Last modified on Dec 13, 2022 11:21

Contributors: O. Bobryshev (DWD)

Issued by: DWD/Bobryshev Oleksandr

Date: 09/06/2021

Ref: C3S_D312b_Lot1.2.2.3-v2.1_202010_PQAR_ECVSurfaceRadiationBudget_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

General definitions

In the scope of the Copernicus Climate Change Service (C3S), two Surface Radiation Budget datasets are combined into one Climate Data Record (CDR) called "CLARA product family".

The first dataset, EUMETSAT's CM SAF CLARA-A2.1, is the core of this CDR. The second dataset includes extra data products that are not included in the CLARA-A2.1 dataset. The core data products are brokered from CM SAF. The extra net fluxes are not included in the brokered CLARA-A2.1 dataset and are calculated specifically within the C3S project as complimentary data for the convenience of users. Their format is as close as possible to the CLARA-A2.1 and the datasets are meant to be used together. Both datasets are frequently updated with Interim Climate Data Records (ICDRs) or simply extensions, generated using the same software and algorithms to cover more recent periods. The ICDR part for all products covers period from 2019 to 2020.

This separation into two datasets is necessary to keep the origin of the data, e.g. licence affiliations: "EUMETSAT's CM SAF" and "C3S", clear for individual products (Table 1)

Table 1: Licence overview of the CLARA product family Surface Radiation Products available through the CDS

The longwave fluxes are not included in the CM SAF ICDR plans for the current phase. To ensure the dataset integrity and continuity, they are calculated within the C3S for the ICDR part. As such, the longwave fluxes change their licence affiliation, namely they are provided within the C3S project for the ICDR part (2019-2020) and are brokered from EUMETSAT’s CM SAF for the TCDR part (1982 to 2018).

Furthermore the CM SAF CLARA-A2.1 dataset has a temporal coverage of January 1982 to June 2019 (as described in [D2]), but TCDR data are only being brokered to the CDS up to December 2018. Data available from the CDS for January 2019 to December 2020 are brokered/derived from the CLARA A2.1 ICDR¹.

In contrast to the original CM SAF CLARA-A2.1 dataset, the brokered service within Copernicus solely provides:

• Level-3 data – excluding the level-2b data format.
• Data on a global equal angle grid – excluding the polar grid format.
• An aggregated version of all satellite data – excluding the provision of the individual satellite datasets.

Scope of the document

This document is the Product Quality Assessment Report (PQAR) for CLARA Product family CDR. This document provides validation results for the Climate Data Record (CDR) of the Surface Radiation Budget.

Executive summary

The brokering service of the CLARA-A2.1 data products includes 37 years (1982-2018) of level-3 data (monthly means) on a regular global latitude-longitude grid (with 0.25° x 0.25° resolution), merged from various polar orbiting satellites. It includes three products: the Surface Incoming Shortwave radiation (SIS), the Surface Outgoing Longwave radiation (SOL), and the Surface Downwelling Longwave radiation (SDL). SIS daily averages are also available within C3S. This CDR is brokered from EUMETSAT’s CM SAF. Therefore, this document refers to the original EUMETSAT CM SAF Validation Report [D1, D8]. It describes the validation methodology and the validation results.

The extra data products (produced specifically within the C3S project) are: the Surface Reflected Shortwave radiation (SRS), the Surface Net Shortwave radiation (SNS), the Surface Net Longwave radiation (SNL), and the Surface Radiation Budget (SRB). Validation results for these extra data products are described within this document.

The CLARA based ICDR data record is a continuation of the products of the CLARA-A2.1 and the extra data products. It extends both products by 2 years (2019-2020). The longwave fluxes are not included in the CM SAF ICDR plans for the current phase. Therefore SOL and SDL change their license affiliation, namely they are provided within the C3S project for the ICDR part (2019-2020) and are brokered from EUMETSAT’s CM SAF for the TCDR part (1982 to 2018). Table 1 provides an overview of the licenses for all data CLARA family products. Validation results for SDL and SOL ICDR data products are described within this document.

