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XBT Fall Rate Discussion

A large fraction of the data on this disc were collected using XBTs. In the early 1990's a group was organized to examine the fall rate of XBTs and to compare what was found to manufacturer's specifications. Results of this work are reported in UNESCO 1994,( Calculation of new depth equations for expendable bathythermographs using a temperature-error-free method (application to Sippican/TSK T-7, T-6 and T-4 XBTs), UNESCO Technical Papers in Marine Sciences, 67, 46pp) and by Rual, et. al. in WOCE Newsletter, October, 1996 as well as in scientific literature. Only a subset of the different classes of XBT probes were tested thoroughly, but generally it was found that depths calculated using the manufacturers fall rate equations were shallower than they really were when compared to coincident CTD data. The corrections are linear but vary from one probe type to another.

Up until this problem was recognized, it was uncommon for information about the type of XBT probe used to accompany the temperature data sent to archive centres. Subsequently, the oceanographic archive centres were alerted to ask data providers to provide this information and advised to store this information with the data. At the same time, WMO modified its BATHY code form (the code used to send XBT data in real-time on the GTS) to allow for the inclusion of information about the probe type, the fall rate equation used and the recorder used. Use of this code form began in November, 1995 but it took two years for more than 80% of the data sent this way to have this information included.

The GTSPP has preserved XBT data in the GTSPP Continuously Managed Database (CMD) from which the data for this disc came. The probe type and fall rate equation information is stored in the CMD if it was provided. First of all, in the global attributes portion of the file (see the format description in the Data Files section), the data_type states what sort of data is being considered. If the value here is BATHY or XBT then the issue of the fall rate comes into play.

Information about the fall rate and probe is associated with the parameter codes, PEQ$, PFR$, and PRT$. These parameter codes are found in the surfacecodes_srfccode variable and the value in the associated surfacecodes_cparm. The value stored is the information in the code table entries of WMO tables 1770 and 4770. For example, where the PFR$ code is found, its value could be 04205 where 042 means a Sippican T-7 probe (table 1770) and 05 means a MK12 recorder (table 4770). Note that 041 is also a Sippican T-7 but with different (older) fall rate equation coefficients.

The U.S. National Oceanographic Data Center (NODC*) has worked with the Integrated Science Data Management (ISDM)* in Canda and the Commonwealth Scientific and Industrial Research Organisation (CSIRO)* in Australia to develop logic for the depth correction process of archived XBT data. It was agreed that Science Centres can make corrections to depths even when they do not have all of the information about probe types, but when they are sufficiently sure that a correction is appropriate. If NODC or MEDS makes a correction, it will do so only when it has checked that no correction has been applied by a Science Centre and only if there is sufficient information to be sure that a correction is appropriate. It was also agreed that corrections to the depth would be applied only to the data that will be placed on the WOCE DVDs. The NODC will not make depth corrections on the archived XBT data. However, if the data centers did make depth corrections, the NODC will preserve the old depths by moving them to a different table, load the new corrected depths by the science centers and use the existing database ID to link the new depths to old ones in the database. In addition, two new codes will be created to retain depth correction information in the surface codes structure. The "DPC$" indicates the status of depth correction and the "FRA$" will retain the conversion factor of 1.0336. The "DPC$" code will have the following states:

  • 01 = Known Probe Type, Needs Correction,
  • 02 = Known Probe Type, No need to Correct
  • 03 = Unknown Probe Type, Not enough information to know what to do, leave alone,
  • 04 = Known XBT Probe Type, Correction was done, and 05 = Unknown Probe Type, but a correction was done.

Having determined which profiles are from XBTs by querying the data type, the XBT probe type and the fall rate equation stored in the XBT archives, the strategy for the fall rate correction is to simply multiply the existing depths by a factor of 1.0336. This will be the technique employed with the multiplication factor stored in the file structure as agreed by the GTSPP team members. A FORTRAN program, dephcorr.f, developed by CSIRO and PERL modules, v3cd_dvd.pl, db.pm, and proc.pm, developed by the US NODC for the depth correction are included in this disk for future references.

However, the international community suggests that the correction to global archives be carried out in cooperation with other data centres around the world to ensure internatinal standards.

A list of XBT Bibliography and XBT/CTD comparison data information is availabe here*.