Last modified on Jan 29, 2024 14:33

Contributors: N. Clerbaux (Royal Meteorological Institute of Belgium (RMIB)), A. Velazquez Blazquez (RMIB)

Issued by: RMIB/Nicolas Clerbaux

Date: 27/11/2023

Ref: C3S2_D312a_Lot1.1.2.5-v1.0_202212_PQAD_ECVEarthRadiationBudget_v1.1

Official reference number service contract: 2021/C3S2_312a_Lot1_DWD/SC1

History of modifications

List of datasets covered by this document

Related documents

Acronyms

List of figures

List of tables

General definitions

Scope of the document

This Product Quality Assessment Document (PQAD) details how the Climate Data Records (CDRs) of the daily Total Solar Irradiance (TSI) are validated prior to data release. These CDRs, part of the Essential Climate Variable (ECV) Earth Radiation Budget (ERB), are generated at the Royal Meteorological Institute of Belgium (RMIB) in the frame of the C3S2-312a-lot1 project as composites of different TSI space instrument records.

The scope of this PQAD document is limited to the presentation of the quality assessment methodology and data used for this assessment. The results themselves are reported in the Product Quality Assurance Reports (PQAR) [D5], which are regularly updated to account for the inclusion of new data in the Interim Climate Data Record (ICDR). However, for the sake of illustration of the methodology, preliminary results are also shown.

Executive summary

The Total Solar Irradiance (TSI) quantifies the amount of solar energy that is received by the Earth. This flux is defined per unit surface perpendicular to the Sun–Earth direction, reduced to the mean Earth-Sun distance (1 Astronomical Unit). The TSI is a fundamental variable governing the climate system, and is recognized as an ECV by the Global Climate Observing System (GCOS). Within the Copernicus Climate Change Service (C3S), a long time composite 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 intercomparisons with independent TSI reconstructions (SATIRE-S and NRLTSI2). 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 [D6] and also in two journal papers [D1] and [D2].

In its version 3.0, the CDR covers the time period from 1^st January 1979 to 31^st December 2020. This record is regularly extended with an Interim Climate Data Record (ICDR) reported as an increasing version “x” number v3.x (i.e., V3.1, V3.2, etc.).

There are no fiducial reference measurements that can be used for a direct validation of the TSI. Instead, the quality is assessed from intercomparisons with TSI records derived by other teams based, when possible, on independent input data. The quality assessment involves an intercomparison with the NOAA NRLTSI2 CDR (Coddington et al, 2015), an intercomparison with the SATIRE-S (Yeo et al, 2014a and 2014b), an intercomparison with the “Community Consensus” TSI composite and also the evaluation of the individual instrument timeseries with the C3S composite.

1. Validated products

The validated product is the version 3 of the daily TSI composite produced by the Royal Meteorological Institute of Belgium (RMIB) for the Copernicus Climate Change Service (C3S) as part of the C3S2_312a_Lot1 contract. This Climate Data Record (CDR) is constructed from timeseries of daily TSI measured by an ensemble of space instruments. Currently 12 instruments are used in the composite as detailed in Table 1-1.

Table 1-1: The instruments used in the C3S TSI v3.0 composite.

The method can be summarized as follow:

• First, the 12 individual timeseries are quality checked by comparison with 2 models of the daily TSI, namely NRLTSI2 (see section 2.1) and SATIRE-S (see section 2.2).

• Second, the measurements of the individual instruments are put on a common absolute scale using optimized radiometric correction factors. The factors are determined to maximize the consistency during the instruments’ overlap periods.

• Lastly, the composite is created as an average of the available measurements, on a daily basis.

The method has been selected according to [D1] and is fully documented in the ATBD [D6]. The product is fully described in the Product User Guide and Specification (PUGS) document [D4].

The TSI timeseries can be accessed via the C3S Climate Data Store (CDS) at https://cds.climate.copernicus.eu

Figure 1-1 shows the version 3.1 TSI composite, including the ICDR interim data from 01.01.2021 onward. The grey curve is the daily value while the red curve shows the 121-days running mean. To illustrate the stability of the record between the solar cycles, a horizontal line at the arbitrary value of TSI=1361 W/m² is shown.

Figure 1-1: C3S composite daily TSI values (grey) and 121-day running mean (red). The 1361.0 W/m² line illustrates the stability of the solar minima.

2. Description of validating datasets

2.1 Naval Research Laboratory TSI version 2 (NRLTSI2)

This CDR was created at the Space Science Division of the Naval Research Laboratory (NRL) in collaboration with the Laboratory for Atmospheric and Space Physics (LASP) of the University of Colorado. The NRLTSI2 CDR is published as part of the NOAA CDR Program and is documented by Coddington et al. (2015, 2016).

