Contributors: N. Clerbaux Clerbaux (Royal Meteorological Institute of Belgium (RMIB)), A. Velazquez Blazquez (RMIB), E. Baudrez (RMIB), S. Dewitte (RMIB), S. Nevens (RMIB) 

Issued by: RMIB/Nicolas Clerbaux

Date: 1828/1211/20202023

Ref: C3SC3S2_D312bD312a_Lot1.2.52.97-v2v1.10_202101202303_PQAR_ECVEarthRadiationBudget_v1.01

Official reference number service contract: 20182022/C3SC3S2_312b312a_Lot1_DWD/SC1

Easy Heading Macro
navigationExpandOption collapse-all-but-headings-1

History of modifications

List of datasets covered by this List of datasets covered by this document

Acronyms

Acronyms

...

List of

...

This Product Quality Assessment Report (PQAR) provides validation results for the second version of the Climate Data Records (TCDR and ICDR) of the Total Solar Irradiance (TSI) part of the Essential Climate Variable (ECV) Earth Radiation Budget (ERB). These records are generated in the frame of C3S 312b-lot1 project as a composite of various TSI records.

The scope of this PQAR document is limited to the presentation of the validation results for the TCDR and ICDR. The methodology is detailed in the Product Quality Assurance Document (PQAD, [D6]).

Executive summary

The Total Solar Irradiance (TSI) quantifies the amount of solar energy that is received by the Earth. The TSI is defined as the amount of solar power that reaches the Earth per unit surface perpendicular to the Sun–Earth direction, reduced to the mean Earth-Sun distance (1 Astronomical unit).

The Total Solar Irradiance (TSI) is a fundamental variable governing the climate system, and is recognized as an ECV by the GCOS. Within the C3S, a long composite TSI record is constructed from measurements of an ensemble of space instruments. The measurements of the individual instruments are first put on a common radiometric scale, and their quality is assessed by intercomparison. Then, the composite time series is constructed as the average of the available measurements, on a daily basis. The full processing is described in the ATBD [D3] and also in two journal papers [D1] and [D2].

In its version 2.0, the C3S daily TSI composite covers the time period from 1^st January 1979 to 31^st December 2018. The record is regularly updated as ICDR with increasing version number v2.x. This PQAR document includes ICDR data v2.2 up to 31^st of October 2020.

The TSI timeseries can be accessed via the Copernicus Climate Data Store (CDS) at https://cds.climate.copernicus.eu.

1. Product validation methodology

There are no reference observations that can be used for a direct validation of the Total Solar Irradiance (TSI). Instead, the accuracy is estimated as an intercomparison with TSI records derived by other teams based, when possible, on other input data.  

The validation methodology is fully described in the Product Quality Assurance Document [D6].

In short, the validation involves :

• the evaluation of the individual instrument timeseries with the composite CDR,
• the intercomparison of the composite CDR with the NOAA NRL TSI v2 daily TSI CDR (Coddington et al, 2015),
• the intercomparison of the composite CDR with the SATIRE-S (Yeo et al, 2014a and 2014b) record (available at http://www2.mps.mpg.de/projects/sun-climate/data.html)

2. Validation results

2.1 Individual timeseries evaluations

A total of 12 different TSI instruments have been compiled to create the C3S TCDR/ICDR as detailed in the ATBD [D3]. To put them on the same radiometric level, a scaling factor is applied for each instrument as detailed in [D6]. The resulting timeseries are said to be “adjusted”.

In Figure 1, Figure 2 and Figure 3, each of the adjusted individual timeseries (described in [D3]) is evaluated by comparison with the C3S and the NRL TSI v2 composites. On these Figures, the red parts of the timeseries are the parts that have been discarded according to Dewitte and Nevens (2016) [D1]. Once the red parts are discarded, the 12 instruments show close agreement with the 2 composites. Table 1 provides the bias, the RMS and the bias-corrected RMS (bc-RMS) between the adjusted individual timeseries and the resulting C3S composite. The last column indicates if the data are used in the TCDR v2.0 and/or the ICDR v2.x. The (adjusted) instrument bias remains below 0.1 W/m² and the bc-RMS improves in time from about 0.17 W/m² down to about 0.05 W/m² with the recent TIM instruments. As an exception, the short SOVAP timeseries (used over 19 Nov. 2010 – 3 Nov. 2013) exhibits higher variability (bc-RMS of 0.190 W/m²).