An executive summary of the evaluation of the CLARA-A2.1 and CLARA-based ICDR surface radiation dataset can be found in CM SAF Validation Report, Section 1 [D1, D8]

Table 2: Summary of the accuracy of the brokered CLARA-A2.1 data products, [D1] Section 1  for SIS, SOL, SDL TCDR, [D8] Section 1 for SIS ICDR. The accuracy estimation of SOL and SDL ICDR extensions is performed within the C3S using the algorithms developed by CM SAF

Table 3: Summary of the accuracy of the extra data products

1. Product validation methodology

In the scope of the Copernicus Climate Change Service (C3S), two Surface Radiation Budget datasets are combined into one Climate Data Record (CDR) called the "CLARA product family".

1.1 Validation methodology for SIS, SOL, SDL

CLARA-A2.1 data products include the Surface Incoming Shortwave radiation (SIS), the Surface Outgoing Longwave radiation (SOL), and the Surface Downwelling Longwave radiation (SDL) datasets. These datasets were validated against reference data records from surface measurements obtained by the Baseline Surface Radiation Network (BSRN, Ohmura et al., 1998, Driemel et al., 2018). The reference dataset is described in detail in CM SAF Validation Report [D1], Section 4.

Therefore, this document refers to the original EUMETSAT CM SAF Validation Report [D1, D8]. It describes the validation methodology and the validation results. The accuracy estimation of SOL and SDL ICDR extensions is performed within the C3S using the algorithms developed by CM SAF

The validation of the SIS ICDR (2019-2020) product is described in CM SAF Validation Report ICDR, Sections 5.2 [D8]. The validation methodology of SOL and SDL ICDR extensions is performed within the C3S using the algorithms developed by CM SAF.

The validation methodology and definitions of the main metrics are provided in the C3S Product Quality Assurance Document (PQAD) [D4], Section 3, which refers to CM SAF Validation Report [D1], Section 5 and 5.1.

1.2 Validation methodology for CLARA family extra data products

The performed validation approach for extra data products is based on the method of error propagation. These data products are the Surface Reflected Shortwave radiation (SRS), the Surface Net Shortwave radiation (SNS), the Surface Net Longwave radiation (SNL), and the Surface Radiation Budget (SRB).

The ICDR part data are validated in the same way as the TCDR

1.2.1 Surface Reflected Shortwave Radiation (SRS)

The accuracy of the SRS is determined based on the accuracy of the Surface Incoming Shortwave Radiation (SIS) from the Surface Radiation Budget brokered from EUMETSAT’s CM SAF CLARA-A2.1 dataset [D1], and the Surface Albedo (SAL), not provided within the C3S.

The accuracy for the SRS is calculated using the fixed accuracy values for the SIS ( ΔSIS = 10 W/m² ) and the relative error of the SAL ( ΔSAL = 25% of the SAL value). These values are the target requirements for accuracy from CM SAF Product Requirement Document [D2]. Resulting global mean accuracy for the SRS is 7.8 W/m² for TCDR and 6.5 W/m² for ICDR. Figures 1 and 2 provide the spatial distribution of the propagated SNS accuracies.

Figure 1: Spatial distribution of the propagated SRS accuracies for 1982-2018 (left) and 2019-2020 (right) years.

1.2.2 Surface Net Shortwave Radiation (SNS)

The accuracy of the SNS is determined based on the accuracy of the SIS and the SAL datasets.

Estimations of the accuracy of the SIS product is given in CM SAF Validation Report [D1] and the accuracy of the SAL product is given in CM SAF Validation Report, Surface Albedo [D6].

The accuracy for the SRS is calculated using the fixed accuracy values for the SIS ( ΔSIS = 10 W/m² ) and the relative error of the SAL ( ΔSAL = 25% of the SAL value). These values are the target requirements for accuracy from CM SAF Product Requirement Document [D2]. Resulting global mean accuracy for the SNS is 13.0 W/m². Figure 2 provides the spatial distribution of the propagated SNS accuracies.

