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AATSR Validation Activities, GAD AATSR Val FR-1 Final ReportSeptember 2003

AATSR Validation ActivitiesGAD/AATSR VAL/FR-1

DEFRA Contract: GA 4/2/193 EPG 1/1/131Final Report

Written by: G.K. Corlett & M.C. EdwardsChecked by: J.J. RemediosApproved by: D.T. Llewellyn-Jones

September 2003

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1 Contents1 Contents................................................................................................................................22 Acronyms.............................................................................................................................33 Introduction and Scope of Document...................................................................................44 Summary of the AATSR Validation Programme................................................................5

4.1 Activities........................................................................................................................54.2 AATSR Data..................................................................................................................6

4.2.1 Availability.............................................................................................................64.2.2 Algorithm verification............................................................................................6

4.3 Summary of validation results.......................................................................................94.3.1 Sea Surface Temperature validation.......................................................................94.3.2 Visible/near infrared channel validation...............................................................13

4.4 Conclusions and Recommendations September 2003.................................................155 Specific Validation Activities at the University of Leicester.............................................17

5.1 Validation using the MAERI Radiometer...................................................................175.1.1 Introduction...........................................................................................................175.1.2 Results...................................................................................................................195.1.3 Summary...............................................................................................................27

5.2 Global and Regional Comparisons of AATSR with other sensors.............................285.2.1 ATSR-2.................................................................................................................295.2.2 AATSR.................................................................................................................32

6 Management Activities & Reporting.................................................................................386.1 Management................................................................................................................386.2 Work-package Compliance.........................................................................................406.3 Reporting & Publications............................................................................................42

7 Phase E Validation.............................................................................................................457.1 Validation....................................................................................................................46

7.1.1 Organisational Activities......................................................................................467.1.2 SST Validation......................................................................................................467.1.3 Land Products.......................................................................................................487.1.4 Other Products......................................................................................................487.1.5 Analysis and Reporting.........................................................................................49

7.2 Summary......................................................................................................................507.2.1 Proposed activities for continued validation in Phase E.......................................50

7.3 Conclusions & Future Work........................................................................................517.3.1 Conclusions...........................................................................................................517.3.2 Future Work..........................................................................................................517.3.3 Benefits.................................................................................................................52

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2 Acronyms

AATSR Advanced Along Track Scanning RadiometerADS Annotated Data SetAIMS Australian Institute of Marine ScienceASTER Advanced Space borne Thermal Emission and Reflection radiometerATSR-2 Along Track Scanning Radiometer 2AVHRR Advanced Very High Resolution RadiometerBT Brightness TemperatureCb CumulonimbusCSIRO Commonwealth Scientific and Industrial Research Organisation DEFRA Department of the Environment, Food and Rural AffairsESA European Space AgencyEOS Earth Observation ScienceGBR Great Barrier ReefGPS Global Positioning SystemISAR Infrared Sea surface skin temperature Autonomous RadiometerJRC Joint Research CentreLST Land Surface TemperatureM-AERI Marine Atmosphere Emitted radiance InterferometerMAVT MERIS and AATSR Validation TeamMODIS Moderate Resolution Imaging SpectroradiometerNCEP National Centre for Environmental Prediction NERC Natural Environment Research CouncilOP Operational ProcessorPI Principal InvestigatorPP Prototype ProcessorRAL Rutherford Appleton LaboratorySCIPIO Satellite Calibration and Interior Physics in the Indian OceanSISTeR Scanning Infrared Sea Surface Temperature RadiometerSPH Specific Product HeaderSST Sea Surface TemperatureTOA Top Of AtmosphereVIP Validation Implementation PlanVS Validation Scientist

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3 Introduction and Scope of Document

This report is the final deliverable of DEFRA Contract GA 4/2/193 EPG 1/1/131.

The AATSR validation programme has operated a successfully for 17 month since Envisat launch and instrument switch-on, with the completion of 14 dedicated cruises, 6 instruments running autonomously and numerous additional validation activities. Validation results show that there are no issues of serious concern at this stage about the accuracy of global AATSR data within the limitations of current validation data; considerable further work is required to establish accuracy at a regional scale. Comparison against global buoy data and SST analysis fields, and in situ data from precision radiometers, demonstrate that AATSR SST retrievals are accurate to within their specifications, ±0.3K, and AATSR visible channel reflectance measurements compare well to those from MERIS.

The report is presented in five main sections that describe: A summary of the validation programme to date, including a synthesis of the main

conclusions that can be drawn at this stage, Section 4 A report on specific validation activities carried out by the University of Leicester,

Section 5 Management activities and reporting, Section 6 A description of proposed ongoing validation activities during Phase E, Section 7 The validation activities performed by the two sub-contractors, RAL and SOC,

Appendix 1 and Appendix 2, respectively.

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4 Summary of the AATSR Validation Programme

4.1 ActivitiesThe AATSR validation programme has operated successfully for 15 months since Envisat launch and instrument switch-on, with a significant number of in situ measurements being collected. Specific activities that have taken place since May 2002 include:

Algorithm verification has been underway since data delivery to Andrew Birks at RAL in June 2002. Ideally, algorithm verification would have been completed before the start of validation. However due to the timetable adopted, these activities proceeded in parallel. The Met Office started receiving near-real-time AATSR data on the 19th August 2002. They have been comparing the AATSR METEO product with buoy data and SST analysis fields almost continuously since that time. There have been small breaks due to the unavailability of the METEO product to the Met Office. The University of Leicester have been comparing AATSR ASST data with SST data from other sensors since the start of L2 data distribution to Leicester at the end of October 2002. I. Barton (CSIRO) has deployed the DAR011 precision radiometer on 5 dedicated validation cruises. The 13 validation points (5 submitted, 8 undergoing further analysis) he has been able to collect on these cruises, in only a few months, is a major achievement. P. Minnett (RSMAS, Uni. of Miami) has three MAERI radiometers. One of these has been deployed almost continuously on a ship in the Caribbean. The University of Leicester is analysing the data collected, and converting and uploading data files to the NILU database. To date, 184 files have been uploaded. The other MAERI instruments have been deployed on 3 cruises of opportunity, in the Arctic, the Mediterranean and from Seattle to Sydney. Unfortunately the last cruise had to be abandoned due to bad weather, but from the other deployments, a significant number of validation points have been collected. The ISAR radiometer has been deployed by the University of Southampton on two ferries, the Val de Loire and the Pride of Bilbao. The first deployment from May 15th to June 26th was much more successful than the second deployment which has suffered from technical difficulties and poor weather. The analysis of this data was hindered by the delay in distribution of AATSR data. Funded through a NERC Enabling Grant, T. Nightingale deployed SISTeR on a cruise in the Indian Ocean, obtaining 4 ATSR-2/AATSR validation points from a possible 17. The vicarious validation work of Smith, Hagolle, Watts and Stammes started when data distribution to the AATSR PIs started at the end of October.

In summary, there have been 14 dedicated validation cruises and 6 instruments running autonomously. Owing to the delay in data distribution and the subsequent time available before the validation workshop for processing, validation results reported at the first workshop, in December 2002, were limited. By 30 September 2003, a small amount of AATSR Level 2 data for validation comparisons is still not distributed. It is expected that the issue of delayed data distribution will be completely resolved in time for the second validation workshop, in October 2003.

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4.2 AATSR Data

4.2.1 AvailabilityAs detailed in the AATSR Commissioning Report (PO-RP-RAL-AT-0511), the end of the main commissioning phase tests was on 19th May 2002. From 19th May 2002 to the validation workshop, AATSR was in measurement mode for most of the time. Data gaps occurring during the commissioning phase are described in detail in PO-RP-RAL-AT-0511, and all data gaps are summarised in Table 4.2.1-1.

Date 2002/3 Instrument non-availability27-29th May AATSR off due to Envisat PL-SOL for all instruments5-11 June AATSR off due to platform Level 3 Protocol Interrupt18 June AATSR switched to low-gain mode over desert15-22 July Repeat of cooler checkout tests22-25 July Instrument out-gassing8-12 September AATSR in standby due to Envisat orbit manoeuvre18-20 November AATSR off as a Leonid Meteor Shower precaution18 December ENVISAT Orbit Manoeuvre31 January - 03 February

Instrument out-gassing

20 -23 February ENVISAT Orbit Manoeuvre15 - 19 March Spacecraft problem18 - 20 May Spacecraft maintenance

Table 4.2.1-1 AATSR Instrument non availability to 31/07/2003

Apart from the Met Office who received the AATSR METEO product from 19th August 2002, the AATSR validation team started to receive AATSR data at the end of October 2002. Data received at this time covered the period from 1st September 2002 onwards. Many PIs did not have the appropriate AATSR data to match in situ measurements, and therefore were unable to attain validation results in time for the Validation Workshop.

To date, AATSR data from 1st September to 2nd January 2003 have been received, although there are data gaps within this time series. Data from earlier in the mission are gradually being provided for specific validation activities.

4.2.2 Algorithm verification To date, algorithm verification has concentrated on 8 AATSR products, comprising the L1b (GBTR), L2 (GST and AST) and METEO products from two orbits from May 21st. As Birks details in his algorithm verification papers for the calibration review and the validation workshop, there have been a number of minor anomalies identified through algorithm verification. Many of these have not affected validation activities, and have since been corrected in both the OP and the PP. There are a number, however, which have been significant for validation.

