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  • Author or Editor: D. C. Webb x
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T. Rossby, J. Ellis, and D. C. Webb

Abstract

To meet the expected need for wide-area acoustic navigation for Lagrangian studies of ocean circulation using RAFOS floats, a new and powerful sound source, a resonant pipe projector has been developed. It consists of a free-flooded open steel pipe with a ceramic-steel driver ring at its midsection. Conservatively demonstrated here at a source level of 195.5 dB re 1 µpa @ 1 m and with an energy conversion efficiency of 85% at resonance (260 Hz), useful operating ranges to 4000 km and beyond are possible, depending on ambient noise conditions. A successful 6-month accelerated test of the complete transducer system was recently completed near Bermuda.

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Andrew R. D. Smedley, Richard C. Kift, and Ann R. Webb

Abstract

This study describes a dual-channel array spectrometer system designed to make high-frequency simultaneous spectral global irradiance and direct solar irradiance measurements covering the visible and ultraviolet wavelength ranges. The dual-channel nature of the instrument allows spectrally integrated quantities (e.g., erythema or vitamin D) to be calculated at a rate similar to broadband instruments while retrieving total column ozone (TCO) from the direct solar channel. The characterization and calibration of the instrument is discussed, with emphasis on temperature stabilization (<±0.01°C) and stray light removal. Focusing on the TCO retrieval from direct spectra, results are compared to a collocated Brewer spectrophotometer during the study period of May 2013–January 2014. Agreement for individual measurements made within 20 min of a reference Brewer direct sun observation on relatively clear example days is <1.5%. For all valid individual measurements, the study found an overall bias of 1.1 Dobson units (DU; 0.4%) and scatter of ±6.7 DU (2.2%) for retrievals obtained at airmass values < 4. A dependence on air mass of 6.3 DU (2.0%) per airmass unit is observed and a correlation of R 2 = 0.954 is found for all individual measurements, although this is reduced to 0.908 for daily means. TCO retrievals are limited to airmass values < 4 primarily because of residual structure in the transmission spectrum that cannot be attributed to other trace gases. These results are encouraging and suggest that similar instrument designs could make a significant and relatively low-cost contribution to surface measurements of atmospheric radiation.

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A. R. D. Smedley, C. P. R. Saunders, and A. R. Webb

Abstract

Oversampling of optical array probes (OAPs) is described as a novel technique to increase the quality of small-particle data available in cloud microphysics. The slice rate of a 10-μm-resolution grayscale OAP is increased by a factor f with respect to that which would produce images with an aspect ratio of 1:1 for the given air velocity and probe resolution. The Fresnel diffraction pattern is simulated and the probe response calculated for normal (f = 1) and oversampled (f = 10) cases. The increased number of pixels imaged by the OAP permits the fraction of 25%–50% shadowed pixels to be used to obtain a much improved estimate of the actual droplet diameter for droplets between 7 and 50 μm in diameter. The process of oversampling is also found to significantly increase the sample area for the smallest particles. Example laboratory particle size distributions are provided. The oversampling factor for an individual probe is limited by the maximum sampling frequency, the airspeed, and the probe resolution.

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R. E. Davis, L. A. Regier, J. Dufour, and D. C. Webb

Abstract

The autonomous Lagrangian circulation explorer (ALACE) is a subsurface float that cycles vertically from a depth where it is neutrally buoyant to the surface where it is located by, and relays data to, System Argos satellites. ALACEs are intended to permit exploration of large-scale low-frequency currents and to provide repeated vertical profiles of mean variables. ALACEs periodically change their buoyancy by pumping hydraulic fluid from an internal reservoir to an external bladder, thereby increasing float volume and buoyancy. Because positioning and data relay are accomplished by satellite, ALACEs are autonomous of acoustic tracking networks and are suitable for global deployment in arrays of any size. While providing only a sequence of displacements between surfacing intervals, ALACEs are efficient in gathering the widely spaced long-term observations needed to map large-scale average flow.

The primary technical challenges met in the ALACE design are maximizing energy efficiency to achieve a lifetime of 50 surfacing cycles over several year., achieving reliable satellite communication with minimal surface buoyancy, and developing overall system reliability in an instrument that cannot be recovered or diagnosed after most failures. This paper describes the ALACE system, design specifications, and some field experiences. The singular failure of a simple dynamical model to predict the surface following behavior of scale models in laboratory tests serves as a cautionary note in using simple models to infer the dynamics of surface floats in various oceanographic applications. The limitations of interpreting the sequence of net displacements between surface positions, including errors caused by surface drift, are also discussed.

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