Improving the SSES inputs to AATSR L2P Products

1. Improving the SSES inputs to AATSR L2P Products Gary Corlett AATSR Validation Scientist

2. Contents • Reminder of current SSES scheme • SSTs from AATSR • What affects the quality of AATSR SSTs? • An alternate scheme using view differences

3. Current SSES scheme for AATSR L2P

9. 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?

10. 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

11. Retrieval Coefficients: 1-km

12. What Affects the Quality of AATSR SSTs? • For December 2005 coefficients • D2-D3 bias (the latitude dependent correction) owing to limited number of channels • Small residual bias (~ 0.15 K at T < 280 K) most likely due to temperature dependent emissivity • These issues are being addressed in (A)RC project • Clouds and aerosols (claerosols) • AATSR cloud screening is over-zealous • AATSR is designed for climate SSTs! • Much better during day owing to 1.6 µm channel • Notable edge effects at night • Dual-view corrects for tropospheric aerosol • But not quite! • Being addressed in (A)RC project

13. Distribution of M-AERI Matchups LEGEND All +ve D-N -ve D-N • Bi-modal distribution of differences • 1 distribution due to -ve dual-nadir SST differences • 1 distribution due to +ve dual-nadir SST differences • Matchups • Day; Overpass ± 60 min; 1x1 Block

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

15. Caribbean SST differences for ‘clean months’ – November to MarchData is derived from 30’ product Day D2-N2 Threshold = -0.03 K Night D3-N3 Threshold = +0.26 K

16. Caribbean SST differences for ‘clean months’ – November to MarchData is derived from 1-km product Day D2-N2 Threshold = +0.25 K Night D3-N3 Threshold = +0.26 K Noyes et al., RSE, 2006

17. Pacific Ocean – ‘clean’ months Good et al., RSE, 2007

18. BT

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

28. Spatial Pattern of AR Differences to Buoys Data from Anne O’Carroll (Met Office)

29. 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 • Link to DT_min needs further work (not suitable for AATSR?) • SST coefficients changed in December 2005 • Data not yet reprocessed so need to wait for clean months! • Need to have NADIR SST in data file • For PCF scheme • N3 is a very good product anyway! • Future scheme should take into account spatial and temporal variation in SST retrievals • OI based bias correction field