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1 ====================================================================== |
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2 R E A D M E . Y O Y O |
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3 doc: Fri Aug 10 07:07:59 2012 |
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4 dlm: Fri Oct 19 11:05:29 2012 |
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5 (c) 2012 A.M. Thurnherr |
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6 uE-Info: 56 0 NIL 0 0 72 3 2 8 NIL ofnI |
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7 ====================================================================== |
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8 |
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9 =Overview= |
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10 |
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11 This README contains notes on how to process data from non-standard |
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12 casts, such as yo-yos (consecutive down-up casts collected at a given |
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13 location without restarting the instruments) and tow-yos (yo-yos |
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14 carried out while vessel is in slow motion). |
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15 |
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16 Processing of yo-yo and tow-yo data requires the following two steps: |
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17 1. Split the data files into individual down-upcast pairs |
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18 2. Process the resulting files as described in [README.ProcessData] |
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19 |
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20 It is important to note that yo-yo and tow-yo casts can be full depth |
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21 (i.e. between the sea surface and the sea bed) or partial-depth. If |
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22 absolute velocities (rather than just vertical shear) are required for |
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23 partial-depth casts, the user must make sure that there are either BT |
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24 data or SADCP data available for velocity referencing of each |
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25 down-upcast pair. Essentially this means that each down-upcast pair |
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26 must extend down to near the seabed or up into the depth range where |
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27 SADCP data are available. |
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28 |
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29 |
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30 =Step 1: Splitting the Data Files= |
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31 |
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32 Both data files must be split between individual down-upcast pairs, i.e. |
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33 whenever the CTD winch switches from up to down. |
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34 |
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35 CTD DATA: First, find the CTD splitting times by plotting depth vs. |
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36 elapsed time. Then, split the CTD data into separate files using any |
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37 text editor. If SeaBird CNV files are use, the same header can be used |
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38 for all output files. |
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39 |
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40 ADCP DATA: There is an ADCP file splitting utility for M$ Windows |
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41 provided by RDI. Alternatively, the length of each ensemble can be read |
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42 from the binary ADCP files and the UN*X utilities "split -b" and "cat" |
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43 can be used to split the files into the required chunks. |
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44 |
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45 |
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46 =Step 2: Processing the Split Data Files= |
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47 |
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48 The split data files, each containing data from exactly one consecutive |
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49 down- and up-cast, can be processed exactly as described in |
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50 [README.ProcessData]. If the CTD time-series data used during |
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51 processing has an "elapsed-time" field all output elapsed times are |
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52 consistent with this input, i.e. the elapsed times of the output casts |
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53 are relative to the beginning of the entire yo-yo or tow-yo cast. |
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54 Otherwise, every down-/up-cast pair gets its own elapsed-time field. |
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55 |
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56 Usually, in standard LADCP processing the velocity data from the down- |
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57 and upcast are combined. While this smears out any information on the |
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58 temporal variability of the velocity field during the cast, down- and |
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59 upcast only profiles are necessarily derived from much fewer samples |
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60 and, therefore, associated with considerably larger uncertainties and |
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61 errors. It has been found, in particular, that the top-to-bottom shear |
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62 in down-/upcast-only profiles is often quite bad. In the context of |
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63 partial-depth yo-yo and tow-yo profiles the severity of this problem |
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64 can be evaluated by comparing two consecutive velocity profiles at |
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65 their "unconstrained" end. E.g., in case of a partial-depth yo-yo near |
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66 the seabed, i.e. constrained with BT data, the uppermost portion of the |
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67 first upcast can be compared to the uppermost portion of the 2nd |
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68 downcast, etc. If the errors are found to be unacceptably high, the |
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69 velocity profiles from the combined down-/upcast data should be used |
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70 instead. Alternatively, multiple simultaneous velocity referencing |
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71 constraints can be applied, e.g. using the shear-inversion method |
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72 described by Thurnherr (JAOT 2010). |
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73 |
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74 |