Contributors: O. Bobryshev (Deutscher Wetterdienst)
Issued by: Deutscher Wetterdienst (DWD) / Oleksandr Bobryshev
Date: 06/11/2023
Ref: C3S2_D312a_Lot1.1.3.3_202311_PQAD_ECVSurfaceRadiationBudget_v1.2
Official reference number service contract: 2021/C3S2_312a_Lot1_DWD/SC1
History of Modification
List of datasets covered by this document
Related documents
Acronyms
List of figures
List of tables
General definitions
Variables | Abbreviation | Definition |
Surface Incoming Shortwave radiation | SIS | The total incoming shortwave (solar) flux, determined at the Earth’s surface. |
Surface Reflected Shortwave radiation | SRS | The total upwelling shortwave flux, determined at the Earth’s surface. |
Surface Net Shortwave radiation | SNS | The net downwelling shortwave flux, determined at the Earth’s surface (equal to SIS – SRS). |
Surface Downwelling Longwave radiation | SDL | The total downwelling thermal (longwave) flux, determined at the Earth’s surface. |
Surface Outgoing Longwave radiation | SOL | The total upwelling thermal flux, determined at the Earth’s surface. |
Surface Net Longwave radiation | SNL | The net downwelling thermal flux, determined at the Earth’s surface (equal to SDL-SOL). |
Surface Radiation Budget | SRB | The total net downwelling radiative flux, determined at the Earth’s surface (equal to (SIS+SDL) – (SRS+SOL)). |
Processing level | Definition |
Level-1 | The full-resolution geolocated radiometric measurements (for each view and each channel). |
Level-2 | Derived geophysical variables at full input data resolution and location as L1 source data |
Level-3 | Geophysical variables mapped on uniform spacetime grid, derived from one or multiple satellites/radiometers, possibly averaged over a specific time period to give for example monthly and daily means. |
Term | Definition |
Brokered product | The C3S Climate Data Store (CDS) provides both datasets produced within the C3S and so-called brokered products. The latter are existing products (data) produced under an independent programme or project which are made available through the CDS. |
TCDR | A Thematic Climate Data Record is a consistently-processed time series of a geophysical variable. The time series should be of sufficient length and quality. |
ICDR | An Interim Climate Data Record (ICDR) denotes an extension of TCDR, processed with a processing system as consistent as possible to the generation of TCDR. |
User requirements | Depending on the different user needs, different product requirements may be applied and they are used to evaluate validation results. This document uses three categories: 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. |
Pygac | Python package to read, calibrate and navigate data from the AVHRR instrument onboard NOAA and Metop satellites in GAC (Global Area Coverage) and LAC (Local Area Coverage) format. Taken from: https://pygac.readthedocs.io/en/latest/ |
Statistical metrics | Definition |
Optimal requirement | This is a level where a product is considered to perform much better than expected given the current knowledge. |
Target requirement | This is the main quality goal for a product. It should reach this level based on the current knowledge on what is reasonable to achieve. |
Threshold requirement | A product should at least fulfil this level to be considered useful at all. Sometimes the term ‘Breakthrough” is used instead. |
Validation metrics | Definition |
Bias | Mean difference between the (monthly or daily) averages of the dataset and a reference. |
Mean absolute difference | Arithmetic mean of absolute values of the difference between (monthly or daily) averaged dataset and a reference. |
Standard deviation | Measure of the spread around the mean value of the difference between (monthly or daily) averaged dataset and a reference. |
Anomaly correlation | Anomaly correlation provides information about the correlation of the anomalies of two time series. A possible bias between them is thereby not considered. |
Fraction of time steps above the validation target values | Fraction of time steps (in this case months or days) of the whole validation period that are outside of a certain target (e.g. target accuracy). Fraction of 0% would imply that all months or days are within the target accuracy. |
Scope of the document
This Product Quality Assurance Document (PQAD) provides a description of the product validation methodology for the CLARA Product family Climate Data Record (CDR), Edition 3 (CLARA-A3), CM SAF Cloud, Albedo, Radiation data record, AVHRR-based, Edition 3 (CLARA-A3), which is a brokered service from EUMETSAT's CM SAF, and its extension: an Interim Climate Data Record (ICDR) derived from the Advanced Very High Resolution Radiometer (AVHRR).
This document mostly refers to the original Satellite Application Facilities on Climate Monitoring (CM SAF) Validation Report [D1] that encompasses an extensive evaluation of the surface radiation Thematic Climate Data Record (TCDR) version 3 product. The ICDR extension of the EUMETSAT CM SAF CLARA-A3 dataset is derived using the same algorithm and processing chain. Additionally, this document refers to the CM SAF Validation Report [D1] with a specific section for the ICDR evaluation.
While the TCDR validation is performed based on reference datasets from surface measurements, the ICDR is evaluated based on a six month overlap with the TCDR and without further references. This document is not part of the official CM SAF documentation but produced solely in the scope of data brokering to the Climate Data Store (CDS).
