Some Considerations concerning the Physical basis of SST Measurements

1. 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 • Deploy a reliable, high-performance radiometer in Space • Apply an accurate and reliable Atmospheric Correction • Appreciate and Allow for the nature of the target Surface • 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 non-linearity at all wavelenghts Notice scan-dependence 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 BodyPeering 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. Cooling the Detectors for Optimum Sensitivity The Reverend Dr Robert Stirling1790-1878cottishclergyman Space-qualified Stirling-Cycle Refrigerator (ATSR-1 & -2, 1991 Designed by john Delderfield and colleagues at RAL)

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: • Constellation of 3-6 high-performance IR SST Radiometers, (Technology challenge) • 1 or 2 wide-swath images for lower accuracy broad coverage • 1 or 2 MW SST radiometers with good calibration(Technology challenge) • Geostationary network for diurnal variability, with higher spatial resolution for coastal processes etc (Technology challenge) • 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