WebEx presentation: Intercomparisons of IASI and AATSR calibrated radiances

[Tim Hewison]

Dear GSICS Colleagues,

We would like to invite you to the final presentation of a study into the assessment of methods into the radiometric inter-comparison of IASI and AATSR, by the University of Leicester. The presentation will take place by WebEx on Wednesday 17th April from 14:00-15:00 CEST (13:00-14:00 BST). Please refer to the WebEx sign in details below and also see below for the abstract.

Kind regards,
Anne and Tim

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Intercomparisons of IASI and AATSR calibrated radiances

By John Remedios, Chris Whyte and David Moore
University of Leicester
Address: EOS, Space Research Centre, University of Leicester, LE1 7RH, UK.;

Intercomparisons of high-quality thermal instruments are essential to provide a verified reference and uncertainty for spaceborne measurements. The IASI and AATSR instruments represent two distinct types of infra-red instruments, but both with very good calibration of the radiances that are observed. IASI is a Fourier transform spectrometer and the Advanced Along-Track Scanning Radiometer (AATSR) was an infra-red radiometer on Envisat. This study examines the absolute levels of agreement and their uncertainties.

For climate applications, it is important to test the sensors in-flight, so as to provide evidence as to the quality of the radiance data which represent a fundamental climate data record. In addition, the instruments can and have provided a foundation for determining calibration corrections for other sensors in space. For example, this is the approach taken by GSICS in using IASI as a reference for thermal infra-red data, a role to which it is well-suited owing to its operational implementation as well as its radiometric accuracy. Finally, the quality of the thermal radiances from the forthcoming SLSTR instrument on Sentinel-3 will need careful examination, and is of particular interest for Eurmetsat and ESA. By comparing IASI and ATSR, this study provides quantitative evidence of performances for these applications.

Comparisons of IASI and AATSR have been performed in two ways. Firstly, a match-up approach has been undertaken utilising both quasi-simultaneous nadir overpasses (QSNOs) and matches with more relaxed time constraints; both brightness temperature and radiances have been tested. The advantage of more relaxed time constraints is that on days where ATSR and IASI overpasses are closely related spatially, large number of matches are obtained. The disadvantages are the change due to scene temperature temporal gradients and cloud evolution. We therefore examine the utility of such match-ups over ocean, and for various types of land cover differentiating between day and night. Secondly, we employ regional averaging approaches of IASI and AATSR data for two scenarios; gridding into defined latitude bins and brightness temperature bins. Temporal measurement coincidence is not taken into account for the monthly averages. These comparisons are particularly useful for investigating measurement drift over time and also have the advantage of including a large number of data. Individual days are also isolated to check biases between days containing a large number of QSNOs and those with fewer matches.

We present results from a range of time matches and regional averages, using the ocean results as a baseline (6 months of data). It will be shown that the IASI and AATSR are within less than 0.1 K of each other at 11 microns with smaller biases in the sea surface temperature range. Larger biases exist at 12 microns with a systematic dependence on scene temperature being observed. The evidence is that IASI is accurate relative to the AATSR to within 0.1K at 11 microns in an absolute sense and is able to distinguish trends and biases in AATSR radiances to significantly better than that.

The results illustrate the importance of spectrometers in space for spectral and radiometric calibration and their ability to provide a foundation for thermal infra-red systems in space to climate quality.


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[Alex Ignatov - NOAA Federal]

Hi Tim, Anne, and Colleagues,

do we have our webex presentations online? to check them prior to the telecon, come back and take another look later, for those who cannot attend..?

thanks, sasha

[Aleksandar Jelenak - NOAA Affiliate]

Sasha,

On Fri, Apr 12, 2013 at 5:37 PM, Alex Ignatov - NOAA Federal
<alex.i...@noaa.gov> wrote:
> do we have our webex presentations online?

What do you mean by "our"? Material from GSICS meetings is available
from this wiki:

https://gsics.nesdis.noaa.gov/wiki/Development/MeetingsAndConferences

Each meeting has its own page with presentations and often even minutes.

-Aleksandar

[Alex Ignatov - NOAA Federal]

Thanks Aleksandar. shame on me, i was not aware of this page.

i was specifically looking at the IASI-ATSR comparison ppt. it might be a good idea to publish slides prior to webex so that we can check them and come prepared.

thanks again!

sasha

[Tim Hewison]

Hi Sasha,

 

This is not a GSICS meeting. It is the final presentation of a study that has been conducted for EUMETSAT by University of Leicester.

 

Furthermore, the presentation will only be available on Wednesday morning before the meeting. That may not help you much due to the time difference, but I will endeavour to distribute it if possible.

 

Cheers,

 

Tim

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