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An Updated PCF Scheme for AATSR SSES

An Updated PCF Scheme for AATSR SSES. Gary Corlett AATSR Validation Scientist. Imaging infrared and visible radiometer on ENVISAT Similar channels to AVHRR & MODIS. Dual view (nadir and 55° to nadir)

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An Updated PCF Scheme for AATSR SSES

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  1. An Updated PCF Scheme for AATSR SSES Gary Corlett AATSR Validation Scientist

  2. Imaging infrared and visible radiometer on ENVISAT Similar channels to AVHRR & MODIS. Dual view (nadir and 55° to nadir) Along-track scanning, two views of same scene at different angles, for better atmospheric correction On-board calibration 2 on-board black bodies for IR calibration VISCAL unit for visible channel calibration 500 km swath 1 km IFOV at nadir Stirling Cycle Coolers, cooling low noise detectors to 80K, for optimum signal-to-noise ratios What is AATSR?

  3. AATSR SST Retrievals • Retrievals by radiative transfer modelling • Usually of the form: where n is the number of channels • For AATSR we have 2 channels during day and 3 during night • 3.7 µm not used during day owing to solar contamination • We have two views, so we have four SST retrievals in total • We also have two product formats • 1-km gridded (swath) • Spatially –averaged (gridded) Four Possible Retrievals: Nadir 2-channel N2 Nadir 3-channel N3 Dual 2-channel D2 Dual 3-channel D3

  4. Retrieval Coefficients: 1-km

  5. Current SSES scheme for AATSR L2P

  6. Example of PCF: Day time AATSR SST PCF Mask

  7. Example of PCF: Night time AATSR SST PCF Mask

  8. AATSR SSES • Current SSES scheme based on heritage of nadir-view only IR sensors • Assume cloud is biggest source of error • Cloud in nadir view at lower BT than surface gives lower estimate of surface T • Therefore test for proximity to cloud and difference to minimum climatology • Provide confidence flag 0 (bad) to 5 (excellent) giving estimate of contamination • For AATSR this does not exploit two important factors • The dual-view means we have two goes at identifying cloud contamination; AATSR cloud screening is over cautious • In reality cloud contamination is small in AATSR, it is clear sky identified as cloud that is the problem! • The dual-view retrieval means that cloud in the forward view gives a warmer estimate of surface T • Propose updated scheme using dual-nadir SST difference to identify cloud (and aerosol) contamination • Based on experience of AATSR and results from AATSR validation program • Give improved uncertainty estimates for operational users

  9. The Affect of Saharan Dust on AATSR SSTs (30’) Nov 2003 July 2003 Remedios et al., 2007

  10. D-N for ‘clean months’: 1-km Threshold = mode + 3 * sigma Day D2-N2 Threshold = +0.25 K Night D3-N3 Threshold = +0.26 K Noyes et al., RSE, 2006

  11. Cloud Contamination at Night Good et al., RSE, 2007

  12. Good et al., RSE, 2007 BT

  13. Schematic of BT Effects Good et al., RSE, 2007

  14. Global Application of D-N Test Good et al., RSE, 2007

  15. Updated SSES using D-N Differences: Day time AATSR SST AATSR D-N SST

  16. Updated SSES using D-N Differences: Day time Current SSES Updated SSES Dark green = Good data Red = Bad data (cloud/aerosol contaminated) Note: Increase in good data (backed-up by validation); change in location of bad data

  17. Updated SSES using D-N Differences: Night time AATSR SST AATSR D-N SST

  18. Updated SSES using D-N Differences: Night time Current SSES Updated SSES Dark green = Good data Red = Bad data (cloud/aerosol contaminated) Note: Increase in good data (backed-up by validation); change in location of bad data

  19. Updated PCF Scheme • D2_N2_U = + 0.14 °C • D2_N2_L = - 1.67 °C • D3_N3_U = +0.67 °C • D3_N3_L = -0.70 °C

  20. Comments • Scheme does not use comparisons to minimum SST climatology • Scheme does not use all levels (0 to 5) • Does it have to? • Scheme needs nadir SST and NR confidence word from N1 file • Do not need to be included in L2P • My recommendation is that they are though! • SSES will still use static biases • Will be updated later this year • Improved SSES following completion of (A)RC Project

  21. Summary • Current PCF scheme used in L2P applied to AATSR data is appropriate for nadir-only IR with poor cloud screening • Does not account for over-zealous operational AATSR cloud screening • Does not account for impact of claerosols on forward view • Can develop improved confidence flag for AATSR • Use D-N difference to identify retrievals affected by claerosols • Threshold = mode + 3 * sigma • SST coefficients changed in December 2005 • Need to have NADIR SST • For PCF scheme • N3 is a very good product anyway! • Future scheme should take into account more spatial and temporal variation in SST retrievals • Climatology rather than static field

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