In NRLTSI2, a model estimates the TSI from the observation of the bright faculae and the dark sunspots on the solar disk. A linear regression between these proxies of solar activity and the TIM/SORCE TSI was established and used in the reconstruction. The model assumes a quiet Sun TSI of 1360.45 W/m² (Kopp and Lean, 2011) as estimated from the TIM/SORCE measurement at solar minimum. The NRLTSI2 starts on 1^st January 1882 and provides data until 30^th September 2020 (at time of writing). New data are regularly added to the timeseries, on a quarterly basis. Figure 2-1 shows the NRLTSI2 daily TSI timeseries (from 1976 onward) as available at

http://lasp.colorado.edu/lisird/data/nrl2_files (at LASP)

https://www.ncei.noaa.gov/products/climate-data-records/total-solar-irradiance (NOAA/NCEI)

Figure 2-1: NRLTSI2 daily values (grey) and 121-days running mean (horizontal line at 1360.45 W/m² to illustrate the stability of the solar minima). Only data from 1976 onward are shown.

2.2 SATIRE-S

The SATIRE-S (Spectral And Total Irradiance Reconstructions, Yeo et al, 2014a and 2014b) is a reconstruction of the TSI over the 1974-present-day period using full-disc magnetograms and continuum images of the Sun. It uses the data from the National Solar Observatory Photospheric magnetogram (NSO KP) (1974-1999), the SOHO/ Michelson Doppler Imager (MDI) (1999-2009) and the Solar Dynamics Observatory (SDO) Helioseismic and Magnetic Imager (HMI) (since 2010). These observations allow estimation of the fractional coverage of: quiet Sun, sunspot umbrae, sunspot penumbrae, faculae and network. A regression between these indices and the TSI is then derived and used in the reconstruction. The SATIRE-S data starts on 23^rd August 1974 and data are provided until 23^rd September 2022 (at time of writing). New data are regularly added to the timeseries. Figure 2-2 shows the SATIRE-S TSI timeseries as daily values (grey curve) and after 121-days running mean smoothing (red curve). An arbitrary horizontal line at a TSI value of 1360.75 W/m² is shown to highlight the change of the TSI between the solar minima. These changes are not fully consistent with other reconstructions and models (e.g. the NRLTSI2). This makes the SATIRE-S reconstruction not fully suitable to assess the C3S composite stability.

The SATIRE-S record can be accessed at http://www2.mps.mpg.de/projects/sun-climate/data_body.html

Figure 2-2: SATIRE-S daily TSI values (grey curve) and 121-days running mean (red curve). The arbitrary horizontal line at 1360.75 W/m² is shown to highlight the change of TSI between the solar minima.

2.3 The Community Consensus TSI composite (CCTSI)

This record is not an independent record as it is based on almost the same ensemble of space instruments. The methodology to create the record is described in Dudok de Wit et al. (2017) and in Montillet et al. (2022).

Figure 2-3 shows the TSI composite created using the methodology of Dudok de Wit et al. (2017). The contributing instruments include the NIMBUS7, ACRIM1, ACRIM2, ERBS, VIRGO, ACRIM3, SORCE, PREMOS, SOVAP, TCTE, and TSIS-1. This list is identical to the one used in the C3S composite, except that the DIARAD/VIRGO is not considered in the community consensus TSI. The timeseries can be accessed at https://spot.colorado.edu/~koppg/TSI/TSI_Composite-SIST.txt

Figure 2-3: Community Consensus daily TSI values (grey curve) and 121-days running mean (red curve). An arbitrary horizontal line at 1361.2 W/m² is shown to highlight the change in the solar minima TSI values.

3. Description of product validation methodology

3.1 Individual timeseries evaluation

First, each of the individual timeseries for the different TSI instruments is visually inspected and compared with the C3S daily TSI composite and the NRLTSI2 reconstruction. This evaluation is performed to confirm the suitability of the data as discussed in the ATBD [D6]. In particular, this part of the quality assessment aims at confirming the rejection of some period(s) of the individual timeseries in the C3S composite. As an example, Figure 3-1 shows the evaluation of the DIARAD/VIRGO timeseries. Following the recommendation of the DIARAD/VIRGO team, the TSI data before 1^st January 1997 have not been used in the C3S composite. Figure 3-1 shows that, indeed, this early part of the timeseries (red and orange curves) does not agree neither with the composite nor with the NRLTSI2 data. The green (daily) and blue (121-days running mean) curves are DIARAD/VIRGO data which are used in constructing the C3S composite. This part shows good agreement with the C3S composite and NRLTSI2.

Figure 3-1: Evaluation of the DIARAD/VIRGO instrument timeseries. The green curve shows the adjusted daily TSI value of the instrument. The red part of the curve shows the daily TSI values which have been discarded following [D1]. The blue and orange curves show the corresponding 121-days running mean. The evaluation is performed with respect to the C3S and NRLTSIv2 121-days running mean values (thin black and red curves).

