adcp measurement problems denmark strait overflow longranger 75 khz 2007 - ongoing broadband 75 khz...
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ADCP Measurement ProblemsDenmark Strait Overflow
Longranger 75 kHz2007 - ongoing
Broadband 75 kHz Quartermaster 150 kHz
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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
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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
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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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Den
mar
k St
rait
Mix
ing
Arra
y
June 24, 4-11pm, 7h Juni 18, 9am-4pm, 7hJune 14, 8am-3pm, 7h
Removing a beam from solution3 different flow situations
a cb
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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.
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New Hypothesis
• Low backscatter intensity in clear overflow water
• Backscatter intensity too low to be measured accurately
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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.
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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?
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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