Sea Surface Temperature (SST) from Space Past and Future…..

The evolution of Along Track Scanning Radiometer (ATSR) into the Sea and Land Surface Temperature Radiometer (SLSTR)

Dr Chris Mutlow Head of Earth Observation and Atmospheric Science RAL Space Science and Technology Facilities Council – Rutherford Appleton Laboratory from Space - Appleton Space Conference 2013

Talk Outline

► Why are we interested in Sea Surface Temperature?

► Why observe it from space?

► Introduction to the Along Track Scanning Radiometers (ATSRs)

► Some of the Achievements of the ATSR Project

► The successor instrument - Sea and Land Surface Temperature Radiometer (SLSTR)

► Summary

2 Sea Surface Temperature from Space - Appleton Space Conference 2013

Why are we interested in SST Why observe from space? ► The oceans cover over 70% of the Earth’s surface; likely to be important!

► change is expected to have a strong signature in ocean temperature

► Most of the energy driving the atmosphere comes from the Earth’s surface - rather than directly from the Sun.

► The oceans transport heat from the tropics to the cold polar regions ► our maritime climate is the result of the Gulf Stream

► Associated with climate change there are expected to be other changes in local SST patterns which will give early warnings ► Warmer tropical Atlantic waters = more Hurricanes

► Measurements from space are the only way to obtain global coverage.

3 Sea Surface Temperature from Space - Appleton Space Conference 2013

How do we measure SST from space?

► By detecting the thermal radiation emitted by the ocean that reaches space.

► Filter-radiometers commonly used working in either: ► Microwave – low accuracy, spatial resolution but … can penetrate ► Infrared – delivers climate accuracy but … can’t penetrate clouds

4 Sea Surface Temperature from Space - Appleton Space Conference 2013

How do we correct for the atmosphere?

► Multi-spectral method – correcting for atmospheric water vapour ► Makes use of varying strength of the absorption by water vapour in different infrared wavelengths ► Least absorption at 3.7μm ► More at 11μm ► Even more at 12μm

► Only technique used by the previous generation of sensors (i.e. AVHRR) ► Not robust enough for Climate Monitoring purposes! ► Doesn’t deal with aerosol well!

► Along track scanning – the real innovation in ATSR! ► Directly measuring the effect of the atmosphere by observing along different length paths to the surface ► Nadir (short) and a Slant (long) path ► Need to maintain surface viewing angle to >55 degrees keep ocean “black” ► Works well for aerosols too!

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Along Track Scanning Radiometer (ATSR)

► Designed in the early 1980’s by UK and Australian scientists – who were then ahead of their time in recognising climate change as an important issue. ► Space-borne imaging radiometer

► Developed to measure sea surface temperature (SST) for Climate Change Detection

► ATSR was the first sensor specifically aimed at this task!

► Later capabilities added for land, , and aerosol

► Three ATSR instruments flown on ESA EO missions:

► ATSR(-1) on ERS-1 (1991-1996) – initial “experimental” instrument ► ATSR-2 on ERS-2 (1995-2003) – new “experimental” visible channels ► AATSR on (2002-2012)

► Series known collectively as (A)ATSR

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ATSR-1, ATSR-2 and AATSR?

► Channels to capture atmospheric state; water vapour and aerosol

► Other sensors rely on external for their operation; ► All ATSRs have infrared channels at 1.6, 3.7, 10.8 and 12µm ► used for SST determination and cloud identification ► ATSR-2 and AATSR have extra visible channels at 0.55, 0.67 and 0.87µm ► used for land, aerosol and cloud applications ► Climate-quality levels of “performance” required the following technical developments over previous sensors: ► Along track scanning ► Low noise detectors → active cooling using Sterling cycle cooler ► On-board calibration → ultra-stable, and accurate blackbodies

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Gulf Stream

The Along Track Scanning Radiometer (ATSR) Mission

• Primary objective to measure Sea Surface Temperature (SST) with an accuracy of 0.3K (1-sigma limit) • Secondary objective thermal and visible data for land studies (e.g. temperature, vegetation): • Provision of a long-term dataset for global climate change studies 8 Sea Surface Temperature from Space - Appleton Space Conference 2013

ATSR viewing geometry – ground track

Forward view is the 2 x shortest path to surface i.e. more absorption

Nadir view is the shortest path to surface

500 km wide Swath

9 Sea Surface Temperature from Space - Appleton Space Conference 2013

ATSR Optical Layout

FPA Field Stop Scan Mirror

M2

Opal UV Filter Baffle

Sun

VISCAL

Off-Axis Paraboloid M1 (f = 540mm) Black-Body

Along-Track View Nadir View 10 Sea Surface Temperature from Space - Appleton Space Conference 2013

The AATSR instrument

Nadir Forward view view aperture aperture

VISCAL This way to Earth Blackbody calibration targets

11 (A)ATSR Data compared to in situ

Good agreement of independent satellite with in situ datasets regarding global sea surface temperature changes over time

Gives high confidence in global SST changes

Satellite only, not tuned Two in-situ only datasets

ESA SST CCI Sea Surface Temperature Climate Change Initiative

ATSRs capture SST variability cleanly : even better with greater SLSTR coverage

ENSO as seen in ATSRs

behemoth.nerc-essc.ac.uk/ncWMS/godiva2.html

Also see animation in exhibition space of CCI data

ESA SST CCI European Space Agency Sea Surface Temperature Climate Change Initiative