Affecting all SST products

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Misalignment of nadir and forward views: One set of parameters that cannot be defined pre-launch is that which corrects for misalignment between satellite and instrument axes. These misalignment parameters must be determined in-orbit to ensure correct geolocation and collocation of forward and nadir views. The pre-launch version of the relevant auxiliary file was in use until 14th November 2002, when the optimised parameters were defined. Dual view SST products processed before this date will therefore be incorrect due to this misalignment; the magnitude of the error will be highly variable. This data will need reprocessing using the new auxiliary file before an accurate validation can be done. The impact of poor misalignment on the spatially averaged METEO product is minimal.

Affecting the AST product

The 17km AST confidence word is anomalous and contains incorrect values.

The 17km dual view SST on ascending arcs differs from corresponding nadir SST by an unrealistically large amount and is clearly invalid. BT differences (nadir-dual) between 5.41 and 19.68 K are observed in one section. PIs were informed of this problem on the 12th November, and advised to use the 10/30 arc minute cells, or, in the case of the 17/50 km cells, the nadir-only retrievals, until further notice.

Both of these problems are under investigation, and were corrected in a processor update in February 2003.

Affecting the AST and METEO products

The latitude/longitude of the 10 arc minute sub-cells was found to be calculated incorrectly when either is negative. The sub-cell, which is a small quadrilateral 10 arc minutes on each side, should be identified by the co-ordinates of its south-west corner, but when either of the coordinates is negative, the co-ordinates of a different corner are computed. The maximum error of a single sub-cell is therefore 10 arc minutes in each co-ordinate. It also follows that the two adjacent sub-cells may be assigned the same coordinates when their common boundary is either the equator or the meridian.

It is expected that since, in general, the SST field changes very little over 10 arc minutes, the impact on validation results will be minimal.

The problem was traced to a simple coding error and corrected in the operational processing facility from mid-January 2003.

Errors in the nadir-view solar angle dataset were found to result in some data records being treated as night time when they were in fact day time. The problem was related to the numbers used for exception values and then a failure to implement cosmetic fill of the solar angles. This problem may impact the METEO product via the cloud detection scheme, but it is unclear whether it would result in some records in the METEO product being erroneously flagged as night. The problem was resolved prior to the validation workshop.

In the METEO product, the two halves of the confidence word appear to be transposed with respect to the confidence word in the AST product. This problem is under

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investigation; when it is corrected, the impact on the BUFR form of the product must be established.

Affecting the visible channels

Calibration of visible channels: under normal circumstances the AATSR visible channels are calibrated automatically within the ground segment using visible channel calibration files (ATS_VC1_AX) provided to the PDS daily by RAL. The system for the daily provision of these files is not yet stable, so since September RAL have been providing weekly updates as an interim measure.

Unfortunately a problem with one of the auxiliary parameters associated with the VC1 file generation process arose meaning that no VC1 files could be generated between 23rd November and the 10th January. An old VC1 file (generated in July 02) was used to process all data acquired within that period, and therefore all visible channel data from this period are not very well calibrated and cannot be used for vicarious validation.

This data will be reprocessed in due course. The Reference DSDs in the SPH of each product shows which VC1 file has been used.

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4.3 Summary of validation results

4.3.1 Sea Surface Temperature validationThe main validation results from the initial validation phase come from the UK Met Office, the University of Leicester and I. Barton of CSIRO.

4.3.1.1 UK Met OfficeThe UK Met Office was the first member of the AATSR validation team to receive AATSR data, with the near real time AATSR METEO product available from 19th August 2002. Near-real time monitoring and validation of AATSR data has been ongoing since that time.

Buoy comparisonAATSR data have been matched up to buoy observations collocated within a 10 arc minute cell and coincident within 3 hours. From ~140 match-ups per week through the period, statistics show that the AATSR skin SST is within ± 0.3 K of the buoy SST (Figure 4-1). There are three exceptions, the first in September 2003, attributable to two match-ups off the coast of San Francisco and possible undetected stratocumulus cloud. The other two, in June 2003, are currently under investigation.

Figure 4-1: Time series of daily global mean difference between AATSR SST and buoy SST

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The mean difference between AATSR SST and buoy SSTs for 4081 match-ups (excluding 3 outliers) was 0.063 K with a standard deviation of 0.522 K. With just the 1863 night time match-ups, the mean was 0.076 K with a standard deviation of 0.431 K. At night a bias of between -0.1 and -0.2 K is expected due to the skin effect. Such a difference is observed between the two data sets. .

Comparison with SST analysis fieldsAATSR data have also been compared to HadISST, a globally complete 1 resolution sea-ice and SST analysis field, produced on a monthly basis. Results show that global differences are generally well below ±1 K. Global statistics are given in Table 4-1.

AATSR Skin SST – HadISSTMean SD

September 2002 -0.07 0.79October 2002 -0.08 0.77November 2002 -0.07 0.79December 2002 N/A N/AJanuary 2003 N/A N/AFebruary 2003 -0.038 0.702March 2003 -0.023 0.728April 2003 -0.001 0.717May 2003 -0.032 0.744June 2003 -0.001 0.817Table 4-1: Monthly mean AATSR SST minus HadISST

Inter-algorithm comparisonIn the day AATSR SST is calculated using the 11 and 12 m channels. The 3.7m channel is excluded as it is contaminated by solar radiation. At night, either a 2-channel or 3-channel algorithm can be used. To investigate the difference between the algorithms, the dual view 2-channel SST was compared against the dual view 3-channel SST at night.

Results showed that the dual view 2-channel SST was ~0.2K cooler than the dual view 3-channel SST. This is consistent with results found for a similar comparison made with ATSR-2 data1 and appears to result from constraints in the accuracy of radiative transfer modelling, coupled with a non-linearity in the relationship between brightness temperature and atmospheric water vapour. The difference between the 4- and 6- channel SST values has been highlighted by the validation team to the recently formed Quality Working Group. An ongoing investigation into the causes and a solution is well-underway.

Summary Comparison of AATSR SSTs from the METEO product to in situ data from buoys and HadISST indicates that both the instrument and pre-launch retrieval algorithms are performing well. In the global mean, AATSR SSTs show excellent agreement to in situ data, to within the specified 0.3 K.

1 L. Horrocks, 2002, 'The 4-channel versus 6-channel debate'. Summary document for the meeting of the ATSR Product Control Board, 4 September 2002

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4.3.1.2 University of LeicesterThe spatially averaged AATSR SST product has been compared with SST data from other sensors MODIS, ECMWF and TMI for September 2002. Comparisons of global monthly mean data are made at half-degree resolution.

Mean Difference (degrees K)

Standard Deviation

AATSR/TMI -0.38 0.55AATSR/MODIS -0.41 0.75AATSR/ECMWF -0.05 1.43

Table 4-2: Comparison of AATSR SST against SST measured by other sensors

The results from the comparisons are shown in Table 4-2. Before conclusions can be drawn about the accuracy of AATSR, more results are needed and the accuracy of these other datasets must be investigated. This further analysis will be part of the ongoing validation in Phase E. However, in order to test the algorithms, a comparison of ATSR-2 and other sensors has been performed. These results are presented in Section 5.2.

4.3.1.3 Barton (CSIRO)The first validation results from a precision radiometer, the DAR011, come from Ian Barton of CSIRO. Table 4-3shows results from validation points obtained on cruises undertaken in Australian waters in September/October 2002, and February 2003. Further validation points have been made available but Ian Barton has requested they not be used until a further qualification has been applied.

The results show excellent agreement between AATSR SST and measured in situ skin SST, well within the design specifications of ±0.3 K.

Datedd/mm

Time(local)

Bulk SST(OC)

Skin SST(OC)

AATSRSST (OC)

AATSR-Skin(OC)

23/09 2208 24.65 24.55 24.39 –0.1621/10 2227 25.92 25.86 25.82 –0.0425/10 2210 26.98 25.76 25.83 0.07

Table 4-3: AATSR SST match-ups between ship (bulk) and radiometer (skin) data, collected by I. Barton

4.3.1.4 Other validation activitiesAdditional in situ measurements have also been collected by a number of validation PIs. Estimates (and actual returned results) for the number of possible validation points collected in the initial validation phase are given in Table 4-4. Further validation results will be obtained using these in situ data before the second workshop when appropriate AATSR data have been received and analysed.

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PI Activity Estimated (actual) number of validation points collected in initial validation phases

Barton DAR011 specific cruises ~ 13 (3)Perth radiometer TBCTownsville radiometer TBC

Minnett Caribbean cruise liner ~ 15Arctic ~ 4-6Mediterranean ~ 3-4Seattle to Sydney 0

Nightingale Indian Ocean Cruise ~ 4 (4)Robinson/Donlon Val de Loire ~ 37 (1)

Pride of Bilbao 0Table 4-4: Number of in situ validation points collected by precision radiometers during the initial validation phase

A plot of the total number of validation match-ups returned from the precision radiometers is shown in Figure 4-2. The data in Figure 4-2 is provided by Barton, Robinson/Donlon & Nightingale. Further data points will be obtained over the next few months as the data backlog is cleared; data from the MAERI instrument will be added following agreement with the University of Miami However, the limited variation in absolute SST should be noted, with only one point occurring outside a small band between 24° C and 28° C; such a limited range constrains the significance of the results and the ongoing validation should seek to expand the range of absolute SST measurements that are validated.

