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Title: Measurements of atmospheric water vapour using a balloon-borne surface acoustic wave frost point hygrometer
Author: Eden, L.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2006
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A frost point hygrometer is described in this thesis which uses surface acoustic waves (SAWs) to detect water vapour condensation. The SAW device crystal is cooled by a thermoelectric cooler until condensation occurs, at which point, water vapour loading onto the SAW device causes changes in the SAW velocity and amplitude. Two methods of determining the frost point temperature are outlined in this thesis. The first uses a feedback loop algorithm to maintain a constant amount of water vapour on the SAW device and hence keep the temperature of the device at the frost point. The second uses heating and cooling cycles to oscillate the SAW device temperature above and below the frost paint. Changes in the SAW properties are then used to detect the condensation-evaporation equilibrium point which is equal to the frost point. The use of SAWs rather than a conventional optical detection system in this hygrometer allows a faster response time, increased sensitivity, and decreased risk of contamination. Between September 2001 and February 2004, the SAW hygrometer was flown on 28 balloon flights, in various configurations. All of these flight data sets show agreement with radiosonde relative humidity measurements to within 1%, using given criteria for extraction of reliable SAW hygrometer measurements. Accurate lower and mid tropospheric measurements have been made on each flight and four flights show accurate upper tropospheric and lower stratospheric measurements with water vapour mixing ratio values of less than 5 ppmv. The Hibiscus campaign flights in Bauru, Brazil, show excellent agreement between the different instruments measuring water vapour on the flight train and the SAW hygrometer. These data are discussed in conjunction with auxiliary data and model backward trajectories. Two different backward trajectory models have been used to assess the possible mechanisms of transport of water vapour above Bauru, one of which is a large scale model and the other is a meso-scale model. A sharp contrast in the air parcel transport above and below the tropical tropopause layer (TTL) is seen, with large scale convection or descent controlling the water vapour mixing ratios below the TTL, and a combination of large and small scale fields controlling the water vapour in and above the TTL.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available