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Design and testing of a new aircraft-based cloud water sampling system




Straub, Derek J., author
Collett, Jeffrey L., Jr., author

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Experimental studies of cloud processing mechanisms necessitate the collection of representative samples of cloud water for chemical analysis. In order to provide samples from clouds that are inaccessible from ground-based sampling stations, a new aircraft-based cloud water collection system has been developed . The objective of the design process was to produce an automated collector that can acquire well-characterized cloud water samples and is portable between multiple research aircraft. Issues such as cloud drop shatter and re-entrainment, structural integrity, system size and weight, material compatibility with the anticipated chemical analyses, and ease of use during field operation w re all considered during the design process. The new cloud water collection system utilizes an axial-flow cyclone to centrifugally separate cloud drops from the air stream. Up to seven individual samples can be stored over the course of a single research flight. An analysis of the axial-flow cyclone was performed with a finite volume based computational fluid dynamics (CFD) code. Solutions were obtained for air flow patterns and cloud drop trajectories. The predicted continuous phase (air) velocity field indicates that the axial-flow cyclone generates a strong rotational ow field with a tangential velocity of 85 ms-'. Based on simulations of cloud drop trajectories, centrifugal force in the rotational flow field is sufficient to quickly move entrained cloud drops to the wall of the axial-flow cyclone duct where they can be removed for storage. Collection efficiency as a function of drop size was ascertained and the 50% cut diameter was determined to be approximately 8 microns. An experimental laboratory calibration involving monodisperse fluorescein-tagged drops verified the numerical modeling results. The system was deployed during the Dynamics an Chemistry of Marine Stratocumulus, Phase II (DYCOM -II) field project in July 2001. The DYCOMS-II campaign served as a testing and evaluation program for the system as well as an opportunity to study the chemical composition of stratocumulus clouds in the remote marine environment. Over the course of the project, 50 samples were obtained during seven nighttime and two daytime flights. Sample pH was measured on-site after each flight. Peroxide, formaldehyde, S(IV), trace metals and major ions (Cr, NO3-, so/-, Na+, NH/, K+, ca2+, and Mg2+) were preserved on site and analyzed after the field campaign. The analyses were used to characterize the composition of the sampled clouds and to investigate cloud processing mechanisms, including the potential for rapid aqueous phase oxidation of S(IV) to sulfate.


December 2002.
Also issued as Derek J. Straub's dissertation (Ph.D.) -- Colorado State University, 2002.

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Cloud physics
Meteorological instruments


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