Executive summary
The TCDR on the Essential Climate Variable (ECV) Surface Radiation Budget is a brokered product of the CLARA-A3 dataset produced by EUMETSAT's CM SAF. The extending ICDR is also a brokered product of EUMETSAT's CLARA-A3 ICDR, also produced by EUMETSAT's CM SAF. Accordingly, this document largely refers to the corresponding EUMETSAT CM SAF original documentation [D1,D3].
The brokering service of the CLARA-A3 data products includes 42 years (1979-2020) 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 five products: the Surface Incoming Shortwave radiation (SIS), the Surface Downwelling Longwave radiation (SDL), the Surface Net Shortwave radiation (SNS), the Surface Net Longwave radiation (SNL), and the Surface Radiation Budget (SRB). SIS daily averages are also available within C3S. This CDR is brokered from EUMETSAT’s CM SAF. Therefore, this document only refers to the validation results from the original EUMETSAT CM SAF Validation Report [D1]. It describes the validation methodology and the validation results. A brief summary of the validated products can be found in section 2 of this PQAD.
All brokered CLARA data products (SIS, SDL, SNL, SNS and SRB) are validated against available reference data sets from surface measurements. The accuracy is defined based on the absolute bias derived from the validation with the reference data and evaluated against the accuracy requirements as given in the product requirements document [D2]. Reference datasets are briefly described in section 2 of this PQAD.
Section 3 contains a description of the validation methodology, while section 4 gives a brief summary of the validation results.
1. Validated products
The validated dataset is the CLARA-A3 dataset produced by EUMETSAT's CM SAF and brokered to the Climate Data Store (CDS). This Climate Data Record (CDR) is also called “CLARA product family” in the CDS.
The data products brokered from CM SAF CLARA-A3 are:
the Surface Incoming Shortwave radiation (SIS) monthly and daily,
the Surface Downwelling Longwave radiation (SDL),
the Surface Net Shortwave radiation (SNS),
the Surface Net Longwave radiation (SNL) and
the Surface Radiation Budget (SRB).
The CM SAF CLARA surface radiation data record is derived from successive AVHRR sensors onboard dedicated polar orbiting platforms, including TIROS-N, NOAA, and Metop satellites. AVHRR observations are used to derive spatio-temporal averaged data sets. The final product is available as monthly averages on a regular latitude/longitude grid with a spatial resolution of 0.25° × 0.25° degrees. Additionally, daily averages are available for the SIS. The TCDR comprises 42 years of data ranging from January 1979 to December 2020. Main characteristics of the CDR are summarized in Table 1-1.
Table 1-1: General characteristics of the brokered CLARA-A3 TOA radiation products
Characteristics | Description |
Originating satellite sensor type | AVHRR Advanced Very High Resolution Radiometer with 4 to 6 VIS-IR channels |
Data class | Earth Observation Data |
Algorithm description reference | CM SAF ATBD Surface Radiation [D3] |
Geographic coverage | Global |
Horizontal grid | 0.25° longitude x 0.25° latitude |
Temporal resolution | Monthly. Monthly and daily for the Surface Incoming Shortwave Radiation |
Main variable | Surface radiation |
Data format | CF-compliant NetCDF |
Algorithm Level 3 processor | PyGAC FDR |
2. Description of validating datasets
The validation for the TCDR is based against surface reference measurements from the Baseline Surface Radiation Network (BSRN) reference dataset. It is described in detail in the CM SAF Validation Report [D1], Section 4 and is summarized in the following.
The BSRN 1 is a Global Climate Observing System (GCOS) recognized worldwide network for global surface radiation measurements. As a GCOS recognized network, all BSRN sites are expected to adhere to high standards for data quality, calibration, and documentation. The BSRN has over 60 stations located in different climatic regions around the world, some stations providing radiation measurements for over 30 years. The data is distributed via the World Radiation Monitoring Center (WRMC) hosted by the Alfred Wegener Institute (AWI) in Bremerhaven, Germany (http://www.bsrn.awi.de/).
However, recent work has identified a problem with the primary reference calibration of surface radiation instruments, which may require a recalibration of instruments and a revision of the current data, including the data in the BSRN archive. This issue was highlighted in a study by Nyeki, et al. 2017, which suggested that the impact on shortwave radiation data is expected to be small. However, the monthly downward longwave radiation data might increase by up to about 3 W/m2.
3. Description of product validation methodology
The validation methodology is described in detail in [D1], Section 4.1 and summarized in the following.
The product validation methodology used for the CM SAF CLARA-A3 SIS, SDL, SNS, SNL, and SRB data sets is based on a comparison with available surface measurements. The validation measures used include bias, mean absolute difference, standard deviation, anomaly correlation, and the fraction of time steps that exceed the target accuracy. To account for uncertainties in the surface measurements and possible errors introduced by calculating the temporal averages from the BSRN observations, uncertainties of 5 W/m2 and 10 W/m2 are assumed for the monthly and daily averages, respectively, derived from the surface observations. Only stations with more than 24 months of data between 1979 and 2020 are considered in the validation. The quality of the data sets is assessed by comparisons with the specified accuracy in the PRD.