Such visual inspections are performed before each release of the C3S CDR and the plots, 12 in total, are provided and discussed in the Product Quality Assessment Reports (PQAR). In addition to the visual inspection, the following metrics are also estimated for each instrument with respect to the C3S composite (see details in the General Definitions section):

• Average bias (W/m²)

• Bias corrected RMS (bc-RMS) difference (W/m²).

The Table 3-1 gives the biases and RMS differences between the instruments (after scaling and selected) and the C3S v3.1 composite.

Table 3-1: Individual timeseries evaluation with respect to C3S v3.1 composite.

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

As shown in Figure 3-2, the daily TSI values in the C3S composite are compared with the NRLTSI2 and SATIRE-S reconstructions, and with the CCTSI composite. A visual inspection of the anomalies is also performed to assess the stability (Figure 3-3). However, the use of SATIRE-S to confirm the TCDR stability should be treated with caution given the apparent trend in the level of Quiet Sun TSI in the SATIRE-S reconstruction (see Figure 2-2) which is not present in the NRLTSI2 data.

In addition to these visual inspections, the following metrics are also estimated with respect to NRLTSI2, SATIRE-S and CCTSI:

• Average bias (W/m²)

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

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

Table 3-2 provides these metrics, evaluated over the CDR period (01.01.1979 to 31.12.2020), for the preliminary data (final numbers will be provided in the PQAR [D5]).

Table 3-2: Bias and bias-corrected Root Mean Square (bc-RMS) differences for the daily values and after 121-days running mean (based on preliminary v3.0 C3S composite).

Figure 3-2: C3S TSI daily values (grey) and 121-days running mean (black). Running means for SATIRE-S (red), NRLTSI2 (green) and Community Consensus TSI (blue) are also shown.

Figure 3-3: Differences between the C3S v3.x composite and: SATIRE-S (green curve), NRLTSI2 (blue curve) and Community Composite TSI (red curve). The curves shown are 121-days running mean average. Also shown with horizontal lines are the mean biases over the CDR (+0.34, +0.31, -0.19 W/m² respectively).

3.3 Key Performance Indicator (KPI) for the ICDR product

The ICDR period (01.01.2021 onward) is regularly evaluated by intercomparison with the independent NRLTSI2 reconstruction. As a 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 3-4. The 2.5% and 97.5% percentiles are +0.120 W/m² and +0.537 W/m², respectively. To reduce the dependency on the level of solar activity, the ICDR verification is performed on the 121-days running mean difference (red curve) and not the daily values (black curve).

The ICDR v3.1 data from 01.01.2021 to 30.09.2023 remains mainly within these percentiles.

Figure 3-4: Difference between the C3S v3.1 composite and NRLTSI2 daily values (black curve) and 121-days running mean (red curve). Also shown with horizontal lines are the 2.5% and 97.5% percentiles on the 121-days running mean difference (+0.120 and +0.537 W/m², respectively).

4. Summary of validation results

The PQAR [D5] provides the full quality assessment of the C3S daily TSI composite v3.0 and its ICDR extension (v3.1). Additional information about requirements and gaps for the ERB dataset on TSI can be found in the Target Requirements and Gap Analysis Document (TRGAD) [D3].

In summary for the v3.0 and v3.1:

• The individual timeseries evaluation (Table 3-1) confirms the values of the adjustment factors and validity periods determined in the ATBD [D6]. After adjustment, the (absolute) biases with respect to the composite remain well below 0.05 W/m² and the RMS difference decreased from about 0.18 W/m² for early instrument (e.g. ERB) to about 0.05 W/m² for the most recent instruments (e.g. TIM). The DIARAD/VIRGO and PMO06/VIRGO instruments, which provide the longest contributions to the CDR (with 8834 and 9063 daily values respectively), have RMS differences with the composite of about 0.12 W/m². The SOVAP instrument on the Picard platform is of significantly lower quality compared with the PREMOS instrument on the same satellite.

• The comparison with the NRLTSI2 shows a significant overall bias of 0.34 W/m². This is much higher than the bias-corrected RMS difference of daily TSI value which is about 0.23 W/m² over the full TCDR period [1979-2020]). When calculated over a 121-day running mean, the temporal variation of the bias is limited to 0.85 W/m² over the entire period covered by the CDR which is more than double the value of the GCOS stability requirements of 0.3 W/m²/decade. It should be noted 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 afterward (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, in opposite directions. For this reason, these timeseries are not used for C3S stability assessment or for the ICDR quality check and 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 space instruments which are currently observing the Sun, in particular the TIM instruments. Concerning the KPI, the ICDR period complies most of the time with the 2.5% - 97.5% percentiles interval.

