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Float to float drift intercomparison in an intermediate current regime

Float to float drift intercomparison in an intermediate current regime. Francisco Machín - ULPGC (*) Uwe Send - IfM Kiel Walter Zenk - IfM Kiel. 1 st Argo Science Meeting Workshop Tokyo, 12-14 November 2003. Motivation. Cycling floats. 1. Hydrographic data (Wong, 2001)

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Float to float drift intercomparison in an intermediate current regime

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  1. Float to float drift intercomparison in an intermediate current regime Francisco Machín - ULPGC (*) Uwe Send - IfM Kiel Walter Zenk - IfM Kiel 1st Argo Science Meeting Workshop Tokyo, 12-14 November 2003

  2. Motivation Cycling floats 1. Hydrographic data (Wong, 2001) 2. Current fields (Davis, 1998; Schmidt, 2001) Goal: discuss sources of underwater speed “contamination” and suggest improvements underwater velocity. Raw drift data vs. known lagrangian references (APEX float) (4 RAFOS floats)

  3. Experimental Site Observations: cooperation between the IfM Kiel and the Federal Maritime and Hydrographic Agency (BSH, Hamburg) Meteor 45 Historical Setting Paillet et al. (1998)

  4. Experimental Site: flow paths (Zenk et al., 2000)

  5. Experimental Site: flow paths Apex: 1500 dbar

  6. Experimental Site: APEX cycle - a closer look

  7. Experimental Site: APEX cycle 1. Surface drift extrapolation a) Estimate surfacing and begin-of-descent times - Message number, - Message block number, - Repetition rate, - Times corresponding to the first and last fixes. Accuracy: 1 s

  8. Experimental Site: APEX cycle 1. Surface drift extrapolation b) Estimate corresponding locations - cubic smooth spline fit Surfacing and begin-of-descent times too long => UW abolished

  9. Experimental Site: APEX cycle 1. Surface drift extrapolation c) Estimate uncertainty surface mean speed: 46.4 cm s-1 mean time to extrapolate : 2.3 h Uncertainty O(3 km)

  10. Experimental Site: APEX cycle 1. Surface drift extrapolation 2. Ascending and descending times - ascending float rate (wf): 8 cm s-1 - mission depth: 1500 m Ascending time = 5.21 h Descending time = 6 h (Webb, 2002)

  11. Experimental Site: APEX cycle - a closer look wf= 8.5 ± 1 cm s-1

  12. Experimental Site: APEX cycle 1. Surface drift extrapolation 2. Ascending and descending times 3. Geostrophic and Ekman shear considerations a) Geostrophic calculations - assuming synopticity between two successive profiles - lnm at deepest level (1500 m) rms geostrophic velocity was obtained Geostrophic displacement = 753 ± 927 m

  13. Experimental Site: APEX cycle 1. Surface drift extrapolation 2. Ascending and descending times 3. Geostrophic and Ekman shear considerations b) Ekman consideration - mean wind speed: 12-16 m s-1 (NOAA) - Ekman spiral velocity, vEk - sEk, displacement by wind stress O(100 m) Uncertainties relative importance Surface drift : Geostrophic : Ekman 78 : 20 : 2

  14. Experimental Site: Velocity comparison Background, at least: - two instruments - ten float days

  15. Experimental Site: Velocity comparison

  16. Experimental Site: Initial and long-term separations a) long-term separation

  17. Experimental Site: Initial and long-term separations b) initial separation

  18. Experimental Site: Initial and long-term separations b) initial separation

  19. Experimental Site: Initial and long-term separations b) initial separation - integral time scale: 8-10 d (Lankhorst, 2002) - mean advection estimate: 4 cm s-1 spatial scale = 28-36 km

  20. Conclusions 1. In a statistical sense underwater velocities are coherent with velocities from the RAFOS reference. 2. Results are coherent with previous estimates from MARVOR floats ( Speer et al . 1999). 3. Geostrophic and wind-induced displacements are a secondary source of uncertainty. 4. Both float types trace the DWBC. 5. RAFOS floats resolve the eddy field, APEX float meanders. 6. More cases are needed to establish a more general procedure to estimate the UW drift.

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