Contributors: E. Carboni (UKRI-STFC RAL Space), G. Thomas (UKRI-STFC RAL Space)
Issued by: STFC RAL Space / Elisa Carboni
Date: 07/06/2021
Ref: C3S_D312b_Lot1.2.5.11-v3.3_202106_PQAD_CCIEarthRadiation_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
Bias (accuracy): Mean difference between TCDR/ICDR and reference data
bc-RMSE (precision): Bias corrected root mean squared error to express the precision of TCDR/ICDR compared to a reference data record
Stability: The variation of the bias over a multi-annual time period
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. These products are brokered to (in case of (A)ATSR) or produced for the Climate Data Store (in the case of SLSTR) by the Copernicus Climate Change Service (C3S).
The TCDR is a brokered version of ESA’s Cloud_cci ATSR2-AATSR version 3.0 (Level-3C) dataset, produced by STFC RAL Space from the second Along-Track Scanning Radiometer (ATSR-2) on board the second European Remote Sensing Satellite (ERS-2) which spanned 1995-2003 and the Advanced ATSR (AATSR) on board ENVISAT which spanned 2002-2012.
The ICDR is derived from the Sea and Land Surface Temperature Radiometer (SLSTR) on board Sentinel-3 and spans from 2017 to present. The validation for the ICDR presented here is over the period from January 2017 to December 2019.
The retrieval algorithm is presented in [D2] and the validation methodology is presented in the Cloud_cci Product Validation and Intercomparison Report [D1]. The same methodology is applied to SLSTR 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.
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.
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 ATSR2-AATSR Earth Radiation Budget CDR, version 3.0, is brokered to the CDS from the ESA Cloud_cci project and produced by STFC RAL. The SLSTR ICDR, version 3.x, is supplied to the CDS via the same route and uses the same processing software and infrastructure as the TCDR.
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). These are level 3 products (monthly means) on a regular global latitude-longitude grid (0.5° x 0.5° resolution).
ESA’s Cloud_cci dataset version 3 on clouds is the Climate Data Record used for the estimation of the Earth Radiation Budget. (https://doi.org/10.5676/DWD/ESA_Cloud_cci/ATSR2-AATSR/V003)
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).
Poulsen et al. (2019) [D3] is the paper describing the TCDR dataset that includes Earth Radiation Budget as well as cloud products and Surface Radiation Budget products.
The Product Validation and Intercomparison Report [D1] is the document that includes validation and intercomparison of the TCDR versus the CERES satellite dataset.
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). (Data available here: https://ceres.larc.nasa.gov/data/ )
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 is described in [D1] section 2.4. In summary, we use the bias (i.e. mean difference between Cloud_cci and reference data) as the metric for accuracy. The bias corrected root mean squared error (bc-RMSE) is used to express the precision of CDR compared to a reference data record, this is also refer know as standard deviation about the mean.
3.1 Comparison with CERES satellite data
TCDR and reference dataset are compared by means of multi-annual mean and standard deviation all for a common time period (2003-2011). The same methodology is applied to ICDR data, covering the period from January 2017 to December 2019.
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 for TCDR dataset is defined as the variation of the bias over a multi-annual time period. It is obtained calculating the linear trend of the bias between 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. TCDR RSF present a stability of -0.15 W/m²/decade and +0.52 W/m²/decade for OLR.
Preliminary validation (using the monthly mean data from January 2017 to December 2019) of ICDR products with CERES present biases consistent with values found through the TCDR validation (2.6 W/m² for RSF and -2.4 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.