1012141618202224262830

0 2 4 6 8 10 12

Sample number

SST

(°C

)

Radiometer - Single AATSR - Single Radiometer - <10 kmAATSR - <10 km Radiometer - <20 km AATSR - <20 km

Figure 4-2: Comparison of precision radiometer validation match-ups

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A plot showing the observed differences is presented in Figure 4-3. The r.m.s. difference of the measurements is calculated as 0.12° C.

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0 2 4 6 8 10 12

Sample Number

Diff

eren

ce (°

C)

Single Pixel <10 km <20 km

Figure 4-3: Difference between AATSR and precision radiometer SST values over match-up points.

Details of the MAERI comparison, not included in the synthesis as the results are still being investigated, are presented in Section 5.1; details of the ISAR comparisons are presented in Appendix 2.

4.3.2 Visible/near infrared channel validation The validation of the visible channel reflectance measurements and the IR brightness temperatures is ongoing. Four methods are currently being pursued:

1. Vicarious validation of visible channel reflectance’s over desert sites and Greenland2. Validation of brightness temperatures over Lake Tahoe, USA3. Inter-comparison of uncorrected visible reflectance measurements with similar

measurements from other sensors including, ATSR-2, GOME and SCIAMACHY.4. Validation of visible measurements using cloud targets.

The onboard AATSR Viscal unit is reported as operating normally, and the application of the visible calibration algorithm in the processor has been thoroughly checked and is reported as correct. The Viscal unit on the previous instrument, ATSR-2, showed high stability until the ERS-2 gyro failure in early 2001; drifts of 2% - 4% over the mission lifetime (since April 1995) have been observed.

The main conclusions from the visible channel validation are: AATSR agrees well with MERIS over desert sites Significant differences between AATSR/MERIS and ATSR-2 are observed over

desert sites in the order of 8% - 13% depending on the channel.

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Results from the inter-comparison show lower discrepancies and better agreement between ATSR-2 and AATSR. However, only a limited number of comparisons have been performed at present owing to the delay in issuing SCIAMACHY data.

Two different conclusions have been presented on any differences between ATSR-2 and AATSR. Reasons for the difference in opinion have not yet been established with the more traditional method of vicarious validation showing a large offset. Any difference between ATSR-2 and AATSR is likely to affect the credibility of producing a long-term SST record even thought the visible channel data does not affect the SST retrieval. It is imperative that the differences, if real, are fully characterised and understood; this objective should be a prime work area for the ongoing validation programme.

Details of the work carried out in this area by RAL are presented in Appendix 1.

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4.4 Conclusions and Recommendations September 2003

There are a number of interim conclusions that can be made at this stage of the AATSR validation programme. It can be said that: -

There are no issues of serious concern at this stage about the global accuracy of AATSR data based on the validation data collated so far. However, certain limitations in the current validation data are apparent and need addressing during Phase E.

Data collection activities in the initial validation phase are near completion, but owing to the delay in data distribution, several important radiometer match-ups are still awaiting analysis.

Validation of the AATSR METEO product from August-June 2002 against buoy data and SST analysis fields, demonstrates that AATSR retrievals of SST are accurate to within 0.3 K globally.

Validation results from I. Barton using the DAR011, confirm these results, showing an agreement between AATSR SST data and in situ skin SST data of <0.2 K. Results from I. Robinson & C. Donlon show similar agreement at a different absolute SST.

Dual view 2-channel SST retrievals are ~0.2 K cooler than dual view 3-channel SST retrievals, as expected and observed previously for ATSR-2. Reasons for the difference are currently being investigated.

The skin effect is seen in the night time comparison of AATSR SST data with buoy data.

An unexpected bias between dual view and nadir SST data during daytime has been reported for both AATSR and ATSR-2, a result which is under investigation. The effect is less than 1 K globally.

Results indicate that the visible/near infrared channels may be measuring reflectance’s which are higher than ATSR-2. Further results are needed, however, before definite conclusions can be drawn.

It is important to establish continuity between the ERS-2 and Envisat missions. Every effort must be made to ensure that accurate ATSR-2 data are available in the overlap period to enable cross validation between ATSR-2 and AATSR to be performed. It is also important that validation of AASTR is performed with an in situ instrument which successfully validated ATSR-2 to ensure a good transfer standard.

Validation must continue throughout the mission performing seasonal validation, regional validation, long term monitoring and the validation of new products. In particular, the team are identifying geophysical regimes for regional validation and recognise the importance of validation over a range of SSTs. A validation plan is in place for Phase E to achieve these aims and will be developed further by October

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The quality of AATSR data strongly suggests that other products from AATSR will be invaluable in climate research conducted by the Hadley Centre and other agencies. Validation of a prototype land surface temperature product is underway. Other products such as cloud/aerosol and vegetation products should be pursued.

At the first validation workshop, in December 2002, the AATSR validation recommended that:

AATSR data should be released on the understanding that the validation programme and product verification are still incomplete.

In order to complete the AATSR validation: High priority must be given by ESA to the processing of the May-August 2002

AATSR match-up data relevant to validation campaigns. By 31 July 2003 most of outstanding data has been distributed

AATSR data for validation must be reprocessed to correct for known processing anomalies including geo-location and other SPR’s that are now closed

A further validation workshop will be is held in October 2003 to close the initial

validation phase

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5 Specific Validation Activities at the University of LeicesterThe University of Leicester has performed AATSR validation in two areas. First, point-to-point match-ups between the MAERI radiometer, operated by the University of Miami, and the AATSR have been performed. Concurrent match-ups between September 2002 and February 2003 have been analysed to date, with the results showing a slightly higher positive bias than expected. Consequently, the MAERI data is not included in the synthesis presented earlier until the analysis has been reviewed by the University of Miami. This review will be performed before the second workshop in October 2003.

Secondly, the University of Leicester has carried out global analysis of AATSR data with other sensors, including, AVHRR, TMI & MODIS and also with ECMWF reanalysis data. Initially, one month of data has been analysed, for September 2003. Further analysis is proposed for the ongoing validation activities as the single month is sufficient to show the clear benefits of this type of analysis. Additional analysis of several years of ATSR-2 data was used to help interpret the AATSR analysis by clarifying annual and seasonal effects.

5.1 Validation using the MAERI Radiometer

5.1.1 IntroductionThe University of Leicester, in collaboration with the University of Miami has begun to analyse a long time series of precision data from the MAERI radiometer. The MAERI is a Fourier-Transform Infrared Interferometer, operating in the infrared from ~3 to ~18µm at a resolution of ~0.5 cm-1. It uses two infrared detectors to achieve this wide spectral range, and these are cooled to ~78oK by a Stirling cycle mechanical cooler to reduce the noise equivalent temperature difference to levels well below 0.1K. The MAERI includes two internal black-body cavities for accurate real-time calibration.

The radiometric calibration of the MAERI is done continuously throughout its use. The absolute accuracy of the infrared spectra produced by the MAERI is determined by the effectiveness of the black-body cavities as calibration targets. During construction, the black-body thermistors are calibrated against thermometers traceable to NIST (National Institute of Standards and Technology) standards. Typical results of the measurements in two clear parts of the spectrum, from each of the two MAERI detectors, are shown in Table 5-5.

Target Temp. LW(980-985 cm-1)

SW(2510-2515 cm-1)

20oC +0.013 K +0.010 K30oC -0.024 K -0.030 K60oC -0.122 K -0.086 K

Table 5-5: Laboratory tests of M-AERI accuracy

The three MAERI radiometers have been deployed on a number of cruises in the past several years that span a wide range of conditions, as shown in Figure 5-4. Most of these deployments are on research vessels; the tracks of two cruises that have been made since the ENVISAT launch, and which will be used for AATSR validation are shown in Figure 5-5.

Figure 5-4: Cruise tracks of M-AERI deployments

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Figure 5-5: Tracks for MAERI cruises on research ships since the launch of Envisat. Left – in the Arctic on the Canadian Coast Guard Icebreaker Pierre Radisson (September 23 - October 18, 2002. Right – in

the Mediterranean Sea on the Italian research vessel Urani

An important MAERI installation for the analysis presented here is on the Royal Caribbean Cruise Lines vessel Explorer of the Seas (http://www.rsmas.miami.edu/rccl/), which operates a weekly cruise schedule from the Port of Miami, and on which a M-AERI has been installed since November 2000. The M-AERI operates continuously while the ship does two alternate circuits round the Caribbean (Figure 5-6) on a bi-weekly basis, leaving Miami on Saturday evenings and returning the following Saturday morning.

In all cases the MAERI instruments are mounted so that the measurements are taken beyond the influence of the ship (Figure 5-7). For the Explorer of the Seas deployment, the skin SST is retrieved form the measured spectra in real-time and transmitted to RSMAS via a satellite link within about ten-minutes. These data are sent to the University of Leicester on a daily basis for inclusion in the AATSR SST validation procedure, and for onward transmission to the NILU data base. In other cases the data are treated off-line, post-cruise.

