Contributors: E. Carboni (UKRI-STFC RAL Space), G.E. Thomas (UKRI-STFC RAL Space)
Issued by: STFC RAL Space (UKRI-STFC) / Elisa Carboni
Date: 26/01/2023
Ref: C3S2_D312a_Lot1.1.2.3-v4.0_202301_PQAD_CCIEarthRadiationBudget_v1.1
Official reference number service contract: 2021/C3S2_312a_Lot1_DWD/SC1
History of modifications
List of datasets covered by this document
Related documents
Acronyms
List of tables
List of figures
General definitions
The “CCI product family” Climate Data Record (CDR) consists of two parts. The ATSR2-AATSR Surface Radiation Budget CDR is formed by a TCDR brokered from the ESA Cloud_cci project and an ICDR derived from the SLSTR on board of Sentinel-3. ICDR uses the same processing and infrastructure as the TCDR. Both TCDR and ICDR data have been produced by STFC RAL space.
These Earth Radiation Budget datasets from polar orbiting satellites consist of two main variables: Outgoing Longwave Radiation (OLR): The outgoing longwave flux, measured at the top of the atmosphere.
Reflected Solar Radiation (RSR): The reflected solar flux, measured at the top of the atmosphere.
Bias (accuracy): Mean difference between TCDR/ICDR and reference data
\( b=\frac{\sum_{i=1}^N (p_i - r_i)}{N} \ \ (Eq. 1) \)
Where: pi is the CDR product, b is the mean bias and ri is the equivalent value from the reference dataset. N is the number of observations.
bc-RMSE (precision): Bias corrected root mean squared error to express the precision of TCDR/ICDR compared to a reference data record
\( bc- RMSE=\sqrt{\frac{\sum_{i=1}^N ((p-b)-r)^2}{N}} \ \ (Eq. 2) \)
Where: pi is the CDR product, b is the mean bias and ri is the equivalent value from the reference dataset. N is the number of observations.
Stability: The variation of the bias over a multi-annual time period
Table 1: Summary of variables and definitions
Variables | Abbreviation | Definition |
Outgoing longwave radiation | OLR | Net total thermal radiation emitted by the Earth, as measured at the top of atmosphere. |
Reflected solar flux | RSF | Net total outgoing shortwave (UV, visible, near-IR) radiation at the top of atmosphere. This is dominated by reflected and scattered solar radiation. |
Table 2: Definition of processing levels
Processing level | Definition |
Level-1b | The full-resolution geolocated radiometric measurements (for each view and each channel), rebinned onto a regular spatial grid. |
Level-2 (L2) | Retrieved cloud variables at full input data resolution, thus with the same resolution and location as the sensor measurements (Level-1b). |
Level-3C (L3C) | Cloud properties of Level-2 orbits of one single sensor combined (averaged) on a global spatial grid. Both daily and monthly products provided through C3S are Level-3C. |
Table 3: Definition of various technical terms used in the document
Jargon | Definition |
Brokered product | The C3S Climate Data Store (CDS) provides both data produced specifically for C3S and so-called brokered products. The latter are existing products produced under an independent programme or project which are made available through the CDS. |
Climate Data Store (CDS) | The front-end and delivery mechanism for data made available through C3S. |
Retrieval | A numerical data analysis scheme which uses some form of mathematical inversion to derive physical properties from some form of measurement. In this case, the derivation of cloud properties from satellite measured radiances. |
Forward model | A deterministic model which predicts the measurements made of a system, given its physical properties. The forward model is the function which is mathematically inverted by a retrieval scheme. In this case, the forward model predicts the radiances measured by a satellite instrument as a function of atmospheric and surface state, and cloud properties. |
TCDR | It is a consistently-processed time series of a geophysical variable 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. |
CDR | A Climate Data Record (CDR) is defined as a time series of measurements with sufficient length, consistency, and continuity to determine climate variability and change. |
Scope of the document
This document provides a description of the product validation methodology for the Climate Data record (CDR) of the Essential Climate Variable (ECV) Earth Radiation Budget. This CDR comprises inputs from two sources: (i) brokered products from the Cloud Climate Change Initiative (ESA’s Cloud_cci), namely those coming from processing of the Advanced Along-Track Scanning Radiometer (A)ATSR) data and (ii) those produced under this contract for the Climate Data Store, specifically those coming from processing of the Sea and Land Surface Temperature Radiometers (SLSTR).