...

Instrument

...

Adjustment
factor

...

Number of daily values

...

Bias
(W/m²)

...

RMS
(W/m²)

...

bc-RMS
(W/m²)

...

Used in TCDR v2.0 (T) and/or ICDR v2.x (I)

...

ERB

...

0.993204

...

3287

...

0.042

...

0.184

...

0.179

...

T

...

ACRIM1

...

0.996232

...

3374

...

-0.042

...

0.176

...

0.171

...

T

...

ERBS

...

0.997864

...

5775

...

-0.035

...

0.098

...

0.092

...

T

...

ACRIM2

...

0.998587

...

3502

...

0.041

...

0.141

...

0.135

...

T

...

DIARAD/VIRGO

...

0.997241

...

8370

...

-0.029

...

0.095

...

0.091

...

T+I

...

PMO06/VIRGO

...

0. 997609

...

7566

...

-0.021

...

0.090

...

0.087

...

T

...

ACRIM3

...

1.000938

...

3900

...

0.045

...

0.090

...

0.079

...

T

...

TIM/SORCE

...

1.001216
(+drift correction [D6]

...

6000

...

-0.001

...

0.041

...

0.040

...

T+I

...

PREMOS

...

1.001085

...

1120

...

0.010

...

0.118

...

0.117

...

T

...

SOVAP

...

1.000518

...

1165

...

0.075

...

0.203

...

0.189

...

T

...

TIM/TCTE

...

1.000633

...

1477

...

0.085

...

0.102

...

0.057

...

T+I

...

TIM/TSIS1

...

1.000450

...

1066

...

0.075

...

0.093

...

0.056

...

T+I

...

Image Removed

Figure 1: Evaluation of the individual daily TSI timeseries for ERB, ACRIM-1, ERBS and ACRIM-2. The green curves show the data used in the C3S composite while the red parts show data discarded from the C3S composite. The blue and orange curves show the 'running mean' (121 days time period) of the individual instruments, respectively for the period kept/discarded from the composite. Black and brown curves show the C3S and NRL TSI v2 composites after '121 days running mean'.

...

Image Removed

Figure 2: Evaluation of the individual daily TSI timeseries for DIARAD/VIRGO, PMO06/VIRGO, ACRIM3 and TIM/SORCE. The green curves show the data used in the C3S composite while the red parts show data discarded from the C3S composite. The blue and orange curves show the 'running mean' (121 days time period) of the individual instruments, respectively for the period kept/discarded from the composite. Black and brown curves show the C3S and NRL TSI v2 composites after '121 days running mean'.

...

Image Removed

Figure 3: Evaluation of the individual daily TSI timeseries for PREMOS,SOVAP, TIM/TCTE, and TIM/TSIS1. The green curves show the data used in the C3S composite while the red parts show data discarded from the C3S composite. The blue and orange curves show the 'running mean' (121 days time period) of the individual instruments, respectively for the period kept/discarded from the composite. Black and brown curves show the C3S and NRL TSI v2 composites after '121 days running mean'.

2.2 TCDR and ICDR comparison with NRL TSI v2 composite

Figure 4 shows the comparison of the C3S and NRL TSI v2 (see PQAD [D6]) daily TSI composites. The two composites are in close agreement, although the period before 1984 shows a higher level of daily TSI difference.

Table 2 provides the bias, the RMS and the bias-corrected RMS (bc-RMS) between the two timeseries. A significant bias of 1.4 W/m² exists. Once corrected for the bias, the daily TSI values are in close agreement, with bc-RMS of only 0.253 W/m² over the TCDR period. Over the ICDR period, the bc-RMS goes down to 0.061 W/m². The better consistency during the ICDR period could be attributed to the lower variability of the TSI at the solar minimum.

...

Table 2: Bias and RMS difference between the C3S and NRL v2 daily TSI composites.

...

CDR

...

Number of daily values

...

Bias
(W/m²)

...

RMS
(W/m²)

...

bc-RMS
(W/m²)

...

TCDR

...

14610

...

1.403

...

1.426

...

0.253

...

ICDR

...

639

...

1.411

...

1.412

...

