A two-channel, tunable diode laser-based hygrometer for measurement of water vapor and cirrus cloud ice water content in the upper troposphere and lower stratosphere

[摘要] The recently developed NOAA Water instrument is a two-channel, closed-path,tunable diode laser absorption spectrometer designed for the measurement ofupper troposphere/lower stratosphere water vapor and enhanced total water(vapor + inertially enhanced condensed phase) from the NASA Global Hawkunmanned aircraft system (UAS) or other high-altitude research aircraft. Theinstrument utilizes wavelength-modulated spectroscopy with second harmonicdetection near 2694 nm to achieve high precision with a 79 cm double-passoptical path. The detection cells are operated under constant temperature,pressure, and flow conditions to maintain a constant sensitivity to H₂Oindependent of the ambient sampling environment. An onboard calibrationsystem is used to perform periodic in situ calibrations to verify thestability of the instrument sensitivity during flight. For the water vaporchannel, ambient air is sampled perpendicular to the flow past the aircraftin order to reject cloud particles, while the total water channel uses aheated, forward-facing inlet to sample both water vapor and cloud particles.The total water inlet operates subisokinetically, thereby inertiallyenhancing cloud particle number in the sample flow and affording increasedcloud water content sensitivity. The NOAA Water instrument was flown for thefirst time during the second deployment of the Airborne Tropical TRopopauseEXperiment (ATTREX) in February–March 2013 on the NASA Global Hawk UAS. Theinstrument demonstrated a typical in-flight precision (1 s, 1σ) ofbetter than 0.17 parts per million (ppm, 10⁻⁶ mol mol⁻¹), with an overallH₂O vapor measurement uncertainty of 5% ± 0.23 ppm. Theinertial enhancement for cirrus cloud particle sampling under ATTREX flightconditions ranged from 33 to 48 for ice particles larger than 8 μm indiameter, depending primarily on aircraft altitude. The resulting ice watercontent detection limit (2σ) was 0.023–0.013 ppm, corresponding toapproximately 2 μg m⁻³, with an estimated overall uncertainty of20%.

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