Development of New SSES for AATSR L2P
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Transcript of Development of New SSES for AATSR L2P
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Development of New SSES for AATSR L2P
Gary Corlett
AATSR Validation Scientist
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Content
Reminder of current SSES scheme SSTs from AATSR What affects the quality of AATSR SSTs? An alternate scheme using view differences Examples Analysis of GHRSST-PP MDB Future work Summary & Conclusions
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Revised (Realistic) Content
Reminder of current SSES scheme SSTs from AATSR What affects the quality of AATSR SSTs? An alternate scheme using view differences Examples Analysis of GHRSST-PP MDB Future work Summary & Conclusions
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What Affects AATSR Data Quality?
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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• Suspected 12 μm calibration error
• In process of analysing flight-spare FPA• 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– Observable edge effects at night
• Dual-view corrects for tropospheric aerosol– But not quite!– Being addressed in (A)RC project
• Relative alignment of nadir and forward views• New results suggest systematic offset• Currently being investigated by AATSR QWG
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Suspected 12 μm Error: Implication for SSTs
Calibration offset seen between AATSR and ATSR-2 12 μm clear-sky SEA BTs• ATS_MET_2P and AT2_MET_2P products analysed• Validation results suggest AATSR is incorrect• Results in increase in latitude dependent D2-D3 bias
Effect on current D3 SSTs is ~ + 0.05 K
Effect on current D2 SSTs is ~ - 0.04 K
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Distribution of M-AERI Matchups
Matchups
• Day; Overpass ± 60 min; 1x1 Block
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
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Nov 2003
July 2003
The Affect of Saharan Dust on AATSR SSTs (30’)
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Caribbean SST differences for ‘clean months’ – November to March
Data is derived from 30’ product
Threshold =
+0.26 K
Threshold =
-0.03 K
Day
D2-N2
Night
D3-N3
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Day
D2-N2
Night
D3-N3 Threshold =
+0.26 K
Threshold =
+0.25 K
Caribbean SST differences for ‘clean months’ – November to March
Data is derived from 1-km product
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Pacific Ocean – ‘clean’ months
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BT
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Schematic of BT Effects
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Relative alignment of forward and nadir views (1)
Nadir View 3.7 micron Forward View 3.7 micron
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Relative alignment of forward and nadir views (2)
Forward minus Nadir View 3.7 micron Forward minus Nadir View 3.7 micron with shift
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Basis of New SSES Scheme
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Requirements for AATSR SSES
Need, per pixel:• Bias
• Retrieval bias, or• Bias to a common standard – GHRSST match-up database
• Standard deviation• What exactly is a single pixel standard deviation?
• Quality flag• Scale of 1-5 agreed in Melbourne
– 1: Cloudy– 2: Bad– 3: Acceptable (defined by provider)– 4: Good– 5: Excellent
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Updated QF Scheme – Theoretical Thresholds
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Updated QF Scheme – Experimental Thresholds
T2U = + 0.04 ± 0.09 °C T2L = - 1.53 ± 0.09 °C T3U = +0.51 ± 0.07 °C T3L = -0.51 ± 0.07 °C
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Updated PCF Scheme - Stratification
Stratification
If 2-channel Retrieval If 3-channel Retrieval
TL2 < D-N < TU2 TL3 < D-N < TU3
D-N < TL2 D-N < TL3
D-N > TU2 D-N > TU3
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Potential SSES Scheme: Summary Effect of clouds and aerosols can be accounted for using D-N SST
differences• Should get well-defined normal distribution
• Can set thresholds to remove data not in normal distribution
Suspected alignment error between forward and nadir views will contribute to D-N differences• Significant positive edge structures still seen after view-difference shift• D-N test is designed for current AATSR data• Will need revising if and when relative alignment is corrected
12 μm calibration error will contribute to D-N differences• D-N test is designed for current AATSR data• Will need revising if and when 12 μm calibration error is corrected
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Examples
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Example 1
AATSR Orbit: ATS_NR__2PNPDE20060516_012117_000049912047_00403_22000_0504.N1
Area of interest: 102 ºE to 118 ºE by 12 ºS to 4 ºS Day time descending node The day time example demonstrates the over-zealous
cloud-screening, as the updated PCF mask is mainly indicating good data, while the larger-scale bad data is most likely affected by aerosol rather than cloud. The data are for 16th May 2006 over Indonesia, and it is noted that the volcano Mount Merapi, in central Java, was fairly active during May 2006.
