an updated pcf scheme for aatsr sses
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Page 1GHRSST-PP, 8th Science Team Meeting, Melbourne, 16th May 2007
An Updated PCF Scheme for AATSR SSES
Gary Corlett
AATSR Validation Scientist
Page 2GHRSST-PP, 8th Science Team Meeting, Melbourne, 16th May 2007
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
Page 3GHRSST-PP, 8th Science Team Meeting, Melbourne, 16th May 2007
Nominal Channel Centre Primary Application
0.55 m Chlorophyll
0.66 m Vegetation Index
0.87 m Vegetation
1.6 m Cloud Clearing
3.7 m SST Retrieval
11 m SST/LST Retrieval
12 m SST/LST Retrieval
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 N2Nadir 3-channel N3Dual 2-channel D2Dual 3-channel D3
n
nnBTaa1
0
Page 4GHRSST-PP, 8th Science Team Meeting, Melbourne, 16th May 2007
Retrieval Coefficients: 1-km
Coefficient N2 N3 D2 D3
Constant -0.34 0.04 2.83 0.70
Nadir 3.7 µm - 1.08 - 2.51
Nadir 11 µm 3.42 0.65 5.42 0.63
Nadir 12 µm -2.42 -0.73 -3.08 -0.73
Forward 3.7 µm - - - -1.47
Forward 11 µm - - -3.20 -0.34
Forward 12 µm - - 1.84 0.40
Page 5GHRSST-PP, 8th Science Team Meeting, Melbourne, 16th May 2007
Current SSES scheme for AATSR L2P
Page 6GHRSST-PP, 8th Science Team Meeting, Melbourne, 16th May 2007
Example of PCF: Day timeAATSR SST PCF Mask
Page 7GHRSST-PP, 8th Science Team Meeting, Melbourne, 16th May 2007
Example of PCF: Night timeAATSR SST PCF Mask
Page 8GHRSST-PP, 8th Science Team Meeting, Melbourne, 16th May 2007
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
Page 9GHRSST-PP, 8th Science Team Meeting, Melbourne, 16th May 2007
Nov 2003
July 2003
Remedios et al., 2007
The Affect of Saharan Dust on AATSR SSTs (30’)
Page 10GHRSST-PP, 8th Science Team Meeting, Melbourne, 16th May 2007
Day
D2-N2
Night
D3-N3 Threshold =
+0.26 K
Threshold =
+0.25 K
Noyes et al., RSE, 2006
Threshold = mode + 3 * sigma
D-N for ‘clean months’: 1-km
Page 11GHRSST-PP, 8th Science Team Meeting, Melbourne, 16th May 2007
Cloud Contamination at Night
Good et al., RSE, 2007
Page 12GHRSST-PP, 8th Science Team Meeting, Melbourne, 16th May 2007
BT
Good et al., RSE, 2007
Page 13GHRSST-PP, 8th Science Team Meeting, Melbourne, 16th May 2007
Schematic of BT Effects
Good et al., RSE, 2007
Page 14GHRSST-PP, 8th Science Team Meeting, Melbourne, 16th May 2007
Global Application of D-N Test
Good et al., RSE, 2007
Page 15GHRSST-PP, 8th Science Team Meeting, Melbourne, 16th May 2007
Updated SSES using D-N Differences: Day timeAATSR SST AATSR D-N SST
Page 16GHRSST-PP, 8th Science Team Meeting, Melbourne, 16th May 2007
Updated SSES using D-N Differences: Day timeCurrent SSES Updated SSES
Dark green = Good dataRed = Bad data (cloud/aerosol contaminated)
Note: Increase in good data (backed-up by validation); change in location of bad data
Page 17GHRSST-PP, 8th Science Team Meeting, Melbourne, 16th May 2007
Updated SSES using D-N Differences: Night time
AATSR SST AATSR D-N SST
Page 18GHRSST-PP, 8th Science Team Meeting, Melbourne, 16th May 2007
Updated SSES using D-N Differences: Night time
Current SSES Updated SSES
Dark green = Good dataRed = Bad data (cloud/aerosol contaminated)
Note: Increase in good data (backed-up by validation); change in location of bad data
Page 19GHRSST-PP, 8th Science Team Meeting, Melbourne, 16th May 2007
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
Page 20GHRSST-PP, 8th Science Team Meeting, Melbourne, 16th May 2007
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
Page 21GHRSST-PP, 8th Science Team Meeting, Melbourne, 16th May 2007
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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