Stability of SST across 3 ATSR Sensors

 ATSR SSTs are low bias and highly stable in time against other data – independent not “tuned” against other data

 This gives ATSRs and future SLSTR a unique role for SST climate data records

Spread of difference

Average difference

ATSR-1 ATSR-2 AATSR vs. Argo floats Satellite vs. global drifting buoy SSTs ESA SST CCI European Space Agency CourtesySea of SurfaceProf Chris Temperature Merchant, ReadingClimate Change University Initiative

ATSRs are used as reference for other meteorological sensors

 This means we can aim for AVHRR sampling …  … with accuracy, stability and independence derived from ATSRs

Dual -view reference SST coverage Tied AVHRR SST coverage

ESA SST CCI European Space Agency Sea Surface Temperature Climate Change Initiative

Sea Surface Temperature from Space - Appleton Space Conference 2013

Example Cloud Products

Cloud fraction 1996-2001 Cloud top temperature 1996-2001

ATSR cloud products

16 Cloud optical depth 1996-2001 Sea Surface Temperature from Space - Appleton Space Conference 2013

Comparison with ISCCP data

ATSR-2 May 1999 Optical depth ISCCP Optical depth May 1999

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ATSR Aerosol Measurements – now in ESA CCI

Thomas et al, ACP, 2010 18 Land Surface Temperature (LST)

Day - LST 340K

Night - LST

220K Courtesy University of Leicester

ESA SST CCI European Space Agency Sea Surface Temperature 19 Climate Change Initiative

Sea Surface Temperature from Space - Appleton Space Conference 2013

ATSR Fires and Gas Flares

Indonesia n Fires

After Casadio et al; ALGO3 persistent hot sites (1991–2009) RSE 2012 Amazon Deforestation

20 Evolution from ATSR-1  SLSTR

1991-2000 ATSR-1 Next Generation 1995-2008 ATSR-2 2015 - SLSTR

2002-2012 AATSR

ERS-1

ERS-2

(A)ATSR Heritage Series ENVISAT Sentinel-3 Sea Surface Temperature from Space - Appleton Space Conference 2013

Sea and Land Surface Temperature Radiometer (SLSTR) for Copernicus ► Part of the payload Copernicus Sentinel 3 Mission ► Equivalent baseline performance to AATSR ► Recognition of LST (land) as being important in addition to SST (sea)

► Differences from AATSR ► Backwards oblique view ► double scanner ► Flip mirror ► Wider swath (improved re-visit) ► Increased dual view coverage ► Extra SWIR (cloud) channels ► Improved fire channels ► Visible channels at 0.5 km resolution

► Launch late 2014/Early 2015

See Coppo et al, J. Mod. Opt, 2010, Donlon et al, RSE, 2012

22 S3 SLSTR: Basic Geometry SLSTR Design Features

• To enable a wider swath SLSTR uses two scan systems (nadir and oblique) and optical paths • A flip mirror (new) is used to select which optical path is directed to the detectors • The nadir swath has a westerly offset to completely overlap the OLCI swath • The oblique view 55°inclination maintains a longer atmospheric path length compared to nadir • better atmospheric correction • Both scan chains view the same blackbody and VISCAL targets S3 SLSTR: Optical path overview The scanning system uses two flat rotating mirrors and one flip mirror - switching between one view and the other

Multi-element detector arrays sample 2 earth sample lines per scan rotation

The scan mirrors rotate on a 300 msec time basis, ~half period of AATSR (~35μs integration time on detectors)

The in flight Blackbody

calibration period is 600 msec.

Each Blackbody calibration source is viewed independently by each scanner once every second scan cycle. Earthshine Alignment Plate Optics International Standards

ATS R

Point source + collimator ATSR-2 Instrument Electronics Blackbody AATSR Source Platform Simulator Copernicus SLSTR

SLSTR Calibration Facility at RAL

→ Facility commissioning completed 19th March 2012

→ Structural Test Model (STM) Integrated & MLI fitted 28rd March 2012

→ Tests with STM in completed last year!

SLSTR Model Testing scheduled to start Q1 2014

The achievements and the future of ATSR?

► The ATSR Programme has delivered the high-quality SST data it was designed to provide! ► (A)ATSR is now regarded as the “gold-standard” for SST determination ► ATSR SST data is now routinely used to calibrate other sensors as demonstrated by GODAE GHRSST Pilot Project ► When AATSR failed the Met Office forecast noticed the loss of skill

► ATSR has matured ► experimental to operational in the journey from ATSR-1 to AATSR, and then SLSTR

► Significant new technology was developed to enable the programme ► Much of this technology has been successfully spun-out to commercial products

► Important new science applications on clouds, aerosols, and land remote sensing have been developed.

► Sentinel 3 will carry the SLSTR to continue and extend the ATSR mission until beyond 2025 26 Sea Surface Temperature from Space - Appleton Space Conference 2013

Acknowledgements

► The many colleagues who worked on the ATSR Programme over the years!

► Chris Merchant, University of Reading.

► Craig Donlon and the ESA S3 Teams at ESTEC and ESRIN,

► Anne O’Carroll and EUMETSAT

► Roger Saunders and the UK Meteorological Office

► David Llewellyn-Jones, John Remedios and Gary Corlett at the University of Leicester

► The SLSTR Teams at Thales (Cannes), Selex Galileo (Florence) and Jena Optronik (Jena)

► Dave Smith, Caroline Poulsen, Gareth Thomas and the rest of the (A)ATSR and SLSTR Team at RAL

► Funding: NERC, STFC, DECC and ESA

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