Contributors: T. Usedly (DWD)

Issued by: Deutscher Wetterdienst / Tim Usedly

Date: 31/07/2024

Ref: C3S2_D312a_Lot1.1.3.4_202407_PQAD_ECV_SRB_SLSTR_v1.2

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

Table 1: Summary of variables and definitions

Table 2: Definition of processing levels

Table 3: Definition of various technical terms used in the document

Table 4: Definition of statistical measures used in the document

B=\frac{1}{n}*\sum_{i=1}^n (y_i - o_i)

MAD=\frac{1}{n}*\sum_{i=1}^n |y_i - o_i|

SD=\sqrt{\frac{1}{n-1}*\sum_{i=1}^n ((y_i-o_i)-(\bar{y}-\bar{o}))^2}

FRAC=100\frac{\sum_{i=1}^n f_i}{n} \text{with} \begin{cases} f_i=1, & \text{if} & y_i \gt T \\ f_i=0, & \text{if} & y_i \le T \end{cases}

Scope of the document

This document provides a description of the product validation methodology for the Sea and Land Surface Temperature Radiometer (SLSTR) v4.0 based Interim Climate Data Record (ICDR) of the Essential Climate Variable (ECV) Surface Radiation Budget (SRB).

The dataset produced by RAL Space and Brockmann Consult (BC) under the Copernicus Climate Change Service (C3S) program ranges from January 2017 to December 2023 and provides an Interim Climate Data Record (ICDR) to the brokered Thematic Climate Data Record (TCDR) from the European Space Agency Cloud Climate Change Initiative (ESA’s Cloud_cci).

The TCDR is a brokered product based on processing of the (Advanced) Along-Track Scanning Radiometer ((A)TSR) onboard ERS-2 and Envisat that was produced by RAL Space for the ESA Cloud_cci program and ranges from June 1995 to April 2012. Detailed validation methodology and results are presented in the Cloud_cci Product Validation and Intercomparison Report [D1].

The ICDR is derived with a five-year gap from SLSTR onboard the Sentinel-3A and -3B satellites covering 01/2017 – 12/2023. Detailed results are presented in the corresponding Product Quality Assessment Report (PQAR) [D2].

Executive Summary

The Sea and Land Surface Temperature Radiometer onboard Sentinel-3A provides data from 01/2017 on. With the launch of Sentinel-3B in 10/2018 not just individual data but also a merged version of Sentinel-3A/3B is provided (see chapter 1). The merged version (until 12/2023) is validated against measurements from ground stations from the Baseline Surface Radiation Network (BSRN). Depending on the temporal availability and variable up to 37 stations were used to provide best possible global coverage (see chapter 2). In addition to the merged SLSTR version, a second version on a different grid (equal area in addition to equal angle) is provided from 07/2022 to 12/2023 and also validated against the same reference dataset as the equal angle version of SLSTR.

A nearest neighbor technique is used to compare gridded satellite data (the points closest to ground stations) with ground stations. Amongst others the uncertainty metrics Bias and Absolute Bias are calculated and further evaluated against requirements defined by the Global Climate Observing System (GCOS) (see chapter 3).

Overall the SLSTR data mostly do not fulfill the requirements by GCOS (table 1); the values of the absolute bias are 13.05 W/m² (13.60 W/m²) for Surface Incoming Shortwave radiation (SIS) for equal angle grid (equal area grid), 13.36 W/m² (15.73 W/m²) for Surface Outgoing Longwave Radiation and do meet the threshold requirement (10 W/m²). Biases for Surface Reflected Shortwave Radiation (SRS) are 14.65 W/m² (14.00 W/m²) and Surface Downwelling Longwave Radiation (SDL) also do not meet the requirement due to a bias of 20.56 W/m² (18.40 W/m²). Comparison with the most continental and representative stations meet the threshold requirement by GCOS, outliers are mainly due the stations at high altitudes, high latitudes or Islands (see chapter 4).