0.061

...

Image Removed

Figure 4: Top: C3S daily TSI composite (green) and NRL TSI v2 composite (red). Bottom : difference between the C3S and NRL TSI v2 daily TSI composites (green). The 121-days running mean curves are also shown (black).

2.3 TCDR and ICDR comparison with SATIRE-S

Figure 5 shows the comparison of the C3S and SATIRE-S reconstruction (see PQAD [D6]) daily TSI composites. The 2 composites are in close agreement. At the very beginning of the TCDR, before 16 Feb. 1980, the TCDR is mostly based on SATIRE which makes the comparison not relevant. This period has been discarded in the present analysis.

Table 3 provides the bias, the RMS and the bias-corrected RMS (bc-RMS) between the 2 timeseries. The bias shows more temporal variability than in the comparison of C3S and NRL TSI-v2. A significant bias of 1.3 W/m² exists. Once corrected for the bias, the daily TSI values are in close agreement, with bc-RMS of 0.287 W/m² over the TCDR period. Over the ICDR period, the bc-RMS decreases to 0.052 W/m². This improvement could be attributed to the lower variability of the TSI at the solar minimum.

...

tables

List of figures

General definitions

b = \frac{1}{N} \sum\limits_{i=1}^{N} (x_i - r_i)

bcRMSD = \sqrt{\frac{1}{N} \sum\limits_{i=1}^{N} (x_i - b -r_i)^2}

Scope of the document

This Product Quality Assessment Report (PQAR) summarizes the validation results for the third version (v3.0) of the Climate Data Records (CDR) of the C3S daily Total Solar Irradiance (TSI). The report also covers the first issue (i.e. v3.1) of its temporal continuation as Interim Climate Data Record (ICDR). These records are generated in the frame of the C3S2 312a-lot1 project as a composite of various TSI records.

The scope of the PQAR document is limited to the presentation of the validation results for the CDR and ICDR. The algorithm is presented in a separate Algorithm Theoretical Basis Document (ATBD) [D1] and the methodology followed for the validation is detailed in the Product Quality Assurance Document (PQAD) [D2]. Information about an appropriate use of the TSI data are given in the Product User Guide and Specifications (PUGS) [D3].

Executive summary

The Total Solar Irradiance (TSI) quantifies the amount of solar energy that is received by the Earth, per unit surface perpendicular to the Sun–Earth direction at the mean Sun–Earth distance. The TSI is a fundamental variable governing the climate system and is recognized as an Essential Climate Variable (ECV) by the Global Climate Observing System (GCOS).

Within the Copernicus Climate Change Service (C3S), a long composite Climate Data Record (CDR) is constructed from different TSI measurements obtained by an ensemble of space instruments. To create the composite, the measurements of the individual instruments are first put on a common absolute scale, and their quality is assessed by intercomparison. Then, the composite time series is created as an average of all available measurements, on a daily basis. The full processing is described in an ATBD [D1] and the technical implementation in a System Quality Assurance Document (SQAD) [D4]. The TSI timeseries (CDR and interim ICDR) can be accessed via the Copernicus Climate Data Store (CDS) at https://cds.climate.copernicus.eu.

This document presents the results of the CDR validation. The results of this exercise show that the CDR complies with the target accuracy requirement of 1 W/m². The long term stability seems also to comply with the target requirement of 0.3 W/m²/decade. However, short term variation of the systematic error could slightly exceed this value.

1. Product validation methodology

There are no reference observations (i.e. Fiducial Reference Measurement, FRM) that can be used for a direct validation of the daily Total Solar Irradiance (TSI) record. For this reason, the validation is based on intercomparison with independent TSI records, derived by other teams and based, as far as possible, on independent input data.

The validation methodology is described and justified in the Product Quality Assurance Document (PQAD) [D2]. In short, the validation involves:

• The evaluation of the 12 individual input instrument timeseries with the C3S composite and the NOAA NRLTSI2 daily TSI CDR (Coddington et al, 2015) data. This evaluation aims at confirming the accuracy of the adjustment factor, the stability of the input data, and the rejection of some periods and/or specific dates (outliers) for some of the early instruments. Results are presented in Section 2.1.