The Thematic Climate Data Record (TCDR) is the product brokered from the European Space Agency Cloud Climate Change Initiative (ESA’s Cloud_cci) ATSR2-AATSR version 3.0 (Level-3C) dataset. This is produced by the Science and Technology Facilities Council (STFC), RAL Space from the second Along-Track Scanning Radiometer (ATSR-2) on board the second European Remote Sensing Satellite (ERS-2) which spanned the period 1995-2003 and the Advanced ATSR (AATSR) on board ENVISAT which spanned the period 2002-2012.
The Interim Climate Data Record (ICDR) is the product derived from SLSTR on board of Sentinel-3 and spans the period from January 2017 to present. Validation of this SLSTR derived product for the period from January 2017 to December 2021 is described in this document.
This document summarizes and refers to the methodology presented in the Cloud_cci Product Validation and Intercomparison Report [D1], used for the validation of the TCDR product. The same methodology is applied to the ICDR dataset.
Executive Summary
The ESA Climate Change Initiative (CCI) Earth Radiation Budget Data Record (TCDR) is a brokered product from the ESA Cloud_cci project, while the extension Interim CDR (ICDR) produced from the Sea and Land Surface Temperature Radiometer (SLSTR) is produced specifically for C3S. The product is generated by STFC RAL Space, using the Community Cloud for Climate (CC4CL) processor, based on the Optimal Retrieval of Aerosol and Cloud (ORAC) algorithm.
The Cloud_cci dataset comprises 17 years (1995-2012) of satellite-based measurements derived from the Along Track Scanning Radiometers (ATSR-2 and AATSR) onboard the ESA second European Research Satellite (ERS-2) and ENVISAT. This TCDR is partnered with the ICDR produced from the Sentinel-3A SLSTR, beginning in 2017, and Sentinel-3B SLSTR beginning in October 2018. In addition to individual products from each Sentinel-3 platform, a combined product that averages data from both SLSTR instruments into single monthly means will also be provided.
The TCDR and ICDR provide level-3 data (monthly means) on a regular global latitude-longitude grid (with a resolution of 0.5°´ 0.5°) and include these products: Outgoing Longwave Radiation (OLR) and Reflected Solar radiation Flux (RSF) at TOA.
This document is divided in different sessions:
- the first section presents a brief description of the Earth radiation CDR products together with reference for further information;
- the second section presents the datasets used to estimate the accuracy of the CDR surface radiation dataset;
- the third section presents the methodology used for the validation with Clouds and Earth Radiation Energy System (CERES) top of atmosphere data.
1. Validated products
The Earth Radiation Budget dataset from polar orbiting satellites consists of: Outgoing Longwave Radiation (OLR) and Reflected Solar radiation Flux (RSF).
The Cloud_cci Earth Radiation Budget CDR is formed by a TCDR, derived from ATSR2-AATSR, brokered from the ESA Cloud_cci project and an ICDR derived from the SLSTR on board of Sentinel-3.
The SLSTR ICDR, both from the individual instruments (version 3.0 and 3.1) and combining both in a single product (version 4.0), is supplied to the CDS via the same route and uses the same processing software and infrastructure as the TCDR. The retrieval algorithm is described in detail in [D2].
The datasets cover the period from June 1995 to April 2012 (TCDR) of satellite-based measurements derived from ATSR2 and AATSR onboard the polar orbiting ERS-2 and ENVISAT respectively, and the period from January 2017 onwards using the SLSTR measurements (ICDR), with Sentinel-3b and combined data becoming available from October 2018. These are level 3 products (monthly means) on a regular global latitude-longitude grid (0.5° x 0.5° resolution). Table 1-1 report the values from [D4]
ESA’s Cloud_cci dataset version 3 on clouds is the Climate Data Record used for the estimation of the Earth Radiation Budget1.
The SLSTR based ICDR extends the coverage, with a five year gap, from 2017 onwards and is only available through Copernicus Climate Data Store (CDS).
The TCDR dataset that includes Surface Radiation Budget products as well as Cloud Properties and Earth Radiation Budget products are described by Poulsen et al. (2019) [D3].
Table 1-1: Summary of the TCDR and preliminary assessment of ICDR accuracy of the Earth Radiation Budget dataset.
Product name | Accuracy [W/m2] | Accuracy for the ICDR – SLSTR-A [W/m2] | Accuracy for the ICDR – SLSTR-B [W/m2] |
Outgoing Longwave Radiation (OLR) | 1.72 | -1.7 | -1.7 |
Reflected Solar radiation Flux (RSF) | 5.72 | 4.9 | 3.2 |
2. Description of validating datasets
The Earth Radiation Budget TCDR and ICDR datasets are compared against Clouds and Earth Radiation Energy System (CERES) Energy Balanced and Filled (EBAF) fluxes Edition 4.1 Top of atmosphere (TOA) fluxes Edition (Loeb et al., 2018)2.
