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Contributors: N. 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
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1Note: In the contract, this deliverable had Delivery ID D2.5.8-v1.1 |
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2Note: In the contract, this deliverable was originally Earth Radiation Budget CERES TCDR v2.0 (OLR, RSF)
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Table 2-1: Evaluation of the adjusted individual timeseries with respect to C3S daily TSI composite. Table 2-2: Bias and RMS difference between the C3S and NRL v2 daily TSI composites Table 2-3: Bias and RMS difference between the C3S and SATIRE-S composites Table 3-1: Summary of KPI results with 2.5 and 97.5 percentiles and number of ICDR months within the rangebias-corrected Root Mean Square (bc-RMS) differences for the daily values and after 121-days running mean. The ‘a’ and ‘b’ parameters of the linear fit for the bias are given in the last 2 columns. |
List of figures
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Figure 2-1: 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’. Figure 2-2Figure 2-2: 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’. Figure Figure 2-3: Evaluation of the individual daily TSI timeseries for PREMOS, SOVAP, TIM/TCTE, and TIM/TSIS1 Figure 2-4: Top: C3S daily TSI composite (green) and NRL TSI v2 composite (red) Figure 2-5: Top: C3S daily TSI composite (green) and SATIRE-S reconstruction (red) |
Scope of the document
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 1st January 1979 to 31st 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 31st 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)
...
2.1 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 2-1 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).
...
. 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’. Figure 2-4: C3S v3.x daily TSI values (grey) and the 121-day running mean (black). The running means for SATIRE-S (red), NRLTSI2 (green) and Community Consensus TSI (blue) are also shown. Figure 2-5: Bias between the C3S v3.x composite and SATIRE-S (red curve), NRLTSI2 (green curve) and Community Composite TSI (blue curve). The curves shown are 121-day running mean average. The parameters of the linear fits y = a + b (x-2001) where y is the bias and x the decimal year, are given in Table 2-2. Figure 2-6: bias-corrected RMS differences between the C3S v3.x composite and SATIRE-S (red curve), NRLTSI2 (green curve) and Community Composite TSI (blue curve). The bc-RMS are estimated on a 121-days running mean time period. Figure 2-7: Difference between the C3S v3.0/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 +0.542 W/m², respectively). |
General definitions
Term | Definition | ||||
Total Solar Irradiance (TSI) | The Total Solar Irradiance (TSI) quantifies the total amount of solar energy that is received by the Earth. It is defined 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 Essential Climate Variable (ECV) by the Global Climate Observing System (GCOS). | ||||
Climate Data Store (CDS) | The front-end and delivery mechanism for data made available through C3S. | ||||
Earth Radiation Budget (ERB) | The difference between the incoming radiant energy to the Earth (directly dependent on the TSI) and the outgoing radiant energy due to reflection and thermal emission. | ||||
Climate Data Record (CDR) | Sufficiently long, accurate and stable time series of a climate variable to be useful to address climate variability and change. | ||||
Interim Climate Data Record (ICDR) | An interim CDRs is an extension of a CDR that meets some timeliness requirements needed in some applications (e.g. “State of the Climate” reports). These preliminary data might not be fully validated and may need to be reprocessed before inclusion in the CDR. | ||||
Fiducial Reference Measurement (FRM) | SI-traceable measurements that provide independent validation of satellite measurements. | ||||
Irradiance | Flux of radiant energy per unit area (usually expressed in W/m² unit). | ||||
Key Performance Indicators | Indicators used to monitor the quality of the C3S services and data quality. | ||||
Bias and bias-corrected Root Mean Squared Difference | The bias (b) is the average value of the difference of the data (xi) with respect to a reference dataset (ri), where N is the number of data points:
The bias-corrected Root Mean Squared Difference (bcRMSD) is the square root of the average of the square of the differences with respect to the reference dataset, once the bias (b) has been removed friom the data points (xi) (therefore “bias corrected”):
|
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
section2_1 | |
section2_1 |
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.
Table 2-1: Evaluation of the adjusted individual timeseries with respect to C3S daily TSI composite. Anchor table2_1 table2_1
Instrument | Adjustment factor | Number of daily values | Comparison with the C3S composite | Comparison with NRLTSI2 | ||
Bias (W/m²) | bc-RMS (W/m²) | Bias (W/m²) | bc-RMS (W/m²) | |||
ERB | 0.992447 | 3342 | -0.023 | 0.256 | 0.232 | 0.322 |
ACRIM1 | 0.995568 | 2536 | 0.004 | 0.053 | 0.252 | 0.321 |
ERBS | 0.997149 | 4969 | -0.000 | 0.075 | 0.379 | 0.187 |
ACRIM2 | 0.997821 | 3501 | -0.006 | 0.173 | 0.396 | 0.214 |
DIARAD/VIRGO | 0.996449 | 8661 | 0.004 | 0.091 | 0.359 | 0.180 |
PMO06/VIRGO | 1.000181 | 8662 | -0.004 | 0.155 | 0.331 | 0.233 |
ACRIM3 | 1.000078 | 4914 | 0.019 | 0.118 | 0.409 | 0.216 |
TIM/SORCE | 1.000256 | 6000 | 0.008 | 0.073 | 0.352 | 0.112 |
PREMOS | 1.000256 | 1296 | -0.003 | 0.041 | 0.306 | 0.151 |
SOVAP | 0.999345 | 1320 | 0.002 | 0.124 | 0.309 | 0.188 |
TIM/TCTE | 0.999771 | 1858 | -0.023 | 0.050 | 0.286 | 0.140 |
TIM/TSIS1 | 0.999535 | 1086 | -0.026 | 0.022 | 0.251 | 0.101 |
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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)
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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
...
