1. Earth Observation Science www.leos.le.ac.uk Some Considerations concerning the Physical basis of SST Measurements David Llewellyn-Jones AATSR Principal Investigator Space Research Centre Department of Physics & Astronomy University of Leicester UK

2. Measuring SST – Mainly from Space Approaching the basic problems: –Calibration –Atmospheric Correction –Which Wavelengths? –The Pesky Target – What are we Measuring?  The Skin and Bulk Issue  Variability and Sampling

3. Measuring SST - Buckets – the Parker Collection

4. Measuring SST –

5. Why do we use Space? Coverage Continuity Consistency All essential requirements for –Climate –Large-Scale Oceanography; and –Meteorology

6. Approaching the Basic Problems 1.Deploy a reliable, high-performance radiometer in Space 2.Apply an accurate and reliable Atmospheric Correction 3.Appreciate and Allow for the nature of the target Surface 4.Also:- We must have a reliable and Consistent in situ reference measurement system in Place

7. What makes a very Good Radiometer? What are we looking at? –Good optical design –Minimum stray light –Well-defined Instrument Field-of-view (IFoV) How much Radiated Power are we receiving? –Rigorous Calibration Procedures –Pre-launch and In-flight What is the wavelength distribution? –End-to-end calibration - if possible Have we got enough sensitivity?

8. Pre-Launch Calibration Issues What must be Measured? IFoV for all channels –Check for stray light Radiometric sensitivity –Overall performance in Representative Thermal Environment (dynamic) Spectral Responses –End-to-end if you can Traceable Standards Must be Used throughout (i.e. Use QA4EO Guidelines)

9. Field of View & Radiometric Response Notice scan-dependence Notice non-linearity at all wavelenghts ATSR-1Measurements by G Mason

10. In-Flight Calibration Stable Reference Targets –At least two of them to deal with detector non-linearity Stable thermal environments Traceable standards

11. An ATSR on-board Black Body Peering into the Void - How Black is Black?

12. Optimum Sensitivity is Essential Detectors need to be at<100˚K Use a closed-cycle refrigerator Basedon the Stirling Cycle

13. The Reverend Dr Robert Stirling 1790-1878 cottish clergyman cottish clergyman Space-qualified Stirling-Cycle Refrigerator (ATSR-1 & -2, 1991 Designed by john Delderfield and colleagues at RAL ) Cooling the Detectors for Optimum Sensitivity

14. Instrumentation – where are we going? Are we getting too complex? And too heavy? Are we experiencing ‘Requirements Creep’ Is a gulf opening between scientists and engineers? This is risky and inappropriate to operational systems For future sensors, can engineers concentrate on suitability for repeat manufacture.

15. Operational and Scientific Users Despite What is sometimes said, operational users do need optimised and improved performance from their observing systems But they are not always in a good position to explore improvements Scientists are keen to do just that So, ideally, operational systems can benefit from strong and pro-active scientific user- communities – and it’s not always easy..

16. What wavelength Region Clear-sky accuracy of IR 3.7-11-12µm combination is undisputed However, coverage limitations due to clouds, require inclusion of Microwave radiometers in a truly Global measurement system. Data-merging systems make this possible R&D on calibration still desirable

17. Understanding the Target -1 : The Traditional Skin-Bulk issue: –Skin is what interfaces with the Atmosphere –and is the basis to the measureable ToA BT –Bulk is in the historical records BUT - –Experience shows we actually looking at both

18. The Pesky Target – What are we Measuring?

19. Understanding the Target - 1: Sampling a 1km Pixel –For a point measurement skin and bulk temperatures are different for little or no wind –A Bulk temperature applies for higher wind- speeds –Often something in between

20. Understanding the Target - 2: We have 2-dimensional non-uniformity at the surface Compounded by vertical non-uniformity beneath the surface Compounded by the fact that vertical T- profile depends on changing surface wind and changing insolation What does this all mean?

21. Horizontal and Vertical Non-Uniformity Unless wind is very stable or non-existent: Mean temperatures of a 1km x 1km surface area is a indefinable mixture of skin and sub- surface temperatures Limited point sampling (e.g. Radiometerscan be unrepresentative Could we put a radiometer on that?

22. Summary Points For Climate Applications, observations must be properly quality-assured (QA4EO) with Reference measurements traceable to accepted standards For calibration procedures and equipment, for retrieval methodologies, this can be achieved. The next generation of sensors needs careful R&D – Requirements creep must stop!

23. An ideal Technology Objective for the Future: 1)Constellation of 3-6 high-performance IR SST Radiometers, (Technology challenge) 2)1 or 2 wide-swath images for lower accuracy broad coverage 3)1 or 2 MW SST radiometers with good calibration (Technology challenge) 4)Geostationary network for diurnal variability, with higher spatial resolution for coastal processes etc (Technology challenge) 5)in situ network of reference sensors, with traceable calibration

24. Summary Points - Resolved Conflicts: In situ obs or Space? –Actually we need both Is it Microwave or IR –Actually we need both Are we measuring Skin or Bulk T? –Actually we need both Should we use Buoys or Radiometers for in situ obs? –Actually we need both Is it the

25. ATSR V2.0: 18 Year SST Climatology