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Figure 5-6: Tracks of the Explorer of the Seas. The western (left) and eastern (tracks) are done on alternate weeks

Figure 5-7: MAERI installed on the Pierre Radisson (top left), the Urania (top right) and the Explorer of the Seas (left). In all cases the sea-viewing measurements are taken ahead of the bow wave and the influence of the ship

5.1.2 ResultsThe following procedure is used to carry out the analysis. The ESOV orbit overpass tool is used to determine which orbit files contained possible spatial match-ups. The output of this program contains a list of all the possible partial spatial match-ups, using the absolute orbit number and UTC date and time to reference the appropriate file. The appropriate AATSR media containing the relevant orbits is then located for data extraction. Temporal and spatial match-ups are then produced accordingly. Here, a match-up is defined as measurement made within +/- approximately 60 minutes and is within the swath width of the AATSR instrument. In addition, a proximity check is included to ensure the measurement was no more than 0.01

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degrees from the position of the point of coincidence; this serves two purposes, first it prevents the match-up from being ridiculous distances from the position of coincidence and secondly ensures that the match-up isn’t close to the edge of the swath width.

The match-up files are determined to be valid if they contain a cloud-free value, for either the nadir or dual SST, in closest pixel of any of the blocks. Furthermore, there is an additional condition applying to the MAERI data; the match-up must contain a valid (non-exception value) for the MAERI skin SST. Results are produced for data within +/- 60 minutes of the point of coincidence and those within +/- 15 minutes of coincidence; results are produced from a single match-up file, with the exception of the specific pixel information within the 3x3, 5x5 and 7x7 blocks, on one line.

Orbit files from the ESA AATSR distribution have been extracted for the period from the 1 st

September 2002 through to 30th June 2003, all these files have been checked for temporal and spatial match-ups and it was found that the following dates contained spatial and temporal match-ups (Table 5-6):

Date of Match-ups Number of match-ups for that date20020903 1120020913 1120020915 520020916 1120020918 1120021125 1120021127 1120021130 1120021201 1120021202 1120021220 320030102 1120030121 1120030203 1120030205 1120030208 1120030210 1120030213 1120030226 1120030311 1120030313 320030401 1120030413 320030414 1120030416 1120030419 1120030420 1220030421 1220030424 1120030507 1120030520 11

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20030522 120030524 1120030606 1120030610 1120030623 1220030625 1120030628 1120030629 1120030630 12TOTAL number of days with match-ups TOTAL number of match-ups37 406Table 5-6: Table 1 – Shows the number of temporal and spatial match-up files associated with each date, as well as the totals at the bottom of each column.

All the above match-ups have been run through the analysis tools, which under the criteria specified above have produced the following results:

Date of valid match-ups Number of valid match-ups for that date20020903 1120021125 1220021201 1120021202 1120030102 320030205 1120030210 1120030213 1120030226 1120030311 520030313 120030414 1120030416 1120030420 1220030424 1120030522 120030630 12TOTAL no. of valid days with valid match-ups TOTAL number of valid match-ups17 153Table 5-7: Table 2 – Shows the number of valid temporal and spatial match-up files associated with each date, as well as the totals at the bottom

The following tables, Table 5-8 and Table 5-9, contain summaries of the above analysis in terms of the number of days with valid match-ups and the actual number of valid match-up files; it shows the low probability of finding a match-up:

Total no. of orbit files in time period

Partial spatial match-ups

Spatial + temporal match-ups within +/- 60 mins

Cloud free spatial and temporal match-ups within +/- 60 mins

Cloud free spatial and temporal match-ups within +/- 15 mins

4324 727 406 153 35

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Table 5-8: Table 3 – Shows a summary of the total number of files at different points in the analysis

Total no. of days in time period

No. of days containing partial spatial match-ups

No. of days containing spatial + temporal match-ups within +/- 60 mins

Cloud free days with spatial and temporal match-ups within +/- 60 mins

Cloud free days with spatial and temporal match-ups within +/- 15 mins

303 303 40 17 15

Table 5-9: Table 4 – Shows a summary of the total number of day containing match-ups or possible match-ups at different points in the analysis.

Each day of analysis is then plotted graphically, as shown in Figure 5-8.

Figure 5-8: Example AATSR-MAERI analysis for

The main features of the analysis are:1. The plot shows the Skin SST measured (in Kelvin) by MAERI (black crosses) and

AATSR (red crosses), for the closest pixel, against the difference in time between the measurements (in decimal minutes). The plot provides an idea of how the two sets of measurements are varying as the time difference converges on coincidence and then diverges.

2. This plot is a variation of the first, showing the difference between the measured SST’s, MAERI SST – AATSR Dual SST, (in Kelvin) for the closest pixel, against the time difference between measurements. This plot improves the users ability to see how variable the difference between the measurements is as the two measurements approach and then move away from coincidence.

3. The third and fourth plots deal with the 3x3 block measurements. The bottom left plot shows the difference between the MAERI SST and the mean of the valid AATSR

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Dual SST’s (in Kelvin), averaged over the 3x3 block, plotted against the time difference between measurements (in decimal minutes). This provides a better picture of how variable the surrounding SST’s are during the measurement, and can also be used to provide an indication of whether the closet pixel measurement is a good indicator of the SST in that area.

4. The bottom right plot shows the difference between the MAERI SST and the highest valid value of the AATSR Dual SST (in Kelvin), within the 3x3 block, against the time difference between measurements (in decimal minutes). This plot simply highlights how well, or not, the highest SST in the block characterises the SSTs in the block.

In addition to the analysis presented above, summaries covering the entire match-ups to date are produced. The analysis is shown in the following figures:

Figure 5-9: Comparison of MAERI and AATSR Differences with MAERI Standard Deviation

Figure 5-9 shows the difference between the MAERI skin SST and the AATSR Dual SST for the closest pixel (in Kelvin) plotted against the standard deviation of the MAERI measurement. The colour of each point has been used to represent the corresponding AATSR Dual SST of the closest pixel. The graph also includes two red lines that indicate the target accuracy of the AATSR instrument (+/- 0.3K). The purpose of this graph is to highlight which closest pixel measurements fall outside the target accuracy and whether there are any trends in the SST’s measured by the AATSR instrument.

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Figure 5-10 represents the same data as Figure 5-9, except that here the colour is used to represent the standard deviation of the valid pixels within the 3x3 block. The reason behind this being that this indicates the state of the SST in the immediate area surrounding the closest pixel, and that it is not practical to apply a statistical analysis to a single pixel.


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The third graph, Figure 5-11, is essential the same as the first two, but here each colour is used to represent the different dates of the match-up. This graph can be used to ascertain if the are specific days in which anomalies arise. The plots 2D plots of the SST for the surrounding area can subsequently be used to gain a greater insight into why these anomalies are occurring.

Figure 5-12: Comparison of MAERI and AATSR Differences with MAERI Standard Deviation for 3*3 Block

Figure 5-12 shows the standard deviation in the 3x3 block for the AATSR Dual SST’s against the standard deviation of the MAERI skin SST measurements, the colour representing whether the point is difference of the MAERI skin SST and AATSR Dual SST for the closest pixel is within the target accuracy of the AATSR instrument. Blue representing points SST differences greater than or equal to +/- 0.3K, and red indicating those outside of this. The graph was produced with the hope that this would help identify any trends in those pixels that were within and outside the target accuracy – as can be seen, no obvious trend was observed.

A fifth way of representing the data is shown in Figure 5-13, a variation on the previous graph, except that in this case the colour is used to represent the difference between the MAERI skin SST and the AATSR Dual SST for the closest pixel (in Kelvin). The purpose of this graph was to explore any trends that might exist between these three variables.

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Figure 5-13: Comparison of MAERI and AATSR Differences with MAERI Standard Deviation for 3*3 Block

From these graphs, in particular Figure 5-12 the user can see which dates contain match-ups that don’t fit with any general trend that might exist. Possible explanations for this anomalous behaviour can be found by looking at Figure 5-10and Figure 5-11, which show that the standard deviation of the 3x3 block, an indicator of the variability of the SST in that region, is higher or lower than usual. Furthermore by looking at the 2D plots of the SST in the surrounding region, with the MAERI cruise track over-plotted, i.e. Figure 5-14; the influence of cloud formations can be more easily determined the cloud is thought to be the white areas in the image. These 2D SST maps provide a greater understanding of the view of the SST from AATSR, hence providing possible explanations for large differences in the measured SST’s.

Figure 5-14: Plot of Cruise track across AATSR swath

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5.1.3 SummaryPoint-to-point validation match-ups between the AATSR and the MAERI radiometer onboard the Royal Caribbean Cruise Ship, MV Explorer of the Seas, have been performed for the period from September 2002 to February 2003. A positive bias of 0.33 K is observed between the two data sets, slightly higher than the required accuracy of AATSR; such a bias is somewhat indicative of a slight regional bias. A bias of this type could be offset by widening the area covered by the data set. A series of statistical tests was applied to the data, but no obvious pattern was observed. Therefore, before the results are synthesised with the other radiometers match-ups, they are being scrutinised by the RSMAS team in Miami and will be finalised for the upcoming workshop.

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5.2 Global and Regional Comparisons of AATSR with other sensorsRemote sensing of the Earth by satellites allows the surface sea temperature (SST) of the oceans to be measured to within 0.3 K. However, there are distinct global variations in the measurements made by the different satellites, and it is necessary for these variations to be investigated to gain a better understanding of the climatic variation, and the ways in which remote sensing can be affected by specific atmospheric conditions. The main focus of this analysis is to categorise different areas of the globe by direct comparison of AATSR with other SST data sets.

