Metadata Limitations and the effect on XBT depth adjustment

A. Gronell and R. CowleyCSIRO Marine and Atmospheric Research

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Why we’re interested:

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Depth correction:

We need entire database to same vertical scale

• We do this by converting all t-7, deep blue, t-6 and t-4 profiles to conform to the Hanawa ‘85 equations

• High or full vert resolution depths are recalculated and replaced.

• Low vert resolution data depths are multiplied by 1.0336

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Depth correction procedures:

• High vertical resolution profiles with ‘known’ probe type : ‘fixes’ profiles with truncated or rounded depths.

• Replace depths with calculated depths. All depths now to .01 resolution

• Low resolution profiles that either need correction (DPC$01) or are of unknown probe type and fit the criteria for correction:

• Multiply depths by 1.0336

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• Inadequate Metadata is currently the biggest hurdle

• GTSPP best copy datasets for IO were downloaded and run through Depth correction software

• Anything that was not straight-forward was revisited

• 37678 profiles were screened

Issues affecting depth correction decisions

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Metadata issues – con.

• Profiles from GTSPP best copy – 37378 profiles

No probe type info 18303 Action depends on date and max depth

PEQ$ missing but determined from elsewhere and added

11442 Action depends on probe type determined

Missing DPC$ field 156 Action depends on date and max depth

• Profiles with DPC$03 or no DPC$ – 18933 profiles

<200m OR >1000m OR post 1996 3960 Do not change depths

>200m AND <1000m AND pre 1996 14973 Change depths

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Metadata problems found in GTSPP best copy dataset

• CTDs that were identified as XBTs – this is a problem if the DPC$ field is 03 and they fit our other criteria

• T10 profiles that go to 1500m (actually t-5 probes)

• MBTs misidentified (probably XBTs) – max depth for MBTs is ~250m

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MBTs that go deeper than 300m

Red profiles are post 1975

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MBT distribution in the Indian Ocean through time

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Other issues

• Number of depths wrong – more or fewer in the parameter field

• Additional or missing depths – in full resolution profiles, depths duplicated or missing

• Deep_depth field value different from the actual deepest depth

• Single point profiles

• Profiles with exactly .6m resolution (depths assigned and not calculated?) – results in error of ~30m at 800m

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Conclusions:

• Codes are confusing – there are too many places to find probe type information

• PEQ$• PRT$• FRE$• PTY$• PFR$

• We should be able to reconcile all these into a sensible PEQ$, even if it’s unknown.

• DPC$ need clarification – • 03 = not enough information, do not correct – but we know

most of these should be corrected.• 02 = known probe type, no correction – what does this

mean? No correction done or no correction necessary?

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Conclusions (con):

• To effectively calculate the corrections needed to fix the fall rate bias of XBTs, we need the best quality baseline data – all using the same base-line fall rate. We therefore need smart methods to get all profiles to the Hanawa equation.

• We can only do this if we have adequate metadata – otherwise, we’re just making educated guesses.