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Comparison of N1 and L2P SSTs
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L2P Quality Control
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N1 D-N & New QF
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Change in QF
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Summary of Main Changes Due to New Scheme
Large number of pixels classified as QF 2 and QF 3 data are now classed as QF 5• Little change for QF 4 and QF 5 (except night time)
A lot of pixels identified as bad (QF 2) by new scheme was previously classed as QF 3
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Analysis of GHRSST-PP MDB
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Definitions of SST D
epth
10m
1 mm
1 m
10 m
(a) Night time situation, light windAATSR 3 channel
-0.3 -0.2 -0.1 0.0
SSTskin – SST10m (K)
Dep
th
10m
1 mm
1 m
10 m
SSTskin – SST10m (K)
0 0.5 1.0 1.5 2.0 2.5 3.0
Infrared sensors (T ~10s)
Contact thermometersShips/Buoys (T ~minutes)
(b) Day time situation, strong solar radiation and light winds
AATSR 2-channel
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Location of GHRSST-PP Match-ups (1)
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Location of GHRSST-PP Match-ups (2)
Drifting Buoys
Others
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Example: Day time “Good” data low wind speed
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Example: Night time “high” data low wind speed
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Statistics from GHRSST-PP MDB Analysis
StratificationStatistics
Number Bias sigma
2-channel, B, L 10762 +0.024 0.41
2-channel, B, H 14345 -0.001 0.35
2-channel, A, L 443 -3.895 3.24
2-channel, A, H 447 -2.230 2.19
2-channel, C, L 419 +0.531 0.66
2-channel, C, H 642 +0.487 0.66
3-channel, B, L 12013 -0.170 0.36
3-channel, B, H 15158 -0.083 0.35
3-channel, A, L 358 -0.901 0.54
3-channel, A, H 348 -0.790 0.52
3-channel, C, L 910 +0.381 0.39
3-channel, C, H 1394 +0.446 0.35
A: Data below lower thresholdB: Data within upper and lower thresholdsC: Data above upper threshold
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New Confidence Values and Statistics
Stratification Case Summary of criteria Bias St. Dev. Confidence Value
Case 1: 2-channel, Middle D-N, Low wind +0.179 0.38 5
Case 2: 2-channel, Middle D-N, High wind +0.179 0.38 5
Case 3: 2-channel, Low D-N, Low wind -2.719 2.88 2
Case 4: 2-channel, Low D-N, High wind -2.719 2.88 2
Case 5: 2-channel, High D-N, Low wind +0.674 0.66 3 (or 4)
Case 6: 2-channel, High D-N, High wind +0.674 0.66 3 (or 4)
Case 7: 3-channel, Middle D-N, Low wind +0.000 0.36 5
Case 8: 3-channel, Middle D-N, High wind +0.087 0.35 5
Case 9: 3-channel, Low D-N, Low wind -0.731 0.54 4 (or 3)
Case 10: 3-channel, Low D-N, High wind -0.620 0.52 4 (or 3)
Case 11: 3-channel, High D-N, Low wind +0.551 0.39 5
Case 12: 3-channel, High D-N, High wind +0.616 0.35 5
Biases have been converted to skin biases by adding 0.17 K
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Summary & Conclusions (1)
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
Have developed improved quality flag for AATSR• Use D-N SST difference to identify retrievals affected by
claerosols• Threshold = mode + 3 * sigma
• Also stratification according to wind speed• Future scheme could take into account spatial and temporal
variation in SST retrievals
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Summary & Conclusions (2) Issues
• Data classed as “others”• No day time wind speed dependence
• May imply no significant diurnal warming has started or diurnal warming cancels out cool-skin effect
• Night-time sub-skin correction• No current agreement on how to correct (consistently)
• Would only recommend QF 5 data if using AATSR as reference• Requires changes to L2P product
• Addition of nadir-only SST for each dual-view SST• Addition of flag to identify 3-channel retrievals
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Acknowledgements
Defra, the UK Department of Environment, Food and Rural Affairs, who funded AATSR to support their programme of climate prediction and research, which in turn provides inputs to their policy-making processes
Funding agencies in Australia who made significant contributions
The European Space Agency
This work was funded by ESA (ESA-NOCS Contract: AO/1-4362/03/I-LG) and Defra (Space ConneXions Contract: 2004-03-001/CPEG10)
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Current SSES Scheme
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Current SSES scheme for AATSR L2P
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AATSR SSTs
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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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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
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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
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Ongoing Work
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Spatial Pattern of AR Differences to Buoys
Data from Anne O’Carroll (Met Office)
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Further Refinement of SSES
The (A)ATSR L2P project is aiming to make a few further improvement to the SSES
The two main ones are
• To use high quality ICOADS in situ data
• Introduce spatial and temporal variation to bias and standard deviations
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L2P Reformatting (2)
10 degree
20 degree
45 degree
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L2P Reformatting (3)
10 degree
20 degree
45 degree
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Example 2 AATSR Orbit:
ATS_NR__2PNPDE20060516_012117_000049912047_00403_22000_0504.N1
Area of interest: 70 ºW to 50 ºW by 25 ºN to 35 ºN Night time ascending node
The night time example in Section 5.1.2 clearly demonstrates the edge of pixel cloud contamination that affects AATSR data, and is clearly not picked up by the current PCF scheme. In addition, as for the day time data, there is no reason to suspect the rest of the SST data, which has been cloud screened twice and retrieved using two different SST retrievals. Therefore the data which passes the D-N test is set as good data and the PCF mask for these pixels is set to 5.
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AATSR and ATSR-2 Simulated Bias: Centre of Swath
From Owen Embury (University of Edinburgh)
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Compare AATSR and ATSR-2 BTs: Case 1
Nadir View Forward View