1. Validated products

The SLSTR-based dataset provides monthly means on a regular global latitude-longitude grid with 0.5°x0.5° spatial resolution for seven variables of the Essential Climate Variable Surface Radiation Budget (SRB): Surface Incoming Shortwave Radiation (SIS), Surface Reflected Shortwave Radiation (SRS), Surface Outgoing Longwave Radiation (SOL), Surface Downwelling Longwave Radiation (SDL) as well as the Net Fluxes (Surface Net Shortwave (SNS) and Surface Net Longwave Radiation(SNL)) and the overall net Radiation named Surface Radiation Budget (SRB).

The record is generated by RAL Space (data from 01/2017 – 06/2022) and Brockmann Consult (07/2022 – 12/2023) solely for the Climate Data Store (CDS) from the Copernicus Climate Change Service (C3S). The Data are provided for each individual satellite: For Sentinel-3A (S3A) from January 2017 to June 2022 and for Sentinel-3B (S3B) from October 2018 to June 2022. In addition, a merged version is provided from 10/2018 on, when S3B was launched. From 07/2022, Brockmann Consult provides a continuation of the merged version until 12/2023. BC provides data for the merged product for two different grids: (1) regular equal angle global latitude-longitude grid (continuation of previous data) and (2) regular equal area global latitude-longitude grid. The equal area projection uses a sinusoidal raster as aggregation raster for the binning process. A final transformation step maps the monthly aggregates into the plate-carree projection. During this projection, data of the sinusoidal raster close to the poles is repeatedly mapped to several plate-carree cells until the angle extension matches the ground extension in kilometers; the measurement data are not altered in this case.

An overview of the various data producers, satellites, grids and time coverages for the ICDRs is shown in Figure 1-1.

Figure 1-1: Overview of data producers, satellites, time coverages and grids for the ICDRs. Products above/below the black line are produced by RAL Space/Brockmann Consult (BC); the data generated from 07/2022 on are provided in two different grids.

The retrieval algorithm is described in detail in [D4].

The PQAD and PQAR cover the merged product for the time periods 10/2018 – 12/2023 on the equal angle grid and 07/2022 – 12/2023 on the equal area grid respectively.

2. Description of validating datasets

The SLSTR ICDR is validated for SIS, SRS, SOL and SDL against a wide range of measurements from ground stations of the Baseline Surface Radiation Network (BSRN). The World Radiation Monitoring Center (WRMC) is the central archive of the BSRN aiming to provide data on short- and longwave radiation fluxes, to e.g. monitor radiative components and their changes, and validate/evaluate satellite-based data. The BSRN provides quality-controlled surface radiation measurements at globally distributed ground stations available in, amongst others, monthly means (Ohmura et al., 1998). A list of stations with geographical information is provided Table 2-1. A global distribution of the stations is provided in Figure 2-1.

Figure 2-1: Location of all used BSRN ground stations

Data is downloaded at the WRMC-BSRN website: https://bsrn.awi.de/

Table 2-1: List of station from the BSRN used for the validation with information on latitude, longitude, altitude and temporal availability

3. Description of product validation methodology

The validation methodology is separated into three parts: Data preparation and application of methodology to compare a gridded satellite base dataset with ground stations (section 3.1), Validation (section 3.2) against available ground stations and Evaluation (section 3.3) against requirements defined by Global Climate Observing System (GCOS).

3.1 Data preparation

SLSTR data is provided at a regular latitude-longitude grid with 0.5°x0.5° spatial resolution and monthly means, and the validation is based on the comparison with monthly means of available surface measurements. A nearest neighbor technique is used forthe comparison, using the ground stations coordinates to extract the nearest grid point of the gridded dataset to calculate the bias and further statistical measures.

The maximum available number of months per stations for the SLSTR data is 63 (10/2018 – 12/2023) and 18 (07/2022 – 12/2023) on the equal angle and on the equal area grid respectively. A criteria is defined that at least 15 months should be available of the BSRN data for comparison with the equal angle grid version.

3.2 Validation

he following uncertainty metrics are calculated: Bias, Mean Absolute Difference (also called absolute bias), Standard Deviation and Fraction of months outside the validation target values (see definitions in General definitions section).

