Contributors: E. Carboni (UKRI-STFC RAL Space), G. Thomas (UKRI-STFC RAL Space)

Issued by: STFC RAL Space / Elisa Carboni

Date: 10/03/2021

Ref: C3S_D312b_Lot1.2.5.12-v3.2_202103_PQAR_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 results for 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 2018.

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.

Poulsen et al. (2019) [D3] is the paper describing the ESA Cloud_cci dataset that includes cloud properties as well as Surface Radiation Budget and Earth Radiation Budget products. This document will mainly refer to the Cloud_cci Product Validation and Intercomparison Report [D1].

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. Table 1 provides a summary of the TCDR accuracies. For the ICDR, an initial validation with CERES (using only the first 24 months of SLSTR data) show biases consistent with the TCDR: with a bias of 2.50 W/m² for RSF and -1.51 W/m² for OLR.

Table 1: Summary of the TCDR and preliminary assessment of ICDR accuracy of the Earth Radiation Budget dataset.

1. Product validation methodology

The validation methodology is described in section 2.4 of [D1]. In summary, we use the bias (mean difference) between the TCDR and reference data as the metric for accuracy. The bias corrected root mean squared error (bc-RMSE) is used to express the precision of the TCDR compared to a reference data record, this is also known as the standard deviation about the mean. For the validation, the CDR dataset is compared with Clouds and Earth Radiation Energy System (CERES) Energy Balanced and Filled (EBAF) fluxes Edition 4.1 Top of atmosphere (TOA) (Loeb et al., 2018) (Data available here: https://ceres.larc.nasa.gov/data/).

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).

The Product Validation and Intercomparison Report [D1] includes the validation and intercomparison of the TCDR Earth Radiation Budget versus the CERES satellite dataset. The same methodology is used for the ICDR.

2. Validation results

The validation results for the TCDR are provided in [D1] section 3 and 5.

Table 1 in the Executive summary provides a summary of the resulting TCDR accuracies.

2.1 TCDR validation with CERES satellite data

The TCDR and reference dataset are compared by calculating multi-annual mean and standard deviation, all for a common time period (2003-2011). Global maps of monthly and multiannual Outgoing Longwave Radiation (OLR) and Reflected Solar radiation Flux (RSF) are computed for the TCDR and reference dataset. The scores (bias and bc-RMSE) are calculated by including all valid data points pairwise in the CERES and the Cloud_cci dataset.

The validation for Outgoing Longwave Radiation (OLR) and Reflected Solar radiation Flux (RSF) at TOA with CERES dataset is described in section 3.3.1, 5.1 and 5.2 of [D1].

Validation of Cloud_cci radiation products with CERES present a bias of 5.72 W/m² and standard deviation of 1.64 W/m² for RSF, and a bias of 1.72 W/m² and standard deviation of 1.12 W/m² for OLR.

General findings:

RSF (from [D1] section 5.1)

• The CDR datasets show very similar patterns to the other Cloud_cci datasets of the global mean RSF. Highest mean RSF is found in the subtropics over land, lowest mean RSF is also found in the subtropics over the ocean.
• RSF show higher temporal variability over land areas.
• The time series plots of RSF show significant seasonal cycles in the global (60°S-60°N) mean with higher values in boreal winter and lower values in boreal summer.

OLR (from [D1] section 5.1)

• Mean global OLR are lowest over Antarctica and highest over the subtropics. Despite visible differences in the spatial resolution, all Cloud_cci datasets show very similar global patterns and comparable mean values.
• Stratocumulus regions are strongly pronounced with the highest TOA upwelling thermal radiation means. In case of the eastern Pacific, a strong gradient between the sea and land surface is noticeable. Stratocumulus regions around Africa and Australia show less differences between land and sea, which is probably due to the different topographic conditions.
• Higher temporal variability is found over land than over the ocean. Subtropical land areas show the largest temporal variabilities, e.g. Southeast Asia. The stratocumulus regions and southern hemispheric storm track region show the lowest temporal variability.
• Time series plots of OLR highlight a significant seasonal cycle in the global (60°S-60°N) mean with maximum values in boreal summer and minimum values in boreal winter.

2.2 ICDR validation with CERES satellite data

The first 24 months of SLSTR products have been compared against CERES following the same methodology as described in [D1]. We estimate the bias, i.e. mean differences, and the monthly mean global average of C3S and the CERES data. To compute the monthly mean global average of both datasets we considered only the valid data between -60 and +60 latitude.

Validation with CERES (using only first 24 months of SLSTR data) shows biases consistent with the TCDR: with biases of 2.50 W/m² for RSF and -1.51 W/m² for OLR.

Figure 1 and 2 show an example of the ICDR monthly products for March 2017 and the equivalent monthly product from CERES.

Figure 1. RSF and OLR from SLSTR (ICDR dataset) for March 2017.

Figure 2. RSF and OLR from CERES dataset for March 2017.

3. Application(s) specific assessment

N/A

4. Compliance with user requirements

There are no direct user requirements for the Earth Radiation Budget defined in the Cloud_cci project. Looking at the GCOS ECV requirements for Earth Radiation Budget

(https://gcos.wmo.int/en/essential-climate-variables/earth-radiation/ecv-requirements) the values for OLR and RSF are 1 W/m² uncertainty, while the TCDR dataset achieves an accuracy of 1.72 W/m² for OLR and 5.72 W/m² for RSF. ICDR preliminary accuracies (estimate with the first 24 months only) are consistent with TCDR accuracy for OLR and slightly lower for RSF.

Table 2 provides an overview of the GCOS requirements for the Earth Radiation Budget and the values achieved by CDS. It should be noted that GCOS requirements are targets and are often not attainable using existing or historical observing systems. The Cloud_cci doesn’t meet the requirement for resolving the diurnal cycle due to the nature of the satellite observations, but exceeds the spatial resolution.

Known limitations [From D1 table 7.1]:

• Higher uncertainties in twilight conditions, especially in the shortwave fluxes, due to limitation in retrieving Cloud Optical Thickness (COT) and Cloud Particle Effective Radius (CER) (input to the radiation calculation) in these condition.
• Partly spare temporal/spatial sampling, partly compensated by introduced diurnal cycles correction
• Somewhat higher uncertainties expected for TOA shortwave fluxes (RSF) for conditions with low clouds frequencies and elevated surface albedo uncertainties.

Table 2: GCOS targets for Earth Radiation Budget ECVs and CDR values.

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.
 

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