Abstract
The NOAA frost point hygrometer (FPH) is a balloon-borne instrument flown monthly at three sites to measure water vapor profiles up to 28 km. The FPH record from Boulder, Colorado, is the longest continuous stratospheric water vapor record. The instrument has an uncertainty in the stratosphere that is < 6 % and up to 12 % in the troposphere. A digital microcontroller version of the instrument improved upon the older versions in 2008 with sunlight filtering, better frost control, and resistance to radio frequency interference (RFI). A new thermistor calibration technique was implemented in 2014, decreasing the uncertainty in the thermistor calibration fit to less than 0.01 °C over the full range of frost – or dew point temperatures (−93 to +20 °C) measured during a profile. Results from multiple water vapor intercomparisons are presented, including the excellent agreement between the NOAA FPH and the direct tunable diode laser absorption spectrometer (dTDLAS) MC-PicT-1.4 during AquaVIT-2 chamber experiments over 6 days that provides confidence in the accuracy of the FPH measurements. Dual instrument flights with two FPHs or an FPH and a cryogenic frost point hygrometer (CFH) also show good agreement when launched on the same balloon. The results from these comparisons demonstrate the high level of accuracy of the NOAA FPH.
Highlights
Water vapor is the most abundant and important greenhouse gas in the atmosphere and contributes to many processes and feedback mechanisms (Dessler et al, 2008)
The concentration of water vapor in the stratosphere is controlled by transport through the tropical tropopause layer (TTL) rendering the stratospheric air extremely dry through a freeze-drying process (Brewer, 1949)
This paper provides detailed information regarding the description and uncertainty of the current digital frost point hygrometer (FPH) along with a brief history of the evolution of the sonde
Summary
Water vapor is the most abundant and important greenhouse gas in the atmosphere and contributes to many processes and feedback mechanisms (Dessler et al, 2008). Worldwide there are more than 800 radiosonde stations launching balloon instruments twice daily, totaling > 500 000 vertical profiles of wind speed/direction, pressure, temperature, and humidity each year. These measurements are predominately used for weather forecasting. NOAA FPH balloon measurements provide high-vertical-resolution profiles Satellite sensors, such as the Aura Microwave Limb Sounder (MLS) (Read et al, 2007), measure water vapor in the UTLS and above (Lambert et al, 2007) with almost daily global frequency but with a vertical resolution of 2–3 km. Comparisons in the laboratory along with dual instrument balloon flights are presented
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