Anchor figure2_1 figure2_1
Figure 2-1: 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’.
Anchor figure2_2 figure2_2
Figure 2-2: 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’.
Anchor figure2_3 figure2_3
...
Figure 2-3: 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
...
2.2 TCDR and ICDR comparison with NRL TSI v2 composite
...
C3S and NRL TSI v2 composites after ‘121 days running mean’.
Anchor section2_2 section2_2
2.2 Comparison with SATIRE-S, NRLTSI2 and Community Consensus TSI (CCTSI)
section2_2 | |
section2_2 |
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).
Table 2-2: Bias and Anchor table2_2 table2_2
...
bias-corrected Root Mean Square (bc-RMS) differences for the daily values and after 121-days running mean. The ‘a’ and ‘b’ parameters of the linear fit for the bias are given in the last 2 columns.
Reference | Bias (W/m²) | bc-RMS daily values | bc-RMS 121d-mean | Linear fit parameter for the bias y = a + b * (x-2001): |
CDR
Number of daily values
(W/m²) |
(W/m²) | a (W/m²) |
b (W/m²/year) |
NRLTSI2 |
0.338 |
0. |
206 |
0. |
105 | 0. |
336 |
ICDR
639
1.411
0.000017 | ||
SATIRE-S | 0.293 | 0.198 |
0. |
178 |
...
Figure 2-4: 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 2-3 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.
...
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
...
Figure 2-5: 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
In addition to the extensive product validation (see chapter 2 for results and chapter 2/3 in [D6] for validation methodology) a second assessment is introduced to evaluate the Interim Climate Data Record (ICDR) against the Thematic Climate Data Record (TCDR) in terms of consistency. Since frequent ICDR deliveries make detailed validation not feasible, a consistency check against the deeply validated TCDR is used as an indication of quality. This is done by a comparison of the following two evaluations:
- TCDR against a stable, long-term and independent reference dataset
- ICDR against the same stable, long-term and independent reference dataset
The evaluation method is generated to detect differences in the ICDR performance in a quantitative, binary way with so called Key Performance Indicators. The general method is outlined in [D7] chapter 3. The same difference between TCDR/ICDR and the reference dataset would lead to the conclusion that TCDR and ICDR have the same quality (key performance is "good"). Variations or trends in the differences (TCDR/ICDR against reference) would require a further investigation to analyze the reasons. The key performance would be marked as "bad". The binary decision whether the key performance is good or bad is made in a statistical way by a hypotheses test (binomial test). Based on the TCDR/reference comparison (global means, monthly or daily means) a range is defined with 95% of the differences are within. This range (2.5 and 97.5 percentile) is used for the ICDR/reference comparison to check whether the values are in or out of the range. The results could be the following:
- All or a sufficient high number of ICDR/reference differences lies within the range defined by the TCDR/reference comparison: Key performance of the ICDR is "good"
- A smaller number of ICDR/reference differences is within the pre-defined range: Key performance of the ICDR is "bad"
3.1 Results
The results of the KPI test are summarized in Table 3-1.
...
p2.5
p97.5
...
0.87 W/m²
1.83 W/m²
...
0.130 W/m²
0.562 W/m²
...
Percentiles were calculated based on the comparison of the TCDR against NRL TSI v2 composite as reference dataset for the variable Total Solar Irradiance (TSI). The ICDR daily means remain in general with in the 2.5 and 97.5 percentiles.
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.
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0.287 | 0.012705 | ||||
CC TSI | -0.202 | 0.228 | 0.162 | -0.205 | -0.008601 |
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.
Figure 2-4: C3S v3.x daily TSI values (grey) and the 121-day running mean (black). The running means for SATIRE-S (red), NRLTSI2 (green) and Community Consensus TSI (blue) are also shown. Anchor figure2_4 figure2_4
Figure 2-5: Bias between the C3S v3.x composite and SATIRE-S (red curve), NRLTSI2 (green curve) and Community Composite TSI (blue curve). The curves shown are 121-day running mean average. The parameters of the linear fits y = a + b (x-2001) where y is the bias and x the decimal year, are given in Table 2-2. Anchor figure2_5 figure2_5
Figure 2-6: bias-corrected RMS differences between the C3S v3.x composite and SATIRE-S (red curve), NRLTSI2 (green curve) and Community Composite TSI (blue curve). The bc-RMS are estimated on a 121-days running mean time period. Anchor figure2_6 figure2_6
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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.
Figure 2-7: Difference between the C3S v3.0/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 +0.542 W/m², respectively). Anchor figure2_7 figure2_7
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 (1st 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-
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year time period) complies with the 0.
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14 W/m²/decade GCOS requirements. The
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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
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(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.
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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.
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.
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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.
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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).
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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)
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. https://www.doi.org/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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This document has been produced with funding by the European Unionin the context of the Copernicus Climate Change Service (C3S) , operated. The activities leading to these results have been contracted by the European Centre for Medium-Range Weather Forecasts, operator of C3S on behalf onof the European Union (Delegation Agreement signed on 11/11/2014 and Contribution Agreement signed on 22/07/2021). All information in this document is provided "as is" and no guarantee ofor warranty is given that the information is fit for any particular purpose. The users thereof use the information at their sole risk and liability. For the avoidance of all doubt , the European Commission and the European Centre for Medium - Range Weather Forecasts have no liability in respect of this document, which is merely representing the author's view. |
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