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_Lot1.2.2.5-v3.3_202106_PQAD_CCISurfaceRadiationBudget_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
Scope of the document
This document provides a description of the product validation methodology for the Essential Climate Variable (ECV) Surface 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).
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) spanned 1995-2003, the Advanced ATSR (AATSR) on board ENVISAT spanned 2002-2012 and SLSTR on board of Sentinel-3 span from 2017 to present (with a validation period from January/2017 – December/2019 in this version of the document). In general, the validation methodology refers to the Cloud_cci Product Validation and Intercomparison Report [D1]. The same methodology is applied to the SLSTR dataset.
Executive Summary
The ESA Climate Change Initiative (CCI) Surface Radiation Budget Climate Data Record (CDR) 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 Surface Radiation Budget is a product of the Broadband Radiative Flux Retrieval (BRFR) module of CC4CL, which uses the cloud properties produced by ORAC to compute broadband radiative flux values.
The Cloud_cci record 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 satellites. This CDR is partnered with the ICDR produced from the Sentinel-3A SLSTR, beginning in 2017, and Sentinel-3B SLSTR beginning in October 2018. The dataset encompasses level-3 data (monthly means) on a regular global latitude-longitude grid (with a resolution of 0.5° X 0.5°) and includes these products: the Surface Incoming and Reflected Shortwave radiation (SIS and SRS respectively), the Surface Downwelling and Outgoing Longwave radiation (SDL and SOL respectively), the Surface Net Shortwave and Longwave radiation (SNS and SNL), and the total Surface Radiation Budget (SRB).
1. Validated products
The Cloud_cci ATSR2-AATSR Surface Radiation Budget CDR, version 3.0, is brokered to the CDS by the C3S 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 CDR.
These Surface Radiation Budget datasets from polar orbiting satellites consist of: Surface Incoming Shortwave radiation (SIS), Surface Reflected Shortwave radiation (SRS), the Surface Net Shortwave radiation (SNS), the Surface Outgoing Longwave radiation (SOL), Surface Downwelling Longwave radiation (SDL), Surface Net Longwave radiation (SNL), and the Surface Radiation Budget (SRB).
The datasets cover the period from June 1995 to April 2012 (TCDR), using 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 (with 0.5° x 0.5° resolution). Cloud properties from the ESA Cloud_cci dataset version 3 (TCDR) are used for the estimation of the Surface Radiation Budget (DOI: https://doi.org/10.5676/DWD/ESA_Cloud_cci/ATSR2-AATSR/V003). The Cloud_cci dataset can be downloaded here: https://climate.esa.int/en/projects/cloud/data/. The SLSTR based ICDR extends the coverage, with a five year gap, from 2017 onwards and is only available through the Copernicus Climate Data Store (CDS).
Poulsen et al. (2019) [D3] is the paper describing the TCDR dataset that includes cloud products as well as Surface Radiation Budget and Earth Radiation Budget products.
The Product Validation and Intercomparison Report [D1] shows the comparison of SIS and SDL against CERES and BSRN.
2. Description of validating datasets
The Surface Radiation Budget TCDR dataset from ATSR2 and AATSR instruments is compared against the ground measurements dataset: the central archive of the Baseline Surface Radiation Network (BSRN) (Data available here: https://bsrn.awi.de/).
Both, TCDR and ICDR from SLSTR instruments, are compared with the Clouds and Earth Radiation Energy System (CERES) Energy Balanced and Filled (EBAF) fluxes Edition 4.1 Top of atmosphere (TOA) and Bottom of Atmosphere (BOA) fluxes Edition (Loeb et al., 2018). (Data available here: https://ceres.larc.nasa.gov/data/).
3. Description of product validation methodology
Validation strategy is described in section 2.4 of [D1].
In summary we use the bias (mean difference between CDR 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, which is also known as the standard deviation from the mean.
The SIS and SDL products of the TCDR dataset are validated against ground measurements and compared with the CERES satellite dataset in [D1]. Ground measurements are considered to be more accurate than satellite measurements and consequently the reported accuracy of these parameters are determined in this way.
The SIS and SDL products of the ICDR dataset are evaluated by a comparison with the CERES dataset and the evaluation is performed within the C3S project.
The SRS, SNS SNL and SRB accuracies for both TCDR and ICDR are estimated by uncertainty propagation as explained below. Table 1 summarizes the methodology used to estimate the accuracies for each product.
Table 1: Summary of methodologies used to estimate the accuracies, for TCDR and ICDR datasets.