The ICDR is compared to the TCDR for a six month overlap period in the year 2020 and thereby validated.
4. Summary of validation results
An error budget (accuracy estimation and trend analysis) for the brokered radiation products can be found in Table 4-1 and CM SAF Validation Report [D1] Sections 4.2-4.6 (for the TCDR); in CM SAF Validation Report, and ICDR in Section 4.7. All data records fulfil or almost fulfil (e.g. due to the difficulty of comparing gridded data with ground stations) the threshold accuracy requirements. Following sections 4.1 – 4.7 briefly summarize the validation results.
Table 4-1: Summary of the achieved accuracy of the brokered CLARA-A3 data products, [D1] Section 1 for SIS, SDL, SNS, SNL and SRB for TCDR and ICDR
Product Name | Product accuracy [W/m2] | Threshold/ Target / Optimal accuracies | |
TCDR | ICDR | ||
SIS monthly means | 7 | 7.5 | 9 / 5 / 3 |
SIS daily means | 17 | 17.7 | 18 / 15 / 10 |
SDL | 7 | 8.6 | 8 / 5 / 3 |
SNS | 10 | 10 | 8 / 5 / 3 |
SNL | 7 | 7 | 8 / 5 / 3 |
SRB | 10 | 9 | 8 / 5 / 3 |
4.1 Validation results for Surface Incoming Shortwave Radiation (SIS) monthly
Validation results are described in detail in CM SAF Validation Report [D1], Section 4.2.1. The CLARA-A3 product on SIS meets optimal accuracy and decadal stability criterions, as specified in the [D2].
4.2 Validation results for Surface Incoming Shortwave Radiation (SIS) daily
Validation results are described in detail in CM SAF Validation Report [D1], Section 4.2.2. The CLARA-A3 product on SIS daily means fulfils the accuracy requirement as stated in the [D2].
4.3 Validation results for Surface Downwelling Longwave Radiation (SDL)
Validation results are described in detail in CM SAF Validation Report [D1], Section 4.3. The CLARA-A3 product on SDL fulfils the accuracy and stability requirements for the TCDR. The CLARA-A3 ICDR product almost meets the threshold accuracy and differences are due to very few regions with large daily variability (e.g. Western Pacific). Differences are additionally affected by the removal of the Metop-C satellite for the ICDR product (compared to Metop-C being part of the TCDR).
4.4 Validation results for Surface Net Shortwave Radiation (SNS)
Validation results are described in detail in CM SAF Validation Report [D1], Section 4.4. Despite achieved accuracy slightly larger than threshold, the CLARA-A3 product on SNS can be considered to be in line with the requirements taking the challenges in comparing spatially-averaged satellite data with measurements from local ground stations into account. With an uncertainty of 5 W/m² for monthly means of SNS from ground stations, about 33% of the available months exceed the threshold accuracy.
4.5 Validation results for Surface Net Longwave Radiation (SNL)
Validation results are described in detail in CM SAF Validation Report [D1], Section 4.3. The CLARA-A3 product on SNL fulfils the accuracy and stability requirements.
4.6 Validation results for Surface Radiation Budget (SRB)
Validation results are described in detail in CM SAF Validation Report [D1], Section 4.4. Despite achieved accuracy slightly larger than threshold, the CLARA-A3 product on SNS can be considered to be in line with the requirements taking the challenges in comparing spatially-averaged satellite data with measurements from local ground stations into account. With an uncertainty of 5 W/m² for monthly means of SRB from ground stations, less than 30% of the available months exceed the threshold accuracy.
4.7 Validation results for ICDR products
Validation results are described in detail in CM SAF Validation Report [D1], Section 4.7.
Results of the comparison show only very small differences leading to the fact, that the ICDR products meet almost all requirements (see section 4.3).
References
References are listed in CM SAF Validation Report Surface Radiation Products [D1], Section 6 and in CM SAF Algorithm Theoretical Basis Document [D3], Section 3.
Driemel, A. et al., 2018. Baseline Surface Radiation Network (BSRN): structure and data description (1992–2017). Earth Syst. Sci. Data, vol. 10, p. 1491–1501.
Nyeki, S. et al., 2017. Revising shortwave and longwave radiation archives in view of possible revisions of the WSG and WISG reference scales: methods and implications. Atmos. Meas. Tech., vol. 10, p. 3057–3071.
Ohmura, A. et al., 1998. Baseline Surface Radiation Network (BSRN/WCRP): New precision radiometry for climate research. Bull. Am. Meteorol. Soc., Vol 79, p. 2115–2136.
Related articles
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SRB CLARA-A3: System Quality Assurance Document (SQAD) (Copernicus Knowledge Base)
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SRB CLARA-A3: Product user Manual (PUM) (Copernicus Knowledge Base)
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SRB CLARA-A3: Algorithm Theoretical Basis Document (ATBD) (Copernicus Knowledge Base)
