an updated pcf scheme for aatsr sses

GHRSST-PP, 8th Science Team Meeting, Melbourne, 16th May 2007

An Updated PCF Scheme for AATSR SSES

Gary Corlett

AATSR Validation Scientist


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


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


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


Current SSES scheme for AATSR L2P


Example of PCF: Day timeAATSR SST PCF Mask


Example of PCF: Night timeAATSR SST PCF Mask


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


Nov 2003

July 2003

Remedios et al., 2007

The Affect of Saharan Dust on AATSR SSTs (30’)


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


Cloud Contamination at Night

Good et al., RSE, 2007


BT



Schematic of BT Effects



Global Application of D-N Test



Updated SSES using D-N Differences: Day timeAATSR SST AATSR D-N SST


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


Updated SSES using D-N Differences: Night time

AATSR SST AATSR D-N SST


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


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


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


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

an updated pcf scheme for aatsr sses

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