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Arctic SST retrieval in the CCI project

Arctic SST retrieval in the CCI project. Owen Embury Chris Merchant University of Reading. SST CCI Phase 1. Combine ATSR accuracy with AVHRR coverage Optimal Estimation (OE) retrieval Cross referenced to ARC SST Diurnal variability adjustment Report SSTs at standard depth and time of day

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Arctic SST retrieval in the CCI project

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  1. Arctic SST retrieval in the CCI project Owen Embury Chris Merchant University of Reading

  2. SST CCI Phase 1 • Combine ATSR accuracy with AVHRR coverage • Optimal Estimation (OE) retrieval • Cross referenced to ARC SST • Diurnal variability adjustment • Report SSTs at standard depth and time of day • Uncertainty estimation • Product specification • NetCDF 4 with classic data model • GDS2.0 compliant

  3. SST CCI Phase 1 • Long-term (Aug 1991 – Dec 2010) • ATSR • OE retrieval • Bayesian cloud screening • L3U (0.05°) • AVHRR GAC • OE retrieval • CLAVR-X cloud screening • Ice detection • L2P • SSTskin at time of observation • SST0.2m at 10:30 local time • Adjusted to nearest am/pm

  4. Bayesian Cloud Detection • Use RTTOV to simulate expected observations from ECMWF NWP • Calculate P(obs | clear) from obs-sim differences • Get P(obs | cloud) from empirical lookup table • Use Bayes to get P(clear | obs, nwp) • P(obs | clear) is dominant factor • ECMWF NWP • RTTOV forward model • Prior error assumptions • Prior SST error is location dependent (0.5 to 1.8 K)

  5. Bayesian Cloud Detection • Can consider the current system as Bayesian “clear-sky” detection • Problem detecting conditions which look like clear-sky in infrared • Seaice • Fog • Potential improvements • Add fog / seaice as extra classes for detection • Needs software refactoring • Use visible channels • Not done yet due to ARC software heritage (ARC needed method applicable to ATSR1) • Daytime only • Review prior SST error assumption in Arctic areas

  6. OE SST retrieval • MAP formulation with prior SST error of 5 K • Reduces influence of prior on retrieval • QC check on χ2 to remove bad retrievals • Calculation of χ2 similar to P(obs | clear) in Bayesian cloud detection • QC check to remove SSTs < 271.35 K • Not applied in pre-release data

  7. Comparison with other datasets • Compare 5 day composite images • Pathfinder v5.2 • ARC v1.1.1 • AMSR-E v7 • OSTIA • CCI • AVHRR L2P • ATSR L3U • Show with OSISAF sea ice concentration • 15% and 85% contour lines

  8. Norwegian and Greenland Seas 2008

  9. Norwegian and Greenland Seas 2010

  10. Ice melt in Beaufort Sea and outflow from Mackenzie river 2008

  11. Ice melt in Beaufort Sea and outflow from Mackenzie river 2010

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