Abstract
Abstract. We compare the performance of five hygrometers fitted to the Facility for Airborne Atmospheric Measurement's (FAAM) BAe 146-301 research aircraft using data from approximately 100 flights executed over the course of 2 years under a wide range of conditions. Bulk comparison of cloud free data show good agreement between chilled mirror hygrometers and a WVSS-II fed from a modified Rosemount inlet, but that a WVSS-II fed from the standard flush inlet appears to over-read compared to the other instruments, except at higher humidities. Statistical assessment of hygrometer performance in cloudy conditions is problematic due to the variable nature of clouds, so a number of case studies are used instead to investigate the performance of the hygrometers in sub-optimal conditions. It is found that the flush inlet is not susceptible to either liquid or solid water but that the Rosemount inlet has a significant susceptibility to liquid water and may also be susceptible to ice. In all conditions the WVSS-II responds much more rapidly than the chilled mirror devices, with the flush inlet-fed WVSS-II being more rapid than that connected to the Rosemount.
Highlights
Water vapour is well established as the most important greenhouse gas and, is crucially important in determining the radiation budget of the atmosphere
As the Total Water Counter (TWC) uses magnesium fluoride windows and a Lyman-α source, both of which degrade over time, the instrument must be calibrated on a flight-by-flight basis using one of the chilled mirror instruments and, does not provide an independent measurement
The offset between pairs of hygrometers during the test period has been calculated in the following way, using only data indicated to be cloud free according to the cloud droplet probe (CDP) (Lance, 2010)
Summary
Water vapour is well established as the most important greenhouse gas and, is crucially important in determining the radiation budget of the atmosphere. Vance et al.: Performance of WVSS-II on FAAM aircraft ity of such routine AMDAR measurements as an adjunct to, or as a replacement for, radiosonde observations for NWP purposes, and positive impacts on forecasts have been reported (Moninger et al, 2006; WMO, 2014b). These studies have two serious limitations: (1) a radiosonde and an aircraft cannot be guaranteed to sample the same air mass throughout their ascents, which increases the uncertainty, and (2) the WVSS-II measurements in question are truncated for transmission (two-digit mantissa, one digit exponent) which “can add substantial error to the moisture reports exceeding 10 k/kg” (Bedka et al, 2006).
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