Articles | Volume 9, issue 9
Research article
05 Sep 2016
Research article |  | 05 Sep 2016

Advancements, measurement uncertainties, and recent comparisons of the NOAA frost point hygrometer

Emrys G. Hall, Allen F. Jordan, Dale F. Hurst, Samuel J. Oltmans, Holger Vömel, Benjamin Kühnreich, and Volker Ebert

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.

Short summary
This work focuses on the balloon borne NOAA frost point hygrometer (FPH) instrument flown at three locations around the world: Boulder, Colorado, Lauder, New Zealand, and Hilo, Hawaii. The ongoing 36-year record is the longest continuous water vapor record with profiles reaching 28 km. Significant instrument updates in 2008 decreased the weight, cost, power consumption, and manufacturing time offering greater precision and ease of use.