1. ADCP Measurement Problems Denmark Strait Overflow Longranger 75 kHz 2007 - ongoing Broadband 75 kHz Quartermaster 150 kHz

2. 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

3. 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

4. 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?

5. Denmark Strait Mixing Array June 24, 4-11pm, 7hJuni 18, 9am-4pm, 7hJune 14, 8am-3pm, 7h Removing a beam from solution 3 different flow situations a cb

6. 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.

7. New Hypothesis Low backscatter intensity in clear overflow water Backscatter intensity too low to be measured accurately

8. Backscatter Intensity Vs Water Speed a c b Backscatter intensity: 60-180 counts Water speed: 0-0.8 m/s a)Low backscatter below plume maximum b)Backscatter not lower below plume maximum c)High backscatter 200 m above ground Connection between low velocity profile and backscatter intensity not consistent.

9. 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?

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