Comparison of the PMOD, ACRIM and IRMB Total-Solar-Irradiance-Composite Time Series

1. Comparison of the PMOD, ACRIM and IRMB Total-Solar-Irradiance-Composite Time Series Claus Fröhlich Physikalisch-Meteorologisches Observatorium Davos World Radiation Center CH 7260 Davos Dorf AGU Fall Meeting 5-9 December 2005

2. Outline • Presentation of the three composites • What are the major differences? Description of the corrections used for the PMOD composite. • How important are these corrections? • Conclusions AGU Fall Meeting 5-9 December 2005

3. How are the composites constructed? • All use essentially the same time series to construct their composites. • The treatment of the data is different: one can use the data as originally published, one can apply corrections for some known, but not yet considered effects, one can also use a combination of these two methods for only some radiometers and at some period of time. • A major problem is to adjust ACRIM-II to the radiometric scale of ACRIM-I. As the HF and ERBE data sets differ during this time period this is the main issue of controversy. • Finally, the different datasets are adjusted at the overlapping periods to get a continuous composite or referred to a common scale and then merged. The absolute value depends on which radiometer or which combination of radiometers is used as radiometric reference. AGU Fall Meeting 5-9 December 2005

4. The original Data and the three Composites • The time-series (mostly daily values) are from: HF/Nimbus-7, ACRIM-I/SMM, ACRIM-II/UARS, VIRGO/SOHO, ACRIM-III/ACRIM-Sat. ERBS has most of the time a value every 2 weeks, so it can only be used for comparisons. The colors of the original time series are repeated in the composites. • The PMOD composite uses alternatively corrected HF and ACRIM-I (without spin-mode) data until after the break in UARS operations in 1992, then ACRIM-II and from February 1996 VIRGO (a combina-tion of PMO6V and DIARAD). The absolute value is from ACRIM-I and II, scaled to SARR via ACRIM-II. • The ACRIM composite uses ACRIM-I, with HF data at the beginning, during the gap before SMM repair and after the end, then ACRIM-II and III. The absolute value is based on ACRIM-III • The IRMB composite uses the same series as ACRIM until February 1996 and then DIARAD/VIRGO. All time series are referred to SARR and then merged. The absolute scale is based on SARR AGU Fall Meeting 5-9 December 2005

5. Differences at the Beginning of the Composites • ACRIM-I, II and III, and the VIRGO investigations have back-up radiometers which can be used to determine exposure dependent changes by comparing the operational radiometers with the less-exposed back-ups. • HF does not have such means and an attempt was made to correct HF for its temporal changes. • The detailed analysis of the behaviour of HF and the so determined corrections provided not only a reliable record around the maximum of cycle 21, but also over its whole period of operation in an internally consistent way. Thus, the use of HF to relate ACRIM-II to the scale of ACRIM-I is no longer a local problem during the ACRIM gap. AGU Fall Meeting 5-9 December 2005

6. Analysis of the behaviour of HF and ACRIM-I • To study the behaviour of HF we need a reliable time series to compare with. • An obvious candidate is ACRIM-I, extended at the beginning (for 15 months) with a locally calibrated proxy model and at the end with ERBS. • Before starting this exercise it was realized that ACRIM-I needed also a re-assessment of the determination of its degradation. AGU Fall Meeting 5-9 December 2005

7. Re-assessment of ACRIM-I degradation • During the analysis of the performance of the PMO6V radiometers, an early increase was detected which is due a change in the aperture absorptance and corresponding increase of the temperture and IR radiation into the cavity. A similar one could be identified for ACRIM-I AGU Fall Meeting 5-9 December 2005

8. Re-assessment of ACRIM-I degradation • During the analysis of the performance of the PMO6V radiometers, an early increase was detected which is due a change in the aperture absorptance and corresponding increase of the temperture and IR radiation into the cavity. A similar one could be identified for ACRIM-I • During the spin mode of SMM the exposure time of ACRIM-I to solar radiation was much less. This was not properly taken into account, so we have used the same algorithms as for VIRGO to re-evaluate the degradation as a function of exposure time. AGU Fall Meeting 5-9 December 2005

9. Re-assessment of ACRIM-I degradation • During the analysis of the performance of the PMO6V radiometers, an early increase was detected which is due a change in the aperture absorptance and corresponding increase of the temperture and IR radiation into the cavity. A similar one could be identified for ACRIM-I • During the spin mode of SMM the exposure time of ACRIM-I to solar radiation was much less. This was not properly taken into account, so we have used the same algorithms as for VIRGO to re-evaluate the degradation as a function of exposure time. • These corrections yield the revised Version of ACRIM-I AGU Fall Meeting 5-9 December 2005

10. Assessment of HF degradation • The HF data set has sudden slips or gradual changes which are due to changes in the beta ange of the S/C orientation. Such slips were identified by several investigations, but there was never a systematic search for them. AGU Fall Meeting 5-9 December 2005

11. Assessment of HF degradation • The HF data set has sudden slips or gradual changes which are due to changes in the beta ange of the S/C orientation. Such slips were identified by several investigations, but there was never a systematic search for them. • For the ‚degradation‘ an early increase, a decrease and long-term increase of the sensitivity are identified. The latter may be of similar origin as the one of DIARAD/VIRGO. AGU Fall Meeting 5-9 December 2005