An important aspect of any analysis of this type is the removal of annual and seasonal biases. Therefore, the initial work has concentrated on a comparison of ATSR-2 data and a single test month of AATSR; the ATSR-2 is used to interpret the AATSR comparison.

Areas were chosen that attempt to subdivide the oceans into: Areas with good agreement between the satellites Areas with seasonal variation Validation areas, where the accuracy and precision of the AATSR satellite is being

assessed

The analysis involved plotting mean SST difference over time and performing a histogram showing the distribution of temperature differences within each area for a chosen month. The data sets used in the comparison are AATSR, ATSR-2, AVHRR, MODIS, ECMWF analysis fields and TMI data. The month of September 2002 was chosen for the analysis of AATSR data. The data sets are compared as monthly means, with a spatial resolution half degree. The areas chosen for the analysis are shown schematically in Figure 5-15 and their respective longitudes and latitudes are show in Table 5-10.

Figure 5-15: Geographical map of areas chosen for analysis

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Box no. Longitude Longitude Latitude Latitude min max min max

0 50 80 5 25 1 -130 -90 -30 0 2 -160 -130 -40 -10 3 -90 -68 -40 -10 4 -135 -108 10 30 5 -100 -68 -9 7 6 -90 -68 -40 7 7 -100 -62 18 30 8 -180 -130 30 55 9 -40 -15 47 6210 -12 5 45 6011 7 28 50 6512 -95 -63 50 7013 -10 42 30 4714 34 44 10 2715 26 50 -36 -1216 5 100 -52 -3817 -55 5 -55 -4018 -60 15 -10 3219 -5 15 -23 -1020 55 70 10 2521 80 98 5 2322 90 127 -10 823 128 150 -12 224 130 144 -20 -1025 100 118 -35 -2326 140 180 10 2127 120 180 30 6028 -60 -10 5 3229 -30 15 -10 1030 3 18 -34 -2331 -40 0 -30 -1032 -180 -144 -10 1033 -80 -53 29 4934 60 90 -32 -1035 -45 -10 33 4636 130 150 30 6037 -150 -90 -5 538 -8 -2 42 48

Table 5-10: Latitude-Longitude Co-ordinates of Areas shown in Figure 5-15

5.2.1 ATSR-2

As detailed above, analysis of ATSR-2 was carried out in order to interpret the AATSR results, particularly if seasonal biases were noted. One example, for box 0, positioned over the Indian sub-continent is shown in Figure 5-17.

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Figure 5-16: Comparison of ATSR-2 minus AVHRR SST for July 2000

Regional images are also produced and two examples of these are shown in Figure 5-17 (a), which shows the area from box 0 around India and Figure 5-17 (b), which shows a zoomed in view of the same region and which represents box 20.

(a) (b)

Figure 5-17: Regional Comparison of ATSR-2 minus AVHRR SST for July 2000. The area shown in (a) refers to box 0, with (b) showing a reduced area called box 20.

Figure 5-18: Time-series of monthly averaged ATSR-2 minus AVHRR over the box 0 region from January 1999 to January 2001

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Figure 5-19: Time-series of monthly averaged ATSR-2 minus AVHRR over the box 20 region from January 1999 to January 2001

The results presented in Figure 5-17, Figure 5-18 and Figure 5-19 show clear seasonal biases in the differences between the ATSR-2 and AVHRR data sets. The remarkable similarity between Figure 5-18 and Figure 5-19 shows that the differences observed over the smaller region (box 20) dominate the differences observed over the larger region (box 0).

A possible reason for the differences can be gleamed from looking at the TOMS aerosol optical index over the region for the same time period.

Figure 5-20: TOMS Aerosol Optical Index for the region covered by box 20 for January 1999 to January 2001

An initial visual inspection of Figure 5-19 and Figure 5-20, show that periods of high aerosol content in the TOMS aerosol index data, correspond to the larger differences observed between ATSR-2 and AVHRR. This conclusion is further verified statistically in Figure 5-21.

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Figure 5-21: Comparison of TOMS Aerosol Optical Index and Observed ATSR-2/AVHRR differences

5.2.1.1 Summary of ATSR-2 AnalysisSimilar analysis to that shown for box 0 and box 20 has been performed for all the boxes in Figure 1. [Note: The results for all the boxes were originally included in this report but have been removed to reduce the size. They are available on request to the AATSR validation scientist.] Various factors are coming into play to produce these SST differences. These factors appear to include aerosols, water vapour and clouds. Certain areas appear to be influenced more strongly by one particular source of SST difference rather than another. Aerosols may be affecting areas around Africa/India and precipitation/clouds may be affecting the Indonesian region. Positive SST differences tell us ATSR2 has a higher SST measurement, negative differences mean AVHRR is measuring a higher SST. The meaning of these differences in relation to the atmospheric corrections employed by the satellites will require further analysis.

5.2.2 AATSRSpecific IDL code has been developed at Leicester to compare global monthly mean SST values from AATSR data to data from ATSR-2, AVHRR, MODIS and ECMWF analyses. As in the ATSR-2 analysis, a global plot of the mean difference is produced, together with a subset of a particular area of interest (which can be specified) and a table showing the average monthly difference, variance and correlation. Analysis over several months can show the relative seasonal variation in the difference field of the retrieved SST fields between two sensors over the whole globe or in a particular region.

Although a comparison of AATSR with other sensors may indicate more about the inadequacies in other datasets than the accuracy of AATSR, it is important to ascertain in which parts of the globe, the AATSR SST retrieval compares well with other sensors and to isolate regions of significant discrepancies for targeted in-situ campaigns. Incorporation of in-situ measurements should be targeted at indications of which sensor is more accurate in retrieving SST in a given location, but also be capable of diagnosing possible reasons for problems in SST retrievals, e.g. surface conditions or atmospheric variability

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The first comparison, between AATSR and ECMWF in shown in Figure 5-22:

Figure 5-22: Global difference between AATSR and ECMWF SST values for September 2002

The statistics of the data presented in Figure 5-22 are: Mean global difference: -0.05 deg K Mean absolute difference: 0.88 deg K Standard deviation: 1.43 deg K Tropical region (+/- 30 degrees)

• Mean difference: -0.40 deg K• Standard deviation: 0.88 deg K• Percentage of values between +/- 0.6 deg K: 54.82

Comparisons with ECMWF show that AATSR and SST from ECMWF analysis fields are generally negligible. However, we do expect the measured skin to be slightly cooler than the ECMWF analysis. The fact that there is no difference suggests that maybe AATSR is marginally too warm. Further months of data are needed before this can be confirmed in case seasonal biases are present.

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The second comparison, between AATSR and MODIS, is shown in Figure 5-23:

Figure 5-23: Global difference between AATSR and MODIS SST values for September 2002

The statistics of the data presented in Figure 5-23 are: Mean global difference: -0.41deg K (MODIS warmer) Standard deviation: 0.75 deg K Tropical region (+/- 30 degrees)


The mean global difference is –0.41 degrees indicating that MODIS is generally reading warmer than AATSR. There is quite a large standard deviation of 0.75 degrees. If you just take the tropical region (half the total number of pixels), the mean difference is the same as the global difference.

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The final comparison, between AATSR and TMI, is shown in Figure 5-24:

Figure 5-24: Global difference between AATSR and TMI SST values for September 2002

The statistics of the data presented in Figure 5-24 are: Mean global difference: -0.38 deg K (TMI warmer) Mean absolute difference: 0.50 deg K Standard deviation: 0.55 deg K Tropical region (+/- 30 degrees)


The comparison of AATSR and TMI shows that TMI is slightly warmer – generally half a degree difference.

One further point for consideration is that generally, the AATSR dual view is expected to be cooler than nadir view in some areas at certain times, including:

Biomass burning off west coast of southern Africa Aerosol from Sahara dust storms

These areas are clearly visible in Figure 5-25, which shows a comparison of AATSR dual minus nadir SST values for September 2002.

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Figure 5-25: Comparison of Dual View minus Nadir only SST values for September 2002

A simple way of testing to see how much of an influence the dual view has on the observed difference sis to compare the MODIS and TMI data with dual view and nadir only SST values. The results from this simple analysis are shown in Figure 5-26:

Figure 5-26: Comparison of AATSR Dual View and Nadir Only SST Values with MODIS and TMI for September 2002

It is noticeable that if you use the AATSR nadir only SST instead of the dual view SST then the comparison with MODIS and TMI is significantly improved.

5.2.2.1 Summary of AATSR analysisThe comparisons were performed for AATSR data and SST from ECMWF analyses, MODIS and TMI data, for September 2002. It is difficult to see patterns with one month’s data and more data and processing needed, including comparisons with AVHRR and ATSR-2. The comparison between AATSR and ECMWF analysis showed negligible difference, which may

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AATSR minusTMI

AATSR minus MODIS


indicate that AATSR slightly too warm. The comparison with AATSR and both MODIS and TMI data showed cooler SST values from AATGSR. However, it is not clear how accurate are TMI and MODIS against buoy data, as this information is not available. It is noticeable that the comparisons between AATSR with MODIS and TMI show better agreement with nadir view only SSTs from AATSR.