• The intercomparison of the resulting C3S composite CDR with the NRLTSI2 and the SATIRE-S (Yeo et al, 2014a and 2014b) records. These 2 products are the reconstruction of the TSI based on Sun observations like the number of Sunspots. These models are based on independent input data and are then highly valuable for the validation. Results are presented in Section 2.2.

• The intercomparison with the Community Consensus TSI record (Dudok de Wit, 2017). This record is however based on mostly the same input data, which limits the usefulness of this record for validation. Results are presented in Section 2.2.

• For the Interim CDR (ICDR), i.e. the data after 01.01.2021, a first validation/verification is performed by comparison with the NRTLTSI2 data. A Key Performance Indicator (KPI) is defined that is regularly monitored. Results for the ICDR period (i.e. 01.01.2021 to 31.12.2022 at time of writing) are presented in Section 2.3.

The description of the different reference records used for validation (NRLTSI2, SATIRE-S, Community Consensus TSI) is given in the PQAD [D2].

2. Validation results

Anchor
section2_1 section2_1

2.1 Individual timeseries evaluations

A total of 12 different TSI instruments have been compiled to create the v3.0C3S CDR and v3.1 ICDR, as detailed in the ATBD [D1]. To put them on a same radiometric level, a scaling factor is determined and applied for each instrument. The resulting timeseries are said to be adjusted.

In Figure 2-1, Figure 2-2 and Figure 2-3, each of the adjusted individual timeseries is evaluated by comparison with the C3S composite and the NRL TSI v2 reconstruction. On these Figures, the red parts of the timeseries are the parts that have been discarded as described in the ATBD [D1]. Once the red parts are discarded, the 12 instruments show close agreement with the C3S composite and the NRLTSI2 reconstruction. Table 2-1 provides the bias and the bias-corrected RMS (bc-RMS) of the adjusted individual timeseries with respect to the C3S composite and the NRLTSI2 reconstruction. As expected, the bias between the adjusted individual instrument and the C3S composite is always very small (less than 0.03W/m²). The biases with respect to NRLTSI2 are always positive, and show more variability (between 0.23 and 0.41 W/m²). The RMS difference with the C3S composite is however less informative, as the C3S composite incorporates the different individual instruments under evaluation. The last column, the RMS difference with respect to NRLTSI2, shows a general improvement over time of the quality of the input data (the bias corrected RMS decreases from 0.32 to 0.10 W/m²). However, part of this improvement could be attributed to a better accuracy of the NRLTSI2 reconstruction with time.

Image Added

Image Added

Image Added

Anchor
section2_2 section2_2

2.2 Comparison with SATIRE-S, NRLTSI2 and Community Consensus TSI (CCTSI)

In Figure 2-4, the C3S v3.0/v3.1 composite is compared with the NRLTSI2 and SATIRE-S reconstructions, and with the Community Consensus TSI (CCTSI hereafter) composite. the following metrics are estimated to compare the C3S product with respect to NRLTSI2, SATIRE-S and CCTSI:

• The average bias (W/m²)

• The bias corrected Root Mean Square (bc-RMS) difference (W/m²) for the daily values.

• The bias corrected Root Mean Square (bc-RMS) difference (W/m²) after 121-days running means.

Table 2-2 provides these metrics, evaluated over the v3.0 CDR period (01.01.1979 to 31.12.2020). The table also gives the parameters ‘a’ and ‘b’ used to estimate the best fit line, between the bias of the C3S product and the relevant reference dataset, estimated as y = a + b * (x-2001), where the value of 2001 is selected approximately in the middle of the CDR (this choice does not impact the value of b but allows to have intercept value a of the order of the overall bias).

To assess the stability, Figure 2-5 shows the variation between the bias of the C3S composite and the 3 reference datasets (121-days running mean) and the linear fits. With respect to NRLTSI2, the bias is stable with no significant long term trend. On the contrary, there is a clear increase of the bias with respect to SATIRE-S and a clear decrease with respect to the Community Consensus TSI record. Finally, Figure 2-6 shows the temporal variation of the bias corrected RMS difference between the C3S composite and the 3 reference datasets. In this graph, the RMS difference is estimated over a 121-day running windows. The figure shows limited and stable RMS difference with respect to SATIRE-S while there is more variation with respect to NRLTSI2 and the community consensus TSI records.

