stratosphere due to the instrument’s sensitivity within a few parts-per-million mixing ratios. The total uncertainty of all frost point or dew point temperature measurements using the CFH are less than 0.2 K under good conditions, and systematic errors are less than 0.1 K, allowing for reliable long-term climate tracking (Vömel, 2016). The derived relative humidity uncertainty can be as low as 2% in the lower troposphere, and 5% in the tropical tropopause. CFH humidity profile observations are compared with corresponding radiosonde and High Resolution Rapid Refresh (HRRR) numerical weather model data. We determine the amplitudes of the radiosonde humidity bias, and check how humidity is represented in the HRRR at the levels of the Upper Troposphere/Lower Stratosphere (UTLS). In addition to radiosonde and model comparisons, we aim to use the CFH instrument data for evaluation of spaceborne humidity profilers and new radiosonde measurements. Currently, CFH launch packages include the IMet and Vaisala RS-92 radiosondes. The replacement of the IMet radiosonde with the new Vasiala RS-41 instrument will facilitate CFH data processing, and allow researchers to further evaluate the RS-41 radiosonde accuracy necessary to support the transition from RS-92 to RS-41 radiosonde baseline measurements. Vömel, H, T Naebert, R Dirksen, and M Sommer. 2016. "An update on the uncertainties of water vapor measurements using cryogenic frost point hygrometers." Atmospheric Measurement Techniques 9: 3755-3768, doi:10.5194/amt-9-3755-2016.