Both, TCDR and ICDR from SLSTR instruments, are validated with the Clouds and Earth Radiation Energy System (CERES) Energy Balanced and Filled (EBAF) fluxes Edition 4.1 Top of atmosphere (TOA) fluxes Edition (Loeb et al., 2018)3.
The CERES product provides long-term shortwave (SW) and longwave (LW) TOA fluxes for all- and clear-sky conditions. The CERES instruments fly on the Terra and Aqua satellites and cover a period from March 2000 to June 2002 for Terra only, and cover combined Terra and Aqua observations from July 2002 to January 2017. The CERES instruments provide global coverage daily, and monthly mean regional fluxes are based upon daily samples over the entire globe.
More detail on the datasets used for the validation are described in [D1] section 2.4.2 and Annex A.6.
3. Description of product validation methodology
The evaluation strategy applied to the TCDR is described in [D1] section 2.4.
The methodology uses the bias between the Cloud_cci product and the reference data to estimate the accuracy of the dataset.
The bias corrected root mean squared error (bc-RMSE) is used to express the precision of CDR compared to the reference data record, this is also referred to as standard deviation about the mean.
In all cases, the same validation approach will be applied to the combined SLSTR product (version 4.0) as is used for the individual platform SLSTR data (version 3.0 and 3.1).
The Product Validation and Intercomparison Report [D1] is the document that includes validation and intercomparison of the TCDR versus the CERES satellite dataset.
3.1 Comparison with CERES satellite data
TCDR and reference datasets are compared by calculation of multi-annual mean and standard deviation for a common time period (2003-2011). The same methodology is applied to ICDR data, covering the period from January 2017 to December 2021.
Global maps of monthly Outgoing Longwave Radiation (OLR) and Reflected Solar radiation Flux (RSF) are computed for the TCDR, ICDR and the reference dataset. The scores (bias and bc-RMSE) are calculated by including all valid data points pairwise in the CERES and the TCDR/ICDR datasets.
The validation methodology for Outgoing Longwave Radiation (OLR) and Reflected Solar radiation Flux (RSF) at the TOA with the CERES dataset is described in section 3.3.1, 5.1 and 5.2 of [D1].
The stability of the TCDR dataset is defined as the variation of the bias over a multi-annual time period. It is obtained by calculating the linear trend of the bias between the TCDR and reference dataset (in this case CERES dataset).
4. Summary of validation results
The validation results for the TCDR are provided in [D1], sections 3.3.1, 5.1 and 5.2. Section 7 of [D1] provides a summary of the validation results together with recommendations for use. Validation of Cloud_cci (TCDR) radiation products with CERES present a bias of 5.72 W/m² and a standard deviation of 1.64 W/m² for RSF, Bias of 1.72 W/m² and standard deviation of 1.12 W/m² for OLR. The TCDR RSF presents a stability of -0.15 W/m²/decade and +0.52 W/m²/decade for OLR.
Figure 4-1 and 4-2 (from [D4]) show an example of the ICDR monthly products for March 2017 and the equivalent monthly product from CERES.
Figure 4-1. RSF and OLR from SLSTR (ICDR dataset) for March 2017.
Figure 4-2. RSF and OLR from CERES dataset for March 2017.
Preliminary validation (using the monthly mean data from January 2017 to December 2021) of ICDR products with CERES present biases consistent with values found through the TCDR validation (4.9 W/m² for RSF and -1.7 W/m² for OLR).
References
Loeb, N.G., Doelling, D.R., Wang, H., Su, W., Nguyen, C., Corbett, J.G., Liang, L., Mitrescu, C., Rose, F.G., and Kato, S.: Clouds and the Earth’s Radiant Energy System (CERES) Energy Balanced and Filled (EBAF) Top-of-Atmosphere (TOA) Edition 4.0 Data Product, J. Climate, 31(2), 895–918, doi:10.1175/JCLI-D-17-0208.1, 2018.
Poulsen, C. A., McGarragh, G. R., Thomas, G. E., Stengel, M., Christensen, M. W., Povey, A. C., Proud, S. R., Carboni, E., Hollmann, R., and Grainger, R. G.: Cloud_cci ATSR-2 and AATSR data set version 3: a 17-year climatology of global cloud and radiation properties, Earth Syst. Sci. Data, 12, 2121–2135, 2020. https://doi.org/10.5194/essd-12-2121-2020
