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 water – in 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 sub-isokinetic 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 first-generation University of Colorado closed-path tunable-diode laser hygrometer (CLH) shows agreement within the 2-σ uncertainty bounds of both instruments.
Highlights
Water is ubiquitous in the Earth’s atmosphere and plays a primary role in the planet’s climate and weather
When water particles are present, the determination of condensed water content” (CWC) from closed-path tunable-diode laser hygrometer (CLH)-2 enhanced total water content (eTWC) measurements entails application of a sampling efficiency correction based on additional observations, including the background water vapor content, the wind field and particle size distributions
During the Instrument Development and Education in Airborne Science (IDEASIV) campaign in September and October 2013 concurrent measurements of CWC were made aboard the GV with the CLH-2 and the Counterflow Virtual Impactor (CVI; Twohy et al, 1997) cloud water instrument
Summary
Water is ubiquitous in the Earth’s atmosphere and plays a primary role in the planet’s climate and weather. The transport of latent heat by water vapor is fundamental to convection and weather processes, and the redistribution of water through evaporation and precipitation is a major factor in shaping terrestrial biomes. Both liquid-water and ice clouds have a profound effect on the radiative budget of the atmosphere (Liou, 1992) through their interaction with solar visible and terrestrial infrared radiation. Despite its potential utility in cloud water measurement, total water instruments are not always used during field studies Logistical constraints, such as size, weight, power consumption and the need for compressed gases, may limit the deployability of some existing total water hygrometers. We describe the design and characteristics of a new total water instrument that operates autonomously, does not require compressed gas and fits in a standard aircraft underwing canister
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