NOAA frostpoint hygrometer (FPH) comparisons, uncertainty measurements, and recent instrument improvements

Accurate measurements of upper tropospheric and lower stratospheric water vapor contribute to many processes and feedback mechanisms and play an important role on the radiative forcing of our climate. The NOAA frostpoint hygrometer is a balloon-borne instrument flown monthly at three sites to measure water vapor profiles up to 28 km. The ongoing 36 year FPH record from Boulder, Colorado is the longest continuous stratospheric water vapor record. The NOAA FPH has an uncertainty in the stratosphere that is < 6 % and < 12 % in the troposphere (2-σ). In 2008, a digital microcontroller version of the instrument improved upon the older versions by incorporating sunlight filtering along with better frost control. A new thermistor calibration technique was applied in 2014. This decreased the error in the thermistor calibration fit from 0.06 °C to less than 0.01 °C over the full range of frostpoint or dewpoint temperatures measured during a profile (-93 °C to +20 °C). Atmospheric chamber comparisons between the NOAA FPH and the direct tunable diode laser absorption spectrometer MC-PicT-1.4 during AquaVIT-2 in Karlsruhe, Germany are presented. Dual instrument balloon flights comparing vertical profiles between the NOAA FPH and the cryogenic frostpoint hygrometer (CFH) also show good agreement providing confidence in the accuracy of the FPH measurements.