improving the sses inputs to aatsr l2p products

Medspiration 4th User Consultation, Villefranche sur mer, 4 December 2006

Improving the SSES inputs to AATSR L2P Products

Gary Corlett

AATSR Validation Scientist


Contents

Reminder of current SSES scheme

SSTs from AATSR

What affects the quality of AATSR SSTs?

An alternate scheme using view differences


Current SSES scheme for AATSR L2P


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


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


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


Nov 2003

July 2003

Remedios et al., 2007

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


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


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

Noyes et al., RSE, 2006


Pacific Ocean – ‘clean’ months

Good et al., RSE, 2007


BT


Schematic of BT Effects

Good et al., RSE, 2007


Spatial Pattern of AR Differences to Buoys

Data from Anne O’Carroll (Met Office)


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

improving the sses inputs to aatsr l2p products

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