A comparison of the 2005 total ozone measurements done with
the Microtops II, Brewer, Toms and Sciamachy instruments

Francis Massen

version 1.0
file name: uccle_2005.html

Summary The year 2005 total ozone measurements made by 4 different instruments, two ground located and two satellite-born, gives statistical meaningful correlation factors mostly exceeding 0.90. A best multiplier of 0.95 is suggested to apply to the meteoLCD Microtops # 5375 readings if the Brewer measurements published as Uccle Woudc mean readings are to be taken as a reference source. The differences between the satellite and Brewer instruments are of a similar magnitude than those between Microtops and Brewer. Only data from January to May 2005 were available from Sciamachy.


1. Reminder on Uccle and Diekirch locations and previous findings

2. Total O3 readings from the 4 instruments:means and standard deviations

3. A possible drift of Microtops during 2005

4. Correlations and linear calibration factors

5. Conclusion
6. Acknowledgments
7. References

1. Reminder on Uccle and Diekirch locations and satellite characteristics:

Please read here for the relevant details. The monthly Sciamachy data files [2] cover the whole globe; only data from Jan to May 2005 were freely available at the time of writing this report (May 2006);
NASA has issued a warning [5] regarding the use of Toms data for longtime drift analysis. The Toms datafile [1] is complete, with 365 measurements corresponding to a daily Uccle overflight. Both Uccle and Microtops datafiles have gaps that are often non-overlapping (see table1). At Diekirch a very mall subset of the measurements were done from end of July to end of August 2005 with the older #3012 model and a different operator; an overlap region with both instruments used allowed to apply a corrective multiplier to make these measurements compatible with model #5375 characteristics.

The conclusion from the years 1997 to 2002 comparison between Microtops #3012 and the supposed correct Brewer DS measurement were to apply multiplier of 0.93 - 0.95 to the Microtops readings to give a good agreement with the Brewer; the 2003 report suggested no need for a multiplier for Microtops #5375 (who started operation in May 2003, and has not yet been recalibrated by the manufacturer).

2. Total O3 readings from the 4 instruments:means and standard deviations

Fig.1 shows the readings of the four instruments; for Uccle there are 2 series: Uccle_woudc represents the mean value of the data published at the WOUDC data archive [6],  whereas Uccle_MKII corresponds to a data file with the Brewer Mark II readings sent by Dr. Hugo De Backer to the author. All data reflect the general yearly pattern with springtime maximum and autumn minimum, but there is quite some spreading and the meteoLCD data seem higher for most part of the year.

Fig.1a gives the overlay with the Uccle total O3 column as published on the RMI website [8] ; the data from all sensors fall practically all into the 2-sigma boundaries (gray area)


 Table 1 shows the number of records available, means, standard-deviations and the mean of the sum of the squared differences with Microtops:

Table 1

January to May 2005

January to December 2005

Instrument number of records mean +/- std mean of sum of squared differences (#) number of records mean +/- std mean of sum of squared differences (#)
Toms 151 332.5 +/- 48.7 1060 365 313.0 +/- 44.3 1252
Sciamachy 60 346.7 +/- 41.3 738 NA NA NA
Uccle_Woudc 237 349.1 +/- 40.3 233 237 324.9 +/- 39.4 426
Uccle_MkII 52 336.3 +/- 46.2 649 181 316.2 +/- 39.2 898
meteoLCD 70 351.0 +/- 47.1 137 334.8 +/- 40.6

(#) differences versus meteoLCD

Table 2 shows the effect of a two restrictions of the comparison: Uccle_Woudc_keep  is restricted to the readings of Uccle and Diekirch done at times less than one hour apart (these readings will have the "keep" suffix), and Uccle_MKII_keep_DS to the similar MarkII readings, with the supplementary condition of a direct sun pointing (DS):

Table 2

January to May

January to December 

Instrument number of records mean +/- std mean of sum of squared differences (#) number of records mean +/- std mean of sum of squared differences (#)
Uccle_Woudc_keep 30 337.4 +/-44.1 218 81 313.8 +/- 38.9 431
Uccle_MkII_keep_DS 07 318.6 +/- 30.3 133 28 301.6 +/- 33.0 708
meteoLCD_keep 30 348.6 +/-45.0 81 331.4 +/- 38.4

(#) differences versus meteoLCD

Conclusion: meteoLCD Microtops readings are closest to the mean Uccle_Woudc data; the cumulative deviation from the direct sun means are noticeably higher, but the relatively small number of available records (28) makes usage of these questionable. The increase of the mean of sum of squared differences between the January-May and whole year data points to an increasing drift of the Microtops during 2005, assuming the Brewer remaining stable (what has been confirmed by Dr. De Backer, personal communication). The Uccle_Woudc readings are the official data published by Uccle; it seems reasonable to focus the comparison between Diekirch and Uccle on these data, and not so much on the (unofficial) MKII readings, which deviate markedly from the former.

