TDL absorption technique using a single wavelength at 1.37µm and a 24 cm path length with 2nd harmonic detection

General

WVSSII was designed for Commercial air carriers by UCAR/SpectraSensors. A measurement is made just inside the aircraft with a TDL absorption technique using a single wavelength at 1.37µm and a 24 cm path length with 2nd harmonic detection. Flow rate inside the measurement cell is 3-6 m/s. Specifications include 0.5Hz averaged data, a claimed sensitivity of 3ppm, and a range of +20 to –70 °CDP. Recent laboratory calibrations suggest that these may not be quite borne out in practice, with lack of sensitivity and over-saturation causing the operating range to be clipped slightly at both ends. Other observed problems include excessive dry-down times when profiling upwards from ground level. A purpose-built certified UCAR inlet on the aircraft skin removes liquid water from the sample stream, to improve measurements in cloud. Although low-profile it is potentially susceptible to icing and has no heater, and this low-profile itself may be problematic, promoting measurement from within the aircraft boundary layer.

Literature

    • Vance, A. K., Abel, S. J., Cotton, R. J., and Woolley, A. M.: Performance of WVSS-II hygrometers on the FAAM research aircraft, Atmos. Meas. Tech., 8, 1617-1625, doi:10.5194/amt-8-1617-2015, 2015. URL, DOI

Contact

    For further details contact Alan Vance, Met Office.