An Updated PCF Scheme for AATSR SSES

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An Updated PCF Scheme for AATSR SSES

• Gary Corlett
• AATSR Validation Scientist

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What is AATSR?

• 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

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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
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Retrieval Coefficients 1-km
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Current SSES scheme for AATSR L2P
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Example of PCF Day time
AATSR SST
PCF Mask
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Example of PCF Night time
AATSR SST
PCF Mask
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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

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The Affect of Saharan Dust on AATSR SSTs (30)
Nov 2003
July 2003
Remedios et al., 2007
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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
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Cloud Contamination at Night
Good et al., RSE, 2007
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BT
Good et al., RSE, 2007
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Schematic of BT Effects
Good et al., RSE, 2007
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Global Application of D-N Test
Good et al., RSE, 2007
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Updated SSES using D-N Differences Day time
AATSR SST
AATSR D-N SST
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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
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Updated SSES using D-N Differences Night time
AATSR SST
AATSR D-N SST
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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
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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

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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

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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