Flight tests, laboratory studies, and intercomparisons of the NSF/NCAR Gulfstream-V VCSEL hygrometer
Abstract
New observation platforms and state-of-the-art science requirements place challenging demands on the development of new instrumentation. Instruments are desired be faster, smaller, lighter while still maintaining excellent accuracy, precision, and even offering new observational capabilities. New measurements understandably require a large body of evidence documenting their performance before much acceptance in the community. In this context, we will discuss the performance of our recently-developed water vapor sensor designed for the NSF/NCAR Gulfstream-V aircraft by showing results from the first flight tests, laboratory experiments, flow modeling studies of the pylon design, and intercomparison campaigns. Water vapor plays critical roles in atmospheric dynamics, radiative properties, and chemistry throughout the troposphere and lower stratosphere, but there are many measurement challenges due to the large dynamic range in concentration, extreme heterogeneity, presence of cloud particles, and efficient adsorption to instrument surfaces. The Gulfstream-V water vapor instrument uses a fiberized vertical cavity surface emitting laser (VCSEL) operating near a wavelength of 1854 nm to measure water vapor at 25 Hz with <3% precision and 5% accuracy over a range from 1 ppmv (-92 C frost point at 50 hPa) to 40,000 ppmv (+30 C dew point at sea level). The instrument operates unattended, has a mass of 5 kg, and consumes 5 W power. Extensive flight testing of the instrument was performed in spring and summer of 2007 as part of the NSF Pacific Dust Experiment (PACDEX) and HIAPER Experimental Flight Tests. The instrument showed excellent response in clouds ranging from stratocumulus to cirrus. Detailed cloud structure was observed in both cases at 25 Hz that can increase the understanding of cloud entrainment/detrainment processes. In addition, a relatively homogeneous flight segment near the tropopause at 14.8 km (135 hPa) allowed for in-flight checks of the instrument precision (as opposed to laboratory conditions). Under flight conditions of ~ 3 ppmv water vapor, 25 Hz measurements had a standard deviation of 2.1%, thereby placing an upper limit on the precision of the system as some of the variability originated from the real atmosphere. Allan deviation experiments in the laboratory at similar concentrations show about 1% precision, but more importantly, suggest that long-term drift for experiments lasting several days is negligible. Determination of the accuracy of the sensor is being conducted by three methods: calibrations using standard dilution of flows, immersion of the sensor at ice saturated conditions in constant-temperature ice/organic baths, and through studies at the upcoming AIDA International Water Vapor Intercomparison campaign (October 2007). In combination, these results will be used to assess the overall performance of the VCSEL hygrometer and identify where outstanding issues remain.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFM.A43D1560Z
- Keywords:
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- 0320 Cloud physics and chemistry;
- 0322 Constituent sources and sinks;
- 0340 Middle atmosphere: composition and chemistry;
- 0365 Troposphere: composition and chemistry;
- 0394 Instruments and techniques