Contributors: O. Bobryshev (DWD)
Issued by: DWD / Oleksandr Bobryshev
Date: 12/02/2021
Ref: C3S_D312b_Lot1.1.2.1-v2.0_201907_ATBD_ECVSurfaceRadiation_Budget_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 onwards.
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
Year | CDR Type | CLARA Product Family | |||
SIS
| SDL, SOL Longwave fluxes | SRS | Net Fluxes | ||
Extra data products | |||||
1982 – 2018 | TCDR | CM SAF (CLARA A2.1) | CM SAF (CLARA A2.1) | C3S | C3S |
2019-onwards | ICDR | CM SAF | C3S | C3S | C3S |
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-onwards) and are brokered from EUMETSAT’s CM SAF for the TCDR part (1982 to 2018).
Furthermore the CMSAF 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 onwards 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 Algorithm Theoretical Basis Document (ATBD) for CLARA Product Family CDR. It describes input and auxiliary data and algorithms used to generate the data products.
The brokered service within Copernicus solely refers to the CM SAF Algorithm Theoretical Basis Document [D1]. It can be found at the CM SAF web site http://www.cmsaf.eu with the copy of the main documents provided at the CDS website.
Extra data products produced within C3S are described in this document.
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 document references the CM SAF documentation [D1] describing the input data and algorithms used to generate the data products.
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). Input data and algorithms used to generate these extra data products are described within this document.
The CLARA Product family is periodically updated with Interim Climate Data Records (ICDRs) generated using the same software and algorithms to cover more recent periods. The ICDR part for all products covers the period from 2019 onwards.
1. Instruments
The Advanced Very High Resolution Radiometer (AVHRR) radiometer is fully described in NWC SAF ATBD [D3] Chapter 4.5.1.
Table 2 shows an overview of successive AVHRR sensors onboard dedicated platforms, used for the generation of the CLARA Product family Climate Data Record (CDR).
Table 2: Different versions and the lifetime of AVHRR on different platforms
2. Input and auxiliary data
This section summarizes the required input data used in the retrieval algorithms.
2.1 Fundamental Climate Data Record (FCDR)
AVHRR Global Area Coverage (AVHRR GAC) is the FCDR used for generation of the Surface Radiation Budget datasets. The detailed description of the algorithm used to generate AVHRR GAC is given in CM SAF ATBD Cloud Products [D6], Section 2.3.
2.2 Specific input and auxiliary data
2.2.1 Surface Incoming Shortwave Radiation (SIS)
The set of the input and auxiliary data used to generate SIS dataset are given in CM SAF ATBD [D1], Section 2.1.1.
The set of the input and auxiliary data used to generate the ICDR part of the SIS dataset are given in the CM SAF ICDR-ATBD (not yet published, will be available on the CM SAF web site http://www.cmsaf.eu). The integrated water vapour is taken from the operational analysis provided by the ECMWF IFS model and was used to replace the ERA-Interim.
2.2.2 Surface Downwelling Longwave Radiation (SDL)
The set of the input and auxiliary data used to generate SDL in the CLARA A2.1 dataset are given in CM SAF ATBD [D1], Section 2.2.1.
SDL is not included in the CM SAF ICDR plans for the current phase, so, for the ICDR dataset (2019 onwards), SDL has been produced within the C3S contract.
Reanalysis data are a key input requirement for SDL product. The ERA-Interim reanalysis dataset (used for the TCDR) ended in August 2019, so for the whole ICDR period (2019-onward) ERA-5 data are used. Other input and auxiliary data used for the ICDR are the same as for the TCDR.
2.2.3 Surface Outgoing Longwave Radiation (SOL)
The set of the input and auxiliary data used to generate SDL in the CLARA A2.1 dataset are given in CM SAF ATBD [D1], Section 2.2.2.
SOL is not included in the CM SAF ICDR plans for the current phase, so, for the ICDR dataset (2019 onwards), SOL has been produced within the C3S contract.
Reanalysis data are a key input requirement for SOL product. The ERA-Interim reanalysis dataset (used for the TCDR) ended in August 2019, so for the whole ICDR period (2019-onward) ERA-5 data are used. Other input and auxiliary data used for the ICDR are the same as for the TCDR.
2.2.4 Surface Reflected Shortwave Radiation (SRS)
The CLARA-A2.1-extra variables are based on the products brokered from EUMETSAT’s CM SAF CLARA-A2.1 [D7] and, additionally, on the Surface Albedo from CLARA-A2 dataset (monthly means), not provided within the C3S.
The SRS product is generated from the Surface Incoming Shortwave radiation (SIS) from the Surface Radiation Budget brokered from EUMETSAT’s CM SAF CLARA-A2.1 and the Surface Albedo (SAL) from the CLARA-A2.1 dataset (monthly means).
The set of input and auxiliary data used to generate the SIS is given in CM SAF ATBD Surface Radiation Products [D1], Section 2.1.1. The set of input and auxiliary data used to generate the SAL is given in CM SAF ATBD Surface Albedo [D7], Section 5.1.
The set of the input and auxiliary data used to generate the ICDR part of the SIS and SAL datasets are given in their respective CM SAF ICDR-ATBD (not yet published, will be available on the CM SAF web site http://www.cmsaf.eu)
2.2.5 Surface Net Shortwave Radiation (SNS)
The SNS product is generated from the SIS and the SAL monthly means.
The set of input and auxiliary data used to generate the SIS is given in CM SAF ATBD Surface Radiation Products [D1], Section 2.1.1. The set of the input and auxiliary data used to generate the SAL is given in CM SAF ATBD Surface Albedo [D7], Section 5.1.