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6 Management Activities & Reporting

6.1 ManagementManagement activities prior to September 2002 have been described in detail in the reports sent to DEFRA. Since September 2002, the main management activities performed by the Validation Scientist (VS) have involved:

Strategic co-ordination and managementThe VS has continued to coordinate and manage the AATSR validation programme, involving 15 projects with international scientists from over 10 institutions for validation of AATSR operational products; in addition there are 6 projects dealing with the prototype land surface temperature product. This has involved numerous communications with validation data providers ESA, members of the Flight Operations Support (FOS) team, and the entire worldwide team of validation PIs.

Representation of AATSR within the Envisat cal/val programmeIt is a pre-requisite of the project that AATSR is represented within the Envisat forum to ensure that AATSR needs and requirements are highlighted and addressed, and to deliver the results of AATSR validation prior to public release of the data. The PI on behalf of the VS made presentations on AATSR validation at the 24th International Geoscience and Remote Sensing Symposium in Toulouse, in July 2003. In addition the VS has been activiely invlvoed in the organization of the second MAVT workshop in October 2003.

Primary interface between the validation team, ESA, FOS and DEFRAThe VS has acted as the primary interface between the validation team, ESA, FOS and DEFRA. Monthly reporting has been carried out since September 2002; the VS collates reports from the PIs and sends an update to ESA who use this information in their Envisat cal/val reports. The VS also informs the validation PIs when there are problems with the AATSR instrument.

Other topics covered through working level communication between VDPs and ESA have been:

VDP data requirements. AATSR data requests need to be made to ESA. This involves specifying the geographic location and dates of in situ measurements and the data products required.

Data distribution. This has been particularly relevant, as the VS and ESA have been working together to formulate strategies in the light of the delay in data distribution. This has included making sample AATSR data products available to the validation team for familiarisation purposes.

Access to ATSR-2 data. Cross validation of ATSR-2 and AATSR is a requirement of the validation programme. The VS has been working with VDPs, NERC and RAL to define data requirements and ascertain the availability of ATSR-2 data to the validation team.

Co-ordination and management of the land surface temperature validation team As a prototype product, coordination and management of the land surface temperature validation team has taken a secondary role to that of operational AATSR products. However, it is still considered an important task. The VS has continued ongoing management of the prototype LST product.

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Collaboration with other programmesKey to the long-term success of the validation programme is the integration of the AATSR validation programme with other scientific programmes. In situ measurements for validation are being carried out worldwide for the validation of other satellite sensors (such as MODIS, ASTER, AVHRR). The cost-benefit to all programmes of joint campaigns and the sharing of in situ data is considerable.

The VS has established links with a number of other programmes, including the MSG Land SAF, MODIS and ASTER. Through this initiative, scientists from each of those programmes are now members of the AATSR land surface temperature validation team.

Cruise planning and participation The VS has continued to explore additional cruise opportunities, contacting the Principal Scientists of NERC cruises to explore the possibility of cruise participation. Regular contact is also made with the MERIS validation team.

Support of industrial programmesThe Earth Observation Group at Leicester is also involved in a project developing level 3 AATSR products (led by INDRA/ESA). The work is being carried out by staff members not supported by DEFRA, but work within the group means an increase in experience and knowledge of AATSR data.

AATSR WebsiteThe AATSR website, developed and maintained at Leicester, is an excellent means of publicising AATSR and disseminating information to the validation team and the wider scientific community. It has been utilised throughout the period but being identified as a desirable work element has meant it is not always kept up to date. The website is identified as an important area for development during the ongoing validation of AATSR.

AATSR data readingThe VS has been involved in testing and developing a number of specific tools for reading AATSR data and performing AATSR validation. These have included:

Testing of EnviView, ESOV, ASC2HDF, IDL2HDF, PDSPixels, USF, FTP links to Esrin/Kiruna, and the Envisat product loader.

Development of IDL code to read and display AATSR data Development of IDL code to compare AATSR data to ATSR-2, AVHRR, MODIS and

ECMWF.

Upload of validation data to the NILU databaseOne of the key requirements of the validation team is to upload correlative data to the NILU database. The VS and Leicester team have uploaded more than 100 data files from the MAERI system to the NILU database. This is a significant task, as the raw data files must be converted to the NILU HDF format, describing the metadata and using a conversion tool.

MAERI dataInformation on the work at Leicester on the MAERI data from the University of Miami is given in Section 5.1.

Collation and Synthesis of validation results

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An important role of the VS is the collation and synthesis of validation results from the validation team as a whole. This synthesis work is disseminated through the weekly reporting of validation PIs. Monthly validation reports for July and August/September have been produced.

As validation results increase with the wider dissemination of AATSR data distribution, this task will become more and more important, culminating in a report to DEFRA indicating the quality of level 2 AATSR and thus supporting or not, the release of data to the general public.

6.2 Work-package ComplianceA summary of the University of Leicester WP tasks and their compliance is given in the following table:

WP Number

Task Compliant

VAL1000 Promote validation activities VAL1000 Coordinate validation activities VAL1000 Maintain AATSR validation website VAL1000 Attend validation and other meetings VAL1000 Produce reports VAL1000 Pursue research interest on related topics VAL3000 Convene and Chair SAG VAL3000 Chair DQG VAL3000 Participate in CCB & PSP VAL3000 Attend FOS Meetings VAL3000 Contribute to Product Handbook VAL3000 Advise DEFRA on conclusions of commissioning report VAL3000 Liaise with project scientist VAL3000 Liaise with user community VAL3000 Liaise with Met Office/Hadley Centre VAL3000 Promote AATSR to scientific community VAL3000 Offer ad hoc advise to DEFRA as appropriate Oct 2002 Coordination of ongoing campaigns Oct 2002 Representation of ESA in ENVISAT Cal/Val programme Oct 2002 Working level communication with ESA Oct 2002 Working level communication with FOS team Oct 2002 Maintain web site

Not possible owing to extra commitment of ESA QWG

Oct 2002 Coordination of LST team Oct 2002 Explore further opportunities for AATSR validation Oct 2002 Synthesis of all validation results Oct 2002 Validation of Level 2 SST with MAERI Oct 2002 Comparison of AATSR with other sensors Oct 2002 Upload to NILU database

Small amount of data still to be

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uploadedOct 2002 Chair MAVT AATSR session Oct 2002 AATSR representation at internal ESA cal/val meetings Oct 2002 Produce first validation report Oct 2002 Weekly reporting to ESA Oct 2002 Update of AATSR validation implementation plan Oct 2002 Work as ESA project correspondent Oct 2002 Organisation with ESA of second workshop Oct 2002 Production of final validation report with error budgets

Not possible until after second workshop. Second interim report issued instead.

A summary of University of Leicester outputs and their compliance is given in the following table:

WP Number

Task Compliant

VAL1000 Summary Report on Final Data Collection Plans VAL1000 Progress Report to ESA June 2002 VAL1000 Monthly Progress Report VAL1000 Final Report VAL1000 Participation in Validation Workshop VAL1000 Contribution to DEFRA GAD Annual Report VAL3000 Minutes of SAG meetings VAL3000 Minutes of DQG meetings VAL3000 Reports for PSP meetings VAL3000 Contributions to Product Handbook Oct 2002 Interim validation report Oct 2002 Presentation for validation workshop Oct 2002 Update of validation implementation plan Oct 2002 Papers of results as appropriate

Non published so far

Oct2002 Final validation report Not possible until after second workshop. Second interim report issued instead.

Oct 2002 Validation Meeting (May/June 2003) Meeting moved to October

Individual reports on work-package compliance of the two sub-contracts are given in the relevant sub-contract report in Appendix 1 and Appendix 2, respectively.

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6.3 Reporting & Publications

During the initial validation phase, the following reports and publications have been completed. This list is not exhaustive as there may have been additional reports and publications by validation team members not circulated to Leicester.

Reports to DEFRA

By the University of Leicester: -

GAD/AATSR VAL/01 Monthly Progress Report for July 2001GAD/AATSR VAL/02 Monthly Progress Report for August 2001GAD/AATSR VAL/03 Monthly Progress Report for September 2001GAD/AATSR VAL/04 Monthly Progress Report for October 2001GAD/AATSR VAL/05 Monthly Progress Report for November 2001GAD/AATSR VAL/06 Monthly Progress Report for December 2001GAD/AATSR VAL/07 Monthly Progress Report for January 2002GAD/AATSR VAL/08 Monthly Progress Report for February 2002GAD/AATSR VAL/09 Monthly Progress Report for March 2002GAD/AATSR VAL/10 Monthly Progress Report for April/May 2002GAD/AATSR VAL/11 Monthly Progress Report for June-September 2002

GAD/AATSR VAL/Q1 Quarterly progress report, July to September 2001GAD/AATSR VAL/Q2 Quarterly progress report, October to December 2001

GAD/AATSR VAL/I-1 Interim Validation ReportGAD/AATSR VAL/I-2 Second Interim Validation Report

By Southampton Oceanography Centre: -

Robinson, I., and Donlon, C., January 2002, Progress report to 31st January 2002, 'AATSR Validation Activities: Southampton Validation Tasks'.

Donlon, C., June 2002, Progress report to May 2002, 'AATSR validation data collection using the Infrared Sea surface temperature Autonomous Radiometer (ISAR)'.

Donlon, C., August 2002, Progress report July-August 2002, 'AATSR validation data collection using the Infrared Sea surface temperature Autonomous Radiometer (ISAR): Status of operations July-August 2002'.