Image Added

Image Added

Image Added

Anchor
section2_3 section2_3

2.3 Key Performance Indicator (KPI) for the ICDR product

The v3.1 ICDR period (01.01.2021 - 30.09.2023) is regularly evaluated by intercomparison with the independent NRLTSI2 reconstruction. As Key Performance Indicator (KPI), it is verified that the TSI difference remains statistically consistent with the differences observed over the CDR period (01.01.1979 to 31.12.2020). To this end, the 2.5% and 97.5% percentiles of the difference are estimated over the CDR period, as illustrated in Figure 2-7. These 2.5% and 97.5% percentiles are +0.120 W/m² and +0.537 W/m², respectively. The ICDR verification is performed on the 121-day running mean difference (red curve) and not the daily values (black curve).

The ICDR data from 01.01.2021 to 31.12.2022 remains well within these percentiles.

Image Added

2.4 Summary of validation results

The quality assessment of the C3S daily TSI composite v3.0 and its first ICDR extension (v3.1) can be summarized as follows.

• The individual time series evaluation (Section 2.1) confirms the values of the adjustment factors and the validity periods determined in the ATBD [D1]. After adjustment, the (absolute) biases with respect to the composite remain well below 0.03 W/m² and the RMS difference with respect to NRLTSI2 goes from about 0.3 W/m² for early instrument (e.g. ERB on Nimbus-7) down to about 0.1 W/m² for the most recent instruments (e.g. TIM).

• The comparison with the NRLTSI2 record shows a significant overall bias of 0.34 W/m². This bias is larger than the bias-corrected RMS difference of daily TSI value which is about 0.21 W/m² over the full TCDR period. After a 121-day running mean is calculated, the (max-min) temporal variation of the bias remains limited to 0.6 W/m² over the more than 4 decades covered by the CDR. This is about the double of the GCOS (GCOS-200,2016) stability requirements of 0.01%/decade (i.e. 0.136 W/m²/decade). It is worth mentioning that most of the variation of the bias is observed over the early part of the CDR (01.01.1979 – 31.12.1982) and better stability is observed afterwards, with a (max-min) variation of the level of 0.4 W/m².

• The analysis of the differences with SATIRE-S and with the community consensus TSI shows significant temporal variation of the bias, but in opposite directions. For this reason, these two time series are not used for the CDR stability assessment or for the ICDR quality check (KPI).

• The results obtained over the ICDR period (1^st January 2021 onward) are in general better than over the CDR period. This is explained by the higher quality of the observations from the space instruments which are currently observing the Sun, in particular the TIM instruments. Concerning the KPI, the ICDR period complies mostly with the 2.5% - 97.5% percentiles interval.

3. Application(s) specific assessments

(no application defined yet for this CDR).

4. Compliance with user requirements

Averaged over the full CDR period, the C3S v3.0 daily TSI composite is 0.38 W/m² higher than the NRL TSI v2 reconstruction, 0.29 W/m² higher than SATIRE-S and -0.20 W/m² lower than the community consensus TSI composite. These results indicate that the C3S absolute accuracy complies with the 0.04% (i.e. 0.6 W/m²) accuracy requirement defined by GCOS (2016). The bias is largely dependent on the choice of the reference instrument(s) selected to homogenize the composite timeseries.

From the results presented in this report, it is not possible to conclude that the stability of the C3S TSI record (i.e. the maximum change of the systematic error over a running 10-year time period) complies with the 0.14 W/m²/decade GCOS requirements. The best stability is observed with respect to NRLTSI2 and no significant long-term change is observed over the 1979-2020 time period. However, some significant variations of the bias are observed of the order of 0.6 W/m² (max-min difference) which is about four time the 0.14 W/m²/decade GCOS requirement for stability.

The validation results reported in this document provide evidence that the accuracy is better than 1 W/m² for the daily mean TSI. This accuracy is not constant in time over the duration of the record as the number and the accuracy of the contributing instruments has varied in time.