The set of the input and auxiliary data used to generate the ICDR part of the SIS and SAL datasets are given in their respective CM SAF ICDR-ATBD (not yet published, will be available on the CM SAF web site http://www.cmsaf.eu)
2.2.6 Surface Net Longwave Radiation (SNL)
The SNL product is generated from the Surface Downwelling Longwave radiation (SDL) and the Surface Outgoing Longwave radiation (SOL) brokered from EUMETSAT’s CM SAF CLARA-A2.1 (see Section 3 below).
The set of input and auxiliary data used to generate the SDL is given in CM SAF ATBD Surface Radiation Products [D1], Section 2.2.1. The set of input and auxiliary data used to generate the SOL is given in CM SAF ATBD Surface Radiation Products [D1], Section 2.2.2.
See sections 2.2.2 and 2.2.3 above for the input and auxiliary data used to derive SDL and SOL in the ICDR.
2.2.7 Surface Radiation Budget (SRB)
The SRB product is generated from the SNS and the SNL generated within this C3S CDR.
See sections 2.2.5 and 2.2.6 above for detailed of the input and auxiliary data used to derive SNS and SNL in the TCDR and ICDR.
3. Algorithms
This section describes the algorithms used to derive the final radiation products.
The algorithm used to generate SIS dataset is described in CM SAF ATBD [D1] Chapter 2.1.1. Flow-charts that summarise all processing steps are given in CM SAF ATBD [D1], Figure 2-1 and 2-2.
The algorithm used to generate SDL dataset is described in CM SAF ATBD [D1] Chapter 2.2.1.
The algorithm used to generate SOL dataset is described in CM SAF ATBD [D1] Chapter 2.2.2.
The SRS is defined as the multiplication of the SIS and the SAL. The level-3 (gridded monthly means) products are used for the SRS generation.
\[ SRS=SIS \ast SAL \quad (Eq. 1) \]The SNS is defined as the difference between the SIS and the SAL. The level-3 (gridded monthly means) products are used for the SNS generation.
\[ SNS=SIS-SRS=SIS(1-SAL) \quad (Eq. 2) \]The SNL is defined as the difference between the SDL and the SOL. The level-3 (gridded monthly means) products are used for the SNL
generation.
\[ SNL=SDL-SOL \quad (Eq. 3) \]The SRB is defined as the sum between the SNL and the SNL. The level-3 (gridded monthly means) products are used for the SRB generation.
\[ SRB=SNS+SNL \quad (Eq. 4) \]3.1 Error budget
An error budget (accuracy estimation and trend analysis) for the radiation products can be found in the Validation Report [D5]. All data records fulfil the optimal or target accuracy requirements for the period 1992-2015. The SDL and SOL data sets should not be used for validation of model data, as these data themselves are based on ERA-reanalysis and model-derived data records (See [D5] Chapter 5).
The Extra data products have no associated uncertainty estimates. Thus, uncertainty information is only available as results achieved in associated validation activities, as presented in the C3S PQAR [D9].
3.2 Known limitations
Known limitations and their implications for SIS are described in CM SAF ATBD [D1] Chapter 2.1.1.4 and are summarised below.
- Limitation due to surface albedo
- Introduction of uncertainty under cloudy conditions
- Error in cloud-detection software
- Application of monthly climatological information instead of real time data
Known limitations and their implications for SDL are described in CM SAF ATBD [D1] Chapter 2.2.1.3 and are summarised below.
- Underestimation of inter-annual variability due to linear regression
- Limited topographic correction due to use of single constant
Known limitations and their implications for SOL are described in CM SAF ATBD [D1] Chapter 2.2.2.1 and are summarised below:
- Assumption of dry-adiabatic temperature gradient for topographic correction
- Predefined emissivity depending on land type classes ignoring soil moisture
Equations 1 and 4 to calculate the SNS, the SNL, and the SRB are based on the definitions of these quantities, so the only limitations are those of the input datasets.
4. Output data
4.1 File format
Information on the file format is provided in CM SAF Product User Manual (PUM) [D2], Section 4 (introductory paragraph).
The CLARA Product Family CDR is provided in the NetCDF (Network Common Data Format). The global attributes follow the Copernicus Common Data Model (CDM) instructions [D8]. The metadata and naming definitions follow the Climate and Forecast (CF) conventions version 1-7 (https://cfconventions.org). The keywords follow the GCMD Science Keywords (Global Change Master Directory), Version 8.1. Data format and data file contents throughout the CDR are made as compatible as possible.
4.2 File contents
Each NetCDF file consists of dimensions, variables and attributes. All data files are built following the same design principles. Each data file contains the following time coordinate and 3-dimensional variables:
- Time
Start of averaging time period [days since 1970-01-01 00:00:00]
- latitude
Geographical latitude of grid-box centre [degrees_north]
- longitude
Geographical longitude of grid-box centre [degrees_east]
- SIS, SDL, SOL, SRS, SNS, SNL, SRB
Parameter grid box mean value, name depends on the parameter
Global attributes are summarized in Copernicus CDM, Section 9 [D8].
Further information on the product-specific file content for CLARA-A2.1 products (SIS, SDL, and SOL) is provided in CM SAF PUM [D2] Chapter 4.1.
References
All relevant references are listed in CM SAF ATBD Surface Radiation Products [D1], Section 3, CM SAF ATBD Surface Albedo [D5], Section 7, and CM SAF PUM Surface Radiation Products [D2], Section 8.
