ADCP Measurement ProblemsDenmark Strait Overflow

Longranger 75 kHz2007 - ongoing

Broadband 75 kHz Quartermaster 150 kHz

The Problem

• Fast decrease of velocity not physical

• According to Boundary Layer theory decrease should be more near to the ground

Vertical Velocity Profile of Overflow Plume

Hypothesis• Beams have sidelobes

(known)• Sidelobes at 70° are of

importance• When instrument is

tilted (strong currents), sidelobes touch ground

• Reflection at ground disturbs measurement

Test• instrument has 4 beams available• 3 beams needed to calculate velocity• instrument tilts to one side• removing the beam that points towards the ground

from solution

could improve velocity profile?

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June 24, 4-11pm, 7h Juni 18, 9am-4pm, 7hJune 14, 8am-3pm, 7h

Removing a beam from solution3 different flow situations

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Conclusion

• Removing a beam from the solution has always the same effect within one measurement period: increase or decrease

• In vicinity of ground effect is stronger• Does it improve the vertical profile? Not

much.

New Hypothesis

• Low backscatter intensity in clear overflow water

• Backscatter intensity too low to be measured accurately

Backscatter Intensity Vs Water Speed

acb

Backscatter intensity: 60-180 counts

Water speed: 0-0.8 m/s

a) Low backscatter below plume maximumb) Backscatter not lower below plume maximumc) High backscatter 200 m above ground

Connection between low velocity profile and backscatter intensity not consistent.

Conclusion

• Backscatter intensity could still be a reason• More help from RDI would be needed to

understand the causes of the problem• Overlap between Longranger and Broadband

in 2012-2013 measurement period at Faroe Bank Channel

• Any suggestions?

The research leading to these results has received funding from the European Union 7th Framework Programme (FP7 2007-2013), under grant agreement n.308299NACLIM www.naclim.eu