Donlon, C., Robinson, I., and Fisher, G., November 2002, Progress report September-October 2002, 'AATSR validation data collection using the Infrared Sea surface temperature Autonomous Radiometer (ISAR): Status of operations September-October 2002'.

Reports to Validation TeamMarch-April 2003May 2003June-July 2003August 2003

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Publications

Edwards, M.C., and Llewellyn-Jones, D., 2000, ENVISAT: The AATSR Validation Programme, RSS2000 Adding Value to remotely Sensed Data, 26th Annual Conference of the Remote Sensing Society, Leicester 12-14th September 2000.

Edwards, M.C., and Llewellyn-Jones, D., 2000, ENVISAT: The AATSR Validation Programme, Proceedings of ERS-ENVISAT Symposium, Gothenburg, Sweden, 16-20th

October 2000.Edwards, M.C., Llewellyn-Jones, D.T., and Lawrence, S.P., 2001, Monitoring global climate change from the Advanced Along Track Scanning Radiometer, RSPS 2001, Geomatics, Earth Observation and the Information Society, London 14-21 September 2001

Llewellyn-Jones, D., Edwards, M.C., Mutlow, C., Birks, A.R., Barton I.J., and Tait, H., 2001, AATSR: Global-Change and Surface Temperature Measurements from Envisat, ESA Bulletin 105, February 2001. Pp. 10-21.

Edwards, M.C., Llewellyn-Jones, D., and Tait, H., 2002, The Advanced Along Track Scanning Radiometer, Validation and early data, Proceedings of the 2002 IEEE International Geoscience and Remote Sensing Symposium and the 24th Canadian Symposium on Remote Sensing, Toronto, Canada, 24-28 June 2002,

Edwards, M.C., Llewellyn-Jones, D.T., and Smith, D., 2002, Monitoring Global Climate Change: the Advanced Along Track Scanning Radiometer. Proceedings of the 1st International Symposium on Recent Advances in Quantitative Remote Sensing Valencia 16-20 September 2002 pp.857 - 862

Noyes, E., Remedios, J.J., Llewellyn-Jones, D.T., and Edwards, M.C., 2002, Comparison of Meteosat-7 and (A)ATSR data over land: a sensitivity analysis. Proceedings of the 1st International Symposium on Recent Advances in Quantitative Remote Sensing Valencia 16-20 September 2002 pp. 656-663

Papers for the Envisat Validation Workshop

Barton, I., Pearce, A., Mahoney, M., Clementson, L., and Edwards, M.C., 2002, Validation of AATSR-derived sea surface temperature in Australian waters.

Birks, A.R., 2002, Algorithm Verification for AATSR: Level 2 Verification.

Donlon, C.J., and Robinson, I.S., 2002, AATSR Validation campaigns using the ISAR radiometer system.

Edwards, M.C., and Llewellyn-Jones, 2002, The AATSR validation programme: an overview.2

Horrocks, L.A., Watts, J.G., Saunders, R.W., and O'Carroll, A., 2002, Validation of the AATSR METEO product sea surface temperature against in situ observations and analyses.

2 A paper giving an overview of the AATSR validation programme to date, presenting similar information as given in this report.

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Nightingale, T., 2002, SCIPIO - Validation of ATSR-2 and AATSR with SISTeR.

Minnett, P.J., and Edwards, M.C., 2002, AATSR SST validation using the M-AERI.

Poulsen, C., Nightingale, T., and Watts, P., 2002, Calibration of AATSR and MERIS reflectance measurements using cloud targets.

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7 Phase E ValidationPhase E, the Exploitation phase refers to the period of the Envisat mission after the initial validation workshop. For AATSR validation, the beginning of Phase E comprised a continuation of the initial validation phase as the team worked to complete the initial validation phase objectives, as defined in the AATSR Validation Implementation Plan, PO-PL-GAD-005 (3). They are:

To determine whether the AATSR instrument is returning an acceptable global skin SST (±0.3 K).

To make an initial assessment of the quality of AATSR SST products in a limited number of sites and seasons, making timely use of the tandem ATSR-2/AATSR mission.

To assess the accuracy of the AATSR data retrieved over land.

Once the initial validation phase objectives are met, validation activities will continue, to meet the aims of an on-going programme: -

To make a detailed assessment of the quality of the AATSR SST data products in an increasing number of sites and seasons.

To monitor the quality of the AATSR data products over the duration of the mission. To validate any new AATSR data products.

As these initial and long-term objectives reflect, it is important that the validation programme carries out seasonal validation, regional validation, long term monitoring and the validation of new products. ATSR-2/AATSR cross validation will also be performed.

By addressing these objectives, the validation programme aims to provide assurance of data quality and accuracy for applications such as climate change research, investigate a varied and representative range of geophysical conditions and seasonal cycles, and monitor any instrumental drifts and other artefacts.

The necessary validation activities required to meet these aims and objectives are detailed in the following section.

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7.1 Validation

7.1.1 Organisational ActivitiesThe following organisational activities are envisaged for ongoing validation:

Co-ordination of validation activitieso Management of national and international validation teamo Collation of validation resultso Upload of correlative data to NILU database.

Involvement of further organisations and results when opportunity arises

Primary working-level interface to ESA for validation team activities, including

Primary working interface to ESA for validation team activitieso Participation in the AATSR Quality Working Group (QWG)o Daily point of contact for AATSR validation including mission planningo Monthly reportingo Data requirements for validation team

Interface of validation team to the instrument and processor teams through the FOS meetings.

Organisation of key validation meetings, for example:o October validation workshop for AATSRo Ad hoc meetings at national level to discuss key technical issues and inform

QWGo FOS deliberations.o Future validation workshops/team meetings/user meetings

User supporto web-siteo Promotion and publicityo Support of exploitation activities

Planning tasks which include a definition study for radiometer campaigns; accounting for cloud coverage and geophysical regimes; inter-instrument (satellite) differences.

7.1.2 SST Validation

7.1.2.1 Seasonal Validation Seasonal validation aims to validate over the course of at least one year to assess the accuracy of the SST retrieval over a range of environmental conditions. Activities that will provide a seasonal validation include:

Global comparisons of the AATSR METEO product to SST data from buoys and SST analysis fields, performed by the Met Office, UK. This will be performed on a routine basis as the Met Office receives the METEO product in near real time.

Global comparisons of the AATSR ASST product to SST data from other datasets (MODIS, AVHRR, TMI, ECMWF, ATSR-2), performed by the University of Leicester.

The operation of autonomous radiometers

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Regular ISAR deployments on ocean-going cargo vessels, regularly passing through areas known, as a result of the analysis activities described above, to show anomalous behaviour.

Occasional high precision measurements by the SISTeR radiometer, again in areas shown to exhibit anomalous behaviour and, of course, where cruise opportunities can be secured.

CSIRO will continue to operate radiometers on passenger ferries running on a daily basis from Perth and Townsville.

The MAERI radiometer, operated by the University of Miami, will continue deployment on the Royal Caribbean Cruise Liner in the Caribbean.

JPL will continue to validate TOA brightness temperatures using radiometers on Lake Tahoe.

Specific repeat cruises such as research cruises using the DAR011 in Australian waters

Diagnostic analysis of results from the above, especially with respect to regional and seasonal variations in performance. Also analysis of Miami data which will be provided on a regular basis from one cruise-path. The analyses will include investigation of self-consistency of AATSR data, such as dual-single-view differences and D4-D6 differences. Also carry out analysis of cloud-cover in order to assess potential efficiency of future in situ measurement campaigns

7.1.2.2 Regional validation at new locationsValidation will take place in an increasing number of geographic regions to investigate in detail a representative range of geophysical conditions and to broaden the range of SSTs that have been validated

Cruises already planned for 2003o Seattle to Sydney, November-December 2003 (MAERI instrument operated by

University of Miami)o Canadian Arctic, September 2003 for 1 year (MAERI instrument operated by

University of Miami, to be deployed for several months out of the year - to be confirmed)

o Southern Ocean, November 2003 (MAERI instrument operated by University of Miami, to be confirmed)

Additional cruises and deployments of autonomous and precision radiometers are also needed, and cruise opportunities will be sought and identified. There will be an emphasis on pre-campaign studies of SST fields to establish geophysical regimes, including water vapour and aerosol distributions, and to determine cloud coverage. Particular target areas of interest for validation include the Southern Ocean, Arabian Sea and West Africa where, for example, atmospheric correction problems due to cloud/aerosol are more challenging for the derivation of SST from AATSR.

7.1.2.3 Monitoring of long-term trendsValidation over the course of the mission will enable the monitoring of instrument drifts and the effects of anomalous environmental conditions etc. For the thermal channels, this will be carried out in Phase E by:

Global comparisons and diagnostic validation of AATSR data through inter-comparisons of data from other sensors, by the University of Leicester

Global comparisons of the AATSR METEO product to SST data from buoys and SST analysis fields, performed by the Met Office, UK.

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Autonomous instruments continuously operating over specific sites Instruments operated by JPL on Lake Tahoe Instruments operated by the University of Miami in the Caribbean on the Explorer of

the Seas.

7.1.2.4 ATSR-2/AATSR Cross validation Establishing the continuity of the ATSR, ATSR-2 and AATSR missions is important for the use of (A)ATSR data in global climate change detection. Cross validation between ATSR-2 and AATSR will be carried out during Phase E by:

Global comparisons of AATSR data and ATSR-2 data, by the University of Leicester In situ data collected from the ERS-2/Envisat overlap period using precision

radiometers. For example:o Data collected by SISTeR radiometer on the SCIPIO cruise in the Indian

Ocean, June 2002.o Data collected by the DAR011 on Australian research vessels in 2002.