12. Assessment of HF degradation • The HF data set has sudden slips or gradual changes which are due to changes in the beta ange of teh S/C orientation. Such slips were identified by several investigations, but there was never a systematic search dor them. • For the ‚degradation‘ an early increase, a decrease and long-term increase of the sensitivity are identified. The latter may be of similar origin as the one of DIARAD/VIRGO. • This yields a HF time series which is corrected over the full period including the slip which is the main difference between the ACRIM and PMOD composites. AGU Fall Meeting 5-9 December 2005

13. Re-analysis and Scaling of ACRIM-II • We went through the whole record and found some discrepancies which needed correction. The first issue was related to the early data and the time when the sensor was switched. Moreover, ACRIM-III was adjusted to ACRIM-II and is now treated as one record. AGU Fall Meeting 5-9 December 2005

14. Re-analysis and Scaling of ACRIM-II • We went through the whole record and found some discrepancies which needed correction. The first issue was related to the early data and the time when the sensor was switched. Moreover, ACRIM-III was adjusted to ACRIM-II and is now treated as one record. • The comparison with HF and VIRGO shows that the result is now quite consistent. AGU Fall Meeting 5-9 December 2005

15. Re-analysis and Scaling of ACRIM-II • We went through the whole record and found some discrepancy which needed correction. The first issue was related to the early data and the time when the sensor was switched. Moreover, ACRIM-III was adjusted to ACRIM-II and is now treated as one record. • The comparison with HF and VIRGO shows that the result is consistent. • The tracing of ACRIM-II to ACRIM-i is based on the corrected HF. AGU Fall Meeting 5-9 December 2005

16. Corrections for the VIRGO Radiometers • Level 1 data show quite different long-term behavior. Note the early increase of PMO6V radiometers and the completely different long-term behavior of PMO6V and DIARAD. AGU Fall Meeting 5-9 December 2005

17. Corrections for the VIRGO Radiometers • Level 1 data show quite different long-term behavior. Note the early increase of PMO6V radiometers and the completely different long-term behavior of PMO6V and DIARAD. • Corrections for exposure-dependent changes are determined by comparison with less exposed radio-meters (leading to level1.8), illustrated for DIARAD. AGU Fall Meeting 5-9 December 2005

18. Corrections for the VIRGO Radiometers • Level 1 data show quite different long-term behavior. Note the early increase of PMO6V radiometers and the completely different degradation behavior of PMO6V-A and DIARAD-L. • Corrections for exposure-dependent changes are determined by comparison with less exposed radiometers (leading to level1.8). • By comparison of the time series of the two radiometer types possible exposure independent changes may be identified (leading to a combined level-2 data set). AGU Fall Meeting 5-9 December 2005

19. Corrections for the VIRGO Radiometers • Level 1 data show quite different long-term behavior. Note the early increase of PMO6V radiometers and the completely different degradation behavior of PMO6V-A and DIARAD-L. • Corrections for exposure-dependent changes are determined by comparison with less exposed radiometers (leading to level1.8). • By comparison of the time series of the two radiometer types possible exposure independent changes may be identified (leading to a combined level-2 data set). • The final result shows also the differences between the determination of the degradation by the VIRGO and IRMB team. AGU Fall Meeting 5-9 December 2005

20. Comparison of the three Composites to ERBS and the Kitt-Peak Reconstruction • Most of the differences identified earlier are now explained by the corrections applied in the construction of the PMOD composite. AGU Fall Meeting 5-9 December 2005

21. Comparison of the three Composites to ERBS and the Kitt-Peak Reconstruction • Most of the differences identified earlier are now explained by the corrections applied in the construction of the PMOD composite. • The comparison with ERBS (corrected for its early increase during the total exposure of 2.6 days) shows the differences even more clearly. Obvious is the problem of the bridging over the ACRIM gap and the neglect of the non-exposure-dependent changes of DIARAD by the IRMB team. AGU Fall Meeting 5-9 December 2005

22. Comparison of the three Composites to ERBS and the Kitt-Peak Reconstruction • Most of the differences identified earlier are now explained by the corrections applied in the construction of the PMOD composite. • The comparison with ERBS (corrected for its early increase during the total exposure of 2.6 days) shows the differences even more clearly. Obvious is the problem of the bridging over the ACRIM gap and the neglect of the non-exposure-dependent changes of DIARAD by the IRMB team. • Essentially the same picture is presented by the comparison with the Kitt-Peak reconstruction which is completely independent and covers the full period of the observations. AGU Fall Meeting 5-9 December 2005

23. Conclusions • The differences between the three composites are due to different treatment of the original data, and how the scaling of ACRIM-II to ACRIM-I is performed. • From the comparison with ERBS and the Kitt-Peak reconstruction, it is obvious that the PMOD composite has the highest correlation and the least scatter. • Whether this is an indication of which one is the most reliable may always be questioned and the author of the corrections may be blamed of having only followed his prejudice. However, the corrections were determined by using all available information about the present state-of-the-art radiometers and their behaviour in space. It is based on the physical understanding of radiometry from space and thus quite objective. • As a user you can make your own choice. All three are available from: • PMOD: http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant • ACRIM: http://www.acrim.com/Data\%20Products.htm • DIARAD: http://remotesensing.oma.be/solarconstant/sarr/SARR.txt AGU Fall Meeting 5-9 December 2005

24. Thank you The presentation can be found on ftp://ftp.pmodwrc.ch/pub/claus/AGU_Fall2005/ CF_AGU2005.ppt AGU Fall Meeting 5-9 December 2005