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iQuam Sensitivity to Reference SST

iQuam Sensitivity to Reference SST. Feng Xu July 28, 2013. Background. iQuam QC: binary check (plausibility, space-time continuity, SST continuity) + B ayesian checks (reference check=RC, cross-platform check=XC)

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iQuam Sensitivity to Reference SST

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  1. iQuam Sensitivity to Reference SST FengXu July 28, 2013 Selection of Reference SST in iQuam

  2. Background • iQuam QC: binary check (plausibility, space-time continuity, SST continuity) + Bayesian checks (reference check=RC, cross-platform check=XC) • RC is the most critical check, which removes the largest portion of suspect data • XC is complementary to RC, further compares SSTs between neighbor platforms • iQuam Quality Monitoring (QM) is based on analyses of delta = “in situ minus reference SST”, global Bias and SD • In principle, QC and QM can use different SSTs • In iQuam, Reynolds daily ¼° v2 product is used for both • Both QC and QM maybe sensitive to reference SST • For better Ref SST, QM statistics should be better (smaller Biases and SDs) • Ideally, QC should be fairly insensitive to Ref SST (as soon as Ref SST is reasonably accurate) • However, for better Ref SST, QC rejection rate may be a little smaller (fewer in situ data identified as bad) Selection of Reference SST in iQuam

  3. Objective, Data, Method • Objective • Select a more appropriate reference field in iQuam • Data • One year (2012) of in situ data (Ships, Drifters, Tropical and Coastal Moorings, ARGO Floats) • Three reference SSTs: Reynolds, CMC, and RTG-HR • Method • Run iQuam v2b, with the same configuration, for 3 reference fields • Compare • QC error rate for (a) Reference Check; and (b) Cross-platform Check • Bias/SD Statistics of “in situ minus reference SST” • Stability in time of Error rates and Statistics Selection of Reference SST in iQuam

  4. Drifter Ship Argo C-Moor T-Moor Selection of Reference SST in iQuam

  5. Drifter Ship Argo C-Moor T-Moor Selection of Reference SST in iQuam

  6. QC Sensitivity Observations • For all in situ data, RTG-HR consistently detects more outliers – this reference field is least consistent with in situ data, out of 3 tested • Reynolds and CMC QCs close for ships, tropical moorings, & ARGO floats • For drifters, Reynolds RC detects fewer outliers than CMC – is Reynolds overly adjusted to drifters? (used as anchor in Reynolds analysis) • For coastal moorings, Reynolds detects more outliers compared to CMC – is CMC more accurate in coastal areas? • The above observations are valid for both Reference and Cross-platform checks Selection of Reference SST in iQuam

  7. Drifter Ship Argo T-Moor C-Moor Selection of Reference SST in iQuam

  8. Drifter Ship Argo T-Moor C-Moor Selection of Reference SST in iQuam

  9. QM Sensitivity Observations • Overall, QM wrt. CMC consistently produces comparable or better statistics, i.e. lower biases and SDs, and more stable time series (with the exception of ship bias -due to ship warm bias?) • For all platforms, QM wrt. RTG-HR produces higher SDs, suggesting presence of additional “internal” noise in this reference SST • Reynolds SST appears to be bracketed by the two reference SSTs Selection of Reference SST in iQuam

  10. Conclusion • iQuam QC is largely insensitive to reference SST • Note that for using RTG-HR as reference, the QC parameters such as reference sigma needs to be adjusted accordingly, to accommodate for larger “internal noise” in the RTG field • Some references may be leaning more towards certain type of in situ measurements • e.g. Reynolds appears closer to drifters (less detection rate) • Ideally, preferable reference field for the use in iQuam should be the one that not assimilate in situ data • Interestingly, CMC shows consistently better agreement with coastal moorings, suggesting a superior CMC performance in coastal areas • Overall, iQuam QM tends to have smaller and more stable biases and SDs of “in situ minus reference SST” when CMC is used as reference • Current CMC time series only go back to 1991. Once CMC time series extend to 1981, CMC will be explored in iQuam Selection of Reference SST in iQuam

  11. Regarding CMC • According to [Brasnett 2008], CMC incorprated all in situ data: Argo floats, ships, drifters, moored buoys • CMC corrected for the warm ship bias • CMC applied stringent QC to all in situ data • This might contribute to the fact that CMC appears superior than other references Selection of Reference SST in iQuam

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