Figure 2: Spatial distribution of the propagated SNS accuracies for 1982-2018 (left) and 2019-2020 (right) years

1.2.3 Surface Net Longwave Radiation (SNL)

The accuracy of the SNL is defined by the accuracy of the Surface Downwelling Longwave Radiation (SDL) and the Surface Outgoing Longwave Radiation (SOL), both datasets are the Surface Radiation Budget brokered from EUMETSAT’s CM SAF CLARA-A2.1 [D1].

Estimations of the accuracy of the SDL and the SOL products are given in CM SAF Validation Report [D1].

The accuracy for the SNL is calculated using the fixed accuracy values for the SDL ( ΔSDL = 8.13 W/m² ) and the SOL ( ΔSOL = 13.77 W/m²  ). Resulting global mean accuracy for the SNL is 21.9 W/m².

Global mean accuracy for the SNL ICDR is 25.0 W/m².

1.2.4 Surface Radiation Budget (SRB)

The accuracy of the SRB is defined by the accuracy of the SNS and the SNL datasets.

The accuracy for the SRB is calculated using the fixed accuracy values for the SNS ( ΔSNS = 13.0 W/m² ) and the SNL ( ΔSNL = 21.9 W/m² ).

Resulting global mean accuracy for the SRB is 34.9 W/m².

Global mean accuracy for the SNL ICDR is 39.2 W/m².

2. Validation results

2.1 Validation results for SIS, SOL, SDL

The TCDR (1982-2018) validation results are fully described in CM SAF Validation Report [D1], Section 5. The validation results for the SIS are provided in CM SAF Validation Report [D1], Section 5.2 and 5.2. The validation results for the SOL are provided in CM SAF Validation Report [D1], Section 5.3. The validation results for the SDL are provided in CM SAF Validation Report [D1], Section 5.4. Considerations for climate applications are provided in CM SAF Data Set Description [D3], Section 6. Conclusions for the validation results are provided in CM SAF Validation Report [D1], Section 6.

Results are summarised in Table 1 in the Executive Summary.

The validation results for SIS ICDR monthly and daily are fully described in CM SAF Validation Report ICDR [D8], Section 5.2.

The validation results of SOL and SDL ICDR extensions is performed within the C3S using the algorithms developed by CM SAF. The summary of validation results for the monthly ICDR SDL and ICDR SOL datasets are shown in Table 4.

At the time of writing (May 2021) only 34 out of 60 BSRN stations have submitted data for years 2019-2020. 12 stations had less than 4 month worth of data and they were discarded. A full list of BSRN stations used in the validation and stationwise results are presented in Tables 5 and 6, Section 5 Appendix of this document. The outgoing longwave measurements are performed not on all BSRN stations. Only 9 stations have submitted data for years 2019-2020. A full list of BSRN stations used in the validation and stationwise results are presented in Table 5 and 6, Section 5 Appendix of this document.

Table 4: Summary of validation results for SDL and SOL ICDR datasets (2019-2020)

2.2 Validation results for CLARA family extra data products (SRS, SNS, SNL, SRB)

The validation methodology for the extra data products is selected in a way to provide the most conservative accuracy estimation, i.e. by assuming that the uncertainties in the input datasets align perfectly and contribute fully to the overall uncertainty. Table 2 in the Executive summary provides a summary of the accuracies for the SRS, the SNS, the SNL, and the SRB. The SRS accuracy is 7.8 (6.5) W/m², the SNS accuracy is 13.0 (14.2) W/m², the SNL accuracy is 21.9 (25.0) W/m², and the SRB accuracy is 34.9 (39.2) W/m², ICDR values are in parenthesis.