Anchor
references references

References

Dudok de Wit, T., Kopp, G., Fröhlich, C., & Schöll, M. (2017): Methodology to create a new total solar irradiance record: Making a composite out of multiple data records. Geophysical Research Letters, 44(3), 1196-1203. https://www.doi.org/10.1002/2016GL071866

Coddington O., J. L. Lean, D. Lindholm, P. Pilewskie, M. Snow, and NOAA CDR Program (2015): NOAA Climate Data Record (CDR) of Total Solar Irradiance (TSI), NRLTSI Version 2. [1984-2020]. NOAA National Centers for Environmental Information. https://doi.org/10.7289/V55B00C1

Available at: https://data.nodc.noaa.gov/cgi-bin/iso?id=gov.noaa.ncdc:C00899

GCOS-200 (2016) : The Global Observing System for Climate Implementation Needs, https://library.wmo.int/idurl/4/55469

Yeo K.L., Krivova N.A., Solanki S.K., Glassmeier K.H. (2014a).Reconstruction of total and spectral solar irradiance from 1974 to 2013 based on KPVT, SoHO/MDI and SDO/HMI observations. Astron. Astrophys. 570, A85 (2014). https://www.doi.org/10.1051/0004-6361/201423628

Yeo K.L., Krivova N.A., Solanki S.K. (2014b) Solar cycle variation in solar irradiance. Space Sci. Rev. (2014). https://www.doi.org/10.1007/s11214-014-0061-7

Related articles

...

CDR

...

Number of daily values

...

Bias
(W/m²)

...

RMS
(W/m²)

...

bc-RMS
(W/m²)

...

TCDR

...

14199

...

1.340

...

1.371

...

0.287

...

ICDR

...

502

...

1.546

...

1.547

...

0.052

...

Image Removed

Figure 5: Top: C3S daily TSI composite (green) and SATIRE-S reconstruction (red). Bottom : difference between the C3S and SATIRE-S daily TSI composites (green). The 121-days running mean curves are also shown (black).

3. Application(s) specific assessments

N/A

4. Compliance with user requirements

The C3S daily TSI record is about 1.4 W/m² higher than SATIRE-S and NRL TSI v2 records, which is out of the 1 W/m² accuracy requirement from GCOS. This (positive) bias is largely dependent on the choice of the reference instrument(s) used to homogenize the composite timeseries.

The validation results reported in this document provide evidence that, if the overall bias is not considered, the accuracy is much better than 1 W/m² for the daily mean TSI. Once bias-corrected, the RMS differences with the SATIRE-S and NRL TSI v2 records are both at about 0.25 W/m².

From the results presented in this report, it is not possible to conclude that the stability of the C3S TSI record (i.e. the maximum change of the systematic error over a running 10-years time period) complies with the 0.3 W/m²/decade GCOS requirements. The most significant decadal variation of the bias is 0.9 W/m²/decade. This change is observed with respect to the SATIRE-S between 1980 (bias about 1.6 W/m²) and 1984 (bias about 0.7 W/m²). With respect to NRL TSI v2 the variation of the bias is limited to 0.8 W/m² over the entire period covered by the TCDR.

References

Dewitte, S. and Nevens, S. (2016). The total solar irradiance climate data record. The Astrophysical Journal, 830(1), p.25.   

Dewitte, S. and Clerbaux, N., (2017). Measurement of the earth radiation budget at the top of the atmosphere—a review. Remote Sensing, 9(11), p.1143.

Odele Coddington, Judith L. Lean, Doug Lindholm, Peter Pilewskie, Martin Snow, and NOAA CDR Program (2015): NOAA Climate Data Record (CDR) of Total Solar Irradiance (TSI), NRLTSI Version 2. [1984-2020]. NOAA National Centers for Environmental Information. doi:10.7289/V55B00C1

Available at: https://data.nodc.noaa.gov/cgi-bin/iso?id=gov.noaa.ncdc:C00899

Yeo K.L., Krivova N.A., Solanki S.K., Glassmeier K.H. (2014a).Reconstruction of total and spectral solar irradiance from 1974 to 2013 based on KPVT, SoHO/MDI and SDO/HMI observations. Astron. Astrophys. 570, A85 (2014). DOI: 10.1051/0004-6361/201423628

Yeo K.L., Krivova N.A., Solanki S.K. (2014b) Solar cycle variation in solar irradiance. Space Sci. Rev. (2014). DOI 10.1007/s11214-014-0061-7

SATIRE-S TIS dataset: http://www2.mps.mpg.de/projects/sun-climate/data/SATIRE-S_TSI_20190621.txt