A fiber-coupled laser hygrometer for airborne total water measurement

Abstract

Abstract. The second-generation University of Colorado closed-path tunable-diode laser hygrometer (CLH-2) is an instrument for the airborne in situ measurement of total water content -the sum of vapor-, liquid-and ice-phase waterin clouds. This compact instrument has been flown on the NSF/NCAR Gulfstream-V aircraft in an underwing canister. It operates autonomously and uses fiber-coupled optics to eliminate the need for a supply of dry compressed gas. In operation, sample air is ingested into a forward-facing subisokinetic inlet; this sampling configuration results in particle concentrations that are enhanced relative to ambient and causes greater instrument sensitivity to condensed water particles. Heaters within the inlet vaporize the ingested water particles, and the resulting augmented water vapor mixing ratio is measured by absorption of near-infrared light in a single-pass optical cell. The condensed water content is then determined by subtracting the ambient water vapor content from the total and by accounting for the inertial enhancement of particles into the sampling inlet. The CLH-2 is calibrated in the laboratory over a range of pressures and water vapor mixing ratios; the uncertainty in CLH-2 condensed water retrievals is estimated to be 14.3 % to 16.1 % (1-σ ). A vapor-only laboratory intercomparison with the firstgeneration University of Colorado closed-path tunable-diode laser hygrometer (CLH) shows agreement within the 2-σ uncertainty bounds of both instruments.

The O2/N2 Ratio and CO2 Airborne Southern Ocean Study

“…Fast-response water vapor measurements were made by the vertical-cavity surface-emitting laser (VCSEL) hygrometer (Zondlo et al 2010). Total condensed water concentrations were measured using the closed-path tunable-diode laser hygrometer (CLH-2) condensed water content probe (Dorsi et al 2014).…”

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

“…The controlled inlet flow of 600 sccm causes the sampling to be subisokinetic, i.e., the sample air velocity inside the inlet (U ) is lower than the free-stream velocity (U 0 ) at sampling altitudes and typical high-altitude research aircraft speeds (140-200 m s −1 ). This subisokinetic sampling leads to an inertially enhanced aspiration efficiency for particles into the inlet (Belyaev and Levin, 1974;Krämer and Afchine, 2004;Eddy et al, 2006;Davis et al, 2007a). As a result, cirrus particle number densities in the sample flow are enhanced by a factor, EF, defined as the ratio of particle number concentration entering the inlet tube to that in the free stream.…”

“…TW inlet particle aspiration enhancement factor (EF) as a function of particle diameter (D p ) calculated using the formulation of Eq. 15in Eddy et al (2006) for the TW inlet geometry and assumed ice particle densities of 0.7 g cm −3 (solid) and 0.9 g cm −3 (dashed). The curves represent conditions near the tropical tropopause (approximately 17.6 km, blue) and near the bottom (approximately 14.4 km, green) of the typical NASA Global Hawk operational altitude range during the ATTREX mission.…”

“…The uncertainty in inlet flow measured by the TW sample mass flow meter is 3 %. Eddy et al (2006) report that the EF parameterization presented (Eq. 15) agreed with their detailed computational fluid dynamics simulations to within 20 % for most Stk values, so the uncertainty in EF contributes significantly to the overall uncertainty.…”

Anvil microphysical signatures associated with lightning-produced NO<sub><i>x</i></sub>

“…These include the presence of relatively high (greater than ∼ 2 cm −3 ) CDP concentrations, the preponderance of small spherical images as the primary images on the CPI, and comparison of the water content from the CDP with simultaneous measurements of total water content from the University of Colorado closed-path laser hygrometer, version 2 (CLH-2, Dorsi et al, 2014). In regions where most of the frozen droplets had aggregated, the CDP concentrations dropped to ∼ 1 cm −3 or less, while the concentrations of particles on the 2DC increased, since the FDAs became large enough to be sampled by that instrument.…”