3. Application(s) specific assessments

N/A

4. Compliance with user requirements

A general overview of user requirements needed for climate monitoring is provided in CM SAF Product Requirements Document [D2]. There are three accuracy categories in the CM SAF PRD document ([D2], Section 5): threshold, target and optimal accuracies. They are defined keeping in mind different target users: operational climate monitoring, global and regional climate modelling and global and regional climate studies, respectively.

Target requirement for accuracy (mean absolute error) are selected as follows:

• SIS monthly means: 10 Wm^-2 (corresponds to target accuracy),
• SIS daily means: 20 Wm^-2 (corresponds to threshold accuracy),
• SDL monthly means: 10 Wm^-2 (corresponds to target accuracy) and
• SOL monthly means: 10 Wm^-2 (corresponds to target accuracy).

Stability requirements for the Surface Radiation Budget TCDR are defined as follows:

• SIS monthly means - 2 Wm^-2dec^-1,
• SIS daily means – 2 Wm^-2dec^-1, and
• SDL and SOL monthly means – 3 Wm^-2dec^-1).

At the time (~2010) when requirements for the CLARA-A2 data record had to be defined in [D2], there was no guidance available for surface radiation products in the available GCOS documents. Instead, requirements had to be set in a dialogue with experts and potential users (e.g., in association with CM SAF User Workshops).

New GCOS requirements for the ECV Surface Radiation Budget are summarized in GCOS-200 [D7], Table 23, page 279 and include requirements for the horizontal resolution, temporal resolution, accuracy and stability. GCOS targets are:

• Frequency: Monthly (resolving diurnal cycle)
• Resolution: 100 km
• Measurement uncertainty: 1 Wm^-2 on global mean
• Stability: 0.2 Wm^-2dec^-1

However, these requirements are only valid for the net fluxes (i.e., SNS and SNL) and not for all individual radiation budget components. However, one could claim that individual radiation budget components should consequently be constrained in the same way as net fluxes.

All products in the brokered CLARA-A2.1 dataset fulfil the GCOS requirements regarding the horizontal and temporal resolution. However, the SNS and SNL products do not fulfil the new GCOS requirements on accuracy and stability which are very stringent. Nevertheless, achieved accuracy and stability results allow consistent quantification of mean values, anomalies, variability and the Earth energy budget in general. We point out the existing uncertainties in the methodology of comparison with area-to point measurements (i.e. satellite-area to point-ground-based reference networks) as important reasons for not fulfilling the new requirements.

The compliance with the user requirements (as defined in the CM SAF PRD document ([D2], Section 5) for the SIS, SDL, and SOL is provided in Table 5.

Table 5: Summary of the requirements and achieved accuracies of the brokered CLARA-A2.1 data products], Section 1 [D1 for SIS, SOL, SDL TCDR, D8 for SIS ICDR]. The accuracy estimation of SOL and SDL ICDR extensions is performed within the C3S using the algorithms developed by CM SAF

The validation methodology for the extra data products is selected in a way to provide the most conservative accuracy estimation, i.e. by assuming that the uncertainties in the input datasets align perfectly and contribute fully to the overall uncertainty. The SRS accuracy is 7.8 (6.5) W/m², the SNS accuracy is 13.0 (14.2) W/m², the SNL accuracy is 21.9 (25.0) W/m², and the SRB accuracy is 34.9 (39.2) W/m², ICDR values are in parenthesis.

Appendix

Table 6: Stationwise validation results for SDL, 2019-2020

Table 7: Stationwise validation results for SOL, 2019-2020

References

Ohmura, A., et al. (1998), Baseline Surface Radiation Network (BSRN/WCRP): New precision radiometry for climate research, Bulletin of the American Meteorological Society, 79(10), 2115-2136.

Driemel, A., et al. (2018): Baseline Surface Radiation Network (BSRN): structure and data description (1992-2017), Earth System Science Data, 10(3), 1491-1501, doi:10.5194/essd-10-1491-2018.

Further references are listed in CM SAF Validation Report [D1], Section 7.

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