Product name | Validation with BSRN | Comparison with CERES | Uncertainty propagation |
Surface Incoming Shortwave radiation (SIS) | TCDR | TCDR and ICDR | |
Surface Reflected Shortwave radiation (SRS) | ICDR | TCDR and ICDR | |
Surface Net Shortwave radiation (SNS) | TCDR and ICDR | ||
Surface Outgoing Longwave radiation (SOL) | TCDR and ICDR | ||
Surface Downwelling Longwave radiation (SDL) | TCDR | TCDR and ICDR | |
Surface Net Longwave radiation (SNL) | TCDR and ICDR | ||
Surface Radiation Budget (SRB) | TCDR and ICDR |
3.1 Validation with BSRN ground base radiative flux
BSRN stations measure direct, diffuse and global downwelling shortwave and longwave fluxes in 1 min temporal resolution. The 1-minute data were aggregated to monthly averages which were used as validation data. Using the TCDR and the reference datasets (in different locations around the world) we compute the bias and standard deviation.
The validation method for Surface Incoming Shortwave radiation (SIS) and Surface Downwelling Longwave radiation (SDL) with BSRN ground measurements is described in sections 2.4 and 3.3.2 of [D1].
3.2 Comparison with CERES satellite data
TCDR and reference datasets are compared by means of multi-annual mean and standard deviation for the common time period (2003-2011). For the ICDR, we used the CERES dataset as a reference and compared the means of multi-monthly mean as well as the standard deviation for the time period 2017-01 to 2019-12. Global maps of multiannual Surface Incoming Shortwave radiation (SIS) and Surface Downwelling Longwave radiation (SDL) are computed for the CDR and the reference dataset. The scores (bias and bc-RMSE) are calculated by including all valid data points pairwise in the CERES dataset and the CDR. The same methodology is applied for TCDR in [D1] and ICDR within C3S.
The validation method for Surface Incoming Shortwave radiation (SIS) and Surface Downwelling Longwave radiation (SDL) with CERES is described in section 5.3 and 5.4 of [D1]. The same methodology will be used to estimate the accuracy of SOL in comparison with CERES.
3.3 Surface Reflected Shortwave Radiation (SRS)
The accuracy of SRS is estimated from the accuracy of Surface Incoming Shortwave radiation (SIS) and Surface albedo (SAL). Applying the error propagation, the accuracy of the SRS product can be estimated as:
\[ \Delta SRS= \frac{\delta SRS}{\delta SIS} \Delta SIS + \frac{\delta SRS}{\delta SAL} \Delta SAL = SAL \Delta SIS + SIS \Delta SAL, \quad (Eq. 1) \]Where \( \Delta SIS \) comes from [D1] and \( \Delta SAL \) is considered as 25% of the SAL value. SAL is estimated as the ratio between Surface Reflected Shortwave radiation (SRS) and Surface Incoming Shortwave radiation (SIS).
\[ SAL = SRS / SIS, \quad (Eq. 2) \]3.4 Surface Net Shortwave Radiation (SNS)
The Surface Net Shortwave radiation (SNS) is calculated using:
\[ SNS = SIS - SRS, \quad (Eq. 3) \]And the accuracy will be estimated as:
\[ \Delta SNS = \Delta SIS + \Delta SRS, \quad (Eq. 4) \]3.5 Surface Net Longwave Radiation (SNL)
Surface Net Longwave radiation (SNL) is calculated [D2] from:
\[ SNL = SDL - SOL, \quad (Eq. 5) \]The accuracy \( \Delta SNL \) will be estimated as:
\[ \Delta SNL = \Delta SDL + \Delta SOL, \quad (Eq. 6) \]3.6 Surface Radiation Budget (SRB)
The total Surface Radiation Budget (SRB) is simply the sum of the short and longwave contributions:
\[ SRB = SNS + SNL, \quad (Eq. 7) \]The accuracy will be estimated as:
\[ \Delta SRB = \Delta SNS + \Delta SNL, \quad (Eq. 8) \]4. Summary of validation results
The TCDR validation results are provided in [D1], section 3.3.2, 5.3 and 5.4. Validation of BOA fluxes against BSRN stations present standard deviations of 24 W/m² and bias of 8.2 W/m² for Surface Incoming Shortwave radiation (SIS) and standard deviation of 14 W/m² and bias of 11.9 W/m² for Surface Downwelling Longwave radiation (SDL). The intercomparison of Cloud_cci radiation products with CERES present a bias of 1.53 W/m², standard deviation of 3.18 W/m² and stability of 0.97 W/m2/decade for SIS. Bias of 10.17 W/m², standard deviation of 1.2 W/m² and stability of 2.8 W/m2/decade for SDL. Intercomparison (using the monthly mean data from January 2017 to December 2019) of ICDR products with CERES showed biases consistent with TCDR and are 2.3 W/m² for SIS, -2.0 W/m² for SRS, 3.1 W/m² for SOL and 9.9 W/m² for SDL.
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, https://doi.org/10.1175/JCLI-D-17-0208.1.
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.
Stengel, M., Stapelberg, S., Schlundt, C., Karlsson, K.-G., Meirink, J.F., Poulsen, C., Bojanowski, J., and Stöckli, R.: Cloud_cci Product validation and Intercomparison Report, http://www.esa-cloud-cci.org/sites/default/files/upload/Cloud_cci_D4.1_PVIR_v6.1.pdf.
