Abstract
Abstract. The recently developed NOAA Water instrument is a two-channel, closed-path, tunable diode laser absorption spectrometer designed for the measurement of upper troposphere/lower stratosphere water vapor and enhanced total water (vapor + inertially enhanced condensed phase) from the NASA Global Hawk unmanned aircraft system (UAS) or other high-altitude research aircraft. The instrument utilizes wavelength-modulated spectroscopy with second harmonic detection near 2694 nm to achieve high precision with a 79 cm double-pass optical path. The detection cells are operated under constant temperature, pressure, and flow conditions to maintain a constant sensitivity to H2O independent of the ambient sampling environment. An onboard calibration system is used to perform periodic in situ calibrations to verify the stability of the instrument sensitivity during flight. For the water vapor channel, ambient air is sampled perpendicular to the flow past the aircraft in order to reject cloud particles, while the total water channel uses a heated, forward-facing inlet to sample both water vapor and cloud particles. The total water inlet operates subisokinetically, thereby inertially enhancing cloud particle number in the sample flow and affording increased cloud water content sensitivity. The NOAA Water instrument was flown for the first time during the second deployment of the Airborne Tropical TRopopause EXperiment (ATTREX) in February–March 2013 on the NASA Global Hawk UAS. The instrument demonstrated a typical in-flight precision (1 s, 1σ) of better than 0.17 parts per million (ppm, 10−6 mol mol−1), with an overall H2O vapor measurement uncertainty of 5% ± 0.23 ppm. The inertial enhancement for cirrus cloud particle sampling under ATTREX flight conditions ranged from 33 to 48 for ice particles larger than 8 μm in diameter, depending primarily on aircraft altitude. The resulting ice water content detection limit (2σ) was 0.023–0.013 ppm, corresponding to approximately 2 μg m−3, with an estimated overall uncertainty of 20%.
Highlights
Water in the upper troposphere and lower stratosphere (UT/LS) plays an important role in Earth’s climate system through aspects of radiative transfer, cirrus cloud formation, and stratospheric ozone chemistry (Kirk-Davidoff et al, 1999; Forster and Shine, 2002; Solomon et al, 2010)
To take advantage of the tropical tropopause layer (TTL) sampling capabilities of the NASA Global Hawk unmanned aircraft system (UAS) platform, we have developed a new, compact, two-channel instrument for the simultaneous measurement of water vapor and total water in the UT/LS, and integrated it onboard the Global Hawk during the recent NASA Airborne Tropical TRopopause EXperiment (ATTREX) mission
The WV sample line is constructed of 0.46 cm ID electropolished stainless steel tubing (WinTech 10, Winter Technologies, Pacific, MO, USA), which was found in our previous laboratory experiments to produce little hysteresis from rapid changes in water vapor at mixing ratios between 1 and 50 ppm (Thornberry et al, 2013)
Summary
Water in the upper troposphere and lower stratosphere (UT/LS) plays an important role in Earth’s climate system through aspects of radiative transfer, cirrus cloud formation, and stratospheric ozone chemistry (Kirk-Davidoff et al, 1999; Forster and Shine, 2002; Solomon et al, 2010). Persistent disagreements among collocated measurements of H2O in the UT/LS have produced uncertainties in our understanding of the cirrus cloud microphysics that control dehydration of tropospheric air in the tropical tropopause region and determine the amount of H2O that reaches the lower stratosphere (Kley et al, 2000; Jensen et al, 2005; Peter et al, 2006; Weinstock et al, 2009). These disagreements among collocated in situ measurements have prompted community efforts to determine the sources of error in the H2O measurements, including the chamber-based AquaVIT-1 intercomparison in 2007 (Fahey et al, 2014) and a recent in situ intercomparison during the Mid-latitude Airborne Cloud Properties EXperiment (MACPEX) in 2011 (Rollins et al, 2014). We describe details of the instrument design and validation, and present instrument performance from recent measurements in the UT/LS obtained during ATTREX
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