3. A possible drift between Microtops during 2005

Fig.2  shows the differences (Microtops - Uccle) between Microtops and Uccle_Woudc_keep readings: over the whole year there is a linear increase of 13.2 from 10.6 to 23.8; the Microtops readings are practically all higher than the Uccle_Woudc_keep data (positive difference)
Fig.3 gives the differences (Microtops - Toms); the increase is about 12 over the whole year, similar to the preceding one.
Fig 4. shows the differences between Uccle_Woudc_keep and Toms; here the difference decreases over the year from  18.5  to 13.6.  
The mean of the 3 preceding differences are 16.7, 33.4 and 15.9: Microtops and Uccle have similar differences as do Uccle and Toms! 
The mean difference between Uccle_Woudc_keep and  Sciamachy is 17.5.
Fig. 5: The statistical distributions of the differences are close to normal.

Conclusion:  All mean differences between Microtops and the other sensors are positive, as are the linear trends over 2005. Probably Microtops had a small positive 12 to14 DU drift (depending on the reference source used) during 2005.

4. Correlations and linear calibration factors

 Table 3 gives the linear correlation factors, which are all significant at the 5% level:

Table 3 Sciamachy UccleWoudc UccleWoudckeep UccleMKII UccleMKII_DS Microtops
Toms 0.90 0.95 0.96 0.90 0.94 0.94
Sciamachy 0.93 0.97 0.78 0.95
UccleWoudc 0.93 0.98 0.95
UccleWoudckeep 0.96 0.97 0.96
UccleMKII 0.88
UccleMKII_DS 0.93

One should note the poorer correlations with UccleMKII; the author has no explanation on this for the moment.

Linear regressions forced through the origin will be computed to give simple calibration multiplicators. Table 4 holds the linear regression slopes for all sensors and subsets (x- values = Microtops, y-values = other sensor); the Goodness of the Fit (GOF) is given between parenthesis

Table 4 Sciamachy UccleWoudc UccleWoudckeep UccleMKII UccleMKII_DS Microtops
Toms 0.98 0.96 (0.94) 0.95 (0.96) 0.97 (0.90) 0.95 (0.94) 0.91 (0.93)
Sciamachy 0.98 (0.93) 0.98 (0.95) 1.02 (0.73) NA 0.94 (0.93)
UccleWoudc 1.02 (0.93) 1.01 (0.97) 0.95 (0.95)
UccleWoudckeep 0.95 (0.96)
UccleMKII 0.93 (0.86)
UccleMKII_DS 0.93 (0.93)

These slopes represent the multiplier to apply to Microtops data to obtain calibrated data, the reference sensor assumed to be correct; a multiplier of 1 would imply a perfectly calibrated instrument. The smaller multiplier for the UccleMKII comes as a surprise, and the restriction to the direct-sun readings does not improve the multiplier (but increases the goodness of the fit). As there are only 28 data to compare UccleMKII_DS and Microtops, the most sensible conclusion would be to to accept the UccleWoudckeep readings as a reference, and to apply a multiplier of 0.96 to the 2005 Microtops results. It should also be noted that the multiplier and GOF for Microtops/Toms and UccleWoudckeep/Toms are the same; Sciamachy/Ucclewoudckeep is higher than Microtops/Ucclewoudckeep, but the GOF's are close.

The next figures 6,7 and 8 show the graphs with the linear regressions

Figure 6 shows all the previous linear regressions for Microtops versus another sensor; the turquoise line is the ideal y = x for identical sensors.
Figure 7 gives the same regressions for Uccle versus the satellites.  
Figure 8 shows the general linear regressions (not forced through the origin); the goodness of the fits are not  better than those corresponding to the simpler regression through the origin; again the best match is for the Ucclewoudc (points closer to the y=x line), compared to  Uccle_MKII or  UccleMKII_DS. 


5 Conclusions

The Microtops II #5375 measured mean total ozone column is best compared to the Uccle mean total ozone measurements as published at the WOUDC, retaining only data whose measurement times differ less than 1 hour with Microtops. The Microtops sun photometer probably suffered from of a small positive drift during 2005. This report suggests to apply a multiplier of 0.95 to give the best correspondence with the Uccle mean Woudc data. Different from previous years, the Uccle_MKII (direct sun or not) readings give a poorer fit (with a suggested multiplier of 0.93), as do the measurements from the satellite born Toms and Sciamachy instruments. Differences between Microtops and the satellites are however of a similar magnitude as those between Uccle and the satellites.


6. Acknowledgements

Special thanks to Dr. De Backer of the Royal Meteorological Institute of Belgium (RMI) for providing the Uccle Brewer data and his continuing help.


7. References:

1. TOM's data are available at: ftp://jwocky.gsfc.nasa.gov/pub/eptoms/data/overpass/OVP053_ept.txt
2. Sciamachy monthly data files (whole globe): http://www.temis.nl/protocols/o3col/o3month2005.html
3. Francis Massen : Influence of intentionally mis-pointing on Microtops II readings [2004]
4. Dr. Hugo De Backer, RMI: personal communication
5. NASA Toms calibration problems: http://toms.gsfc.nasa.gov/news/news.html#nov18
6, The Uccle (station53) 2005 WOUDC data.
7. The meteoLCD (station 412) 2005 WOUDC data.
8. RMI website with Brewer total O3 graph

All calculations and graphs have been made using Statistica 7.1 by Statsoft.


History of versions:

1.0        original version 02 May 2006

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 Please send all mail to francis.massen@education.lu