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Martin Visbeck

Abstract

Lowered acoustic Doppler current profilers (LADCPs) have matured from an experimental instrument to an operational hydrographic tool to study ocean dynamics. The data processing, however, is still in a rather primitive state. First, a method to estimate bottom-track velocities using the standard water profile data was developed. Then inverse solutions are presented that enhance the standard data processing by adding external constraints such as bottom-referenced velocity profiles. Depending on the depth of the profile and the ADCP range the inclusion of bottom-track data can reduce the local velocity errors by a significant factor. The least squares framework also allows for simplified error analysis of the LADCP system and some of the trade-offs are discussed.

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Martin Visbeck and Jorgen Fischer

Abstract

Surface data obtained from 153-kHz acoustic Doppler current profilers deployed in the Greenland Sea at about 350-m depth during the winter of 1988/89 were investigated under several aspects. First a method is described to improve the instrument depth measurements using the binned backscattered energy profile near the surface. The accuracy of the depth estimates is found to be significantly better than 0.5 m.

Further, improvements of wind speed estimates were found by using the ambient noise in the 150-kHz band in favor of the surface backscattered energy as suggested by Schott. Limitations of the ambient sound method at low wind speeds are presented when thermal noise overwhelms the wind-induced noise.

Finally, a method to detect the presence of sea ice above the ADCP is presented by cross correlating the surface backscatter strength and the magnitudes of all Doppler velocity components. The resulting time series of ice concentration are in overall good agreement with Special Sensor Microwave/Imager estimates but allow for higher temporal resolution. Further, in the vicinity of the ice edge, enhanced high-frequency ambient noise in the 150-kHz band was observed.

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