The corresponding Product Quality Assessment Report (PQAR) [D2] provides results on bias and absolute bias for each variable and station as well as maps with stations in green/red whether they meet/don’t meet the threshold requirement by GCOS. In addition, a correlation of SLSTR and BSRN data for all months is provided with metrics on correlation coefficient.

3.3 Evaluation

The Absolute Bias is used as evaluation against the requirements defined by the Global Climate Observing System (GCOS) in The 2022 GCOS ECVs Requirements (GCOS 245) [D3].

GCOS defines three requirements depending on users needs:

• Goal (G): The strictest requirement, indicating no further improvements necessary

• Breakthrough (B): Intermediate level between threshold and goal. Breakthrough indicates that it is recommended for certain climate monitoring activities

• Threshold (T): Minimum requirement

It should be mentioned, that these requirements are rather intended towards potentials and resolutions of climate models. Thus, GOCS requirements are not identical to the users needs outside the climate modelling community. Also, they are often not attainable using existing or historical observing systems.

Table 3-1 names the requirements for horizontal and temporal resolution as well as accuracy for SIS, SDL and SOL

Table 3-1: Summary of requirements for SIS, SDL and SOL based on GCOS [D3]

4. Summary of validation results

A brief summary of the validation results is provided in sections 4.1 to 4.4. Figures 4-1 to 4-4 show bias and absolute bias for each station and variable. Section 4.5 shows the evaluation results compared to the GCOS requirements. Detailed results can be found in the corresponding Product Quality Assessment Report (PQAR) [D2].

4.1 Surface Incoming Shortwave Radiation

Figure 4-1: Bias (black) and absolute bias (white) for SIS between satellite-based SLSTR data and ground stations of the BSRN. The green area marks the threshold accuracy defined by GCOS.

Figure 4-1 shows bias (black dots) and absolute bias (white triangle) for SLSTR data compared to every available ground station. Most of the station’s biases are within the threshold requirement (10 W/m²). Outliers are due to stations with high altitude (Boulder, Izana, Sonnblick, Yushan) or stations on islands (Lanyu Island).

4.2 Surface Reflected Shortwave Radiation

Figure 4-2: Bias (black) and absolute bias (white) for SRS between satellite-based SLSTR data and ground stations of the BSRN. The green area marks the threshold accuracy defined by GCOS.

Figure 4-2 shows proportion wise more outliers for SRS compared to SIS considering the number of available stations. On average a small negative is seen with the same stations outside the 10 W/m² threshold requirement. The proportion of negative values for the bias is higher compared with SIS.

4.3 Surface Downwelling Longwave Radiation

Figure 4-3: Bias (black) and absolute bias (white) for SDL between satellite-based SLSTR data and ground stations of the BSRN. The green area marks the threshold accuracy defined by GCOS.

The comparison with SDL reveals a positive bias for most of the stations (31/37 stations). Most of the station’s biases are outside of 10 W/m² and there are three significant outliers in Izana (Spain, 2373 m), Sonnblick (Austria, 3109 m) and Yushan (Taiwan, 3858 m). These stations are located at higher altitudes compared to the other stations. Concordia Station (3233 m) has the highest negative bias.

4.4 Surface Outgoing Longwave Radiation

Figure 4-4: Bias (black) and absolute bias (white) for SOL between satellite-based SLSTR data and ground stations of the BSRN. The green area marks the threshold accuracy defined by GCOS.

Most of the months/stations tend to have a small negative bias which is compensated by measurements from Georg von Neumeyer and Izana Stations with positive biases.

4.5 Evaluation with GCOS requirements

Table 4-1 summarizes the uncertainty metrics for each variable. Absolute Bias is lowest for SIS, while SDL has the hightest. Most of the biases are positive, except for SRS and SOL on the equal area grid. Between 42-60% of the station months are outside the 10 W/m² threshold.

Table 3-1: Summary of requirements for SIS, SDL and SOL based on GCOS [D3]

References

Ohmura, A., et al. (1998), Baseline Surface Radiation Network (BSRN/WCRP): New precision radiometry for climate research, Bulletin of the American Meteorological Society, 79(10), 2115-2136. DOI:https://doi.org/10.1175/1520-0477(1998)079<2115:BSRNBW>2.0.CO;2

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