Earth Observation Science www.leos.le.ac.uk

Some Considerations concerning the Physical basis of SST MeasurementsDavid Llewellyn-JonesAATSR Principal InvestigatorSpace Research CentreDepartment of Physics & AstronomyUniversity of LeicesterUK

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 IssueVariability and Sampling

Measuring SST - Buckets – the Parker Collection

Measuring SST –

Why do we use Space?

• Coverage• Continuity• Consistency• All essential requirements for

– Climate– Large-Scale Oceanography; and – Meteorology

Approaching the Basic Problems1. 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

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?

Pre-Launch Calibration IssuesWhat 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)

Field of View & Radiometric Response

Notice scan-dependence

Notice non-linearity at all wavelenghts

ATSR-1Measurements by G Mason

In-Flight Calibration• Stable Reference Targets

–At least two of them to deal with detector non-linearity

• Stable thermal environments• Traceable standards

An ATSR on-

board Black Body

Peering into the Void - How

Black is Black?

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

The Reverend Dr Robert Stirling

1790-1878

cottish clergyman

Space-qualified Stirling-Cycle Refrigerator(ATSR-1 & -2, 1991Designed by john Delderfield and

colleagues at RAL)

Cooling the Detectors for Optimum Sensitivity

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.

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 . .

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

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 recordsBUT -–Experience shows we actually

looking at both

The Pesky Target –

What are we Measuring?

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

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?

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?

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!

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

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

ATSR V2.0: 18 Year SST Climatology