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

Issued by: RMIB

Date: 18/12/2020

Ref: C3S_D312b_Lot1.2.5.8-v2.1_202010_PQAD_ECVEarthRadiationBudget_v1.0

Official reference number service contract: 2018/C3S_312b_Lot1_DWD/SC1

History of modifications

List of datasets covered by this document

Related documents

Acronyms

Scope of the document

This Product Quality Assessment Document (PQAD) details how the Climate Data Records (CDRs) of the daily Total Solar Irradiance (TSI) have been validated prior to data release. These CDRs, part of the Essential Climate Variable (ECV) Earth Radiation Budget (ERB), are generated in the frame of C3S 312b-lot1 project as composites of different TSI 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) [D6], which are regularly updated to account for the inclusion of new data in the Interim Climate Data Record (ICDR).

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 ECV by the Global Climate Observing System (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 SQAD [D5] 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.

There are no reference observations 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 other input data.

In short, quality assessment involves an intercomparison with the NOAA NRLTSIv2 daily TSI CDR (Coddington et al, 2015), an intercomparison with the SATIRE-S (Yeo et al, 2014a and 2014b) record (available at http://www2.mps.mpg.de/projects/sun-climate/data.html), and also evaluation of the individual instrument timeseries with the composite CDR.

1. Validated products

The Total Solar Irradiance (TSI) quantifies the amount of solar energy that is received by the Earth.

TSI is defined as the amount of solar power that reaches the Earth’s top of the atmosphere per unit surface area perpendicular to the Sun–Earth direction at the mean Sun–Earth distance. The product is fully described in the Product User Guide and Specification (PUGS) document [D4].

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 PQAD document includes ICDR v2.2 data 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.

2. Description of validating datasets

There are no reference observations that can be used for a direct validation of the C3S Total Solar Irradiances. The validation is therefore performed as intercomparison with two other datasets which are described in this section.

2.1 SATIRE-S

SATIRE (Spectral And Total Irradiance REconstructions) uses observed changes in dark sunspots and bright faculae, and some other manifestations of the solar magnetic field, to model the spectral and total solar irradiance.

The SATIRE-S record (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. These observations allow estimating 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 provides data until 16^th May 2020 (at time of writing). New data are regularly added to the timeseries. Figure 1 shows the SATIRE-S daily TSI timeseries which is available at http://www2.mps.mpg.de/projects/sun-climate/data/SATIRE-S_TSI_20200516.txt.

Figure 1: SATIRE-S timeseries as available from http://www2.mps.mpg.de/projects/sun-climate/data/SATIRE-S_TSI_20200516.txt *. Green curve shows daily values and blue the 121-days running mean. The dashed black line at 1361 W/m² is shown to highlight* the variation of the solar minima ("quiet Sun") TSI in the SATIRE-S timeseries.

2.2 Naval Research Laboratory TSI version 2 (NRLTSIv2)

This CDR was created by Judith Lean (Space Science Division, Naval Research Laboratory), Odele Coddington and Peter Pilewskie (Laboratory for Atmospheric and Space Physics, University of Colorado). The NRLTSIv2 CDR is published as part of the NOAA CDR Program and is documented by Coddington et al. (2015, 2016).

In NRLTSIv2, 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 NRLTSIv2 daily data starts on 1^st January 1882 and provides data until 30 June 2020 (at time of writing). New data are regularly added to the timeseries, on a quarterly basis. Figure 2 shows the NRLTSIv2 daily TSI timeseries (from 1975 onward) as available at http://lasp.colorado.edu/lisird/data/nrl2_files.

Figure 2: NRLTSIv2 daily TSI timeseries (green curve) and 121-days running mean (blue curve) from 1975 onward. The dashed black line at 1360.45 W/m² is displayed to show the assumed quiet Sun TSI (Kopp and Lean, 2011) used in the NRL TSI v2 model.

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 NRLTSIv2 record. This evaluation is performed to confirm the suitability of the data as discussed in the ATBD [D3]. 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 shows the evaluation of the DIARAD/VIRGO timeseries. Following recommendation of the DIARAD/VIRGO team, the TSI data before 1^st January 1997 have not been used in the C3S composite. Figure 3 shows that, indeed, this early part of the timeseries (red and orange curves) does not agree with the composite or the NRLTSIv2 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 NRLTSIv2.

Figure 3: Illustration of the individual timeseries evaluation, here for the particular case of the  DIARAD/VIRGO instrument. 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 [D3]. The blue and orange curves show the corresponding 121-days running mean. The evaluation is performed wrt the C3S and NRLTSIv2 121-days running mean values (thin black and red curves).

Such visual evaluations are performed before each release of the C3S CDR (TCDR or ICDR) and the plots are provided 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:

• Average bias (W/m²)
• Root Mean Square (RMS) of the difference (W/m²)
• Bias corrected RMS (bc-RMS) difference (W/m²).

3.2 Comparison with SATIRE-S

The daily TSI values in the C3S composite are compared with the SATIRE-S reconstruction. The bias, RMS and bc-RMS are estimated over the TCDR and ICDR periods.

A visual inspection of the anomalies is also performed to detect possible stability problem. However, the use of SATIRE-S to confirm the TCDR stability should be done with caution given the apparent trend in the level of Quiet Sun TSI in the SATIRE-S reconstruction (see Figure 1) which is not present in the NRLTSIv2 data.

3.3 Comparison with NRLTSIv2

The daily TSI values in the C3S composite are compared with the NRLTSIv2 reconstruction. The bias, RMS and bc-RMS are estimated over the TCDR and ICDR periods. A visual inspection of the anomalies is also performed to detect possible stability problem.

4. Summary of validation results

The PQAR [D6] provides the full quality assessment of the C3S daily TSI composite v2.0 (01/01/1979 – 31/12/2018) and its ICDR extension v2.2 (01/01/2019 -31/10/2020).

In summary for TCDR v2.0 and ICDR v2.2:

• The individual timeseries evaluations confirm the values of the adjustment factors and validity periods determined in the ATBD [D3]. After adjustment, the (absolute) biases with respect to the composite remain below 0.1 W/m² and the RMS difference goes from about 0.18 W/m² for early instrument (ERB) down to about 0.04 W/m² for the most recent TIM instruments. The DIARAD/VIRGO and PMO06/VIRGO instruments, which provide the longest contributions in the CDR (with respectively 7871 and 7515 daily values), have RMS difference with the composite of about 0.09 W/m².
• The comparison with the NRLTSIv2 shows a significant overall bias of 1.4 W/m², much higher than the bias-corrected RMS difference of daily TSI value which is about 0.25 W/m² over the full TCDR period (1979-2018). After a 121-days running mean, the most significant temporal variation of the bias remains limited to 0.8 W/m²/decade ( or ~0.06%/decade) over the entire period covered by the TCDR which is higher the GCOS stability requirements of 0.01 %/decade (~0.14 W/m²/decade).
• Comparison with SATIRE-S provides similar results in terms of overall bias and RMS. However, a significant temporal variation of the bias is here observed which could indicate a problem of stability. However, this change is likely due to change in quiet Sun TSI level in the SATIRE-S record.
• The results obtained over the ICDR period (1^st January 2019 onward) are in general better than over the TCDR period. This is explained by the higher quality of the space instruments which are currently observing the Sun, in particular the 3 TIM instruments.

References

Coddington, O., 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

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.

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.

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

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

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