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  • Author or Editor: A. Fong x
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Derek A. Fong and Stephen G. Monismith

Abstract

The accuracy of an acoustic Doppler current profiler (ADCP) used with an internal bottom-tracking system is considered. The boat speed measured using bottom tracking is extremely accurate, comparable to the speeds measured by a high-resolution, real-time kinematic global positioning system (KGPS). The accuracy in the direction of boat motion reported by the bottom tracking is limited to the accuracy of the internal compass of the ADCP. Directional differences (after correcting for local magnetic declination) are about 3° between the ADCP bottom tracking and KGPS. An error of this magnitude is shown to result in a maximal measurement error in water velocity of less than 6%.

Nonetheless, an unexplained water velocity error is observed that is significantly larger than can be explained by a simple compass error. Repeated transects in opposing directions show a bias in measured water velocities in the direction of boat motion. The bias cannot be explained by an error in the compass or the bottom-tracked boat velocities. The difference in recorded velocity between two repeated transects with the boat moving in opposite directions exhibits an error of up to ±5 cm s−1 that has vertical variability.

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Nicholas J. Nidzieko, Derek A. Fong, and James L. Hench

Abstract

A field experiment was conducted to directly compare the effects of different sampling modes on Reynolds stress estimates calculated from acoustic Doppler current profilers (ADCPs). Two 1.2-MHz ADCPs were deployed concurrently over a fortnightly cycle: one collected single-ping measurements using mode 1 and a second ADCP employed the fast-ping rate mode 12 with subping-averaged data recorded at the same sample rate as the first ADCP. While mode 12 clearly has a lower noise floor for the estimate of mean velocities, it has been an open question whether the averaging of subpings leads to a biased estimate of turbulence quantities, due to the temporal averaging inherent in this approach. Using the variance method, Reynolds stresses were estimated from the two ADCP datasets and compared with stresses computed directly from the velocity records obtained with a pair of fast sampling acoustic Doppler velocimeters (ADVs) collocated with the ADCPs. Mode-12 stresses were more accurate than mode 1 in comparison to ADV-derived stresses, and mode 12 exhibited much lower measurement uncertainty than mode 1. Mode 1 appears to overestimate stresses by 20% in this study. The lower noise floor associated with mode 12 suggests that the variance method may be used with mode 12 to resolve smaller stresses than would be possible with mode 1.

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Gary A. Morris, Walter D. Komhyr, Jun Hirokawa, James Flynn, Barry Lefer, Nicholay Krotkov, and Fong Ngan

Abstract

This paper reports on the development of a new technique for inexpensive measurements of SO2 profiles using a modified dual-ozonesonde instrument payload. The presence of SO2 interferes with the standard electrochemical cell (ECC) ozonesonde measurement, resulting in −1 molecule of O3 reported for each molecule of SO2 present (provided [O3] > [SO2]). In laboratory tests, an SO2 filter made with CrO3 placed on the inlet side of the sonde removes nearly 100% of the SO2 present for concentrations up to 60 ppbv and remained effective after exposure to 2.8 × 1016 molecules of SO2 [equivalent to a column ∼150 DU (1 DU = 2.69 × 1020 molecules m−2)]. Flying two ECC instruments on the same payload with one filtered and the other unfiltered yields SO2 profiles, inferred by subtraction. Laboratory tests and field experience suggest an SO2 detection limit of ∼3 pbb with profiles valid from the surface to the ozonopause [i.e., ∼(8–10 km)]. Two example profiles demonstrate the success of this technique for both volcanic and industrial plumes.

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