References

Coddington, O., Lean J.L., Lindholm D., Pilewskie P., Snow M., and NOAA CDR Program (2015): NOAA Climate Data Record (CDR) of Total Solar Irradiance (TSI), NRLTSI Version 2. https://doi.org/10.7289/V55B00C1

Coddington, O., Lean, J.L., Pilewskie, P., Snow, M. and Lindholm, D. (2016): A solar irradiance climate data record, Bulletin of the American Meteorological Society, 97(7), pp.1265-1282. https://doi.org/10.1175/BAMS-D-14-00265.1

Dewitte, S., Crommelynck, D., and Joukoff, A. (2004): Total solar irradiance observations from DIARAD/VIRGO, J. Geophys. Res., 109, A02102, https://doi.org/10.1029/2002JA009694

Dewitte, S., Janssen, E. and Mekaoui, S., (2013a): Science results from the SOVA-Picard total solar irradiance instrument, In AIP Conference Proceedings (Vol. 1531, No. 1, pp. 688-691) https://doi.org/10.1063/1.4804863

Dewitte, S. and Nevens, S., (2016): The total solar irradiance climate data record. The Astrophysical Journal, 830(1), p.25. https://www.doi.org/10.3847/0004-637X/830/1/25

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. https://doi.org/10.3390/rs9111143

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://doi.org/10.1002/2016GL071866

ERBE Science Team (1986): First data from the Earth Radiation Budget Experiment. Bulletin of the American Meteorological Society, 67, 818--824. https://doi.org/10.1175/1520-0477(1986)067<0818:FDFTER>2.0.CO;2

Froehlich, C., Crommelynck, D.A., Wehrli, C. et al. (1997): In-Flight Performance of the Virgo Solar Irradiance Instruments on Soho. Solar Physics, 175, 267–286. https://www.doi.org/10.1023/A:1004929108864

Hickey, J.R., Stowe L.L., Jacobowitz H., Pellegrino P., Machhoff R.H., House F., Vonder Haar, T.H. (1980): Initial solar irradiance determinations from Nimbus 7 cavity radiometer measurements, Science, 208, 281--283. https://www.doi.org/10.1126/science.208.4441.281

Kopp, G., Lawrence, G. & Rottman, G. (2005): The Total Irradiance Monitor (TIM): Science Results. Solar Physics, 230, 129–139. https://www.doi.org/10.1007/s11207-005-7433-9

Kopp, G. and Lean, J.L., (2011): A new, lower value of total solar irradiance: Evidence and climate significance. Geophysical Research Letters, 38(1). https://doi.org/10.1029/2010GL045777

Kopp, G. (2016): Magnitudes and timescales of total solar irradiance variability. J. Space Weather Space Clim, 6, A30. https://doi.org/10.1051/swsc/2016025

Kopp, G. (2020): TSIS TIM Level 3 Total Solar Irradiance 24-hour Means, version 03, Greenbelt, MD, USA: NASA Goddard Earth Science Data and Information Services Center (GES DISC), https://doi.org/10.5067/TSIS/TIM/DATA306

Montillet, J. P., Finsterle, W., Kermarrec, G., Sikonja, R., Haberreiter, M., Schmutz, W., & Dudok de Wit, T. (2022): Data Fusion of Total Solar Irradiance Composite Time Series Using 41 Years of Satellite Measurements. Journal of Geophysical Research: Atmospheres, 127(13), e2021JD036146. https://doi.org/10.1002/essoar.10508721.2

Schmutz W., Fehlmann A., Finsterle W., Kopp G., Thuillier G. (2012): Total solar irradiance measurements with PREMOS/Picard, AIP Conf. Proc, 1531, 624. https://doi.org/10.1063/1.4804847

Willson, R.C., Gulkis S., Janssen M., Hudson H.S., Chapman G.A. (1980): Observations of Solar Irradiance Variability, Science, 211, 700 - 702. https://doi.org/10.1126/science.211.4483.700

Willson, R. (1994): Irradiance Observations of SMM, Spacelab 1, UARS, and ATLAS Experiments. International Astronomical Union Colloquium, 143, 54-62. https://doi.org/10.1017/S0252921100024532

Willson, R.; Mordvinov, A. (2003) : Secular total solar irradiance trend during solar cycles 21–23. Geophysical Research Letters, 30, 1199. https://doi.org/10.1029/2002GL016038

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. https://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., https://doi.org/10.1007/s11214-014-0061-7

Related articles

• Page:
  ERB TSI: Algorithm Theoretical Basis Document (ATBD)
• Page:
  ERB TSI: Product User Guide and Specification (PUGS)
• Page:
  IV data version 1.3: System Quality Assurance Document (SQAD)
• Page:
  IV data version 1.3: Product Quality Assurance Document (PQAD)
• Page:
  IV data version 1.3: Product Quality Assessment Report (PQAR)