There are currently no cruise opportunities identified for the SISTeR radiometer in Phase E. With a heritage of ATSR, ATSR-2 and AATSR validation, an objective of Phase E will be to identify further opportunities for AATSR validation with the SISTeR radiometer.

7.1.3 Land ProductsSmith (RAL) and Hagolle (CNES) will continue to study and compare top of the atmosphere visible (TOA) radiances from AATSR and MERIS, for a range of stable sites. Radiances will be also compared against other sensors and ground data collected by Prata (CSIRO) in Australia. These activities will achieve regional and seasonal validation of the visible channels, and through long term monitoring, will detect any instrument drifts.

At the current time, there is a validation team validating the prototype land surface temperature product. Third parties would only collect these data, but the AATSR VS would need to provide the usual coordination and communication functions. The team members currently validating the product in different locations are:

F. Prata, CSIRO (Australia, Greenland, Canada, Borneo, Spain, Sahara, Siberia, Kamchatka)

S. Hook, JPL (Lake Tahoe) C. Coll, University of Valencia (Spain) J. Stroeve, University of Colorado (Greenland) J. Sobrino, University of Valencia (Spain, Finland)

7.1.4 Other ProductsVegetation indices (NDVI), for example, are already output as an AATSR product and therefore require characterisation.Other potentially valuable new products could include aerosol/cloud physical parameters, land properties incorporating improved atmospheric correction (vegetation indices, fire products), and ice products (e.g. ice/cloud separation).

Additional activities

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There are a number of additional validation activities, which do not form part of the AATSR long-term validation plan, but fall into Phase E, running over from the initial validation phase. Although they are being performed primarily for the validation of other instruments, they still provide useful data for AATSR validation. They include:

Vicarious validation of visible/near infrared reflectances over cloud targetso This activity, carried out at RAL, started in the initial validation phase and will

continue into Phase E. The AATSR and MERIS visible and near infrared data are being compared over cloud targets.

Inter-comparison of AATSR with MERIS and SCIAMACHYo This activity, carried out at KNMI and RAL, will compare visible/near infrared

AATSR data with data from MERIS and SCIAMACHY.

Throughout Phase E, further opportunities to carry out AATSR validation will be sought. These may come from ESA Science AO's that use AATSR data or through further collaboration with the MERIS validation teams and validation teams from instruments on other platforms.

7.1.5 Analysis and ReportingThe regular reporting from the Validation PIs to the Validation Scientist that was established in the initial validation phase will continue throughout Phase E. The Validation Scientist will carry out a continual appraisal of results and will communicate these on a regular basis to the PI, and where appropriate, the instrument and software teams in accordance with agreed procedures. Systematic reviews are carried out on a regular basis within the project team and it is envisaged that wider-ranging reviews involving ESA and appropriate members of the Envisat team, covering all aspects of AATSR validation, will be carried out at appropriate intervals.

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7.2 Summary

7.2.1 Proposed activities for continued validation in Phase E

The UK Met Office will continue to validate the AATSR METEO product throughout the period.

The University of Leicester will continue to carry out validation co-ordination, analysis of MAERI data and global comparisons of AATSR SST with data from other sensors until at least the end of current contract with DEFRA in August 2003. After that work is contingent on further funding from DEFRA.

Barton has two validation cruises planned for the DAR011, in the Gulf of Carpentaria and out of Darwin. In addition there may be other 'piggy-back' validation cruises not yet defined (TBD).

One of the MAERI instruments will continue to operate on the Royal Caribbean Cruise Liner. Data collection is funded by NASA although data analysis and validation is performed by the University of Leicester in collaboration with P. Minnett, and hence funded by DEFRA. This activity will continue given the availability of funding. The other two MAERI instruments will be deployed on cruises in the Mediterranean, Canadian Arctic, from Sydney to Seattle and from Australia to Japan.

The ISAR instrument will continue to be deployed on the Pride of Bilbao given the availability of funding. ISAR may also be deployed on another ship, the Falstaff, operating between the United States and Europe (TBD).

F. Prata and S. Hook will continue to deploy autonomous instrument on sites in Australia and Lake Tahoe respectively.

CSIRO autonomous instruments will operate on passenger ferries from Perth and Townsville.

The long term monitoring of desert sites by D. Smith will continue throughout the period (funding TBC). Long term monitoring by O. Hagolle will also continue on a best efforts basis.

The AATSR validation activities of Watts and Stammes are of a finite length and expect to be completed mid-2003.

J. Nieke is carrying out validation activities mainly for MERIS but also for AATSR, had a field campaign in the Arctic for April 2003. Further campaigns beyond this one are TBC.

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7.3 Conclusions & Future Work

7.3.1 ConclusionsThe AATSR validation programme has performed very well so far, despite problems with data distribution from ESA. The data collection activities in the initial validation phase are near completion. Furthermore, preliminary analysis has been able to demonstrate that AATSR retrievals of SST are accurate globally to within 0.3 K, although much further work is required to ensure that this is the case over a longer time period and in different geophysical regimes/ ranges of SST data.

It is vitally important that the AATSR validation programme continues its momentum in the next year to complete the initial validation phase and provide the validation data and a validation report to support the use of AATSR data by the wider scientific community. A plan has been established for the implementation of this work in Phase E (Section 8) and this has been circulated to ESA for incorporation within the overall ENVISAT validation programme. It is estimated that the initial validation phase will be completed by October2003 when a further AATSR validation workshop will be held.

7.3.2 Future WorkIt is clear that the next stage of the validation programme must support climate science, such as that of the Hadley Centre, by establishing a more complete validation of accuracy in differing geophysical regimes and over a range of temperatures. This will allow both global and regional trends of SST to be examined with consequent improvements in the detection of spatial patterns of change.

It is suggested that the emphasis of the validation programme needs to be changed to reflect this situation. The main difference should be that rather than concentrating on as many high quality in situ measurements that we can acquire on an opportunistic basis, there is much more emphasis on analysis of the validation results we do have and using the results of the analysis to determine priorities for more targeted validation campaigns in the future, which address problems associated with specific geographical or meteorological conditions. This is not to say that no routine validation measurements should be made. It is still essential to carry out routine global monitoring, as a diagnostic function, in order to test for small drifts or anomalous behaviour in certain areas.

This global monitoring activity should be complemented by a limited number of regular measurements at higher accuracy, such as are provided by autonomous radiometers. Long-term monitoring of AATSR SSTs will also be a necessary priority and will be achieved by inter-satellite comparisons and through continued deployment of autonomous radiometers. Organisationally, data processing issues will continue to be identified and corrections implemented through the new Quality Working Group (QWG). Finally, it is recognised that validation/characterisation must extend to other AATSR products such as NDVI, land surface temperature and clouds which either are or will become publically available.

Activities to meet the long-term validation programme objectives are needed throughout the AATSR mission. A number of significant tasks have been identified, in Section 7.2.1., but it is clear that in the coming months, support to ensure their continuation must be actively pursued.

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7.3.3 BenefitsThe formal ESA specifications, as formally agreed with ESA, require that AATSR’s SST values achieve an absolute accuracy of better than +/-0.5ºC, with /-0.3ºC adopted by the project as the target accuracy.

This level of accuracy would allow reasonably accurate calculations of ocean-atmosphere heat transfer and reasonably effective accurate tracking of major SST anomalies, such as El Niño, which is typically of 3-4ºC magnitudes. For the accurate detection of global change in the SST fields, the requirements can be more stringent, raising a number of questions concerning the consistency of the data.

Firstly, there is a requirement for great stability and freedom from drift. A paper by Allen et al (Nature, 1993) showed that, given the current expectations at the time for anthropogenic changes in average global SST, an instrumental drift of the order of 0.1ºC/decade is desirable for the most efficient detection of global change. This is at the limit of what can be meaningfully measured. Therefore there is a need for regular checking of AATSR’s SST accuracy, to the highest level of precision.

Secondly, although AATSR is generally meeting the ESA specifications, it is also the case that, within the specified accuracy of 0.3ºC. There are variations of accuracy from regions to region. Regions that are difficult include the South-West Atlantic, where there are often high concentrations of low aerosols, the tropics in general where there are heavy loads of water vapour, the Southern Ocean, where correlative data are extremely sparse. Also, it is clear from the data we have that there are latitude dependent effects in the current retrieval scheme which possibly have their roots in shortcomings in our knowledge of radiative transfer.

Thirdly, change in global SST is not only detected by monitoring global averages. It can also be detected, perhaps more rapidly than is the case with global averages, by inspecting patterns such as gradients across ocean basins or differences between ocean basins. Such techniques, referred to as ‘finger-printing, would demand high accuracy and precision, such as only AATSR has the potential to achieve, in the region of 0.1-0.2ºC.

Obviously, it is crucial to the fingerprinting approach that not only does AATSR achieve that level of accuracy, but that it does so consistently in the entire principal regions of global oceans.

For this reason it is foreseen that there is a need for an on-going validation programme for AATSR, to monitor for the detection of drift and to carry out carefully targeted campaigns, using autonomous systems complemented by occasional high-precision ‘point’ samples, throughout the AATSR mission.

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